Fourfold success - Twenty years of Audi quattro
Twenty years ago Audi launched a vehicle whose name has become synonymous with an unique success story: the Audi quattro, first exhibited on March 3rd, 1980 at the Geneva Motor Show, carried off innumerable victories in rallies across the globe. Later generations of the legendary "original quattro" also went from victory to victory on the racetrack.
However, it wasn't only in the area of motorsport that the Audi quattro's debut signalled the breakthrough of a new idea. The permanent four-wheel drive affirmed its supremacy in series production too, demonstrating its lead in traction, safety and everyday driving enjoyment.
In the Audi quattro all four wheels are driven permanently. The advantage: in the ideal case each wheel needs no more than half the propulsive power required in a conventional single-axle driveline. Because the total force transmitted by each wheel - the aggregate of propulsive and lateral locating force - is limited, an Audi quattro can transfer a correspondingly greater lateral locating force to the road surface: a necessity for guaranteeing excellent cornering stability and outstanding straight-line running on both wet and dry surfaces.
Of course, an Audi quattro is particularly capable of demonstrating its traction advantage on wet roads, gravel and snow, on slippery surfaces or on partially iced roads. Conventional cars fitted with purely electronic control systems can also prevent the driven wheels from spinning. However, this method works only be reducing the propulsive force.
Not so in the Audi quattro, where the self-locking Torsen centre differential ensures that the engine's power is distributed uniformly to front and rear wheels. Any sudden differences in speed - for example if only the front axle hits a patch of black ice - sees the system responding like lightning: up to 75 percent of torque is then redirected to the wheels turning more slowly - in this case, the rear wheels. Torsen is a purely mechanical system.
In transverse-engined cars - such as the A3, S3 and the TT- a special variation on the quattro technology has been used since the autumn of 1998. The Haldex electro-hydraulic clutch responds particularly quickly to changes in traction conditions and ensures that drive distribution is immediately optimised for both front and rear axles.
In 1977 Audi technicians paved the way for the success of the quattro technology. The then chief technician at Audi, Dr. Ferdinand Piëch - today chairman of Volkswagen AG's Board of Management - suggested building a high-performance sports car based on the Audi 80 floor pan. The basic version of the 200 bhp five-cylinder turbo engine powered the Audi 200, the basic body was taken from the Audi 80 Coupé. This mixture saw Audi entering uncharted waters: up till that point in time four-wheel drive vehicles were almost as clumsy, high off the road and inelegant as commercial vehicles negotiating rough terrain.
From the very start the Audi engineers opted for permanent four-wheel drive. The driver doesn't have the bother of switching into this mode, but has optimum traction available at all times should an unforeseen critical situation arise. The special trick in the technological operating principle for the quattro: a hollow-shaft within the transfer case fulfils both the task of power distribution and compensation between the axles.
From 1984 onwards the driver team at Audi was joined by the German Walter Röhrl; in his debut event, the Regensburg-born double world champion took first place in the Monte Carlo rally. The Audi Team ended the season with a double title: alongside the constructors' world championship the drivers' championship went to Stig Blomqvist. As in the previous year various national titles accompanied this success.
The Sport quattro version - with shortened wheelbase - and the martial-looking, quattro S1 with spoiler continued the "four rings" victory trail on the world's rally circuits. Among the most spectacular successes were without doubt the multiple victories chalked up in the legendary hill-climb race held on Pikes Peak in Colorado, USA.
The bowl tracks of the North American championships also played host to Audi quattro triumphs. Participation in the Trans Am STC championship came to a end in 1988 with eight individual victories, the drivers' and the constructors' titles for the Audi 200 quattro saloon. At the wheel - as well as US drivers - were top German drivers Walter Röhrl and Hans-Joachim Stuck.
These two drivers also drove the amazingly powerful Audi V8 quattro cars which were entered for the German Touring Car championship in 1990. Once again the record proved impressive: eight victories and Stuck won the driver's championship. A chapter in the success story that is a long way from being over, as the title victory in 1999 by the A4 quattro driven by Christian Abt amply demonstrates.
Even more impressive than the sporting triumphs is the quattro principle's success in series production models. More than 820,000 quattro cars have been built so far. 66 model versions with quattro drive are available in the current Audi model programme, powered by four, six and eight-cylinder, spark-ignition and TDI engines. The S and RS models - the most powerful and sporting Audi model variants - are equipped with this driveline too. It's no surprise then that every fourth Audi now sold is a quattro.
Audi 80/90 Quattro, Audi UrQuattro 1981-1987
-First Quattro generation. Permanent all wheel drive with free differentials.
Manually (2 rotating switches on console) locking rear and center differentials.
Audi 80/90/100 Quattro 1987-1994 - Quattro II.
Permanent all wheel drive. Torsen center differential with automatic
distribution of torque between front and rear axles. Manually activated
(electro-pneumatic system) rear locking differential which automatically unlocks
when speed exceeds 25 km/h
Audi V8/A8 Automatic 1989-1994 - Quattro III - torque distribution via electronically controlled multi-plate hydraulic transfer clutch in the center. Torsen differential in the rear axle. Quattro was for the first time used with automatic transmission. The gearbox is a good source of high pressure oil, that can be used to activate the clutch.
Until the all wheel drive Audi TT arrived the Quattro abbreviation meant the same Awd system regardless which car carried the badge. Now Quattro is just a trademark and there is a lot of confusion. Audi TT Quattro used Haldex Awd system, Audi A3 Quattro uses viscous coupling system similar to Awd system used on VW Golf Syncro. But there is "real" Quattro system used on VW Passat 4motion, although adjusted in some other way, as the representatives of V.A.G. said. But this new beautiful name is nothing but another trademark. VW Golf IV 4motion uses Haldex system.
Audi A3/S3 Quattro, Audi TT Quattro - Haldex Awd-system, Electronically controlled Haldex multi-plate hydraulic clutch, situated near the rear differential. Normally front wheel drive vehicle. It detects the need of 4wd at 1/8 of the wheel spin.
Audi A4/A6/A8/S4/S6/Allroad Quattro 1994-... with
manual transmission - Quattro IV - Torsen center differential. Electronic
traction control (EDL) on front and rear axles that applies brakes to spinning
wheels, which has effect of locking front and rear differentials. Works at
speeds up to 40 kph (25mph). On more powerful versions S4/S6 - up to 80 kph
(50mph). A8 uses EDL and ASR, which reduces engine output. It is almost
impossible to lose traction with this system under any circumstances.
Allroad can feature low gearing (optional)
X5 - full-time all wheel drive with free differentials. 38 front / 62 rear torque distribution and ADB-X (Automatic Differential Brake) electronic traction control performs functions of locking differentials. Hill Descent Control (HDC)
330iX 2000y. - same as X5, 32/68 torque distribution in normal conditions. ADB-X traction control.
Escalade - Based on Tahoe/Yukon/Denali platform. NVG 246 "AutoTrac" four wheel drive system. 2Hi, Auto 4wd (100% of torque is transferred to rear wheels until it detects a need for extra traction. Dry multi-plate clutch is used to transfer torque to the front), 4Hi (50/50 split), 4Lo, all button activated. 2.72:1 low range gear.
Tracker (Suzuki Vitara) - Lever-operated
two.speed part-time transfer case. 1.82:1 low gear. From 1999- new
vacuum-actuated front axle disconnect, which eliminates backing-up to disengage
the 4wd system (the front hubs)
Corporate IFS open differential axle up front, corporate rear live axle equipped
with limited slip (open differential rear on Tracker).
Tahoe - NVG 246 "AutoTrac" four wheel drive
system. 2Hi, Auto 4wd (100% of torque is transferred to rear wheels until it
detects a need for extra traction. Dry multi-plate clutch is used to transfer
torque to the front), 4Hi (50/50 split), 4Lo, all button activated. 2.72:1 low
range gear.(3,73 optional).
GM's 10 bolt IFS front with Central Axle Disconnect (CAD)
GM's corporate 10-bolt live axle with g80 limited slip (opt) and 3.73:1 gears.
Silverado - 2 versions can be used: manually actuated part-time 4wd or "AutoTrac" (See Tahoe)
Sierra XR 4x4 - full-time all wheel drive, 40% front / 60% rear power distribution.
HRV - normally front wheel drive vehicle. Torque transfer to rear via electronically controlled multi-plate clutch.
Transfer case:
All HUMMERs use a New Process Gear (NPG) 242 2sp transfer case. It is a full
time transfer case incorporating an open differential between the front and rear
driveshafts (DS). It has 4 modes, High (H), High Locked (HL), Low (locked) (L),
and Neutral (N). The ratios are 1:1 in high, 2.72:1 low.
The input shaft and rear output shaft are co-linear. The front is a left handed
output, chain driven.
In H, the normal operating position, torque is evenly split front and rear via
the transfer case's differential. This center differential is open and allows
the front and rear driveshafts to turn at different rates to allow turning on
high traction surfaces. In HL, the differential is locked (manual locker),
forcing the front are rear driveshafts to turn at the same rate. This is
equivalent to a part-time 4wd system. The L position forces the input shaft to
turn a planetary gear assembly at the front of the case. The ring gear is
machined into the front case. Low adds a gear reduction of 2.72:1. The
differential is always locked in low. The N position disconnects the input from
the output shafts totally. The differential is open in this position.
A sensor at the rear of the transfer case (TC) is used to determine speed.
Another sensor determines if the TC differential is locked or unlocked,
illuminating a status light.
The TC is cooled via transmission fluid flowing through an intercooler inside
the TC.
The engine, transmission, and transfer case are rigidly connected together. The
assembly is canted slightly to the right.
Differentials
The differential case is a Dana (AMC) 20, hypoid (top entry), same as in some
older Jeeps (AFAIR). The differential has a ratio of 2.73:1. HUMMERs use
Zexel-Gleason Torsen torque sensing/biasing differentials. These are not a
limited slip design or locking design. The bias ratio is 3.8:1 AFAIK. Since the
suspension is independent, the differential case is connected to the frame.
