When it comes to vehicle handling, the first function of the tyre is for the full footprint to remain in contact with the surface of the road for as much of the time as possible. The tyres are the only link the vehicle has with the road. The behaviour of these tyre footprints can determine how safely a vehicle performs.
The surfaces of the road will all give different amounts of grip. The compound of the rubber determines the amount of interlock between the tread and the road surface.
Under a sliding condition, braking, wheelspin or break away, the interlocking rubber is actually ripped off the tyre, and this is what leaves skid marks on the road. Tyre-width, suspension settings, wheel-rim width, temperature, tyre pressure, and load all influence tyre grip.
Grip is responsible for traction while accelerating and braking and it also generates cornering force, the only difference in all these cases is that the direction of the force is different. A tyre with high grip will corner fast, accelerate rapidly without spinning the tyre and brake quickly.
A tyre has a maximum amount of traction force available, which can act in any direction. It can be entirely cornering force, entirely acceleration force, entirely braking force or a combination of the three. To increase cornering force, acceleration force has to decrease when the tyre is at the limit of adhesion.
Understeer is the condition when, during cornering the vehicle continues, or tries to continue, in a straight line because the front tyres have reached the limit of their adhesion before the back tyres have.
This is the opposite of understeer in that it is the rear of the vehicle that slides out when the limit of adhesion is reached while the front tyres are still gripping the road.
For the average driver, oversteer is best avoided. The reason for this is that, in an oversteer situation, the average driver's natural instinct is to react incorrectly by attempting to steer excessively against the breakaway. This, in turn, causes the rear of the vehicle to respond by breaking away in the opposite direction, usually more severely than the initial breakaway. This leads to what is often referred to as "fish-tailing" until the vehicle becomes so "side-on" to its direction of travel that it begins to "roll".
The natural instinctive reaction to understeer is usually correct, that is, to apply more steering force in the desired direction. If grip is lost at that point, the vehicle continues in its direction of travel in the "straight-ahead" position, regardless of the direction in which the front wheels are pointed.
In order to remedy either of these two situations, cornering force must be increased by decreasing acceleration - not braking - and attempting to reduce the radius of the turn gradually. Understeer is basically stable. In moderation it can easily be controlled by the driver and is quite acceptable . . . when mixing tyres with different cornering powers, always fit the lower cornering power tyres (more easily distorted) on the front wheels. This way the change in vehicle behaviour caused by the "mixing" will be towards understeer." It is the general aim of motor manufacturers to produce cars with slight understeer.