This is a quick one as you can search a lot of previous information about roll couple. I learned something about the relationship between roll centers and the center of gravity that may appear subtle, but is in fact not.

As the distance between RC and CG is increased, a car will roll more because the invisible lever arm is longer. The length of this lever increases when folks lower cars, especially those with Mac struts up front - Mini Cooper. The subtly here is that the force or weight transfer, due to a lower RC, is better proportioned between the inside and outside tires. In other words, the inside tire can perform more work.

Just an updated FYI

 

The roll couple DECREASES with lowering, until a certain point... that point is determined by the angle of the lower control link on the mac-strut. The following diagram shows what happens when you have lowered the car TOO much... the increasing roll-couple.

In reality, this only starts to happen when the angle between the strut and lower control arm is greater than 90°. If you lower your car this much, you can get control arms with lower pivot points on it to reduce the angle of the control arm relative to the strut, therefore reducing the roll-couple.



RM2k5

 

There are a host of 'things' that will change RC and therefore roll couple. Adding camber plates,for example, to a mac strut and dialing in 3 degrees neg camber will affect the location of roll centers, and, instantaneous centers - the length and location of the swing Arm pivot point as indicated in the drawing. This pivot point describes the arc the wheel moves thru; in the case of the drawing, it moves farther away which means the front end's camber gain or loss maybe improved or hurt as the wheel travels to complete compression. The right angle with regard to the center line of the of strut determines the new location...which by the way alters SAI and therefore scrub.

Simply lowering a mac strut will alter roll couple, without altering the angle of the strut tower because the angle of the lower control arms are now different, as was stated. But, what the drawing does not show, and I should have written again in my opening post, is that roll centers move around. Too often folks look at that drawing, which is a typical drawing found in many books, and think that roll centers are static points. With exception to the a live rear axle, roll centers move around with all other modern suspension designs. The center of the differential is the roll center location in a live axle.

So, although the RC location may not appear to move much in a static front on view after lowering a mac strut by one inch for example, the RC may follow a new arc which drops its location aggressivly as a car is intitially turned. This may or may not be desirable.

A better way of getting a feel for this is to write the relationship this way; the center of gravity rolls over the roll center. When the two are seperated by a longer lever arm, the car rolls more. But, if the new lower RC location has been carefully defined, the transfer of weight between the inside and outside tires is more evenly proportioned.

Incidentally, if geometry of the tie rods and steering rack axis has been designed to always point at the instantaneous center, bump steer will not be a major problem. It's when the location of the instantaneous center defines and arc substantially different than that of the angle and therefore arc the tie rod follows as the wheel moves up and down.