Will, you have two questions:
1. The rebound rubber on both two and four seaters is directly under the axle screwed to the chassis. The chassis has a curved dropped section directly under the axle, this is to allow more rebound travel. If you look between the axle and the chassis you will see/feel the rebound rubber. I will try to find a photo and get that loaded.
2. Spring aids and bump stops are quite different, made from different materials to do different things.
A bump stop is designed to take occasional big loads to limit suspension travel in bump and is a material with a lot of damping so as to slow the axle's upward motion. If the damper has 4" of travel, so 3" in bump and 1" in rebound then the bump stop will be around 1" long. It will be maybe 10 times stiffer than the spring and solid with no voids in the material.
A 'spring aid' is designed to take load much of the time. It is a closed cell foam, the gas in the cells are there to give some of the spring effect. It has very little damping as the damping effect should be provided by the damper! It is a spring and maybe a similar rate to the coil or leaf spring it is intended to stiffen. If the damper has 4" of travel, so 3" in bump and 1" in rebound the 'spring aid' may well be 2.5" long! So only 1/2" to 1" of damper rod exposed. A 'Spring aid' is very progressive, so a soft initial rate. You will see 'spring aids' on the SUPLEX front suspension units on the M3W and on the Bilstein dampers on the last of the 4 seaters. Photo to follow.
Cheers Peter. SSL. (Seems much of what I wrote on leaf springs was appreciated, pleased by that)
