Caster is the forward or backward tilt of the steering axis. Caster is measured in degrees from the steering axis to vertical as viewed from the side. Backward tilt from the vertical line is called “positive caster”, while forward tilt is called “negative caster”. The distance from the intersection of the steering axis centerline with the ground, to the center of the tire-to-road contact area is called “caster trail”. The caster angle affects the straight-line stability and the caster trail affects the wheel recovery after cornering.
If the wheels are given excessive positive caster, the straight-line stability is improved, but cornering becomes difficult.
Straight-line Stability and Wheel Recovery
Straight-line stability due to caster angle When the steering axis rotates, during cornering, if the wheels have caster angle, the tires are inclined relative to the ground and jack-up torque is generated that attempts to lift the vehicle body as in the figure. This jack-up torque functions as a recovery force that attempts to return the vehicle body to the horizontal and maintains the straight-line stability of the vehicle.
Wheel recovery due to caster trail If the wheels are given caster angle, the contact point of the line extending from the steering axis is forward of the center of tire-road contact. Therefore, since the tires are pulled from the forward direction, the force pulling the tires holds down the force attempting to destabilize the tires and maintains straight-line performance. Also, when the tires face sideways due to steering or disturbance during straight-line travel, side forces (F2 and F’ 2) are generated. These side forces act as rotation forces around the steering axis due to the caster trail and are forces attempting to return the tires to their original positions (recovery force). At this time, if the caster trail is long, for the same magnitude side force, a larger force works to return the steering wheel. Therefore, the longer the caster trail, the greater the straightline performance and recovery force.
Nachlauf and Vorlauf Geometry
In general, the caster angle must be increased in order to increase caster trail. However, even if the caster angle remains the same, the caster trail can be set as desired by offsetting the steering axis to the front or rear from the wheel center. Nachlauf geometry enables caster trail to be increased by offsetting the steering axis to the front from the wheel center. Vorlauf geometry enables caster trail to be decreased by offsetting the steering axis to the rear from the wheel center. In actual vehicles, various settings are made by Nachlauf and Vorlauf geometry in order to match vehicle characteristics.