4x4 Explained

Introduction to All Wheel Drive systems - By Eliot Lim

Tourque Splits

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.

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