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DIY: Do It Yourself
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Old 05-04-2014, 05:58 PM   #1
Phoenix330ci
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RTAB's and Limiters

So I just got done with this job and took the car for a ride and it handles great now. I believe I had original Rtab's and the two other bushings since the car was new.

Just a couple of observations.

1. I have over 125K on the car and no one has ever changed these three bushings (per side). The condition of the old parts looked almost new, especially the two smaller bushings. The lower one I can bearly make out some stress cracks, but underload I'm sure they opened a little. The Rtab was the same way, not bad looking, a couple of stress cracks but not a total failure. I expected the RTAB to be toast when I took it out. The upper bushing, the one that already looks like the M3 one, I didn't see anything wrong with it.

2. The RTAB in my 2004 was the new style RTAB to begin with. Thought that interesting because I expected to see the old style.

3. Replaced all bushings with M3 parts plus the limiters.

4. The limiters, ok these were a very tight fit but with a little pushing and turning I was able to make it all fit.

5. Ok everyone talks about preload. With poly I know you dont worry about this. Just using the straight rubber RTAB I can see where this is critical. However using the rubber RTAB and the Limiters I double checked the alignment of the housing and it lines exactly with a seam line in the trailing arm. I lined it up as it came out. However here is the interesting part, after aligning and torquing the bolt down to 81 Ft-lbs, I noticed the housing could still spin (not easily) but it will spin. The limiters stay in place, the bushing stays in place, however the housing can slide on the limiters. So I'm going to say that spending the extra time linining up the housing doesn't have to be done because the housing will still turn on the limiters and not place a twist or load on the bushing??? Anyone else notice this??

If others have done this set up and have found this to be true then please chime in. The limiters I am using are the black nylon type of material that came as a set from OEMBimmer, the RTAB is new style Lemforder.

I marked all the bolt positions but I am still going to take this in this week for a four wheel alignment as I'm pretty sure it is correct but I want to be positive.
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Old 05-04-2014, 06:12 PM   #2
Megalocnus
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My experience (Z4M bushings + Vorshlag limiters) is that after torqueing the bolt, the RTAB housing can still move a bit in each directions, but only by a few degrees.

Going to install RTABs again this month.
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Old 05-04-2014, 06:48 PM   #3
Cavi Mike
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Quote:
Originally Posted by Phoenix330ci View Post
However here is the interesting part, after aligning and torquing the bolt down to 81 Ft-lbs, I noticed the housing could still spin (not easily) but it will spin. The limiters stay in place, the bushing stays in place, however the housing can slide on the limiters.
Which is it? Is the bushing only spinning, or is it sliding? They are not the same thing. The bushing should not be able to slide side-to-side or it defeats the whole purpose of the bushing.

As far as it spinning - of course it can spin. What else would it do? The job of the bushing isn't to prevent the arm from rotating. And of course there was no point in lining anything up - I don't even know what you were trying to line up anyways. The slots on the outside of the bushing?

It appears you don't fully understand the function of the trailing arm or even these bushings.

Last edited by Cavi Mike; 05-04-2014 at 06:49 PM.
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Old 05-05-2014, 01:31 AM   #4
Phoenix330ci
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Originally Posted by Cavi Mike View Post
Which is it? Is the bushing only spinning, or is it sliding? They are not the same thing. The bushing should not be able to slide side-to-side or it defeats the whole purpose of the bushing.

As far as it spinning - of course it can spin. What else would it do? The job of the bushing isn't to prevent the arm from rotating. And of course there was no point in lining anything up - I don't even know what you were trying to line up anyways. The slots on the outside of the bushing?

It appears you don't fully understand the function of the trailing arm or even these bushings.
The bushing does not slide sided to side along the bolt axis, there is no room for that. What I meant by sliding, the housing slides (rotates around the bolt axis) easier on the limiter as the coefficient of friction is lower between the limiter and the inside of the housing in comparison to the rubber on the housing (the original condition)

The housing rotates on the limiters, and like above to some degree. If you read all the articles on RTAB's then you would understand what I am talking about on lining up the housing with the trailing arm. Lining it up is called preload. Check the technical documents because BMW even has a procedure on lining up the housing. Many articles about this. The point of my thread is that using limiters the housing can rotate, to some degree without twisting the rubber in the bushing. This ends up being similiar to being able to do this with poly bushings. It has been mentioned in various post that once you torque the bolt down you can no longer rotate the housing. Yes the housing will rotate, but it twists the rubber in the bearing when not using limiters which is why people say you need to preload or you will wear the bushing prematurely.

I'm just trying to find out from others that may have used the limiters if they were able to rotate the housing a little while using limiters.

So as far as knowing what these bushings do, I understand them, not only do they allow rotation of the trailing arm they also allow flexure along the axis which the limiters 'limit' while braking or in heavy accelerartion, therefore adjusting toe in/out during those times as the force is applied to the trailing arm.
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