(Taken from
Gerald's
HUMMER Page)
Vehicross - TorqueOnDemand (tm) system, developed by Borg-Warner. Full time four wheel drive system, distribution of torque to the front wheels is infinitely variable from 0 to 50%. Computer-controlled we multi-plate clutch works as quickly, as in 20 ms. 2,48:1 low gear. Limited slip differential rear.
Amigo (Opel Frontera) - Lever-operated part-time two-speed transfer case with Neutral position. 4x4 button to engage the front axle (CAD). 2.05:1 low range gear. Front - corporate IFS with open differential. Dana 44 solid rear axle with limited slip.
Trooper 1993 - Part-time transmission. Manual front free hubs, automatic hubs - optional. Rear manually lockable differential - optional. 2,28:1 low gear ratio.
Grand Cherokee 1999 - Quadra-Drive four wheel
drive system (standard on "Limited", optional on "Laredo"). Quadra-Trac II
transfer case with speed-sensing differentials that offer on-demand four wheel
drive capability in any condition, at any speed. The differential is the
hydromechanical system based on geroter oil pump and clutch pack, located within
the transfer gearbox between the driveshafts that carry power to the front and
rear axles. One part of the pump, the rotor, is driven by the front driveshaft
while the other part, it's case, is attached to the rear driveshaft. The system
reacts faster than the viscous coupling unit of the previous Jeep AWD system.
The transfer box can be also locked in low-range mode to provide equal power to
the front and rear axles.
Progressive Vari-Lok gerodisc differentials front and rear, using the same
geroter pump system. Almost 100% of torque can be transferred to any wheel.
NVG 247 AWD transfer case with lever-operated 2.72:1 low range gear.
Dana 30 non-disconnect live axle at front, aluminum central section Dana 44 at
rear.
Grand Cherokee 1993 - Full-time four wheel drive, automatic torque distribution via viscous coupling in the center. Automatic differential lock rear (????). 2,72:1 low gearing, must be activated while vehicle is stopped.
Sportage - Part-time four wheel drive. WARN vacuum-operated front wheel hubs, similar to Ford Explorer and Ranger. Rear corporate solid axle with a clutch-type limited slip differential.
Niva 1977-... - permanent all wheel drive with free differentials. Low gear. Manually activated (lever) locking center differential.
Diablo VT - permanent all wheel drive with Viscous Coupling Locking Center Differential
Delta HF Integrale 1986-1990 - Full-time all wheel drive, 47 front/ 53 rear power distribution (in normal conditions) via Ferguson central differential and Torsen differential rear.
Discovery II - Permanent four-wheel drive. Four-wheel electronic traction control. Active cornering enhancement system. Hill Descent Control (HDC). Full-floating live axles front and rear. Unlike on the MB ML-class - it needs only 1/4 - 1/2 of wheelspin to detect the need of traction control activation.
Freelander - normally front wheel drive vehicle. Power distribution to rear via viscous coupling.
Series I 1948-1954 - there was a 4wd system without center differential in the beginning, with freewheeling device up front for tire-scrub reduction. From 1950 - dogleg clutch is used to disengage the front axle.
Range Rover 1970-1996 - Full-time all wheel drive. Manually lockable center differential. (50/50)
Range Rover 1998- - Full-time all wheel drive with viscous control unit in the center and traction control (ETC)
RX300 - sold with front only or all-wheel drive. Viscous coupling at center, normally 50/50 torque distribution and up to 95% either axle. Rear limited slip is optional. No low gearing.
Navigator - 2H - Auto4wd - 4H - 4L. Similar to Escalade?????? Locking differential rear (opt.)
Pajero 1982-1991, L200 - Part-time transmission. Rear wheel drive with disengageable front axle. No center differential. Low gear. Some were equipped with limited slip differential in the rear.
Pajero II, Pajero Pinin (1991-) - Super Select
II system featuring:
* 2H mode - vacuum disconnected front front axle. The driveshaft does not
rotate. Switching from 2H to 4H is allowed at speeds up to 100 kph (65 mph).
From 4H to 2H at any speed.
* 4H mode - 33/67% distributed power front/rear, up to 100% to either axle if
slippage occurs. Shifting from 4H to 4HLc is allowed at speeds up to 100 kph (65
mph). From 4HLc to 4H at any speed.
* 4HLc mode - fixed 50/50 torque distribution (locked center differential). Full
stop required to switch to 4LLc and back
* 4LLc mode - fixed 50/50 torque distribution (locked center differential) with
1.9:1 low gear. (Pinin - 1,548:1)
W124 4matic - normally rear wheel drive vehicle. Power is progressively transferred via multi-plate hydraulic transfer clutch to front wheels when slipping occurs. Torque distribution in this case is 65% to 35% rear to front. ABS sensors are used to detect wheelspin. If more traction is necessary, computer locks another clutch in rear axle. If brake pedal pressed, all clutches disengage to allow the ABS to work properly. On takeoff/acceleration the front axle normally engages, proactively, regardless whether wheel slip is detected or not.
ML - permanent all wheel drive with 3 open
differentials. 4ETS electronic traction control, that applies brakes to wheel,
that is about to spin, thus transferring torque to wheels, that have traction.
M-Class 4ETS kicks in up to about 36 MPH (60 km/h) and if engagement conditions
are maintained beyond 60 km/h during acceleration, control is effective to up to
48 MPH (80 km/h). Read a very extensive article
about 4ETS
---
Two-speed AWD variant of the Borg-Warner 44-06 transfer case. Open
differentials. Button-operated low-range locks to 50/50 torque split. (2,64:1
low gear).
G-class - from 1991 - full-time all wheel drive with 3 manually lockable differentials (buttons). Differentials lock after the vehicle has moved some distance. 2.16 low gear ratio.
Patrol GR 1998 - Part-time 4wd system. LSD rear - standard, Locking differential - optional. 2,02:1 low gear.
PickUp 1998 - Part-time 4wd system. LSD rear - standard, Locking differential - optional. 2,02:1 low gear.
Frontera (Isuzu Amigo) 1999 - Lever-operated part-time two-speed transfer case with Neutral position. 4x4 button to engage the front axle (CAD). 2.05:1 low range gear. Front - corporate IFS with open differential. Rear - Dana 44 solid axle with limited slip.
911 Carrera 4 - from 1989- three differential system using computer-controlled hydraulic clutches to help distribute power to the four wheels. From 1994 - mechanical limited slip at rear, viscous coupling in the center. From 1999 - viscous coupling just behind front differential, transferring from 5 to 40 % up front.
959 - permanently locked multi-plate hydraulic clutch in the center. Unlocked only if turning at low speed, for example while parking. Torque distribution in this vehicle is adjusted even before wheelspin occurs. Gathering information from numerous sensors (i.e. g force sensor, accelerator pedal position, speed, and even turbo pressure) while accelerating, computer changes normal 40front/60back torque split, sending up yo 80% to the rear wheels, even if all four wheels are rotating with the same speed.
Seat León,
Seat Alhambra -
Haldex Awd-system
Skoda Octavia 4x4 - Haldex Awd-system
Legacy/Legacy Outback -
With manual transmission: Normally 50/50 front-to-back power split. A mechanical viscous center differential is used to vary the power split between axles when there is a loss of traction.
With automatic transmission: Normally front wheel drive (90/10). Uses traction sensing computer input to actively vary the front-to-back power split via a hydraulic multi-plate transfer clutch.
Shift on the Fly: this refers to an less
refined and effective, older Subaru 4x4 system that allowed the driver to engage
the four wheel drive as the car was moving. Last used on the 1994 Loyale, but
also used on older GL, DL, Subaru, Brat etc. Not currently used on any Subaru.
All are now All-Wheel-Drive. At one time Subaru even offered a hi/lo dual range
4x4 system..
Following text was taken from Subaru official web site.
Couldn't be explained better than this.
"We often refer to the All-Wheel Drive (AWD) capabilities of a Subaru,
but don't specifically mention the type of AWD system used. Because of this,
many Subaru owners assume that the same All-Wheel Drive system is used in all
Subaru models. In fact, two different types of Subaru AWD systems are available
across the model line. The system type is dependent upon which transmission the
vehicle has, manual or automatic. Although the two transmission and All -Wheel
Drive types are different, both systems automatically distribute power to all
four wheels as needed to maintain traction.
For example, if the front wheels start to slip, power is instantly directed to
the rear wheels, and if the rear wheels start to slip, power is instantly
directed to the front wheels. When a vehicle is braking or accelerating, it
causes the weight to shift, thereby reducing traction. Subaru AWD transfers
power from the wheels that slip to the wheels that grip.
Viscous Coupling Locking Center Differential AWD For 5-Speed Manual
Transmissions
In vehicles with the 5-speed manual transmission, the All-Wheel Drive uses a
viscous coupling in a center differential inside the transaxle case. It contains
a series of opposing discs attached to the front and rear output shafts,
surrounded by a silicone fluid. In normal operation, power is distributed
equally between the front and rear wheels (50/50 power split). Loss of traction
at either the front or rear wheels causes a rotational difference between the
front and rear discs in the viscous unit, which then shears the silicone fluid.
The shearing action heats the fluid, causing it to thicken. As the fluid thickens, the discs lock together to transfer power from the slipping wheels to the wheels with the best traction. When traction is regained, all the discs turn at the same speed, restoring the 50/50 power split. The process is quick and unnoticeable to the driver and passengers.
The 5-speed All-Wheel Drive system is simple, compact and virtually invisible in operation. Its traction adds a significant margin of safety on all road surfaces.
What Is A Differential?
A differential is the driving-axle gear assembly located in the center housing
between the driving wheels on rear-wheel drive vehicles, or as part of the
transaxle on front-wheel drive vehicles or between front and rear differentials
on all-wheel drive vehicles. The differential transmits power to the wheels
while allowing each wheel to rotate at different speeds, such as when turning a
corner.
Active Power Split AWD For 4-Speed Automatic Transmissions
Subaru vehicles equipped with the 4-speed Electronic Automatic Transmission
(4EAT) feature a different type of All-Wheel Drive than with the 5-speed manual
transmission. Instead of a viscous coupling center differential, an automatic
transmission-equipped Subaru features an electronically managed continuously
variable multi-plate transfer clutch located in the transaxle's tailshaft. Power
transfer is governed by slippage in the clutch plates, which use a special
friction material that easily handles the loads generated during power transfer.
