Alignment Angles: Caster: Notes
Caster is the forward or rearward tilt of the steering axis centerline, as viewed from the side of the vehicle, and is measured in degrees. When the steering axis centerline is exactly perpendicular to the road surface, the Caster is considered to be at zero degrees.
When the top of the steering axis centerline is tilted rearward (toward the bulkhead), the caster is considered to be "positive". When the top of the steering axis centerline is tilted forwards, the caster is considered to be negative.
Most vehicles have a caster angle from zero degrees to a positive angle. Negative caster is not very desirable because is reduces the vehicle stability especially at high speeds. BMW vehicle always have a positive caster angle. Positive caster promotes high speed stability and provides feedback to the driver.
Positive caster also promotes "steering return" which increases driver comfort and safety. Caster angles which are more negative (or not very positive) can increase the effect of wheel "shimmy", create sensitivity to high winds and creates poor steering return.
When the caster angle differs greatly from side to side, the vehicle may "pull" to the side which has the "least positive" caster.
One of the main characteristics of positive caster is the positive effect on high speed steering stability. However, low speed steering effort is increased. This characteristic is counteracted by the power steering system and in particular the Servotronic system. Servotronic will be discussed in later training modules.
Caster is not a directly measured angle. It can only be measured by sweeping the steering through an angle of 20 degrees in both directions. This is important to know during the alignment procedure.
On BMW vehicles, caster can only be measured, there is no adjustment. However, caster measurement can be used to detect and diagnose alignment concerns. Caster which is out of specification could indicate damaged or worn components.