Subject:
Slip Angle:
The sidewall of the tire is always flexible. The advantage of this is that this part deforms over obstacles on the road surface. The disadvantage is that during steering, the sidewall causes reduced stability. The tread wants to follow the steered direction, but the wheel itself will, due to the lateral force, be positioned in a different direction. The slip angle is the angle between the original and the actual driving direction. This is clearly visible in the image below.
The original driving direction is straight ahead. There is no steering. The large red arrow marked “F wind” indicates the crosswind; it pushes against the car. The sidewalls of the tires deform due to the lateral force.
Due to the deformation, the driving direction of the car changes. It follows with the crosswind the arrow of the actual driving direction. This causes the driver to steer counter to maintain the original driving direction. A larger F wind results in a greater angle between Actual and Original driving direction. This angle is therefore referred to as the slip angle.
Here are two situations:
- The wheel in the normal (rolling) situation.
- A lateral force acts on the wheel. The lateral force pushes against the hub and the center of the rim, as now indicated by the arrow direction. The tread “sticks” to the road surface. The sidewall of the tire provides the flexibility between the tread and the rest of the tire. The height of the sidewall and the tire pressure greatly influence the deformation of the sidewall.
It is understandable that a soft sidewall does not benefit the driving characteristics of the car. The higher the sidewall (and the softer the tire pressure), the more it deforms. The tire size 195/65R15 has a high sidewall and the size 225/40R18 a relatively flat sidewall. The tires have the same rolling circumference, so they are comparable. The 15″ tire will lean more in the corners than the 18″. This makes the 18″ a suitable tire for sportier driving. There is also more feedback between the steering wheel and the road surface. The downside of the flatter, stiffer sidewall is that comfort deteriorates because the car becomes stiffer. The sidewall is too flat to have as good a cushioning effect as the 15″.
