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BMW = Fahrvergnügen
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Discussion Starter · #1 ·
I've been reading a lot about rear trailing arm bushings lately. Quite a bit about bushings in general, in fact. Lately, my car's felt a little bit off. Kind of hard to explain what it is. It seems reminiscent of the looseness I got when my FCABs went a couple years ago except I think it's coming from the back. There's no weird noises or obvious vibrations, so I've got very little to chase except a vague feeling. Based on what I've read and from what I can guess based on the geometry of the rear suspension and what appears to be the function of that bushing, I would expect that failed RTABs would cause the rear toe to drift around a bunch, probably toeing in on acceleration and out on deceleration, and shifting around as road surface changes. I've heard it described as "the back trying to steer the front." I'm not certain that's what I'm feeling, but whatever. I do feel like the car tends to tramline over changes in road surface a little. For example, the highway I use every day is working on resurfacing a bunch of bridges. They've reduced it to one lane, the right lane, and have shifted that lane off to the shoulder. They've got those grooves cut into the shoulder to wake up sleeping drivers, so they just temporarily patched them over, so there's a strip there you have to cross. I feel like the car pulls across those, especially the passenger side, and I think it's more rear than anything. There's no looseness in the steering.

My wife's car needed an oil change, so I was already out in the garage. While it was draining, I lifted the back of my car and looked at the RTABs. They're not obviously failed; I wasn't able to pull on the wheel and get any meaningful deflection. They look fine, inasmuch as you can see them when they're up in the cavity they live in. There's not any chunks hanging out or anything and I couldn't see any marks anywhere like the arm had been hitting the bracket. I took a larger screwdriver, maybe a foot long, and pried against the bushing in the cavity. I was able to get it to deflect fairly easily parallel to the axis of the bushing. It should deflect some, though. I guess I don't know how much is abnormal. My Bentley manual was zero help, though it does have a clever idea for preloading a new bushing, which was kind of cool. I think the bushing on the passenger side was easier to deflect than the one on the driver side. I quickly checked the other suspension connections and didn't notice anything odd going on.

My rear differential bushing has several hairline cracks in it, because of course it does. Explains the clunk I've been getting on shifts. Subframe mounts still look good, though.

Anyways, besides pulling the arm down and taking the bracket off, anybody think of any good ways to check the RTABs besides what I did? The car's 15 years old and has 65k on it, so it wouldn't surprise me at all if the bushings were starting to go, but I don't want to replace them unless they need it. That's a waste of an afternoon and $200.

While we're on the subject, I'm still unclear on what the best choice for me would for RTABs should I discover I need new ones. To give you an idea, I put in a set of Powerflex purple FCABs and Meyle HD control arms when they went and I really like what they did to the front end. I had been planning to replace the RTABs with Powerflex purples, but they seem to not exist anymore, something about premature failure, which is a bit distressing. All they've got left are the black ones, which are 80D or something similarly nutty. I tend to shy away from "recommended for track only" kind of hardware. I ran across the AGK 90A ones on Pelican Parts. People seem to like them, though they're reviled by the "you can't use poly for the RTABs" crowd. It sounds like Meyle HD and a set of limiters (probably ECS ones) would be the other reasonable choice. The poly ones would be much easier to install and I really like what the FCABs did to the front. Both solutions cost about the same.

What do you guys think?
 

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Bad RTABs will shave the inner tread off the tires within a few months. Just get a tread depth meter and measure it every week to two weeks.

When my RTABs were done the car was always tail happy on throttle or brakes in even a subtle corner.

I installed Bimmerworld solid sealed mono balls in place of rubber or poly bushings. There is no loss in ride quality and when I'm on the throttle the car just tracks where the steering wheel is pointed like the car should.
 

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I've been reading a lot about rear trailing arm bushings lately. Quite a bit about bushings in general, in fact. Lately, my car's felt a little bit off. Kind of hard to explain what it is. It seems reminiscent of the looseness I got when my FCABs went a couple years ago except I think it's coming from the back. There's no weird noises or obvious vibrations, so I've got very little to chase except a vague feeling. Based on what I've read and from what I can guess based on the geometry of the rear suspension and what appears to be the function of that bushing, I would expect that failed RTABs would cause the rear toe to drift around a bunch, probably toeing in on acceleration and out on deceleration, and shifting around as road surface changes. I've heard it described as "the back trying to steer the front." I'm not certain that's what I'm feeling, but whatever. I do feel like the car tends to tramline over changes in road surface a little. For example, the highway I use every day is working on resurfacing a bunch of bridges. They've reduced it to one lane, the right lane, and have shifted that lane off to the shoulder. They've got those grooves cut into the shoulder to wake up sleeping drivers, so they just temporarily patched them over, so there's a strip there you have to cross. I feel like the car pulls across those, especially the passenger side, and I think it's more rear than anything. There's no looseness in the steering.

