FRONT SUSPENSION BASICS (MACPHERSON -TYPE)
In this chapter we explain front suspension basic terms and vehicle dynamics terminology per SAE J670.
Below you will find short description of following:
- Caster trail
- King pin inclination angle / KPI / KPA
- Scrub radius
- Toe angle
- Bump steer
Camber angle is the angle between the wheel plane and chassis vertical axis from vehicle front/rear projection.
Caster angle is the angle between the steering axis line (drawn through top mount pivot and control arm outer joint) and chassis vertical axis from vehicle side projection:
Caster trail is measured at ground surface as distance between the steering axis and vertical line drawn through wheel center.
King pin inclination angle / KPI / KPA is the angle between the steering axis (as above) but from the front/rear projection and vehicle vertical axis.
Scrub radius is measured at ground surface as distance between the steering axis and wheel center line.
Roll-center definition is a bit more complex and it cannot be measured with hand tools. First a line is drawn from top mount pivot perpendicular to steering axis in front projection, second line goes through outer and inner joints of the control arm and the third line starts from wheel contact center. When all three lines are connected on each other on one point, the height of the point on the third line at the vehicle center plane defines roll-center height.
Toe angle is measured between wheel axis and vehicle center axis in top/bottom projection.
Bump steer is change of toe angle when suspension travels up or down. Ideally we want to have zero bump steer to have predictable steering behaviour in turns and on uneven surfaces.