tommudd wrote:
Old rule of thumb
for every inch of lift an inch of bumpstop
Geometrically, it would seem to me that if your lift is from a spacer type lift, a longer bumpstop would be beneficial, on a one to one ratio, since the distance between the control arm and the rest of the body (i.e., like the top of the wheel well where the bumpstop mounts) would be greater. That would limit the angular motion of the control arm to the same extreme as the stock setup. But if you ground out the control arm pockets to gain additional flexibility, it seems it would be counterproductive. Using a shorter bumpstop would allow the control arm to take advantage of the larger pocket openings, and since the top of the tire would be lower relative to the fender, it should be safe to use the shorter bumpstops.
However, if your lift comes from extending things below the control arm, like the OME or the JBA, the distance between where the control arms sit relative to the rest of the body (like to where the bumpstop mounts) remains unchanged. Therefore, it would seem that, all other things being equal (including tire size), the stock bumpstops would be the proper height; the angular motion of the control arm would also remain the same. This setup would also keep the control arms from taking full advantage of the bigger control arm pocket openings; for that, you would need shorter bumpstops, which does not seem practical since the stock ones are pretty short already.
Now, if you are going to put bigger tires in, (with either type of lift or even with the stock suspension) then additional bumpstop height might be needed, since the top of the tire, all other things being equal, would be closer to the fender when the control arm is at the bumpstop. But, my guess would be that the ratio there would
nominally be one inch of additional bumpstop for every half inch of tire increment (but should be further adjusted based on the ratios of the lengths from the control arm pivot to the bumpstop and from the control arm pivot to the top of the tire).