Cornering: cornering force vs slip angle

Everyone says that the best way to improve your car's handling, the best mod to do at the very beginning is and always is the tire. Good tires stand for excellent handling and bad tires mean terrible cornering.

What is the story of tires and cornering?

Cornering force vs slip angle

In general when a car goes through a corner, there is a centrifugal force out there, and you could feel being thrown towards the outside. In order to successfully drive out the corner, the car would need some lateral force to help support the car push against that centrifugal force. That is the so called cornering force. Due to the cornering force as well as the acceleration force on the tire parallel to the tire direction, the force would be directed towards some angle from the tires' direction (assume that ideally all the tires have the same properties, which point to only one direction). The angle is the slip angle. You can notice obvious slip angles in any racing event when race cars make turns. Here is a top view of the deformation of the rubber contact patch on the tire. The slip angle is the angle between the arrow and the vertical line (tire direction).

The following figure not only depicts vividly the distortion of the tread on the tire but also points out the side force (centrifugal force) and the cornering force. The cornering force always tries to pull back the car from skidding as you can see.

Tire distortion and cornering force

The cornering force and the slip angle kinda supports each other. This is like the 'chicken or egg' problem. What on earth is the relationship between these two?

Cornering force vs slip angle

Shown above, the racing tires have high peak curves. The largest cornering force means racing tires enable really fast cornering speed. Racing tires could hold you there! The maximum force is reached earlier than the ordinary street tires. However, street tires are flatter curves, which indicates it is easier for drivers to get aware of the change of the slip angel when cornering. The driver won't lose control of the vehicle because it gives time for the driver to feel the gradual change. The racing tire really outperforms the street ones but its thin top won't give you a second chance once you harshly pass the peak force limit. You may feel the tires lose traction and start skidding. There is few warning before it happens.

Nowadays, the racing tires could be manufactured so that it won't drop that hard when it goes beyond that peak point.

Grip

In real life we don't need to talk about cornering force or slip angle. They are just there and grip comes as a more handy term for describing tires. Grip is traction or the tractive force between two surfaces. Continuing with the above example, racing tires are normally high grip tires. Racing tires don't have much tread on it for the reason that the bigger the contact patch is, the larger the traction the tire could offer. Compared with racing tires, street tires have complicated tread so that they handle driving in the rain perfectly. Hydroplaning is a consequence of worn tire tread. In other words, you can drive on fantastic soft, no-tread tires on dry racing tracks fast and furious but too dangerous on wet, water covered roads. You won't be able to control your car's direction, no matter how hard you pull your steering wheel!

Check your tires often

Last but not least, if you are racer, you probably have already been taking care of your tires for a long time. But if you are not, don't forget to check your tires quite often. Here, 'check' means:

1. Check the tread of all four tires to make sure they are not worn out. Use coins to do that.
2. Check band and sidewalls of all four tires so that no scratch or severe damage is there. Replace the tires of damages as soon as possible.
3. Check tire pressure every 2~3 weeks, or every time you are ready for a road trip.