What Is Slip Angle?
A slip angle is the difference between the steering angle and the direction in which the tyre foot print is taking. Tyre slip angles play a major part in steering systems and have a large effect on steering geometry such as Ackermann and toe.
When a tyre runs at slip angle, the tread and carcass of the tyre distort in the vicinity of the tyre/track contact patch. Essentially the tread points in the direction of motion and the carcass distorts to accommodate this.
Every tyre has its own slip angle curve that helps to explain how the tyre grip levels change with relation to slip angle.
The diagram below is a tyre slip angle vs cornering force graph.
It can be seen that for this particular racing tyre the cornering force or lateral force increases up to a slip angle of between 5 and 6 degrees in each scenario before grip tails off. At this point, the car would go into understeer if these were the front tyres being observed.
It is worth noting that as the normal load on the tyre increases, so too does amount of lateral force available from the tyre at any one slip angle. If the above model represents the front of a car then it shows that a car with 50/50 loading on both tyres mid corner has more overall grip than a car that transfers 80% of its mass onto the outside tyre and leaves the inside tyre with only 20% mass. This imbalance causes less cornering force available; however, it is a common scenario in fast circuit racing.
Most racing tyres at the maximum lateral G often get up to 7 or 8 degrees of slip angle. Dirt tyres, used in rally or speedway, can get up to a 40 degree slip angle.