My wife's car needed an oil change, so I was already out in the garage. While it was draining, I lifted the back of my car and looked at the RTABs. They're not obviously failed; I wasn't able to pull on the wheel and get any meaningful deflection. They look fine, inasmuch as you can see them when they're up in the cavity they live in. There's not any chunks hanging out or anything and I couldn't see any marks anywhere like the arm had been hitting the bracket. I took a larger screwdriver, maybe a foot long, and pried against the bushing in the cavity. I was able to get it to deflect fairly easily parallel to the axis of the bushing. It should deflect some, though. I guess I don't know how much is abnormal. My Bentley manual was zero help, though it does have a clever idea for preloading a new bushing, which was kind of cool. I think the bushing on the passenger side was easier to deflect than the one on the driver side. I quickly checked the other suspension connections and didn't notice anything odd going on.

My rear differential bushing has several hairline cracks in it, because of course it does. Explains the clunk I've been getting on shifts. Subframe mounts still look good, though.

Anyways, besides pulling the arm down and taking the bracket off, anybody think of any good ways to check the RTABs besides what I did? The car's 15 years old and has 65k on it, so it wouldn't surprise me at all if the bushings were starting to go, but I don't want to replace them unless they need it. That's a waste of an afternoon and $200.

While we're on the subject, I'm still unclear on what the best choice for me would for RTABs should I discover I need new ones. To give you an idea, I put in a set of Powerflex purple FCABs and Meyle HD control arms when they went and I really like what they did to the front end. I had been planning to replace the RTABs with Powerflex purples, but they seem to not exist anymore, something about premature failure, which is a bit distressing. All they've got left are the black ones, which are 80D or something similarly nutty. I tend to shy away from "recommended for track only" kind of hardware. I ran across the AGK 90A ones on Pelican Parts. People seem to like them, though they're reviled by the "you can't use poly for the RTABs" crowd. It sounds like Meyle HD and a set of limiters (probably ECS ones) would be the other reasonable choice. The poly ones would be much easier to install and I really like what the FCABs did to the front. Both solutions cost about the same.

What do you guys think?
New RTAB's stops rear steer, makes for a much better drive
 

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BMW = Fahrvergnügen
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446 Posts
Discussion Starter · #4 ·
Bad RTABs will shave the inner tread off the tires within a few months. Just get a tread depth meter and measure it every week to two weeks.

When my RTABs were done the car was always tail happy on throttle or brakes in even a subtle corner.

I installed Bimmerworld solid sealed mono balls in place of rubber or poly bushings. There is no loss in ride quality and when I'm on the throttle the car just tracks where the steering wheel is pointed like the car should.
I was hoping for something a little more immediate and a little less destructive. The tires seem to be wearing evenly, though they are wearing a lot faster than I'd hoped. I had an alignment done last summer and it was fine I've driven probably 5k since then, maybe 2500mi since I rotated the tires. If they're wearing more on the inner edge, that'd be something to go on, especially since my alignment sheet said (going from memory here) that I had near as much 0 rear toe as doesn't matter and only a small fraction of a degree of negative camber.

Maybe I should just replace the damned things and be done with it.
 

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Any alignment means nothing when bushings are worn out and causing the wheel/tire to change toe when in motion. When mine were done, no amount of twisting and shaking the wheel would hint at anything loose. If you have a GoPro, you can also mount it somehow to watch the RTAB. Lighting it up will be the creative part since the RTAB housing is in a cave. Then you can see if there is excessive movement with how you accelerate and decelerate.
 

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BMW = Fahrvergnügen
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446 Posts
Discussion Starter · #6 ·
Any alignment means nothing when bushings are worn out and causing the wheel/tire to change toe when in motion. When mine were done, no amount of twisting and shaking the wheel would hint at anything loose. If you have a GoPro, you can also mount it somehow to watch the RTAB. Lighting it up will be the creative part since the RTAB housing is in a cave. Then you can see if there is excessive movement with how you accelerate and decelerate.
You know, you make a good point. Unless the bushings are basically disintegrated, they'd likely hold static toe just fine, so an alignment would show up relatively normal.

It's interesting that these bushings can fail in a way that defies normal inspection. While strapping a GoPro to the bottom of the car and watching the lateral deflection of the trailing arm is a clever idea, I don't have one (and likely wouldn't be able to borrow one from a friend if they knew what I was going to do with it!), it's pretty silly that you've got to go to such lengths. Seems the moral of the story is "if the back feels at all loose, change the RTABs." I think I'm going to do just that, as soon as I get my winter car sorted.

That brings me to the next question: what RTABs did you go with? I'm currently leaning toward Meyle HD and some ECS limiters. If I go poly, I think I'll get the softer AKG ones. I'm having trouble slogging through the years long debate about poly vs. rubber bushings for RTABs. Like I mentioned in my original post, I've got PowerFlex purple FCABs I've had for several years and maybe 20k that I really like. That tends to give me a bit of a poly bias. I'm just looking to collect more objective opinions.
 
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