The Transmission Control Module (TCM) controls the All-Wheel Drive multi-plate clutch. Under normal driving conditions, the power split is biased toward the front wheels. The active All-Wheel Drive system can adjust the power split in an instant, depending on many input factors. If the front wheels begin to lose traction, the TCM increases hydraulic pressure on the clutch, reducing slippage of the plates to transfer power to the rear wheels. As the front wheels regain traction, the TCM reduces pressure on the clutch, increasing slippage of the plates and transferring power to the front. The TCM monitors inputs from speed sensors on the front and rear output shafts and from the throttle position and transmission shifter position sensors. These factors cause the TCM to select a software "map" that determines how aggressively the TCM adjusts the power split between the front and rear wheels.
Viscous Limited-Slip
Rear Differential
For even greater traction capability, select Subaru vehicles feature a viscous
limited-slip rear differential. If one rear wheel starts to lose traction, the
differential automatically transfers the power to the other wheel. A viscous
limited-slip differential not only helps traction in very slippery on- and
off-road conditions, but it's also a cornering aid. With some vehicles, tight
cornering can cause the inside rear wheel to lift, which diminishes traction and
can compromise safety. The viscous limited-slip differential automatically
transfers power to the rear wheel with the best traction, providing improved
cornering and superior stability.
Benefits Of Subaru
All-Wheel Drive
Regardless of whether you have an automatic or a manual transmission, the
benefits of AWD are the same. Both Subaru AWD systems give you greater
stability, handling and traction because power is transmitted to all four wheels
automatically in response to changing road conditions.
Watch future issues of Drive for more information about the third Subaru All-Wheel Drive system."
Vitara - see Chevrolet Tracker
RAV-4 (1996 and on) - (Recreational Active Vehicle with 4-wheel drive). Permanent all wheel drive. Models with manual transmission also feature a center differential lock.
4Runner Limited (1999) - multi-mode four wheel drive system (full-time all wheel drive system with 4x2 capability). VF3AM transfer case. Lever-operated AWD, 4Hi, 4Lo (2,48:1), N modes. Corporate 8-inch IFS front axle is engaged using Toyota's ADD (Automatically Disconnected Differential), which is similar to other companies' CAD systems. Electro-mechanically operated rear differential lock (optional) in 8-inch solid axle.
Land Cruiser FJ 100 - Full-time two-speed four wheel drive system with manually (button) lockable differentials - center and rear (opt.) The case is similar to previous generation Land Cruisers
Land Cruiser 80??? 1993 - Full-time all-wheel drive system with viscous coupling, which is locked when in low gear (2,49:1 ratio). Front and rear manually lockable differentials.
Passat V6 4motion - "The 4MOTION system is all about continuous distribution of power to all four wheels, all the time, at all speeds. In normal driving conditions, the drive ratio is 50-percent to the front and 50-percent to the rear. On low-grip surfaces, the wheels with the higher level of adhesion receive more of the power — up to a front-rear ratio of 67-percent to 33-percent or vice versa. In addition to front and rear power allocation, 4MOTION distributes power left to right by way of an electronic differential-locking system. This feature detects and limits individual wheel spin and redistributes the torque to the side that needs it most. Coupled with front-to-rear power distribution, 4MOTION’s lateral capabilities make it possible for one wheel of traction to propel the Passat."
Volkswagen Golf 4motion, Volkswagen Bora 4motion,
Volkswagen Beetle Rsi, Volkswagen Sharan -
Haldex Awd-system
850 AWD - Viscous Coupling Locking Center
Differential, electronic traction control on front wheels, limited slip rear
differential.
V70 AWD - Viscous Coupling Locking center differential, Volvo's TRACS system applies brakes to spinning wheels, thus transferring torque to wheels, that have grip. TRACS system works on speeds up to 25mph. more information on Volvo V70
Volvo S60 AWD,
Volvo V70 AWD,
Volvo XC 70,
Volvo XC 90,
Volvo S60R AWD,
Volvo V70R AWD -
Haldex Awd-system
Currently the Haldex
Awd-system is introduced on the following vehicle models
- Volkswagen Golf 4motion
- Volkswagen Bora 4motion
- Volkswagen Beetle Rsi
- Volkswagen Sharan
- Audi A3/S3 Quattro
- Audi TT Quattro
- Skoda Octavia 4x4
- Seat León
- Seat Alhambra
- Volvo S60 AWD
- Volvo V70 AWD
- Volvo XC 70
- Volvo XC 90
- Volvo S60R AWD
- Volvo V70R AWD
Copyright © 1997,1999 by Eliot Lim. This article may be distributed freely, provided it is distributed in its entirety.
By Eliot Lim
Third major revision: March 2 1997
Last update: February 26 1999 (all other versions obsolete)
This article was originally written in the fall of 1992. Back then as is now, there was a general lack of good information regarding all wheel drive vehicles and how they differ from traditional four wheel drive vehicles. I have updated the original article to mention current vehicles. This article has been very well received by the internet community.
It is important to get the definitions down first, since for any four wheeled vehicle, all wheel drive and four wheel drive literally mean the same thing. Generally speaking "all wheel drive" implies permanently engaged or automatically engaging four wheel drive and "four wheel drive" implies manually engaging, part time four wheel drive. The auto industry usually abide by these definitions but not in all cases. The now departed "all wheel drive" Ford Tempo and Subaru Justy were really part time manually engaging systems, like the older Subaru GLs. The term on demand four wheel drive is quite ambiguous. It can either mean that it is a part time manually engaging system or a part time automatically engaging system!
The automotive media shares a lot of the responsibility for the confusion. Factual errors are common, so is the careless use of the various terms interchangebly.
For this article I shall be using the terms loosely and will be more specific where necessary.
A differential is a mechanical set of gears which takes input torque from a driveshaft and splits it evenly to two output axles, allowing them to rotate at different speeds. A differential in a front wheel drive or rear wheel drive car allows both wheels to apply power to the road and yet be able to rotate at different speeds so that the car can turn without resistance.
A permanently engaged four wheel drive system needs to have three differentials to enable it to apply power to four wheels and be able to turn without resistance: The front, rear and center diffs. (diff = short for differential) This is because the distance traveled by the turning front wheels is not the same as the distance traveled by the non articulating rear wheels.
Power leaving the gearbox first goes to the center diff, which then splits it via the driveshafts to the front and rear diffs. Manually engaging part time four wheel drive systems in most cases do not have a center diff, so they cannot be used in the dry. When four wheel drive is engaged in such a system, the front and rear axles are locked together and will rotate at exactly the same speeds. The difference in front and rear wheel speeds have to be scrubbed off by the tires.
This is a core design issue in all wheel drive technology because these have a profound effect on the cars' road behavior. Consider the case of the simplest all wheel drive car with 3 "free" diffs. The car can be rendered immobile if any one of the four wheels lose traction because basic differentials equalize the torque output. A simplistic way to look at this is to think that the basic "free" differential sends power to the axle with the least grip, so if one wheel loses grip, all the power is sent there, leaving nothing for the remaining three. In reality, the differentials are equalizing the power distribution, so everything is equalized to zero in this case. Remember that a four wheel drive vehicle has twice as many wheels as a two wheel drive to lose grip and mobility on. And since four wheel drive vehicles would tend to be used more in bad conditions, it is quite important to have some form of differential locking. Every full-time four wheel drive car on the market today has some form of diff locking. A good way to understand this concept is to trace the evolution of the very early systems to the state of the art.
Audi was the first manufacturer to successfully sell high-performance, permanent four wheel drive with the quattro, released in Europe in 1981 and in the US in 1983. (The car goes by the more popular name turbo quattro coupe in the US and more recently, the Ur Quattro around the world). The cars were very successful in rallying , winning several world titles and it set the automotive world ablaze because four wheel drive was never previously associated with ultra high performance. Even though the 1966 Jensen FF was the first vehicle to have full time four wheel drive (and also anti lock brakes) the car was a commercial failure and it was left to Audi to break through the public consciousness and go into the history books for launching the full time four wheel drive revolution.
During the 1980s Audi decided to spin off four wheel drive and the quattro name to its entire range of cars. The first generation quattros had simple locks for the center and rear diffs, which locked one or both of them solid (no speed difference) to dig one out of deep trouble. When the center diff was locked, it meant that one had to lose grip on one rear and one front wheel to become immobile. When both the center and rear diffs were locked, one had to lose both rear wheels and one front wheel to get stuck. The locks on these Audis were manually engaged and were quite cumbersome since the driver already had to worry about shifting and steering in addition to this. Audi found that many drivers forgot to disengage the locks once they got going again.
Thus development went in the direction of automatically locking differentials. First on the scene was the viscous coupling (VC for short) which used a silicone liquid in a casing designed so that minor speed differences were allowed between the two axles but increased slip would lead to a rapid increase in the viscosity of the fluid which would then lock up the coupling. The viscous coupling was used in two radically different ways:
Some manufacturers used regular differentials in conjunction with the VC where the VC functioned as a diff lock that acted automatically when conditions needed it. The current Mitsubishi Eclipse GSX and current manual transmission all wheel drive Subarus use this scheme. The departed BMW 325ix and Toyota Celica turbo all-trac also used it.
Audi, during the development of the original quattro, also played with VCs and came up with a completely different way of using a VC. In this implementation, the VC is used as the center diff, resulting in a part-time, automatically engaging four wheel drive system. In this implementation, the car is basically front wheel drive, with the rear wheels coasting along and minor speed differences absorbed by the VC when the vehicle was turning. When front wheelspin occurred, the speed difference would increase to the point where the VC with its viscous liquid churning would start transferring some of the torque from the front to the rear wheels and thus the vehicle would become four wheel drive. Note the difference between this system and the former. The latter is auto-engaging part time four wheel drive, while the former is full-time auto-diff-locking four wheel drive.
