4x4 Explained

Introduction to All Wheel Drive systems - By Eliot Lim

Part time, manually engaging versus full time/part time auto engaging

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.

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