Understeer & Oversteer

An explanation of these terms.

These are basically the technical terms for front and rear wheel skids, respectively.
A tyre loses adhesion when forces acting on the vehicle exceed the coefficient of friction between the tyre and the road surface.

Many things can cause this, such as excessive speed for the road conditions, sudden braking, fierce acceleration and harsh or excessive steering. A car’s condition can also play an influential role, particularly if the tyres or brakes are worn.

Skids don’t just happen. They are a product of poor concentration and failure to take road conditions into account. If you take a corner too fast, for example, the front tyres may not have enough grip to cope with what you are asking them to do, and the car will tend to continue straight on (a car without anti-lock brakes will also slide straight on, irrespective of any steering input, if you lock up the wheels under emergency braking).
This is understeer, so called because the car is literally turning Less than you want it to.

Oversteer is slightly more complicated, because front-wheel drive and rear-wheel drive cars handle differently. In a FWD car, lifting off the power too sharply in mid-corner will cause a transfer of weight towards the front of the vehicle; the back end will go light and the rear tyres will lose grip.
The same thing applies to a rear-wheel drive car. You can also break the adhesion of the rear tyres by applying too much power while cornering. Either way, the back end will slide wide and the car might start to spin. This is known as oversteer because the car is turning more than you want it to.

To recover an understeering car, you must ease off the power and/or reduce the steering input until the front tyres regain sufficient grip to negotiate the corner.

More than 90 per cent of all accidents involve a skid. It is harder to contain an oversteering car, particularly if you are inexperienced. You can apply “opposite lock” by turning “into” the skid (in the same direction the tail is moving — turn left to counter oversteer in a right-hand bend, and vice versa). Because you are effectively pointing the front wheels in the direction you want to go, this is a more natural reaction than it sounds, but many drivers do it too aggressively. If that happens, the car might start to oversteer in the opposite direction when the tyres regain grip, and you are likely to find yourself “fishtailing” down the road.

In a FWD car, more power might help to pull the car out of oversteer, in a RWD car, you must counteract the cause of the skid; more power will exacerbate power oversteer, but lifting off will cause a weight transfer to the front and further reduce grip at the rear.

Remember that a car is much more likely to spin out of control in damp, wet or icy conditions, but never forget that you could encounter an unexpected hazard —such as a diesel spillage, mud, sand, loose gravel or fallen leaves — on an apparently dry surface.