The part time automatically engaging system was never put into production by Audi but was instead spun off to VW, which did put it to market as the syncro system. The simplicity of this implementation has drawn a very wide range of manufacturers to use it as well, from all the minivan implementations, many of the newer SUVs to exotics like the current Porsche 911 turbo and Carrera 4 and the Lamborghini Diablo VT (these have permanent drive on the rear wheels, of course). Volvo is a new player in this field and its latest all wheel drive offerings also use this scheme, with an unusual cocktail of limited slip devices thrown in, namely a traction control system at the front and a regular mechanical limited diff in the rear. Several magazines have found this system to be in need of further refinement.
Next came the torsen (stands for TORque SENsing) differential, which was embraced by Audi in its second generation quattro system. Audi was approached by FF development (owners of the VC patent) during development of the original quattro back in the late 1970's but the VC was rejected for reasons that will become clear shortly. The torsen diff was invented by an American company (Gleason corp.) and had all the advantages of the VC and none of its disadvantages. It is a fully mechanical device of worm gears and a worm wheel whose workings are quite difficult to describe with words and probably beyond the scope of this article. However, the torsen's characteristics is the issue that is of interest here. The torsen differential will split torque 50:50 in a no-slip condition. However, when one axle slips, the torsen diff will send more torque to the axle with more grip, in other words, it works in an exactly opposite way to a conventional diff. Torque splits of up to 80:20 are available, depending on the pitch of the worm gears. And since it is a completely mechanical device, the locking action is instantaneous and progressive as opposed to the VC, which has a very slight lag for the viscous fluid to heat up and suddenly lock. The torque sensing characteristics of the torsen also allows it to be proactive in preventing wheel spin rather than reactive, in correcting a wheel spin situation. The torsen diff is thus "more sensitive" to slip than the VC. Its locking action is also more progressive. (Porsche also rejected the VC in the 964 Carrera 4 because they felt that the VC was too difficult to control and that it had exponential rather than linear locking characteristics.)
More importantly, the torsen does not lock or inhibit speed differences under braking, thus allowing all 4 wheels to rotate independently at their own speeds when no power is applied. The torsen diff only locks in a power application situation while the VC locks both during acceleration and braking. The torsen has a torque sensing characteristic while the VC has a rotational sensing characteristic.
The VC's rotational sensing characteristic initially caused lots of problems for the engineers. Anti lock braking systems rely almost entirely on speed differences between the 4 wheels to detect a locking wheel. Thus, when the transmission tries to force 4 wheels to rotate at the same speed, it creates serious difficulties for the ABS system.
The engineers had to use a variety of hacks to get around this problem. Mitsubishi delayed ABS for a while for its first generation GSX, then finally decided to make ABS and rear VC limited slip mutually exclusive options. The VW syncro system simply disconnected four wheel drive the moment the brake pedal was stepped on via a secondary clutch. Most other vehicles using this implementation of VC have a very similar disengage feature. The very successful World Rally Championship Lancia Delta Integrale even went as far as to apply a little bit of power (via the engine computer) to reduce the drag of the VC when the brakes were applied! Some very crude systems used a overrun device that is conceptually similar to the bicycle crank. This meant that while four wheel drive was disengaged during braking it was also inoperative when reverse was engaged!
The easiest hack was to reduce the effective viscosity of the fluid in the coupling, so that the drag was reduced. This also meant that the VC's locking effectiveness was reduced, which is probably quite acceptable for a vehicle intended primarily for paved roads. The VC's attraction is its simplicity and cheapness, not its sophistication.
In the late 1980s Porsche and Mercedes were treading slowly and came out with all wheel drive vehicles of unparalleled complexity. Mercedes' 4Matic system used the ABS sensors to determine wheelspin. In the dry, the Benz was a rear wheel drive car. When the wheel sensors determined that the rear wheels were spinning, a signal was sent to the computer to start engaging a hydraulically actuated multi plate clutch to send power to the front wheels. Clutch engagement was progressively altered by the computer. When the computer determined that even more traction was needed, a second clutch would start locking the rear diff. When the brake pedal was pushed, both clutches disengaged instantly to allow ABS to work without interference.
The Mercedes 4Matic was a part time, automatically engaging four wheel drive system. The reason given by Mercedes why they went to great pains to design a part-time four wheel drive was that they did not want to upset their loyal clientele with a full-time four wheel drive, which because of the driven front wheels, would "change the traditional feel of a Mercedes". One could also speculate that they were too proud to use anything less complicated than Audi, which in the marketplace is considered "lower". In practice, the 4Matic system worked no better and no worse than the other crop of full-time four wheel drives, but its cost and complexity made it look bad. This original 4Matic system has been ditched and the latest 4WD Mercedes is now a full time system, including the system to be used in the "M" class SUV. The Nissan Skyline GTR uses a system that is conceptually similar to the original 4Matic.
Porsche used a similar system of locking clutches (though they are implemented quite differently) as the Mercedes in the limited production, state of the art 959, but the center diff (which is actually just a hydraulic clutch) was engaged at all times except when parking so that the steering would be easier to turn. Torque split in the 959 varied with load and conditions. (via the progressive locking of the clutch). Unlike all other implementations of all wheel drive, the 959's torque split varied under no slip conditions. i.e. In every other all wheel drive system, the split is fixed until slip occurred, after which the various limited slip devices would begin to alter the split. In the 959, the all wheel drive computer is fed information from many sources, including the throttle position, steering angle, g force accelerometers and even the turbo boost gauge. In a straight line, under maximum acceleration, the system will send up to 80% of the power (from a normal 40 front/60 rear split) to the rear wheels, even if all 4 wheels are turning at exactly the same speed. This was by far the most complex and sophisticated all wheel drive system ever built.
The 959 was followed by the 964 which was first introduced in 1989 as the 911 Carerra 4. Porsche claimed that this was an evolution of the system used in the 959 and is even more advanced. However, this was a fixed split system like all the others, with computer controlled clutches acting as limited slip devices. The 964's trump card, however was that the speed sensors and accelerometers were used with the computer controlled locking rear differential to cure the 911's natural tendency to oversteer if the throttle was suddenly lifted off in a turn. The rear diff would start locking when the computer detected that oversteer was imminent. A locked rear diff would induce understeer, which in turn countered the oversteer. Through the use of all wheel drive and smart differentials, Porsche was able to tame a formerly unruly beast into a much more docile animal. This, according to their chief engineer was their main reason for implementing all wheel drive in the 911, as the 911 with its rear biased weight distribution is not in a real need of extra traction.
In 1993 Porsche updated the 911 with a brand new rear suspension. Even the rear wheel drive version was tamed and thus the justification of using a highly complex computer controlled all wheel drive system disappeared. The four wheel drive version of this 911 (alias the 993), has a much simpler, lighter and cheaper part time automatically engaging VC system such as those found in the Golf syncro and most minivans. However, the smart rear differential that fought the deadly oversteer was retained to quell any remaining tendency to oversteer. The new watercooled 911 (aka 996) C4 uses essentially the same system as the 993 C4, but with additional computer controlled stability assistance tweaks. It is somewhat disappointing to see that Porsche, once the clear technological leader in this field retaining the viscous coupling in its latest AWD offering while many other new AWD offerings such as the new VW Golf 4Motion and 1999 Jeep Grand Cherokee resort to more sophisticated designs.
Subaru deserves mention here because in the automatic version of the Legacy and Impreza (including the Outback variants), it uses a computer controlled system much like those found in the Mercedes 4Matic, automatic Audi A8/V8 and the earlier Porsches. Subaru has been offering this sophisticated system for a long time in a relatively inexpensive car. Much more recently other makers have started offering conceptually similar systems. The Honda CR-V, the 1999 VW Golf 4Motion and its derivatives such as the Audi TT now use a system that is conceptually similar.
The Audi A8 (as well as the automatic version of the Audi V8) also used a computer controlled clutch to lock the center differential, in a manner similar to the systems just described. The automatic transmission supplied a ready source of hydraulic pressure to lock a pack of clutches, so it was tapped. This system represented Audi's first successful mating of automatic transmission with quattro all wheel drive. Current quattro models with automatic transmission use a center torsen differential with the exception of the A8.
For all its technological glitz in the 1980s, all wheel drive cars eventually turned out to be a commercial failure with the exception of the niche brands like Audi and Subaru. In the late 1980s there were all wheel drive offerings from every major manufacturer as it was the latest fad. Many of these manufacturers have since dropped their all wheel drive models in favor of highly profitable truck based sport utility vehicles or SUVs in short. For the cars a more simple and inexpensive alternative was there waiting to be exploited.
Anti-lock braking systems have speed sensors on two to four wheels to detect differences in speed between wheels so that the computer could intervene and "pump" the brake on the locking wheel. By a few simple extensions to the system, it could be made to brake a spinning wheel, thus effectively transferring power to the one with grip. More sophisticated systems would reduce engine power to further slow the wheelspin, but generally speaking, traction control is merely an optimization of two wheel drive using ABS technology.
Current versions of Audi quattros (dubbed quattro IV) use all wheel drive in conjunction with 4 wheel traction control. Under no slip conditions, power is delivered 50-50 to the front and rear via a center torsen differential, which would take care of limiting slip between the axles. The traction control system would take care of limiting slip between each wheel of a given axle. Thus for the first time, the quattros have to lose traction on all 4 wheels before they become immobile.
The prior generation of quattros had center torsen differentials (except A8/V8) and manually lockable rear diffs which locked it solid. This featured automatic unlock at speeds exceeding 15 mph to aid the forgetful driver. The V8 quattro had a torsen diff in the rear and either a computer controlled clutch in the center (for automatic transmission) or a torsen (manual transmission).
The new Mercedes ML320 (and ML430) SUV uses a relatively simple implementation of open differentials and four wheel traction control. This implementation has been criticised from several sources as being inadequate. The main disadvantage of the M class AWD implementation is that excessive demands are placed on the brakes under extreme conditions and the system can also be prone to hunting. Zexel engineers found that if a torsen center differential is added to this system which would act before the onset of wheelspin, TCS brake activation would be reduced by over 50%. The presence of this data suggests that Mercedes might have gone a little too far in cutting costs by eliminating limited slip or torque sensing capability in its center differential.
