The front tire caster angle is the forward or backward tilt of which component?

• A. The steering wheel
• B. The kingpin axis
• C. The axle centerline
• D. The steering axis

Answer: (D)

Caster angle refers to the tilt of the steering axis relative to vertical when you look at the wheel from the side. The steering axis is the imaginary line through the suspension’s pivot points that the wheel rotates about as you steer. In many cars that axis runs through the upper mounting point of the steering knuckle (or strut) and the lower ball joint, and in older designs it’s described by the kingpin axis. So the front tire caster angle is the forward or backward tilt of that steering axis. This is not related to the steering wheel, nor to the axle centerline; it's about the line about which the wheel pivots during steering. Positive caster helps with straight-line stability and steering feel, while zero or negative caster would reduce that stability.