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Thread: Poll: Did Polyurethane Rear Subframe Bushings Prevent Diff Mount Failure?

  1. #26
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    Another vote for IE greens. Just had my subframe out for some other work and the 5+ year old IEs were still perfect as was the trunk floor.

  2. #27
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    I will throw my voice of support in for the IE green bushings. These things made such a huge difference when installed, it feels like a different car. A lot of the loosey goosey feelings from the rear of the car in cornering are gone. Car is at 172k miles and so far *knock on wood* I havent seen any signs of trunk fkoor failure.

    Suspension: Bilstein Sports, GC Coilover conversion, IE Subframe Bushings, IE RTABs, M roadster sways, M roadster front control arms, M roadster Front calipers/hubs.
    Engine/Driveline: M54B30 intake Manifold, Dinan CAI, ZHP Cams, Headers, Underdrive pulleys, Conforti Software, 3.46 diff.

  3. #28
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    Z3speed4me is offline Coupe Cartel Forever! BMW CCA Member
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    I had greens might try powerflex this time around, it's really the same thing, just definitely get the rubber out one way or another.

    ~Ken~ '99 M coupe THE "original" TT Stage 3 - HTA3586R; 701 whp 672 wtq @ 26.5 psi ; NeverSell - CoupeCartel

  4. #29
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    Quote Originally Posted by Codeman View Post
    For anyone interested in doing this at home here's a thread about making your own bushing removal tool.

    http://forums.bimmerforums.com/forum...or-uber-cheap!
    For what it's worth I had one tack weld "popped" when I got the car at 80k miles. This was a great negotiating standpoint. I re-welded it and installed Powerflex purple bushings about 10k miles after that. I bought the Powerflex bushes because IE was going through the bad batch ordeal during that time. I didn't use a puller. I just heated the mount up until the rubber started smoking and pulled it out. I did this with the subframe still in the car, one side at a time. granted I burnt the shit out of my hand on the first one but gloves took care of it on the 2nd. It took some finagling between lowering sides but it worked and saved me from dropping the diff out completely along with the half shafts. I have 109k on it now and everything is holding solid. I've always had a 1.5mm crack on the diff mount ear where it transitions into the boxed in crossmember and it has never spread "knock wood" http://i.imgur.com/nXmmplr.jpg . I do however have a Mr. Randall Forbes sub frame kit in a box waiting. I'll also add that I don't do this car any favors by babying it. I drive it like the hooligan car that it is meant to be. It also was in a wreck before I owned it and has had a bumper tie to crossmember bar replaced on the passenger side under the cubby hole in the trunk. It seems that some are more resilient than others, mine is a 98.
    Last edited by Ruckusnuts; 10-06-2013 at 04:09 PM.

  5. #30
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    Did AKG poly sub bushings and RTABs at 70k. Left the stock diff bushing in place. Didn't have any popped spot welds before but alignment was drifting a bit due to worn RTABs. All is well now at 84k and hope it stays that way! Very noticeable difference in handling as well.

    BTW used the home made bushing tool for the subs and bought one for the RTABs. Sure made the job easier.

  6. #31
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    I installed the Purple Powerflex bushings and new OEM diff bushing last year @ 119,000 miles. OEM bushings were worn and spot welds are intact. I will keep an eye on it this summer. After the install, the drive has gotten ten times better. Love it.

    Sent from my Lumia 928 using Tapatalk

  7. #32
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    IE Greens in September 2006 at 44k miles. Now at 72k miles and no signs of impending doom.

    i have a new axle carrier with IE weld in camber and toe waiting to go on, so I'll have a look then to see if the IE greens have aged at all.

  8. #33
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    I've got powerflex purple RSFB' and RTAB' and although I can't say much comparatively speaking (only had about 2 weeks seat time with OEM bushes before the transition) I'd say they're well worth the investment as can be deduced from the replies in this thread. This spring I plan to look over the entire rear subframe/diff area with fingers crossed that everything still looks good. I've got a friend who has a completely stock coupe, one year older than mine and 20k km more, and we've got a few road trips planed together this year.. I might just drive his car a few times back to back with mine to get a better feeling for the difference (see: improvement) in my suspension setup.
    Quote Originally Posted by bmxmett318 View Post
    FYI: On my 95 M3 I run an AFE intake and my car sounds like "Chewbacca." lol

  9. #34
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    Quote Originally Posted by DekaB View Post
    Any insight on the AKG subframe bushings that raise the carrier? Going to be doing the entire rear end and I have not found anything comprehensive as far as bushing choices.
    Quote Originally Posted by JWin View Post
    Not sure what you mean by "raise the carrier" as I've just installed AKG street bushings and didn't notice anything out of the ordinary.
    Quote Originally Posted by mcoupemindy View Post
    AKG makes a set of shorty subframe bushings and a bump up bushing for the differential that effectively moves the subframe about 1/4" up to the unibody. Is it to combat negative camber gain when lowering and keep the axles at optimal angle.

    The disadvantage is that weld in adjusters couldn't be used as they would interfere with the gas tank.