The subject of torque splits has been quite misunderstood. Basically, every four wheel drive vehicle with the exception of the Porsche 959 has a fixed split when there is no slip. For the full time systems, 50-50 is common, but also not unusual is 60+% rear 30+% front. The latter is usually found on cars that started life as rear wheel drive vehicles, while the former on cars that were originally front wheel drive.
For the part time VC systems this ratio is usually quoted as 95% front, 5% rear. Some have argued that the 5% constant rear drive would qualify it to be considered a full time system. Regardless of the merits of this argument, the fact remains that the main reason why there is a dribble of power going to the rear wheels is because a little "slip" is deliberately engineered into the driveline to keep the VC tight, so that when the front wheels spin there is little or no lag before the rear wheels start driving. The VC in this implementation always thinks that the front is slipping slightly relative to the rear even if all four wheels are running at exactly the same speed. Slightly different final drive ratios are used to achieve this.
The conventional idea of slip suggests a scenario where one or more wheels are spinning when the vehicle is operated under slippery conditions. There is however an additional concept of slip to consider. Recall that the front wheels travel a greater distance compared to the rears in a turn. Thus to a limited slip device sitting on the center differential, the front axle is "slipping" relative to the rear. The limited slip action thus directs more power to the rear in a turn. For nose heavy vehicles such as Audis, this effect reduces the amount of driving the front tires need to do, thus allowing them to be used for increased turning power. This small dynamic optimization in torque distribution allowed Audi to greatly reduce the terminal understeer experienced in the first generation cars.
Consider the case of the Mercedes ML 320 SUV where with four wheel traction control and an open center diff. When one end loses grip completely, the system would transfer some power to the other end. Theoretically speaking, if one were to jack up the rear end of the vehicle off the ground, the system could potentially transfer 100% of the power to the front, making it a front wheel drive vehicle, and vice versa. In reality since traction control merely pumps the brakes rather than lock the spinning wheels completely, less than 100% of power can be transferred to the front.
The point to note is that quoted torque splits like 37/63 only apply when there is no slip. Given the extreme example above of one axle being jacked up off the ground, a AWD system with any type of limited slip devices can theoretically go from its nominal split of say 50/50 (or whatever it may be) to 0/100 or 100/0 depending on how solidly the center limited slip device or 4 wheel traction control system locks. Mercedes does not quote the percentage locking factor on its traction control system, so one cannot really tell what its true variations of torque splits are under extreme conditions. Part time manually engaging systems with no center differential as well as early full time systems such as the first generation quattros with manual locks can have the variation of going between 100/0 front/rear and 0/100. These extreme variations also mean that no speed differences will be allowed between axles, which is why most modern systems never achieve 100% transfer of power. A 80% locking ratio would allow the speed differences of turning wheels to occur without interference.
A system that can lock the center diff solid would also mean that each axle will have to be engineered to be able to handle 100% of the engine's output, when in reality it would be loaded no more than 50% most of the time. This would lead to a virtually indestructible system with a life that would far exceed the rest of the car. The downside is that the doubling of rotational masses would make the car sluggish when moving off the line, affecting automatic transmissions variants the worst because these tend to have a higher (numerically lower) 1st gear.
An emerging trend in vehicle dynamics development is the use of "stability management systems", which use much of the ABS and computer controlled AWD hardware to further optimize all available grip. The more sophisticated AWD systems will reproportion the power distribution according to available traction at each wheel, leading to a very safe and neutral feel when powering out of a turn. However these systems do not work if the driver lifts off the accelerator pedal completely in a turn.
Recall that Porsche progressively locked the rear differential to handle such a situation. The latest 996 Carerra 4 will in addition to this, selectively brake individual wheels as needed to stabilize the car when it is driven in a clumsy manner to its limits. Specifically it will brake the inner rear wheel to correct an understeering situation and the outer front wheel for an oversteer. This is done independent of driver input. Stability management systems have started to appear in other more expensive car models and will undoubtedly trickle down and be as common as ABS one day.
Many potential buyers of all wheel drive cars wonder if the extra "stuff" would mean more problems or if the system would lead to heavy penalties in fuel consumption. Real world experience has shown that all wheel drive systems are not known for any kind of teething problems. The probability of an extra set of driveshafts failing has turned out to be as probable as a V8 engine failing because it has double the number of cylinders over an inline 4. This is a good analogy because with the power split over more wheels, the drivetrain is less stressed.
Those implementations that rely on ABS wheel sensors to lock differentials would be as likely to suffer from problems as any car with anti lock brakes. i.e. no greater than average.
In fact, many of the suspicions of all wheel drive come from the world of manually engaging part time systems where attempts were made to make four wheel drive engagement less cumbersome, with features such as automatically locking hubs and/or "shift on the fly four wheel drive". An all wheel drive system is always engaged and is actually simpler because it eliminates the need of these convenience "features" and their associated parts, which are the usual source of problems.
Accusations that four wheel drive wastes a lot of gas is only applicable to part time manually engaging systems. A full time system with a center diff has none of the tire scrubbing waste of the former. Furthermore, research by Audi showed that as tractive loads built up, the tire losses of two wheel drive exceeded the losses caused by the extra weight and inertia of a full time four wheel drive system. Tire losses were found to rise disproportionately with load. Consider the extreme case of the "burnout" or wheelspin scenario, where 100% of the tractive energy is converted to burning rubber rather than propelling the vehicle.
Part time, manually engaging four wheel drive systems also make it extremely difficult to have a decent suspension set up. For cars with front-wheel steering the front wheels have to travel a greater distance than the rears in a turn. Because there is no center differential, the rears would therefore have to scrub its excess speed and in doing so, lose some adhesion for cornering. With less grip in the rear, the vehicle becomes oversteery which to an average driver does not constitute safe behavior. The result is that a lot of positive camber is applied to the front, making the front wheels resemble an upright "V". The effect is that the front wheels now have a smaller contact patch and thus less grip in a turn. Remember that all this tweaking is to make sure that the vehicle is semi neutral in four wheel drive mode. When four wheel drive is not engaged, which is typically the majority of the time, one is left with a hopelessly understeery vehicle, because the front still assumes a rear end that has to scrub off excess speed. Anti lock braking, if offered will also be inoperative in four wheel drive mode, just when it is needed the most.
It does not take much to see that this is a very suboptimal implementation compared to all wheel drive systems, which are on the other extreme of being able to dynamically alter the division of power to each axle depending on which end is sliding. The behavior of a full time or part time auto engaging system is completely predictable and is therefore optimizable to dramatic effect.
Average consumers also tend to dismiss the need for good handling. The line "I am not going to race this vehicle" is repeated often enough. However, even if we were to judge vehicles solely as appliances, good handling does enter the equation. A good handling vehicle, such as the many excellent all wheel drive examples mentioned, will hide the difficulty of negotiating a turn, making it seem more effortless. The average driver would then feel more comfortable and confident and will therefore shed less speed entering a curve, leading to less momentum being lost, which in turn means that the vehicle does not have to consume energy reaccelerating back to its original speed. In other words, it would be a more energy efficient appliance. This point is hardly ever raised when discussing the appliance value of vehicles.
It is unfortunate that old fashioned part time, manually engaging systems are still being sold on many SUVs today with high prices that are a match for their mediocrity. There is no reason, from a conceptual point of view that these vehicles should not have an all wheel drive system. It is this author's opinion that consumer ignorance and a uncritical media are the main reasons for the slow progress in the truck/SUV market.
It is false that a permanently engaged system is incapable of handling the rigors of off roading as well as the antiquated part time system. The Range Rover has been on the market since 1976 and it has had a full time system with a center differential since the very first one rolled off the production line. Likewise, the ultimate off roader, the Hummer uses a permanently engaged system with torsen differentials rather than solidly locked axles and part time manual engagement. Both of these vehicles are held in the highest regard with respect to their off roading capabilities.
The 1999 Jeep Grand Cherokee is significant for being the first mainstream mass market SUV to feature an AWD system that is more advanced than most of its peers. The Grand Cherokee uses hydraulically controlled progressive locking differentials on both front, rear and center resulting in a system that can deliver all available torque to any one wheel if it is the only one with grip. Unfortunately this highly advanced AWD system is merely an option and consumers who are suspicious and distrustful of technology would end up buying the much cruder 4WD/AWD system which is not necessarily more reliable because of its multitude of selectable drive options.
Audi and Subaru continue to successfully market all wheel drive vehicles and both marques actively compete in motorsports to show the worth of their technology. The Audi A4 quattro has been particularly successful in demolishing its two wheel drive competition in seven major touring car series last year despite severe weight handicaps. The Subaru Impreza turbo has been extremely successful in World Championship Rallying. The Mitsubishi Eclipse GSX has been less successful in the marketplace, with the vast majority of buyers opting for the front wheel drive version. Die hard Porsche enthusiasts prefer the rear wheel drive version of the 911 to the all wheel drive version. Four wheel drive is increasingly outlawed in competition because they tend to do too well.
With the success of sport utility vehicles the market for high performance all wheel drive vehicles will remain tiny. One could only hope that competition among the makers will eventually force all the SUV makers to bring their technology up to all wheel drive levels. This is starting to happen but at a very slow pace.
VW recently redesigned the Passat and based it on Audi A4 mechanicals. Since it was using a stretched version of the A4's floorpan, it made economic sense to use Audi's quattro system for the all wheel drive model, rather than to create a unique all wheel drive floorplan using the syncro system. Thus the "syncro" moniker becomes merely a generic term to distinguish an all wheel drive VW from a two wheel drive variant. This is not the first time that VW has used Audi quattro mechanicals. In the mid 1980's, Americans could buy the VW Quantum Syncro, which not only used the Audi 4000CS quattro's floorpan, but also the trademark inline 5 cylinder engine, mounted longitudinally ahead of the front axle.
For the 1999 model year VW has upgraded its AWD system from the viscous coupling based system to a computer controlled clutch developed by the Swedish company Haldex. The advantages include added simplicity since the extra clutch needed for braking can now be eliminated. Also, more precise control of the torque splits and a greater transfer ratio is possible. Derivatives of the AWD fourth generation Golf, (now renamed the "Golf 4Motion") such as the Audi TT and the A3 quattro will use this system. Note that this is still a part time, automatically enganging four wheel drive system. The quattro name, which used to have special significance is now being diluted by marketing expediency. Considerable confusion has arisen from the naming corruption by the two companies.
