Rear roll center question

[Peter Mc Mahon]

I have been thinking about roll centers lately [mostly rear] and I have some thoughts

I would put rear suspensions into 2 different catagories, fixed and floating

An example of a fixed rear suspension would be a triangulated 4 link, I believe that the angles of the arms [due to triangulation] make a fixed roll center at a static position.

An example of a floating rear suspension would be a 3 link [no lateral locating device here], the roll center is floating around until it gets locked down with a locating device

So, going back to the triangulated 4 link, if I add a watts link to it and lower the pivot point of the adjuster to lower the roll center, I am conflicting with what is already there, I am going to create bind because you can't have 2 roll centers on the same vertical plane. In a nut shell, the roll center is already defined and fixed so you can't [shouldn't] change it.

However on a floating suspension, think 3 link again, we have to add something for lateral location. When we do this we can choose the height of the roll center because there is not one there already.

Now, this brings me to my main point, I have read that if we use say a panhard bar we should keep it equal from each end so the pivot points are the same so the car will act equal in left and right corners.

So lets say we do that, and lets say that each end is adjustable in height so we can control our roll height.

I think conventional wisdoms says that if one end is 11" off the ground and the other end is 12" off the ground the roll center is 11.5"

I am having trouble wrapping my head around this as the vehicle does not pivot off the center, it pivots off the bolt at each end.

I am thinking that there are 2 roll centers [they don't conflict as they are not on the same vertical plane] 2 distinct roll centers with corresponding moment arms measured from the center of gravity to the pivot of the bolt on each side. NOT measured to the middle of the bar.

So, without going off in too much detail here, if the geometry of the locating points of your rear suspension lock in the position of it [think triangulation] there is no need for a lateral device as it will only cause bind if the roll center is anything other than what is already there. However if your locating points do not lock it in position, you will require a locating device and that device will dictate the roll center.

Now all of that is based upon a static height, if I am not too far off base we can get into when it is compressed etc.

Am I out to lunch?