Handling Traits

Introduction

The handling trait of a car depends on a great many things, including the weight distribution, to which wheels the car gets power, the suspension settings, and similar. But under most circumstances, a typical front wheel drive cars default to understeer, that is, the front wheels lose grip (“washes out”), and the car’s nose drifts wide or, in extreme cases (such as on an icy road), ploughs straight on.

When a front wheel drive car starts to understeer under power, this is because the front tyres are overloaded - that is, they cannot cope with the acceleration that the driver is trying to place on them. As an example, if a car is driving around in a circle at a given speed, the tyres are constantly accelerating in the direction of the turn. If the car increases speed, there will come a point where the car cannot drive any quicker in a circle of that size. Any attempt to increase speed will probably result in much wailing of tyres, and although the speed of the car may increase, the radius and diameter of the circle will also increase. We’ll call this the corner speed.

In the real world, most cars will not be able to demonstrate this, but for other reasons - unless the car has a dry sump, the oil would slop to one side of the engine bay, which would be A Very Bad Thing.

Rear wheel drive cars can also understeer and for the same reasons. Assuming that the rear wheels have sufficient traction, the front tyres of the car will also lose grip. However, because the front wheels are not carrying the load from the engines, the theoretical corner speed will be higher than an otherwise identical front wheel drive car.

Four wheel drive cars can also understeer, again because the front tyres are overloaded.

When a car is said to oversteer, this is because it is the rear tyres that lose grip first, and consequently start to skid. Generally speaking, front wheel drive cars will only oversteer if they are decelerating, that is, power off oversteer or braking oversteer. As an example, imagine the same front wheel drive car used above, but in this case, instead of maintaining speed, we will lift off the accelerator pedal and let the car slow down. Immediately, the car starts to slow down because of the extra friction caused by it going around a corner. This causes the centre of gravity to shift forward, which in turn increases the effective weight over the front tyres (giving them more grip) and reduces the weight over the rear tyres (reducing their grip). If the level of grip that the rear tyres now has is less than that required, they will not be able to corner as well as the front tyres - in order words, they will start to slip to the outside of the turn. If the driver then re-applies power, one of two things can happen. One is that the car picks up speed, stopping the skid. The other is that the car will adopt a neutral cornering attitude, where all four wheels are slipping. I’ll detail neutral handling cars later.

Try the same technique in a rear wheel drive car and you might initially get the same result - less grip at the rear will cause the tail to start to slide. But rear wheel drive cars can also overload rear wheel grip under load, thus causing power on oversteer.

Finally, neutral handling simply means that both the front wheels and the rear wheels lose grip at the same time, and the car slides to the outside of the corner. As a rule of thumb, many cars can be driven to exhibit neutral handling, but few seem to do it “right from the box.”