Contact information for the wheel, reported by WheelCollider.
Friction for the WheelCollider is computed separately from the rest of the physics, using a slip based tire friction model. This allows for more realistic behaviour, but makes wheel colliders ignore standard PhysicMaterial settings.
The way to simulate different ground materials is to query WheelCollider for its collision information (see WheelCollider.GetGroundHit). Usually you get the other collider the wheel is hitting, and modify wheel's forwardFriction and sidewaysFriction based on ground's material.
The other members of WheelHit structure are usually queried for information purposes or special effects. E.g. a "slipping tire" sound can be played if forwardSlip or sidewaysSlip exceed some threshold.
See Also: WheelCollider.GetGroundHit.
The other Collider the wheel is hitting.
The point of contact between the wheel and the ground.
The normal at the point of contact.
The direction the wheel is pointing in.
The sideways direction of the wheel.
The magnitude of the force being applied for the contact.
Tire slip in the rolling direction. Acceleration slip is negative, braking slip is positive.
Tire slip in the sideways direction.