Are four driven wheels really better than two? Manufacturers would have us believe so, not the least reason is that four-wheel vehicles can be more profitable than two-wheel drives. A slew of marketing imagery from companies like Land Rover and Mitsubishi has given the four-wheel drive (4wd) vehicle an image of go-anywhere invincibility. However, not all four-wheel drive systems are born equal, so we’ll show you some of the basics of 4wd, the benefits and disadvantages of having 4wd, and what to look for in a four-wheel drive vehicle.
The idea behind 4-wheel drive systems is simple: by turning four wheels instead of two, a vehicle can continue to propel itself forward even if two wheels lose their grip. First of all, any 4wd system needs to have three basic components: a transfer case, an two differentials: one between the two front wheels, and the other between the rears.
The transfer case divides the turning force from the engine, known as torque, and sends it to front and rear differentials. The front and rear differentials then send the torque to each of the drive wheels. The differentials also allow a difference of speed between the left and right wheels, such as when the vehicle is turning. When a vehicle turns, the outside wheel goes faster than the inside wheel because it’s following a larger-radius circle. Most vehicles also have a center differential that allow for a speed difference between the front and rear wheels.
Four-wheel drive used to be a feature only of military vehicles and large pickup trucks. Audi was the first automaker to successfully introduce four-wheel drive in a car not with the intention for the owner to take it off-road, but simply to improve its roadholding characteristics. They gave it the system the brand name Quattro, and indeed they branded the car itself Quattro. “All-wheel drive” was coined as a marketing term to denote full-time 4wd, usually for improved roadholding instead of off-roading. Many cars are now available with four-wheel drive, described variously as Quattro, Full-Time, Real-Time, All-Trac, etc. There are two main types of four-wheel drive systems: part-time and full-time.
A part-time four-wheel drive system powers only two wheels when sufficient grip is available. When those two wheels (either the front or the rear) start to lose their grip, then the system either automatically distributes torque to the other set of wheels, or the driver has to push a lever or button to engage the other two wheels. On some vehicles, a low range can be selected to further increase the torque being transmitted to the wheels.
The CR-V is one example of a automatically-engaging part-time system. The “Real-Time 4WD” system usually powers only two wheels—the front. When the fronts start to lose grip, say on mud or sand, the system’s computer can detect the speed difference between the front and rear tires and automatically engage the transfer case. A pump activates and hydraulically engages a multiplate clutch, sending torque to the rear wheels as well. Some driving tests have shown that the Real-Time system does help in slippery conditions, but is ultimately not meant for off-roading. Deep sand can easily bog down the system, when the fronts are already deep in the loose sand before the rear tires can help out. The Mercedes Benz 4-matic is another example of an automatic part-type system but with all torque being sent to the rear, and the fronts engaging only when needed. 4Matic also uses a hydraulically-activated multiplate clutch.
There are several advantages with part time systems. Since they propel only two wheels most of the time, there is less transmission loss in the system due to friction. This can lead to better fuel economy versus a full-time system. Audi, however, claims that full-time 4wd is actually more fuel efficient, as there is less traction loss due to wheelspin.
Also, the car can retain the handling characteristics of a front or rear-wheel drive car. Drivers who enjoy controlling the car direction using the throttle as well as the steering can still have the “pure” feel of a rear-wheel drive car, with the fronts ready to help out in slippery conditions.
As previously mentioned, 4wd vehicles have three differentials: front, center and rear to allow speed differences between wheels. Some differentials allow unlimited speed differences between the wheels—these are termed “open” differentials. The problem with open differentials is that they allow one wheel to spin faster than the other. If one wheel is stuck in mud or raised off the ground, the open differential sends all the torque to that wheel, thus causing it to spin uselessly, while the other wheel doesn’t turn at all. In the case of mud, the wheel just digs in deeper and the vehicle gets more stuck. You may have experienced this in a muddy parking lot.
To avoid this situation, most 4wds have limited-slip differentials. The simplest of these differentials is called a viscous coupling: the diff transfers torque via a silicone fluid in the casing. The silicone allows a speed difference between the two wheels, but if the speed difference increases, the fluid gets churned and gradually becomes harder, to limit the slip between the two wheels. Another type is the Torsen (short for torque-sensing) differential. The Torsen is a mechanical device consisting of worm gears and a worm wheel. The Torsen usually splits torque 50:50 but when one wheel starts slipping, it sends torque to the wheel with more grip, or the opposite of what an ordinary diff would do. Being mechanical, Torsen reacts quicker than a viscous coupling and helps to prevent wheel slipping rather than just reacting after wheel slip takes place.
The latest systems such as those on the Mercedes M-class and BMW X5 use open differentials. A computer uses the ABS sensors to monitor differences between wheel rotation speeds, and if it senses wheel spin, it orders the brake at the offending wheel to activate. This slows down that wheel and transfers torque to the other wheels that actually have grip. Theoretically, these cars can get moving even if only one wheel has grip. Such systems save on weight and complexity by not bothering with a limited-slip differential.
Four-wheel drive is no longer limited to pickups, SUVs and station wagons. Some of the highest-performance cars in the world use 4wd. These cars have so much torque available that they can easily cause the rear tires to break their grip in hard acceleration or cornering. The availability to send some of that torque to the fronts when needed is an added measure of safety and improves the performance, too. One of the first supercars to have 4wd was the exotic Porsche 959. The 450 bhp from its twin-turbo engine was sent to all four wheels for good measure. This was not just for show. The 959 bagged 1st, 2nd and 6th places in 1986’s Paris-Dakar rally.
Sports cars that retain their rear-wheel drive “feel” while being able to send some propulsion forward include the Porsche 911 Turbo, Nissan Skyline GT-R, and Lamborghini Murcielago. For today’s World Rally Championship cars, 4wd is a must. Thus championship-winning cars like the Lancia Delta Integrale, Mitsubishi Lancer Evolution and Subaru Impreza Turbo all have 4wd. Lucky owners of their roadgoing counterparts can play like McRae with the unflappable grip that the cars’ computer controlled-4wd systems can provide.
Notably, several Formula One cars that were propelled via four-wheel drive systems were introduced, such as the Lotus 63 from 1969. None however achieved much success. The added weight of such systems, and the success of today’s computer-managed traction control systems somehow negate the need for 4wd in today’s F1.
Four-wheel drive has indeed come a long way from the World War II Jeep. It has made its way into the fastest and most fun-to-drive cars on the planet—wherever they may be driving.
Sources:
Introduction to All-Wheel Drive by Eliot Lim
How Stuff Works
Audi
Honda
Porsche
Rally Live
World Rally Championship.Net
Toyota
By Jason Ang | Photos Courtesy of Mercedes-Benz, Toyota, Peugeot, Subaru, Porsche, Honda, Ford
Originally Published October 2001 Issue
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