Rtabs or “Rear trailing arm bushings” are the only bushings in the rear of a BMW that have a huge effect on toe movement or “side to side” movement.
So why is there a bushing? Don’t you want the toe to stay the same? Not in all cases and here’s why you should carefully select your bushing material when upgrading this area of the car. A little bit of flex in the rear end is a good thing – when we’re talking side to side. I always say go as stiff as you want with the “back and forth” bushings like the subframe and differential mounts but keep the side to side a little softer. So when your car is at the limit of traction, you’re entering a sweeping corner at 100mph and you need those rear tires to stick – The stock bushing has a tendency to allow both rear tires to flex in the direction you’d like them to go. From behind, this looks like the car is driving a different way than the wheels are going – its kind of crazy. Now if those bushings don’t allow any flex there is that much less room between traction and no traction. So if you’re building a drift car – go as stiff as you want. But if we’re trying to keep the rear end following the front end we should allow a little bit of movement.
For most street cars and weekend track cars we recommend replacing the bushings with an upgraded “rubber’ bushing which keeps the feel as close to stock as possible. We also add in a Urethane spacer on each side of the bushing to limit the travel a bit but it still has some flex. Using this urethane spacer is a must and ensures the bushing will last forever.
If your car is going to be running slicks or R compound tires we recommend upgrading to a urethane rear trailing arm bushing. Because now you have a further limit of traction and waaay more grip than a street tire, we need a bushing that can handle that. Because the tire is much stickier it has the power to flex this urethane bushing when needed versus a street tire that will lose traction before that can happen. The downside of these bushings (like any upgrade bushing) is the noise they’ll make – even with lubricated they’ll squeak and clunk. This isn’t such a huge concern on the race track when you can’t really hear any of this anyways but if installed in a street car it can be a nuisance when stuck in traffic or going over speed bumps when you car sounds like a squeaky door from a horror movie.
If we’re building a drift car and WANT the rear end to lose traction when it’s told to then go as stiff as possible or even use a solid bushing/spherical bearing.