The Physics of Grip

Grip is a wonderful thing. It keeps you on the road, it allows you to accelerate, turn corners, and then stop again. However, there is a limit to tyre adhesion and this, sir, is the limit of your car. It's vital to have a grasp of the basics of grip to help you drive on the limit.

Think about the area of tyre that is actually in contact with the road. It's only a small patch of rubber which provides all the grip. (see Figure 1)

Figure 1: The contact patch of a tyre

In simple terms, when driving, a contact patch is most likely to experience either longitudinal or lateral forces. Longitudinal forces are experienced when accelerating or braking, and lateral forces occur when cornering (see Figure 2).

Figure 2: Tyre contact patch forces

The contact patch can only accept a finite amount of these forces before the limit of adhesion is reached. Put another way, if you're braking or accelerating hard, there is less adhesion available for steering (Figure 3). This is also usually the time when a tree decides to jump out in front of you.

Figure 3: Grip vs driving forces

If the sum of lateral and longitudinal forces exceeds the available grip, then wheel spin, wheel lock or sideways sliding will occur. In bar C in Figure 3, the driver is braking hard, then applying a degree of steering lock. If the driver tries to turn too tight a corner and exceeds the limit of grip on the front tyres then understeer will result.

Weight transfer to your advantage

The grip levels of any tyre contact patch can be artificially increased by applying more weight to that wheel (this is the basis of aerodynamic down-force assistance). Therefore front engine cars tend to have more grip at the front wheels and vice versa. In addition, forward, rear, and sideways weight transfers affect the availability of grip at each wheel individually.

Increasing grip

Levels of grip can be artificially increased (or reduced) by several factors.

Firstly, the weight acting on a particular wheel directly affects the amount of grip that is available. More weight equals more grip, but obviously you don't want to be adding lots of additional weight to your racing car! This is where aerodynamic aids come in handy - a properly developed spoiler can generate significant down force at speed, which can help increase grip, but bear in mind at lower speeds you'll be no better off.

Secondly, the size of the contact patch is directly proportional to the amount of traction it can develop - the simple solution here is to fit wider tyres.

Figure 4: Wider tyres = larger contact patch = more grip