The Slip Angle
You may know the basics of how a tire works, but there's more to it than just rubber rolling along the ground. Tires are elastic and deform, and that's part of how they grip. However, the nature of how this happens is very important. In this article we'll explore the basics of how tires deform.
Here is a drawing showing what the tread of a tire does during a left turn:
The angle between the direction the tire is pointing and the direction the tire is moving is called the slip angle. No matter how gentle you turn, accelerate, or brake, the tire will be generating a slip angle- be it small or large. The larger the slip angle, the more grip that tire will have- to a certain point. A tire generating 100lbs of cornering force may be doing so at a one degree slip angle, while the same tire generating 200lbs of cornering force may be doing so at a two degree slip angle. The curve goes up almost linearly until it reaches a peak, and then grip tapers off beyond that. At that point, the tire is being pushed too hard and will wear and heat very quickly instead of gripping. Some tires taper off very rapidly while others taper more gradually- these tires are said to feel very "progressive" and predictable when driven at the limit.
Different tires deform different amounts and thus reach their peak amounts of grip at various slip angles. A street tire might see its peak grip at 15 degrees of slip, but a racing tire might see its peak grip at five degrees of slip or less. Wider, more aggressive tires deforms less and thus work best at smaller slip angles. Beyond this threshold, the tire can lose grip very quickly. This is why "drifting" a car is usually very slow- the tires are well beyond their optimal slip angles for grip.
This is also where we get the definitions of understeer, oversteer, and neutral steer. A car that understeers will have greater slip angles in the front than the rear. A car that oversteers will have greater slip angles in the rear, and a neutral vehicle will have similar slip angles at all four tires.
Beneath the tire, a high slip angle causes a few things to happen. The tread will try to bend so with the direction of the slip angle since it is trying to grip the asphalt your are slipping along. Look at the illustration below where the tread slips off towards the trailing edge; we want to delay this as much as possible for optimal grip. This shows tread deformation in the contact patch:
The goal here is really to have a short enough contact patch that the tread can conform along the slip angle. We want to delay that drop off near the trailing edge of the contact patch. Wider tires accomplish this with a shorter contact patch whereas racing compound tires are optimized for very low slip angles.