We always say for the Rear Trailing Arm Bushing on the E36/46 BMW rear suspension, either "go spherical" or "stay rubber and use limiters".
After seeing too many issues with poly RTAB and LCA bushings when used in street cars and dual use cars, I just can't recommend them for those situations. Polyurethane is too stiff a material (generally, although there is a large range of durometers available) to allow for the proper articulation that this (and the front LCA) bushing needs during suspension travel. Poly tends to only work well for static bushings or mounts (diff mounts, motor mounts), or ones that pivot in one direction only (a single axis suspension bushing). The RTAB goes through a dual axis of rotation and using a material that has very little flex will add bind in this joint. This can cause erratic rear suspension characteristics, especially with a certain range of spring rates and/or on cars that see use on roads or tracks that aren't perfectly smooth.
Note: Many BMW road course drivers/racers use poly successfully in this RTAB joint (and the LCA) on dedicated race cars with relatively high (or extremely high) spring rates. Some swear by them and will espouse their perfect use and wonder why everyone doesn't jump right on the "put poly everywhere!" band wagon. Remember, bone crushing race car spring rates tend to limit suspension travel substantially, and the bind that can happen with "normal" suspension travel will likely not be seen on these cars when used on a smooth race course with very high spring rates. I'm not saying they are wrong, just that poly works in some situations but not all.
On our 400hp E36 that has 305mm R compounds and makes enough grip to rip your face off, we use rubber RTABs and limiters. There's still "plenty of suspension travel" in the rear of our car (too much, actually) and no bind - its still fairly effortless to drive with no weirdness in the rear suspension, and it puts down power exceptionally well (maybe too well). Its by no means a perfect car, not hardly, but rear suspension bind is not a problem. We've used similar rubber+limiter set-ups in all all manner of E36 and E46 street, race, and dual use cars with zero drama.
Going to spherical bushings in the RTAB is an expensive, somewhat race-only solution, but its the best solution to re-do the RTAB joint if your racing class and/or budget allows. Sphericals work best in many other multi-axis bushing locations, such as the camber plate upper mounts and rear shock mounts - and we recommend them for use in these locations even on "enthusiast" street driven BMWs. Designed and sized properly, spherical bearings can last a very long time in these two locations (maybe indefinitely) with no banging or rattling. The RTAB bushing location, however, is very near ground level (under the car) and would be more directly exposed to road salts/rain/grit than an RSM or camber plate location - which may impact it's long term durability (becomes a wear item like other rod ends and sphericals that also see weather/dirt). Also, installing sphericals into the ends of factory control arm is "a bit more difficult" (that's an understatement).
So if you do not have the resources to use a spherical end in the RTAB location, replace the OEM RTAB with a new M3 rubber bushing and use our's or some other vendor's RTAB limiters. There is pretty broad agreement on the design/shape of BMW RTAB limiters. We chose a self lubricating, ultra high density polymer material for a variety of reasons, while others use Delrin or Nylon (both of which are similar to what we use) or even aluminum for their limiters. These all tend to work similarly and they do allow for the proper articulation at this bushing location, yet limit the deformation that this rubber bushing would normally see. Its what we think is "the best compromise solution". As with much of the BMW front and rear suspension, its all about the best compromise.
Do replace the RTAB bushing every few years or every 30-40K miles, as it is a common wear item. Front LCA bushings are as well, of course. These multi-axis bushings take a beating in normal use and tend to come off of cars torn and cracked after a number of miles.
Just my $.02.
Liverpool
