Toe & Caster

Toe

Toe is defined as the difference of the distance between the leading edge of the wheels and the distance between the trailing edge of the wheels when viewed from above. Toe-in means the front of the wheels are closer than the rear; toe-out implies the opposite.

For a rear-wheel-drive vehicle, the front wheels normally have a slight amount of toe-in. When the vehicle begins to roll, rolling resistance produces a force through the tire contact patch perpendicular to the rolling axis. Due to the existence of the scrub radius, this force produces a torque around the steering axis that tends to cause the wheels to toe-out. The slight toe-in allows for this, and when rolling, the wheels align along the axis of the vehicle. Conversely, front-wheel-drive vehicles require slight toeout. In this case, the tractive force of the front wheels produces a moment about the steering axis that tends to toe the wheels inward. In this case, proper toe-out absorbs this motion and allows the wheels to parallel the direction of motion of the vehicle.

Caster

Caster is the angle of the steering axis from the vertical as viewed from the side. Positive caster is defined as the steering axis inclined toward the rear of the vehicle.

With positive caster, the tire contact patch is aft of the intersection of the steering axis and the ground. This is a desirable feature for stability.

When the wheel is turned, the cornering force acts perpendicular to the wheel axis and through the contact patch. This creates a torque about the steering axis that acts to center the wheel. Obviously, negative caster results in the opposite effect, and the wheel would tend to continue turning about the steering axis. The most common example of positive caster is a shopping cart. The wheels are free to turn around the steering axis, and when the cart is pushed straight ahead, the wheels self-align to the straight-ahead position.