Caster works with another aspect of steering geometry known as kingpin inclination angle or KPA for short. Another name that can be interchanged with this is steering axis. This is the angle of the line from vertical drawn through the same joints as caster but viewed from the front of the car instead. The KPA is always sloping towards the center of the car and expressed as degrees. KPA is not adjustable on production cars but commonly is on race cars.
|Kingpin Inclination Angle or KPA is demonstrated by the red dotted line. This is the view when looking from the front of the vehicle. For a MacPherson strut car the KPA is measured by drawing a line from the top strut mount through the lower ball joint to the ground. The point where the line hits the ground is called the Dave Point because Dave Coleman could not find an official name for this point so he named it after himself.
Caster and KPA greatly affect straight-line stability. Increasing positive caster projects the Dave point (the point where the steering axis projected line meets the ground) further away from the tire’s contact patch. This creates what is known as caster trail. When the tire contact patch is behind the steering axis, the steering wants to stay centered behind the axis much like how a shopping cart’s casters naturally line up in the direction of travel. I wonder if this is why they call it caster?
|For grip driving positive caster is used to increase negative camber gain as the wheel is turned. The feel is typically improved turn in and straight line stability. Sometimes you can increase positive caster which will allow you to reduce static negative camber for better braking. Too much positive caster is felt as mid turn and later understeer and or bumpsteer. You must use caution using more than 8 degrees or so positive caster, especially with wider tires.
The distance between the Dave point and the tire contact patch forms a moment arm (engineer speak for a lever arm) creating a self aligning torque, lets call this SAT. The greater the positive caster angle, the bigger this moment is and the greater the SAT. It takes more of a deflecting force to overcome the SAT, which the driver perceives as greater straight line stability and on center steering feel, both of which are good things.
|Perhaps the motorsport where caster and KPA are the most critical is drifting. Drift cars need a strong self steering effect as drivers use it to feed in large amounts of opposite lock quickly. Look at the large amount of wheel tilt seen in Dai Yoshihara’s S13. Dai prefers strong self steering so this is all dialed into the suspension. Drifting is unique in the fact that front grip is mostly important at shallow drift angles as the throttle is used to steer the car to a degree at larger angles. This amount of KPA and caster would probably be detrimental for front traction in grip driving.
Increasing the SAT also increases the self steer effect. This is when the steering is turned, if the steering wheel is released, it will tend to self center in the direction of travel. This is handy in drift, dirt track and rally cars as it can be used to help a driver countersteer faster than he could by hand simply by releasing the steering wheel. SAT also helps a driver as it increases steering feel. If a car is starting to understeer, the steering will get lighter in the driver’s hand. If the front has good bite, the steering will feel heavier.