An RC car's kingpin is a shaft or pin around which the steering block (which holds the axle and wheel) rotates.
The kingpin is often used to change the caster angle. By leaning the kingpin back at the top, when viewed from the side, it increases the car's caster. On pan carsit is sometimes used to set the camber angle as well.
On on-road pan cars, the kingpin also plays a role in damping. The steering block rotates on the kingpin, and the space between the kingpin and the steering block is used to create damping by filling the space with heavy oil or grease. Learn more about damping.
Sometimes the center of rotation of the steering block is slightly offset from the kingpin, which is known as trail.
The kingpin will often lean inwards (towards the chassis) at the top, creating kingpin inclination. Inward lean is a positive inclination. This works with the scrub radius to affect the responsiveness and stability of the car.
Scrub radius is the distance between the center of the tire where it hits the ground and the rotational axis of the tire, which is where a line drawn through the kingpin hits the ground (see diagram below.)
More KPI results in a lower or more negative scrub radius, and less KPI gives higher or more positive scrub radius.
Positive scrub radius (kingpin axis inside tire center):
- Increased on-power steering
- Less off-power stability
Negative scrub radius (kingpin axis outside tire center)
- Less on-power steering
- More off-power stability