CMP Monoball RTA bushes. The Pro's & Con's
The E46 BMW 3-series is considered to be one of the most engaging and satisfying to drive BMW's ever made peaking at the E46 M3 CSL.
The old school semi trailing rear suspension is simple, a little on the heavy side but, incredibly effective. For anyone looking to enter the car scene or entry level Motorsport this is a great chassis to start with.
In the back end of the E46 (and E36) chassis you have two simple adjustments with each component performing a single function. Adjusting the camber and rear toe on an E46 is simple in comparison to more modern BMW's multi-link suspension.
One downside to the old school suspension is that the large steel trailing arm transfers all braking force, accelerating force and scrub forces through a single rubber bushing. This bushing naturally suffers a large amount of elastic deflection which accelerates the deterioration of the rubber and thus compromises the bushing functionality creating undesirable changes in toe and forward movements. If you've increased the power output, brake performance or fitted wider, sticker or greater offset tyres then the stock bushing simply isn't up to the task even when brand new.
As this bushing wears the back end is known to feel numb and begin to 'rear steer' which is the phenomenon where the bushing is deflecting sufficiently that the rear toe is literally steering the back end in a different direction. This phenomenon is best felt under hard acceleration and braking.
This is where our Monoball RTA bearings come in. Within the purple 6061 aluminium housing is a brand new BMW OEM bearing made by lemforder (No. 3067901). This bearing is the same diameter as the bearing between the E46 RTA and upper control arm that supports the weight of the vehicle meaning it's more than up for the task and and was originally used in the rear suspension of the E9X/E8X/X1.
Pro's
based on experience with these bearings, we anticipate the average life to be between 100k-150k km (60k-100k miles) making it very likely the last RTA bushing you ever install and in the off chance you find yourself somewhere your shouldn't be causing damage to the bearing, it can be easily pressed out and replaced without the need to remove the housing.
Being a bearing, this part allows free rotation about all axis eliminating suspension binding (resistance to suspension travel). Given the E46's RTA mount pocket is also prone to rip from the chassis in extreme cases eliminating the prying effect non bearing bushing apply to the mount is a good way to help prevent its occurrence.
The significant performance gain from this bushing is that the rear suspensions toe geometry remains constant. Meaning the rear wheels are always pointing in the exact direction you set them to giving you greater control, predictability and precision when driving.
The bearings also do not require flex to occur to transfer accelerating or braking forces to the chassis increasing the car's rate of response to your driving inputs.
Con's
Now the negative to this modification is that essentially the rear wheels are solid mounted directly to the chassis. This does not cause any vibration or noise however, does increase the harshness when travelling over pot holes, reflectors etc.
We personally believe it is a great modification for any E46 despite the increase in harshness and have run this exact setup paired with our solid subframe bushes for the last few years on our daily driven M3 and could never imagine going back.