In yet another twist, Subaru has for many years been quietly offering radically different AWD systems in the same car, depending on the transmission choice. The manual transmission Legacies and Imprezas use a full time system that is split 50-50 with viscous couplings for limiting slip. In the automatic transmission versions, however, the system is a part time, computer controlled, automatically engaging system in some models and a full time uneven torque split with computer controlled locking in other models.
Mitsubishi has quietly continued to offer its four wheel drive GSX, now with rear limited slip and ABS together in the same package, even though its relatively low sales figures mean that it is far from a profitable model. Advances in ABS technology mean that the coexistence with VC systems is far less troubling than before. Porsches, for example have different ABS specifications depending on whether the model is four wheel drive or two wheel drive.
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Four-Wheeling in the Land of Hiawatha The search for snow, traction and truth takes us to Michigan's fabled Upper Peninsula to ponder the virtues of three pairs of all-wheel-drive wagons and cross-over SUVs • Photos by John Lamm |
 If you were looking for a place to drive in the sand, you would probably pull out a map of North Africa and call for hotel reservations in, say, El Qatrun or Kidal. If there are any hotels in those towns. Or phones. But if it's snow you're after, you could do a lot worse than look to Michigan's Upper Peninsula, otherwise known as "the U.P."
This is a place where residents frequently have to dig down through the snow to reach the tops of their mailboxes, and where the snowmobilers of February outnumber the tourists of August. Arctic winds howl across the largest body of fresh water in the world, picking up moisture from the (slightly) warmer water and immediately dropping it as snow. Lots of it, sometimes. In the winter of 1968-1969, 298 inches of snow fell on the shores of Keweenaw Bay.
The U.P. is Snow Central. It's also a wild and beautiful country of state forests, lakes, trout streams and rugged coastlines, lodged like an arrowhead of rocky land between Lake Superior and Lake Michigan. The Finns, Scandinavians and Cornish who came to fish, mine or cut timber are said by archaeologists to be the 11th distinct culture to settle on the U.P., preceded most recently by the Menominee and Ojibway. Seems humans have always enjoyed a good blizzard. One of those hardy Finnish-Americans, luckily for us, is our own Managing Editor, Ellida Maki, who grew up on the U.P. in Ishpeming. When we were casting about for a place to do an all-wheel-drive comparison, Ellida suggested Upper Michigan. "My brother Bill can show us around," she said. "He knows all the good roads." By which she meant all the bad roads. Exactly what we were looking for.
So the staff decided to fly from sunny California, pick up our fleet of awd test cars in windy Chicago and drive up the west coast of Lake Michigan into the North Woods. As a good Wisconsin boy who sometimes camps and sails in the U.P., I was invited along, and so was our Detroit Editor, Matt DeLorenzo, our role ostensibly to offer winter survival tips, or maybe just oil the muskrat traps. So I packed my fur hat and Sorrel boots and awaited pickup by photographer John Lamm, another erstwhile Wisconsinite who was back in the area visiting his family. John collected me in the Acura MDX and we headed for a rendezvous with the Newport Beach gang. I suggested as our meeting place a motorcycle shop called Corse Ducati, just north of Milwaukee. This is a beautiful new shop, with a built-in Italian deli and good cappuccino. While we were there, I bought a set of carbon-fiber mufflers for my Ducati 996 and stuffed the sizable box into the back of the Acura. Henceforth, each time we switched cars the box was referred to as "these @#$%&*%$#! mufflers again!" North we motored, toward the shores of Gitche Gumee, by the shining Big-Sea-Water, to the Land of Green Bay Packers, past Lambeau's frozen tundra, to the Vince Lombardi Exit, where we found a warm motel, 'neath the frozen moon of winter, with clouds of roof steam rising. Early in the morning, we stepped out into the parking lot and I got my first good look at all the cars gathered together. An impressive group, three wagons and three cross-over SUVs, paired off against each other in three different price ranges, from the affordable to the lavish. Our plan was to live with these vehicles long enough to find out which features and overall designs would most endear themselves to a group of test drivers after five days on rough and snowy roads. To further put all six through their paces, we planned a visit to the Brimley Development Center, a winter proving ground near Sault Ste. Marie, built by Continental-Teves for the testing of electronic traction control systems on snow and ice. There, on the BDC's winter handling circuit and vast expanse of hard-packed snow, we would be able to slither and spin to our hearts' content without hitting pine trees or telephone poles. Or being apprehended in the school parking lot, as some of us were in high school while attempting this very same kind of scientific research with our parents' Mustangs. At the top of the heap were the BMW X5 and the Audi A6 Allroad Quattro. Let's take a quick look at each: BMW X5 ($38,900): BMW calls the X5 a "Sports Activity Vehicle," perhaps to take away the workaday connotations of "utility" and to imply that driving the big Bimmer itself is a form of sport, like kayaking. With the optional 4.4-liter V-8, it probably is. But even the 225-bhp 3.0-liter inline-6 in our test car is plenty snappy and gives this 2 1/4-ton SUV reasonably zesty performance. The X5 has traction-enhancing systems of the You-Name-It school: full-time awd, traction control, Dynamic Stability Control, and even a Hill Descent Control to slow it to a controllable pace on very steep drops. Added to that, you have the usual BMW luxury interior on a par with a 5 Series sedan. Audi A6 Allroad Quattro ($39,900): Audi has been a pioneer in the awd wagon business, and the Allroad is essentially a Quattro wagon with a 4-position pneumatic ride-height control that turns it into a viable alternative to the conventional SUV. Its standard engine is the Quattro's optional 2.7-liter turbocharged V-6, rated at 250 bhp and mated to either a 5-speed Tiptronic automatic transmission or a 6-speed manual. Ours had the Tiptronic. In the next step down the cost chain, we have the Acura MDX and the Volvo V70. Acura MDX ($34,370): A late arrival to the SUV wars, after marketing Izusu's Trooper as the SLX, Acura has finally built its own, with the intent to concentrate on a more carlike ride and handling package. The MDX is built on a strengthened and stiffened Honda Odyssey minivan platform, propelled by a torquey, 240-bhp 3.5-liter sohc V-6 and a 5-speed automatic transmission. Its Variable Torque Management awd system controls the amount of power going to the rear wheels, giving full engagement below 6 mph and releasing it at 18 mph. Electromagnetic clutches at the rear control how much power goes to each wheel.
Volvo V70 Cross Country ($36,500): Like the Audi, the Volvo is a high-water wagon, with a ride-height control that can lift it to 8.5 in. of ground clearance, and all-wheel drive. It also has scratch-resistant matte-black plastic body panels front and sides for beating the bush, and comes only with a 197-bhp 20-valve 2.4-liter dohc turbo-charged inline-5, mated to a 5-speed automatic transmission, with "Geartronic," a separate manual selection gate. Making up our lowest-price pair are the Toyota RAV4 and the Subaru Impreza Outback. Toyota RAV4 ($16,215-$17,815): Newly redesigned, the RAV is finally more likely to be described as handsome, rather than "cute." With the new bodywork has come a new engine as well, an all-aluminum 2.0-liter 16-valve 4-cylinder with variable valve timing. It's 21 bhp more powerful than the old one, now rated at 148 bhp, with an extra 10 lb.-ft. of torque as well. Our test car had a 4-speed automatic transmission, but a 4-speed manual is also available. Suspension and steering have also been redesigned for a less choppy ride, and the interior has been restyled. Subaru Impreza Outback ($18,695): This is Subaru's little Outback and the smallest vehicle in the test. It's essentially an Impreza wagon with the ride height raised for awd off-roading, and it uses Subaru's naturally aspirated 2.5-liter opposed-4, now rated at 165 bhp (a 16-percent improvement over the previous version), with either a 5-speed manual or 4-speed automatic transmission. Our car had the manual gearbox, the only one in the test. The interior has been modernized and upgraded as well. So, with this little group we pulled out of Green Bay early in the morning and all-wheeled north toward Ishpeming. Roads in the southern part of the U.P. were mostly clear. But the previous week had brought just enough snowfall to bring out the snowmobilers, who were everywhere, their headlights like lanterns bobbing through the deep woods, or crowded around gas pumps and running on trails along the road. We cruised past the ski lodges and old iron mines of Iron Mountain and motored up Highway 95 though hills covered with that beautiful northwoods mixture of white birch, evergreens, hardwoods and exposed rock, dotted with lakes. By noon, we were in Ishpeming, Ellida's old hometown, where we picked up her brother, Bill, and went to lunch. At Sherri's café, we had pasties (pronounced past-ees) for lunch, a specialty brought to the U.P. by Cornish miners. This is a sort of folded-over pot pie of meat, carrots and potatoes, useful because it made a self-contained meal that fit in a miner's lunch pail. A burrito or chimichanga of the North, if you will, without the incendiary hazards of spice, which could be dangerous in a mine shaft. Hearty stuff. Lida got to telling us about Finnish humor in the U.P. and said the two guys who always appeared in Finnish jokes were Eino and Toivo. Bill said, "Did you hear about the Finn they arrested on terrorist charges in Ishpeming?" "No," we said. "His name's Eino bin loggin'." Bill led us on a scenic route through some of the snowy back roads in the vicinity, and when the shadows in the forest grew long, we headed for our hotel in nearby Marquette, a lovely port town on the hills overlooking Lake Superior. Ishpeming and Marquette, I explained to our spellbound group (at least those who stayed awake), was home ground to one of my favorite authors, the late John Voelker, whose pen name was Robert Traver. Voelker was an attorney and judge who wrote two fly-fishing classics, Trout Madness and Trout Magic, as well as the best-selling novel, Anatomy of a Murder. The last was a courtroom drama, based on a real murder case in Big Bay, Michigan, and it was made into a superb movie starring James Stewart and Lee Remick, directed by Otto Preminger. Nominated for seven Academy Awards in 1959, including Best Picture, it was filmed on location in Ishpeming and Marquette. Ellida and Bill told us they used to see Voelker driving around town in his now-famous old fishing cars (a Ford Model A, then a pickup truck) when they were young. I suppose you have to like trout fishing — or good movies — to appreciate all this, but we were on holy ground as far as I was concerned.
