The Ultimate Handling Guide Part VIII: Understanding Your Caster, King Pin Inclination and Scrub
By Mike Kojima
Understanding what caster does and how to use it is a powerful tool in the box of a suspension tuner. Since caster is not adjustable on nearly all FWD cars and usually not adjustable for many late model cars as well, we saved its discussion until now. Discussions of caster pertain to the front suspension only as caster comes into play as steering angle is induced.
|Caster in a car is like fork rake on a bike. They have about the same effect on both.|
Caster is the angle from vertical of an imaginary line drawn when looking at a car from a side view through the ball joints of a multilink suspension car to the ground. On a MacPherson strut car the caster angle is the angle of the line from vertical drawn from the top strut mounting point through the lower ball joint to the ground. Think of caster as the fork rake angle of a bicycle or a motorcycle.
|This is positive caster in a car. It is the angle about which the steering pivots in relation to the front and rear of the car.|
Positive Caster is when the angle is sloping backward toward the rear of a car like fork rake on a bicycle or motorcycle. Negative caster is when the angle slopes forward. Negative caster in the steering geometry just doesn’t work so we won’t discuss it any further. Vehicles do not use negative caster.
|This illustration shows positive caster, negative caster which no car ever has to our knowledge and the shopping cart caster analogy which we will discuss on the next page. You should be getting the idea by now!|