    It is an interesting idea.
    I checked in to them & they are "red" bushings, denoting race use. The "black" bushings are for street use & they don't supply that option for that bushing set.

    The 3 piece set at the top of the page.

    http://www.akgmotorsport.com/catalog...ion%2FBushings


    AKG rear end lift.JPG

  10. #35
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    Quote Originally Posted by Randy Forbes View Post
    Just stick with the OE rubber bush in the differential cover/bodymount location..
    I need to change old OEM bushing for differential cover and I have new Powerflex poli bush. Do you recommend better to use Powerflex poli or OEM rubber bush for differential? All other bushes will be changed to Powerflex.

  11. #36
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    The diff mount needs to be soft - stick with rubber (OEM) there. The RSBs need to be stiff - use polyurethane there. This combination should force the loads to the strong parts of the frame rather than to the weak parts of the sheet metal trunk floor unibody mount.
    See Posts 12&14 in this thread. Also look over this thread: http://forums.bimmerforums.com/forum...cement-results
    Last edited by Blacklane; 03-05-2014 at 07:28 AM.

  12. #37
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    Quote Originally Posted by Blacklane View Post
    The diff mount needs to be soft - stick with rubber (OEM) there. The RSBs need to be stiff - use polyurethane there. This combination should force the loads to the strong parts of the frame rather than to the weak parts of the sheet metal trunk floor unibody mount.
    Look over this thread: http://forums.bimmerforums.com/forum...cement-results
    Thanks for answer!

  13. #38
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    Just to add my experience to the thread. I put my polyurethane bushings in at about 138k miles. Now at 178k, and still holding strong.

    I did the recommended polyurethane IE sub frame bushings, and an oem diff bushing.

    Suspension: Bilstein Sports, GC Coilover conversion, IE Subframe Bushings, IE RTABs, M roadster sways, M roadster front control arms, M roadster Front calipers/hubs.
    Engine/Driveline: M54B30 intake Manifold, Dinan CAI, ZHP Cams, Headers, Underdrive pulleys, Conforti Software, 3.46 diff.

  14. #39
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    IE green here and I agree with all the positive comments. My suspension has been the focus of my mods and the best advice came off of this forum. Lots of solid reasoning and experience shared on the pros and cons of each of the options. I recently picked up a second M Coupe that has had the suspension tweaked a wee bit and as I bang around in it I am reminded of how these cars handled originally. No slouch, to be sure, but lots of opportunities exist to get the car handling the way you want. Insofar as the subframe so far - so good, but I'm still at pretty low mileage. I've already figured out that I'll auto-train it to Florida from N. VA for my Forbesification session.



    2006 Z4M Roadster, 2002 M Coupe, 1984 533i, 2-1996 318ti's, 2013 328i M Sport
    Member and officer, ZSCCA

  15. #40
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    Replaced what I believe were the original subframe and diff bushings at 248k miles on my 1.9. No popped welds or damage. All three were replaced with Powerflex. My choice to go poly was simply for ease of installation. There are strong arguments on both sides whether to use OEM or poly, and while the theories are good, neither has actually been proven. No one knows the true root cause of subframe failure. The only thing we know for certain is how to fix/prevent it permanently (thanks Randy).
    Last edited by Kyle Anderson; 02-27-2014 at 09:50 PM.

  16. #41
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    upgrading to poly next week...
    subscribed

    - - - Updated - - -

    upgrading to poly next week.
    subscribed.

  17. #42
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    As a parallel question, I would love to hear what direction people have gone in with their RTABs. Poly been popular/preferred by many of you there as well? I was always under the impression that the only bushing in the rear which should be kept OE rubber is the Diff. mount bushing, to which I concur. How many of us changed our RTABs (whether to Poly or just new OE rubber) at the same time as the RSFB, or how many just did the RSFB's and perhaps Diff. mount but never looked into the RTAB's?
    Would be great to hear from everyone who's already chimed in, with their input on their RTAB's as well.
    Last edited by MTRD3; 02-28-2014 at 05:07 AM.
    Quote Originally Posted by bmxmett318 View Post
    FYI: On my 95 M3 I run an AFE intake and my car sounds like "Chewbacca." lol

  18. #43
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    I have IE red RTABs that have been on the car since 10k miles (now around 55k). I have new AKG bushings in my tool box ready to replace them next time I'm compelled to do work down there.

  19. #44
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    Quote Originally Posted by Kyle Anderson View Post
    No one knows the true root cause of subframe failure.
    Failure mechanism is not complicated. When the car accelerates a downward force is imparted into the trunk floor via torque reaction through the differential. Harder acceleration gives higher loads. Highest loads are at tyre limit of adhesion. Specifically around 550kg downward force onto the lugs at the rear of the diff going into the trunk floor. Is the truck floor strong enough to withstand 550kg load? Nominally yes - there is a C-section cross member running across the rear of the floor welded to it effectively forming a box section. This box section is strong enough to take 550kg. However there are too few welds connecting the C-section to the floor. The welds are highly stressed and fail by fatigue. Time to failure is a variable effected by the driving style of the owner. More high accelerations gives lower fatigue life. Once the welds start to fail the box section loses strength, stresses rise and it will degrade quickly. When all the welds are broken you are left with just the C-section beam trying to withstand the force which it is not strong enough to do.

