By Jake Venter
At first, off-road vehicles relied on four-wheel drive to ensure that excessive power would not cause wheel spin. Later, limited-slip differentials and locking differentials were introduced to couple the wheels together and so reduce the prevalence of wheel spin.
The computer age has made it possible to use electronic controls to brake a spinning wheel, or reduce throttle openings and retard ignition timing to produce the same effect. This advance has led to worthwhile extra accessories such as hill descent control and trailer towing control. The former was first introduced by Land Rover, and has reduced problems associated with going down a steep incline on a loose surface. The latter has reduced the danger of coping with a swaying caravan.
Advanced electronic off-road driving aids, such as Land Rover’s Terrain Response system, have become popular. On the Discovery 4, this system employs a full-time 4WD system plus
an electronically controlled limited-slip diff to offer the driver a choice of modes that vary the front-to-rear torque ratios. These are: general (full-time 4WD), grass-gravel-snow, mud-ruts, sand and rock crawl modes.
Hybrid 4x4s, such as the Lexus RX450h, will find a niche market. The Lexus has one electric motor at the front coupled to a petrol engine and another electric unit at the rear. Normal drive is through the front wheels, but the rear motor comes into action automatically when wheel spin is detected at the front.
Torque vectoring differentials, also known as active differentials, have started to make an appearance, but they are expensive and complicated. They are able to exercise some measure of individual control over the torque being sent to each driven wheel without having to wait for wheel spin to occur. At present, most of them are fitted inside the rear axles of 4WD vehicles.
Some theory is needed to understand why they are desirable. The body of a cornering car rotates around a vertical axis through the centre of gravity. This movement is called yawing, and it results from the forces generated by the four tyres in response to the steering wheel movement. The total torque produced in this way around the vertical axis is called the yaw torque.
Each wheel contributes a different torque to the total because each wheel carries a different vertical load. For example, on a front-heavy car, the outside front tyre carries the heaviest load, but on a rear- heavy car, the outside rear tyre carries the heaviest load.
Torque vectoring differentials are able to supply additional yaw torque to increase vehicle stability, and we’ll take a look at two of them. Both are based on the ZF company’s Vector Drive design.
Dynamic Performance Control (DPC) is an addition to BMW’s xDrive, and is fitted to X6, X6 M and X5 M 4WD models. It not only curbs wheel spin but also introduces additional yaw torque to improve stability during cornering. It comes into action as soon as the hardest-working tyre has reached its traction limit.
For example, if the outside front tyre is close to the breakaway point, the system applies more torque to the outside rear tyre and less to the inside rear tyre. This supplies an additional turning torque that increases stability by not asking the outside front tyre to generate more cornering force than it is able to.
The system is controlled by a computer that uses the ABS speed sensors as well as a yaw sensor to keep track of what the car is doing. There is a planetary gear set on each side of the differential, and when intervention becomes necessary the normal ratio of 1:1 is changed to 1,25:1 on the side of the wheel that needs more torque, with the result that the other side gets less torque. This produces the necessary extra torque in the right direction.
Audi’s version, available on the S4, is called the Active Sport Differential (ASD). It combines their front-to-rear torque-splitting Quattro system with the ZF vector drive to achieve similar results to the BMW unit. Because the S4 is not an off-road vehicle, this system is used on the rear wheels to curb the understeer that normally accompanies 4WD.