The Science of Race Tires - by Paul Haney, CART.com Contact Patch Deformation & Slip Angles We take it for granted, but the tire is a marvelous device. Those old-time, steel-rimmed cart wheels didn't generate much traction. It was the...
The Science of Race Tires - by Paul Haney, CART.com
Contact Patch Deformation & Slip Angles We take it for granted, but the tire is a marvelous device. Those old-time, steel-rimmed cart wheels didn't generate much traction. It was the horse pulling the cart around a corner, not the steel tires. Rubber is elastic; it deforms when pushed or pulled. This elasticity allows rubber tires to generate both traction and cornering force.
A rubber tire tread has the ability to mechanically key into the irregularities of the road surface, greatly increasing the traction the tire can generate. A high grip level without sliding reduces heat generation and improves wear. Cornering forces come from the flexible structure of the tire, the rubber and fabric in the construction and tread. It's the elastomeric nature of a tire that makes it work.
A car traveling straight down the road has its tires pointing forward. To turn the car, say, into a left-hand turn, the driver moves the steering wheel to the left, which rotates the front tires. For an instant after he first starts to move the steering wheel, the car still goes straight ahead. The wheel and tire need to complete almost a full revolution before steering input affects direction.
After the wheel and tire have rotated toward the turn, the very front of the tire patch contacts the road just a little to the left of where the car is headed. The angle between where the tire is pointed and where the car is headed is called the slip angle. This slip angle is how a tire generates turning forces, and it can't happen if the tire is rigid.
As the weight of the car comes on the leading edge of the contact patch, the tread rubber grips the road with a combination of adhesion, friction and mechanical keying. The car wants to continue in a straight line, but every little increment of tread that rotates into the contact patch is a little further to the left.
The action of the tread is similar to what happens when you change direction when you're walking. In a left turn, you point first one foot and then the other toward the left, and as your weight comes onto each foot, your body rotates in the same direction. Each step turns your body a small amount, and your path becomes an arc.
The tread rubber deforms and passes forces to the fabric construction of the tire, which also deforms. The tire tread and construction resist this deformation with a force that pushes against the road surface. The result is lateral forces on the front wheels that pass to the car through the suspension links and change the direction of the car.
If the rear wheels were on casters, these forces on the front tires would spin the car, causing the back end to swing out. But the rear tires are fixed -- they resist the spin with a slip angle and lateral forces of their own. As a result of the steering input and the tire slip angle, the car changes from moving in a straight line to traveling in an arc.