    If bmw had put more spot welds into the floor the problem would not have occurred.

  20. #45
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    This problem affect cars of all age, power, and driving style. Conversely, there are also high power cars, track cars, and abused dailys with very high miles and no problems. If it were only seen on the higher-powered and abused cars, a weak design would be easy to point out. It doesn't explain why it occasionally happens to average cars though. I believe some unknown manufacturing variability is to blame, and when coupled with abuse, makes it more likely to show up in higher power cars. We have no 100% guaranteed way of looking at a car and knowing whether or not it will experience failure, therefore we don't know the complete root cause. We probably never will.

  21. #46
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    Quote Originally Posted by Deano1712 View Post
    Failure mechanism is not complicated. When the car accelerates a downward force is imparted into the trunk floor via torque reaction through the differential. Harder acceleration gives higher loads. Highest loads are at tyre limit of adhesion. Specifically around 550kg downward force onto the lugs at the rear of the diff going into the trunk floor. Is the truck floor strong enough to withstand 550kg load? Nominally yes - there is a C-section cross member running across the rear of the floor welded to it effectively forming a box section. This box section is strong enough to take 550kg. However there are too few welds connecting the C-section to the floor. The welds are highly stressed and fail by fatigue. Time to failure is a variable effected by the driving style of the owner. More high accelerations gives lower fatigue life. Once the welds start to fail the box section loses strength, stresses rise and it will degrade quickly. When all the welds are broken you are left with just the C-section beam trying to withstand the force which it is not strong enough to do.

    If bmw had put more spot welds into the floor the problem would not have occurred.
    Being argumentative isn't in my nature__I much prefer to avoid conflict, than to incite it__BUT, I have seen examples of cars with no broken spotwelds, showing evidence of the trunkfloor buckling. The embossments directly above the differential mount should be flat across all the raised area (I've had a couple of uninstalled trunkfloor assemblies that I could compare to).

    I have also encountered several cars where the trunkfloor had no visibly stressed spotwelds__certainly none that'd completely fractured__but the actual differential mounting bracket had started cracking.

    There may be several factors that contribute to the ultimate failure of the trunkfloor and differential mount (two__2__separate faults) and build design and the specification of material is at the top of my list. Driver input (read as heavy right foot) and also road condition are definite contributors, but in my opinion, should've been accounted for in the design stage.

    So Deano, while more spotwelds would certainly have kept the "C" section xmbr connected to the floor, at least for a longer period of time, that still wouldn't have prevented the differential mounting bracket attached to it from cracking and failing at high stress points.

    To summarize, I've seen (and repaired/reinforced) cars with trunkfloor failure and zero damage to the differential mounting bracket, and I've also repaired/reinforced cars with a stress cracked & damaged differential mounting bracket showing no failed spotwelds in the trunkfloor. While I cannot say for absolute certainty, I expect the the latter example(s) would've exhibited visible signs of buckling in the stamped embossments of the trunkfloor.

    Two (2) problems; most times simultaneous, but sometimes just one (1) or the other.

  22. #47
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    Since this is intended to be a survey, here is the current status:

    As of 28 Feb 2014:
    Respondents who found a diff body mount failure after installing poly bushings: 0
    Respondents who found a diff body mount failure after installing bushings, but didn't look before: 1
    Respondents who had some damage but have seen no further damage since installing poly bushings: 4
    Respondents who had no damage before installing poly bushings and still don't: 10
    People who had damage with original rubber bushings installed: 75-80 (all Randy Forbes)
    Respondents who a just happier with their handling after installing bushings: 10

    I'll still take updates.
    Last edited by Blacklane; 03-02-2014 at 06:16 PM.

  23. #48
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    i know this is a RSB thread, But I'm curious as to what people are using (with good results) for FCAB's? I currently have IE RSB's & RTAB's. Am in process of installing TCK DA's. Thanks for any help!

    p.s. i did try searching but couldn't find anything useful
    Last edited by ticklemejoviemo; 04-26-2014 at 08:52 AM.

  24. #49
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    Quote Originally Posted by ticklemejoviemo View Post
    i know this is a RSB thread, But I'm curious as to what people are using (with good results) for FCAB's? I currently have IE RSB's & RTAB's. Am in process of installing TCK DA's. Thanks for any help!

    p.s. i did try searching but couldn't find anything useful
    AKG centered black polyurethane bushings. Well priced and easy to install.
    -Abel

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    - 2016 Mini Cooper S

  25. #50
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    It depends on your objectives. I went with OEM rubber, but upgraded to the solid M-style FCAB. Others might want stiffer polyurethane at the expense of increased vibration, noise, & harshness. Still others might want off-set holes to increase caster. In fact, there are so many choices that this question deserves to be another thread. I would ask the members how they chose their FCABs and how it turned out.
    Last edited by Blacklane; 04-26-2014 at 10:13 AM.

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