So of course we checked into the grand old Landmark Hotel, just across from the Marquette County Courthouse, and the desk clerk handed me a key to a room on the fourth floor. When I got to my room, a brass plate on the door said "John D. Voelker Room." Inside, there were pictures on the wall of the famous author himself, smoking cigars with a bemused Jimmy Stewart during the filming of the movie. On the mantle sat all of Voelker's books in hardcover, and there was a framed book of old trout flies on the wall. I went to sleep that night re-reading Trout Madness and wishing it were summer. We got an early start in the morning and drove all the way up to the Keweenaw Peninsula, a thumb of land that reaches northeast into the cold and forbidding waters of Lake Superior. We stopped at the college/mining town of Houghton, where Michigan Tech was in the midst of its Winter Carnival, in which the engineering students of various fraternities and clubs build huge and complex snow sculptures. The most spectacular works were a snow replica of Disney's Magic Kingdom — castle, monorail and all — and a huge Buddhist temple. Built more to my own skill level was a slightly lumpy copy of the Stonehenge, called "Snowhenge." Conceived by a journalism fraternity, no doubt, and built by English majors. Your lesser projects leave more time for beer. We forged northeast through Calumet, where former mining riches had left a legacy of beautiful stone civic buildings and homes, and drove to Copper Harbor, at the tip of the peninsula. Stopping on the northern shoreline to take some lighthouse pictures, we looked out on the gray, wind-swept waters of Lake Superior, and Tom Bryant remarked that the lake had a malevolent look to it, cold and unforgiving. We were all thinking the same thing. No single body of water, save, perhaps, the North Atlantic, seems to express the indifference of Nature quite as eloquently as Lake Superior on a cold, windy day. Even the name sounds chilly and aloof. It is said the lake never gives up its dead because the cold water (averaging about 40 degrees year around) preserves drowned sailors, so they never rise to the surface. Gordon Lightfoot's song about the wreck of the Edmund Fitzgerald is more than just a folk song in these northern port towns. On that cheerful note, we all retired to a cozy lakeside restaurant in Eagle Harbor and had coffee and hot soup. Much as I like sailing, there is much to be said, at times, for landlubbing. And heated car seats. Evening found us back in Marquette for a fine dinner of whitefish at the Landmark Hotel, and the next day we drove across the peninsula to Sault Ste. Marie and the nearby Brimley Development Center. We cruised east on Highway 28 through Seney, the town where Ernest Hemingway's fictional character, Nick Adams, supposedly detrained and hiked cross-country to go trout fishing in the Two Hearted River. Our friend John Voelker, of course, believes Hemingway actually fished in the nearby Fox River because (a) you can't walk to the Two Hearted River from Seney; (b) no fly fisherman in his right mind ever publicizes his favorite trout stream, and (c) The Big Two-Hearted River makes a more romantic short story title than The Big Fox River. We'll never know, but you have to love any historical debate that draws in both fly fishermen and American Lit professors. By noon we'd reached the BDC and had a tour of the Continental-Teves winter testing grounds, an impressive combination of handling circuit, circle track, ice rink, drag strip and sprawling snow field, all surrounded by north woods. It looks like Brooklands, as imagined by Dr. Zhivago. While Road Test Editor Patrick Hong ran the cars through their testing regimen, the rest of us spent the afternoon and most of the next day driving the awd collection on different combinations of snow and ice. And, after four days on the road and our time at BDC, we'd formed some pretty firm impressions of the vehicles. Starting from the top of the food chain again — and leaving the hard numbers to Patrick's instrumentation — here are our subjective findings:
BMW X5
And it does feel a bit bulky. Like so many SUVs, the BMW looks bigger on the outside than it feels on the inside, as though much of its size is dedicated to door thickness. That said, it has large luggage space at the rear, with 60/40 fold-down rear seats for skis, etc. The rear seats themselves, however, are hard and flat, and a tight fit for large adults; 6-footers will find just-adequate leg room, their knees just touching the front seats. The X5 feels solid and strong, but a bit heavy on the road. Ride is on the sporty side, not very supple and rather choppy on the rural roads in which the U.P. abounds. All the controls, however, from the switches to the shifter, exude smoothness and refinement. The only controls everyone disliked were on the radio/trip computer in the center console, variously described as "ridiculous" and "maddening." Most of us gave up and left the radio off. If we could figure out how to turn it off. Overall, the words that appeared most often in the BMW notebook were stylish, safe, refined and solid. Audi A6 Allroad Quattro If the BMW was criticized for being too firm and choppy, the Audi disappointed several drivers for being softer than they had expected, lacking the sporty, rally-car edge of the old Quattros. Steering is heavily boosted also, and brake pedal feel is soft.
The interior of our test car was done in a beautiful gray-green leather and nearly everyone agreed that the handsome interior and exterior styling were the best of the group (though a few disliked the brushed-metal door guards). The engine is smooth and likes to rev, the turbocharged V-6 building power nicely when you pass on the highway, but still with enough torque to bat the wagon around on snowy roads. Speaking of which, this is the only vehicle we got stuck in the snow on the trip, when one of our crew backed onto a snowmobile trail next to a grocery store and sank through the crust. The Audi high-centered and spun its wheels helplessly, until we activated full ride-height, proof that you can never have too much ground clearance in the North Woods. On our snow-field testing, however, the Audi did very well — at least with the ESP off. It swung its tail out a little more than the others, but held a drift well and never got more than a few degrees out of shape. Overall, the Audi is a class act, a plush awd wagon, served up with civility, style, interior space and dozens of thoughtful touches, all at a substantial price. Check out the complete specifications and test results.
And to start with our next pair, we have the: Acura MDX
Acura has tried to make this SUV carlike, and they have more or less succeeded, though it might more accurately be described as "minivan-like" (the view over the rounded hood has something to do with this impression), but with a heavier-duty off-road spin. It is, after all, based on the Odyssey platform, to which extra metal and a stronger engine have been added, but there's not a lot of hard-core Jeep in its DNA. On the road, the MDX is smooth and refined, with a nice balance between ride and roll stiffness, and it feels crisp without being stiff and jouncy. The engine is smooth and quiet, reasonably torquey, and more than adequate to move the Acura down the road in tight passing situations. Grip on the snow is excellent, and the MDX is surprisingly agile and quick on its feet for a large vehicle. Rear luggage space is excellent, and the rear seats are comfortable and roomy, though probably better for two adults than three. Jump seats in back can just accommodate adults, but children will be happier there. The MDX is what might be called a well-rounded package, an ideal family vacation SUV with few irritants and no sharp edges. It works smoothly, quietly and with deep competence, without drawing attention to itself. Volvo V70 Cross Country Volvo has been making sensible utility wagons for a long time, so the melding of the potent V70 series with ride-adjustable awd would seem like a natural for the Nordic company. And it is.
A few of our editors were unimpressed by the matte-black plastic cladding on the nose and sides, but others noted that, for driving in brush country, it makes perfect sense. Its appeal depends on how far off-road you intend to go. The only other drawback we could spot is a lack of leg room in the rear seats. A tall passenger's legs will be pressed directly against the front seatbacks. Other than that, the interior is full of user-friendly controls and features (three cupholders in the front!) that can be operated at a glance. The heater controls, for instance, are a no-brainer. No operator's manual needed. The Volvo's handling and traction in the snow were excellent, and the highway ride was crisp and direct without being harsh. It's a chassis of impressive balance, pushed along by a nice drivetrain. That 2.4-liter turbo, by the way, got the best average mileage of any car here, hitting a high of 28 mpg on the highway. After our second day on the road, I asked everyone at dinner for a first choice, the one vehicle each person would like to drive home from Michigan and then keep in the family for a daily driver. All eight drivers and co-drivers in our little band chose the Volvo. It's simply a good blend of handling, luggage capacity, ergonomics and common sense. Check out the complete specifications and test results.
And on to the smaller, less expensive end of the awd measuring stick: Toyota RAV4
More than that, it's fun to drive. The short wheelbase can make it slightly more choppy over the rough stuff than its larger brethren here, but not disconcertingly so, and that same shortness translates into an almost dune-buggy-like agility on tight forest roads. You can throw the RAV4 around without fear that you're releasing some unrecoverable amount of mass in the general direction of the treeline. That compactness, of course, exacts some penalty in interior space. Rear leg room is just adequate, and the luggage bay is small, but with the rear seats folded forward, a fair amount of gear can be hauled. The interior and dash designs have some techno-robot touches that will probably appeal to young buyers, but merely look cheap and instantly dated to us older types. Nevertheless, the controls and switches are probably the simplest and most logical of the group. It's an easy interior to live with. Even the radio can be turned on and off by someone with very little training. The RAV4 has very light steering — some thought overboosted — but it's also quick. That, together with excellent traction in the snow and a willingness to change direction made the Toyota a ball to drive on the BDC snowfield. Words like "light and lively" and "quick and spunky" were sprinkled through the notes. The RAV is clearly the sportiest of the sport-utility vehicles in this group, and the most fun to drive. Subaru Impreza Outback If the RAV4 was the most lively of the SUVs, the Subaru is the absolute sports car of the entire group. Light, responsive, revvy and agile, it makes the other two wagons — and the two larger SUVs — feel like middle-age fuddy-duddies. It's even more fun to throw around than the Toyota, thanks to its lower ride height.
A few of our crew thought the Subaru interior somewhat cheap, while others deemed it remarkably good for this price range. In any case, its controls are well-thought-out and logical, the instruments big and easy to read. One thing that is not big is the rear leg room. The rear seats are for children only, or emergency adult use. Front seats, however, offer great lateral support, and the rear luggage bay is surprisingly large, with a built-in plastic protective liner. The Subaru worked beautifully on the snow field, where, as one driver noted, "it's a snow-driving tool. Point and squirt, modulate the throttle, and this car is easily the most fun to drive of the entire group." Check out the complete specifications and test results.
