Thought I would begin a thread about Roll couple given the number of other threads, questions etc about how low is too low.

Very simply, every car has a center of gravity. More appropriately, centers of gravity since the CG is not always the same for the front and rear. Every car has front and rear roll centers as well. The relationship or distance between the two is called Roll Couple.

Lets consider just one end of a car for simplification. Imagine a CG at 18" above the ground with a RC at 9" above the ground. This is the important part - the distance between the two is 9". Now, if this hypothetical car is lowered by 2", the CG will be 2" lower. However, the RC may actually be lowered by 3" or 4" or more increasing the distance between CG and RC. Since the CG acts on the RC as a torque, the longer lever arm in the lowered car will actually cause the lowered car to roll more! Think of the CG as a pedulum sitting some distance above the RC. If this lever arm is lengthened, the CG will act on the RC with greater force. We tend to assume that a lowered car will always handle better because we are focused only on CG, and not its affect on RC.

If one is concerned with good ride characteristics, lowering, or lowering too much, may not prove out to be a great path; because, in the above example, stiffer springs and dampers are required to overcome the greater torque acting on the RC. Or, you simply accept less roll resistence.

If one is concerned with great handling, then one must understand a longer lever arm will have some deleterious affect on handling.

Lowering affects how springs and dampers work, it affects camber curves, it affects tow and therefore the potential for bumpsteer effects thru out the entire driving envirnoment.

This is not to say lowering is a bad thing. You should know, however, what lowering does so you can compensate for it in your overall handling package.

 

i believe there is a thread either here or on mini2 that has the actual mini geometry....my recollection is that lowering a mini is a good thing, i.e., there isn't the roll lever increase you mention.

 

Well that's good news...a bit surprising with a Macphearson strut though. I didn't bother to perform the search...here I go. I can never get onto mini2's web site. I've contacted those folks a bunch of times. Can't be bothered anymore.


Thanks for the info.

Michael

 

Found; geometry for the rear, thanks to cristo. Nothing for the front and no Center of gravity info.

 

Post your findings! Dont leave us in the dark

 

these are quoted from the following post (which has some crude
sketches on posts 21 and 22:
https://www.northamericanmotoring.co...ad.php?t=31491

The roll center for the front seems to start at the midline about 3" above
the ground, and as the car leans, it scoots to just under the inside tire
contact patch.

I just took some measurements with a tape measure, and used a piece of
graph paper and a pencil with a good eraser, and got out my copy of
Puhn's book to double check the theory. I then shortened and compressed
opposite sides of the struts on paper and saw how it shifted things and
redrew. It's not rocket science, and if you're a little off on the exact
center of the ball joints and pivot points, it only makes a very small
change in the location of the roll center.

Regarding the rear suspension flat, 3" compressed (lowered),
6 degree roll, and 3" extended (raised) ... Flat, the roll center is 10" above the ground in the
center....At 3" compressed, it's 5" above the ground. At 6 degree roll, it's
9 inches up and 1 1/2" to the side. The camber of the tires is not precise in these diagrams.At 3" extended, the roll center is 13" from the ground in the center.