‘Understeer’ and ‘oversteer’ are bandied around when car enthusiasts talk to one another. As is often the case with such terms, they can be used wrongly, largely because enthusiasm can outshine knowledge. But understeer and oversteer aren’t too difficult to understand.
Both these terms have two definitions, one technical and one real-life. Beginning with the technical definition will help you understand how the real-life one applies. There won’t be questions later but there will be very simple, final definitions of both understeer and oversteer. So pay attention at the back.
If you were looking down at a car’s wheel from the top as the car takes a corner, you’d see that the wheel follows the curved path the driver wants it to follow. However, you’d also notice that the wheel itself is pointing along a more tightly curved path. This is because the forces acting sideways cause the tyre to distort and this happens at any speed over about 10mph.
The difference between the wheels’ actual path and the direction in which it is pointing is measurable. Imagine a line drawn along the actual path and one drawn through the centre line of the wheel and tyre. The wheel centre’s line will be at an angle to the actual path’s line; it will be heading towards the inside of the corner.
This difference is called the ‘slip angle’ and it’s the key to understanding understeer and oversteer. If the slip angles at the front and rear of the car are identical, the car is in a ‘neutral’ state. If the front wheels’ slip angle is greater than that of the rear wheels’, the car is understeering. Lastly, if the rear wheels’ slip angle is the greater, the car is oversteering.
Now for the real-life definitions. In real-world terms, understeer is that feeling that your car isn’t cornering as tightly as you expected: you have to steer more into the bend. Oversteer gives a feeling that the car’s tail is trying to overtake you, you have to steer less into the bend.
In practise, road cars are generally set up to understeer, because it’s safe and predictable. This, however, is highly simplistic. Many different aspects, from tyre pressures to the way the weight is distributed in the vehicle, affect a car’s dynamics. However, some cars are ‘understeerers’, largely because they’re front-heavy.
Rallying is a particularly good example of a driving style where oversteer is used to advantage. Rally drivers tend to induce the tail of the car to slide outwards on the way into a corner, so that they can apply maximum power in a straight line on the way out. But don’t forget that any car can over and under-steer. For example, if you’re driving on snow or ice and you feel the car’s tail heading towards the outside of a corner, that’s oversteer.
Should you still have failed to grasp the notion of these two steering conditions, you can always rely on the simple definitions…
Understeer: when you can see what you are going to hit.
Oversteer: when you can’t.