Unlike other suspension bushing locations that only have one axis of rotation, swapping in excessively stiff replacement bushings using materials such as Polyurethane can and will cause suspension bind. The stock rubber M3 bushing is fairly compliant and effective when brand new but wears quickly over time, and doesn't cope with the added forces from race tires in perfect condition. Added wear adds excessive toe change under suspension movement, acceleration and braking. It is compounded greatly with additional power and grip from racing compound tires. If your stock RTABs have more than 20K miles and/or you've used the car for more than a season of racing, they are likely shot.
Many racers just blindly install polyurethane into this and all bushing locations as a "fix" for a worn OEM rubber RTAB bushing. Polyurethane is a bad choice for a bushing material if it has to deflect - such as in a multi-axis suspension bushing location like the RTAB. Rubber bushings + limiters or a complete replacement with a custom steel spherical bearing are the only two textbook choices for this location. We cannot count the number of times we have seen RTAB or front LCA failures on BMWs that were using poly in these areas - an expensive repair and dangerous racing situation. At the very least poly will add bind during significant suspension movement, and this can make the handling feel odd (unexplained oversteer) as well as add significant stress to the suspension-to-chassis mounts (which is how they can fail over time).
If you have a street car or dual purpose BMW you should always stick with OEM bushings and RTAB limiters, and even many race prepped BMWs use this setup with excellent results.
