What role for roll centres?

Our suspension guru returns to a favourite topic as he considers whether lateral migration of a roll centre is crucial or simply irrelevant

- MARK ORTIZ

Q There is one thing that has been bugging me for some time now. And that is, just how do you prevent lateral migration of the roll centre?

THE CONSULTANT

People who have read my material for any length of time will know that I don’t think the ‘kinematic roll centre’ (KRC), or front view force line intersection, actually is the roll centre in an independent suspension. That is, lateral force acting through the linkage, times KRC height, does not precisely equal the geometric roll/anti-roll moment except in a very small percentage of cases, and does not even come close to that in some cases. Also, when the car is subjected to a z axis (road-vertical) acceleration, as in a banked turn or a dip or crest, and the KRC is offset laterally from the sprung mass c.g., that offset does not create a roll moment.

It is desirable to minimise changes in the suspension’s properties as the suspension moves, including changes in the front view force line slopes or the individual wheel suspensions’ jacking coefficients. Additionally, it is true that if the force line slopes don’t change at all, their intersection point won’t move at all. And, all other things held equal (depending on exactly what we mean by that), it is loosely true that more force line slope change produces more lateral KRC migration.

Use the force