So. After all this driving, sliding, discussion and note-taking, a couple of patterns emerge from the trip. First, it's remarkable how well all these vehicles work in the snow, from the largest to the smallest. Awd, ABS and various traction-control systems have made them all capable of lane-change, braking and cornering feats that would have put the average 4-wheeler deeply into the trees or Armco a decade ago. Second, there appears to be an inverse relationship between the cost and weight of a vehicle and the amount of fun you can have driving it. The cheapest and lightest vehicles here are the most enjoyable to drive in a spirited fashion. They also get slightly better fuel mileage than all but the Volvo, though not as much better as you might expect. Third, the wagons are generally less ponderous and more pleasant to drive than the SUVs, with equal or superior seating and luggage space for a given wheelbase, and they also get better fuel mileage. The SUVs, however, tend to have better towing capacity for those with boats, trailers, etc., and better ground clearance for serious off-roading. No surprise there. But now that the wagons are offering variable ride height, that latter advantage is less pronounced. All depends on how much utility and/or sport you need. As mentioned, the Volvo wagon was everyone's idea of the ideal road companion on this trip. It's that rare combination, a Grand Touring utility wagon that goes through the snow and is a pleasure to drive. With our testing done, we left the little town of Brimley on the shining Big-Sea-Water, heading south along the highway, through Hiawatha's famous forest, through Manistique and Escanaba, down into Wisconsin, with a stop for pie and coffee, to the town that beer made famous, where we stopped to take our rest in a lodge along the I-road. The rest drove to Chicago, to fly home to California, and I drove home across Wisconsin in a driving winter snowstorm. Different Diffs The six vehicles in this story are not purpose-built rally cars, but they are designed to function in the same type of environment. Each of the wagons and SUVs here has some form of all-wheel drive that allows it to overcome the lack of grip due to dirt, snow or water on the road. In addition to modern traction control, the differential is also responsible for managing available grip. A differential's function is to allow the driven wheels to rotate at different speeds in a corner, while dividing the driving force appropriately. In an awd car not only is there a speed difference from left to right, but also front to rear. So front, center and rear differentials are needed. Differentials come in different forms and vary in their operation. The basic idea is the open type shown in a nearby drawing. Open diffs are the most common form of differential, but they have the side effect of always providing the same amount of torque to each wheel. So if you are cornering hard and lift a driven wheel off the ground, all the engine power will go to spinning the wheel in the air with none to the wheel on the ground. Gradations of this also happen when driving on a slippery surface. With an open diff, the tire with less grip determines the power that can be applied. Not a good thing when you are racing and want to apply power in a corner, or when you are driving with half the car in snow and the other half on pavement.
The opposite of a diff is a spool, meaning both wheels turn at the same speed no matter what. A spool would have no problem going straight, but imagine the havoc of trying to turn. Spools resist a car turning, thus they are mostly found in drag racing where going straight is all that is desired. A locking diff is an open diff until a switch is thrown and the unit locks together to become a spool. Locking diffs are used only for low speed and serious off-road duty because they give the utmost in grip, but the least amount of steering stability. This brings us to the LSD, Limited Slip Differential. LSDs fit conceptually between a spool and an open diff; two common types are the viscous limited slip and the gear type. Either directs power to the wheel with more grip, not less. Tradeoffs still exist. LSDs act like an open diff until elements in their design force them to start locking. Unlike locking diffs, LSDs never fully lock, but instead are characterized by their bias ratio. A "3:1" bias ratio means that an LSD can deliver three times the torque of the wheel with the lesser traction. The viscous LSD depends on a speed differential of rotating plates attached to the halfshafts. Under normal straight conditions, the plates are rotating together, and there is no speed differential. When one wheel starts to spin faster (because it is on ice, for example), a speed differential grows between the plates. A special fluid surrounding the plates adds friction that binds the plates together, thereby resisting the speed differential and transferring torque to the wheel that isn't slipping. A gear type is also referred to as a torque-bias or torque-sensing (Torsen) differential. Within is a gear set that, when driven backward due to a torque differential, resists the rotation, the resistance coming from the meshing angle of the gear teeth. One benefit about a torque-biasing diff is that its gears react so quickly that actual wheelspin is not needed to transfer torque. This fact means that it responds before a loss of traction has occurred. The downside to a torque-biasing diff, besides its expense, is that it functions like an open diff when one wheel is in the air. Thus, this diff is suited more for sports cars than wilderness-adventure vehicles. Toyota's RAV4 and the Subaru Impreza come standard with a center viscous LSD. We noticed that the Subaru had less wheelspin and better traction than the Toyota. However, the RAV4 had a more stable quality, whereas the Subaru tended to drive sideways without great provocation. Volvo's Cross Country has a system called TRACS that under normal conditions is 95-percent front-wheel drive. Upon wheel slippage, a clutch is engaged to transfer more torque to the rear wheels. For further control of wheelspin, brakes can be applied independently at each wheel. When a wheel is slipping and all the torque is being transferred through the open diffs to that wheel, that wheel's brake is applied and torque can be passed to the wheels with traction. A similar system called VTM-4 is used on Acura's MDX. At speeds less than 18 mph the system can control torque independently at the rear wheels through a set of electronically controlled wet clutches. Normally the system is disengaged and is front-wheel drive, but unlike the Volvo it doesn't wait for wheelspin to engage. Instead it is proactive: Depending on throttle and acceleration, the rear wheels will be brought into play. From our notes it seemed the MDX launched better in the snow, but as soon as 18 mph is exceeded the system would disengage and it would be back to front-wheel drive. The MDX seems much better suited to low-speed deep-snow duty, while the Volvo is more adept on high-speed snow-dusted roads.
BMW's X5 uses two open diffs, the center being a fixed planetary gear set that sends 62 percent of torque to the rear; this, to retain a sporty rear-wheel-drive dynamic. Traction control in the BMW is called AST and it uses individual brake and throttle modulation to control wheelspin. Audi's Allroad is more traditional and uses a Torsen center diff, but also controls wheelspin with individual brake modulation. Looking back at our notes, I cannot tell which system worked better, but it was generally agreed that the BMW was better in the snow, not because of its awd system, but tires. Though identified as M+S, the Audi's tires had an aggressive low-profile appearance. As one editor put it (and most agreed), the Allroad felt slithery. — Shaun Bailey In My Opinion ... The Audi Allroad is by far the most visually aggressive and intriguing, and inspired me to want to go for a drive anytime. In the middle group, nothing comes close to the driveability of the Volvo XC, with its quick steering and positive feedback. The performance of the rough-and-tumble rally-bred Subaru Impreza wagon and its manual gearbox make it my favorite. — Shaun Bailey, Road Test Assistant The Volvo V70 XC stands out as the best all-weather sports-utility vehicle. Great, predictable handling and traction with plenty of cargo space. The Subaru provides the most fun driving in slippery conditions. The BMW X5 with its traction and power edges out the Audi Allroad, which has lost the crisp road manners of its Audi brethren with this iteration of the Quattro. — Richard M. Baron, Art Director In the most expensive category, my choice is the Audi. Beautiful styling, inside and out, complements the excellent handling and fine ride character. In the middle-price category, the Volvo V70 XC is the clear winner (and the one I would buy out of all six) for everyday utility and driving pleasure. If a bargain is in order, the Toyota RAV4 jumps to the forefront. — Thos L. Bryant, Editor-in-Chief Even though its all-season tires don't do justice to the Audi Allroad's adjustable suspension and Quattro all-wheel drive, I still find a warm spot in my heart for this wagon. In the mid-price segment, the Volvo XC stood out as the perfect companion for a romp through Michigan's version of Scandinavia. And the Toyota RAV4 won me over with its spunky styling. — Matt Delorenzo, Detroit Editor For everyday driving or long-distance road trips, I prefer the size, general ergonomics and cabin configuration of the stylish and beautiful Audi, but as I do tow a boat and a race-car trailer, the BMW X5 gets my vote. In the middle, the Volvo is my pick, and in the low-price range the Subaru gets the "sports car masquerading as a small wagon" award. — Peter Egan, Editor-at-Large To drive these all-wheel-drive cars and sport utes in the Upper Peninsula was an experience. The nod here goes to the BMW with its confidence-inspiring handling. Between the Acura and the Volvo, the V70 XC wins my vote for its better all-around performance. And while the Subaru Impreza is fun, the Toyota RAV4 is surprisingly nimble and sure-footed. — Patrick Hong, Road Test Editor
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As a translation of BMWness into SUV-dom, the X5 is pretty successful. It
has all the smells, sounds and feel of a high-quality BMW sedan, including
the smooth, ripping sound of that glassy straight-6. Nice styling, too,
everyone thought, a good morphing of the BMW look into the bulkier off-road
idiom. 
But if the raised ride height and suspension tuning have given the Allroad
more of a friendly "family" wagon feel, the Audi is also a very civilized
place from which to observe the wilderness. The superbly comfortable seats
are heated, front and rear, and the rear-seat room is much better than the
X5's — four large adults could happily tour in this wagon — and there's a
commodious rear luggage bay.
When John Lamm picked me up in the MDX, I asked, "How is this thing?" and he
said, "Perfect. Beautifully engineered, functional and a little bland."
Which more or less sums up our overall impression.
With its uncluttered, handsome styling that says wagon, without any pretense
of being anything else, good luggage space, largely sensible controls
(except for the radio, as usual), a smooth and powerful turbocharged
inline-5 and 8.5-in. maximum ride height, it makes a pleasant touring
partner for the snow country.
If
there were a Most Improved award in the group, it would go to the RAV4. This
new version has much more handsome lines than the old one, more horsepower
and better suspension. Moreover, it has a feeling of substance and solidity
that was missing before.
The
Subaru's flat-4 engine is busier than the others and sounds a bit more
wound-up at highway speed, but some of us (this writer, for instance) like
that boxer snarl. The manual transmission snicks easily through its gates,
the steering is quick and the pedals are well located. You have to work the
Outback a little harder than the others when the pace picks up, but it's
enjoyable work. Wakes you up, like a strong cup of coffee.
