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Differential Mount Cover Bushing

Discussion in 'Z3 Roadster & Coupé' started by Wasatch7, Sep 9, 2014.

  1. Wasatch7

    Wasatch7 Regular Member

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    So my diff mount bushing is destroyed and it is making a clunking noise when I shift. I am going to replace it, but I am wondering should I get a stock rubber one or like a poly or a 80A Powerflex one. The OEM one just looks like it could fail again pretty easy but I need to have the best option for my trunk floor that is starting to fail. Whta should I do???
     
  2. vintage42

    vintage42 Dedicated Member
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    The best option for taking stress off the trunk floor subframe it to install urethane bushings in the differential carrier (which is confusingly called a subframe), and an OE soft rubber bushing in the differential mount. Or at least maintain the latter. But if the trunk floor welds are already failing, it's too late for bushing strategies. The only sure solution is the Randy Forbes kit.
     
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  3. Wasatch7

    Wasatch7 Regular Member

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    Yeah it's two spot welds have popped, but that's what I was planning on doing when I got the money. Any places to get the oem mount bushing? And some nice stiff subframe bushings?
     
  4. oldcarman

    oldcarman Zorg Guru (V)
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    Please, what does the Forbes kit do exactly, for us uninformed? Cost is? Difficulty to install? Etc? Thanks, I've been meaning to ask as you've mentioned this several times previously. JIM
     
  5. zedonist

    zedonist Guest

    IMHO I would just get the popped welds repaired, and fit original bushes, all stiff bushes will do is make the joint stiffer and transmit more stress through the boot floor and hence accelerate any issues in the area. Fix the issues and replace with new oem bushes, and you will have a car that lasts for another 15 years.

    The Randy Forbes kit is an enthusiasts attempt to strengthen the area uneccessarily by welding a girder in place (bigger is better right!) when understanding root cause of the initial failure would have suggested replacing like for like, it's your money guys and your choice. But that is all the RF kit does is reduce your bank balance nothing else.

    :popcorn: Waits to watch the fire works............
     
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  6. hard top

    hard top Zorg Expert (I)
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  7. vintage42

    vintage42 Dedicated Member
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    Popped spot welds have not been successfully repaired in the long term by rewelding or bolting. The Randy Forbes kit is proven.

    The OE soft rubber bushings, with their voids, allow movement of the differential carrier (aka rear subframe) around the bushing pins. The movement is then taken by the differential mounting bushing. That is one source of stress that tears the differential mount and the trunk floor channel that it is attached to. Rigid urethane bushings in the ends of the differential carrier stop its movement around the bushing pins.
    http://forums.bimmerforums.com/foru...-Subframe-Bushings-Prevent-Diff-Mount-Failure

    The other source of stress that tears the differential mount and trunk floor is the torque coming from the drivetrain. The big six-cylinder Z3s with manual transmissions, doing full power upshifts, hard braking downshifts, clutch dumps and burnouts -- all put more stress on the differential mount and trunk floor than they were designed for.
     
  8. zedonist

    zedonist Guest

    Ooh like fishing this one,

    Question 1 - How is the Randy Forbes kit proven?

    Answer 1 - It is not, it has no calculations or testing associated with it, the material used has not been substantiated, and the welding is done by Fred in a shed without controlled conditions

    Question 2 - Why are the OEM bushes a source of stress?

    Answer 2 - They are not, the differential is held in place by four bolts to the axle carrier, if it was to twist then it would twist the rear beam out of alignment, as this does not happen, then it cannot possibly twist the boot floor. The actual use of the rear differential bush is as a damper, removing the voids causes excessive noise, and consequently any movement to be transferred to the super structure ergo adding stress

    Question 3 - Why are the six pots so much more powerful that they tear the boot floor apart?

    Answer 3 - They are not, there is evidence to support that boot floor issues have occurred across the full range during a specific period, as mentioned in answer 2 the differential is contained by the rear axle and cannot therefore twist to this magnitude. According to the vehicle issue database the main issue with the boot floor during this period, was recorded as a welding process issue, and that a number of bodies did not receive complete welds in this area as a consequence. Torn diff carrier hangers are a result of a faulty part, there are no design issues with this area of the car.

    I would suggest that if cars are failing after 15 years that this is down to stress corrosion, as in most pictures the welds can be seen to be rusty, but I would not rely on a photo to tell you root cause.

    Can someone please post something about the metal being too thin, and that the RF kit is dual eared, have a response for these as well.
     
  9. Wasatch7

    Wasatch7 Regular Member

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    Can anyone tell me where I can locate a OEM diff bushing? I am having a hard time finding one
     
  10. zedonist

    zedonist Guest

    You can buy it from the dealer just as cheap as on ebay, plus it's cheaper to buy it already in a new back plate, what engine and year have you got I will give you the part number?
     
  11. zedonist

    zedonist Guest

    Differential cover incl of bush part number - 33111428238
     
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  12. vintage42

    vintage42 Dedicated Member
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    The Forbes kit has been proven after more than ten years and hundreds of installations. Forbes says a good body shop can install the kit, but also has a list of experienced installers. Still, cars are shipped to him in Florida from all over the country, especially in winter.

    This is a good illustration of the axle carrier and differential showing how they are bolted together as essentially one unit, attached to the body at three bushings:

    [​IMG]

    The soft rubber and voids in the OEM bushings allow movement of the axle carrier around the pins. The pins are two points of contact between the body and the carrier. Cornering and road forces move the pins around in the bushings.
    The third point of contact between body and axle carrier and differential is the differential mount on the trunk floor subframe channel. The movement allowed by the two soft OEM axle carrier bushings ends up in the differential mount, which contributes to tearing it off the channel.
    Changing to urethane bushings, on the pins holding each side of the axle carrier, stops movement of the carrier in the pins, which takes stress of the differential mount. Urethane bushings have been shown to be beneficial:
    http://forums.bimmerforums.com/foru...nt-Diff-Mount-Failure&p=27396114#post27396114
    The urethane bushings do harshen the ride. They come in street and track grade hardness.

    The drivestrain exerts forces on the differential which is mounted to the trunk floor channel. One effect of the power can be seen in the squat of the rear end on take off. The differential mount, and the trunk floor channel, were designed and welded for the Four, and sometimes experience failure under the higher power of the Sixes, particularly M models that are accelerated hard or tracked.
    http://forums.bimmerforums.com/foru...nt-Diff-Mount-Failure&p=27394422#post27394422

    The caption on YouTube for this video says:
    "M Roadster cracked diff mount. For those who don't know, this is quite a common fault on the M Roadster, but most people don't know they have a problem. The design of the diff mount can't handle the 350Nm of torque from the 321BHP engine. 0-60 in 5 seconds puts a strain on the car and something has to break somewhere, and this is the weak point."

    View: https://www.youtube.com/watch?v=2ArgX4muhsc
     
    #12 vintage42, Sep 10, 2014
    Last edited: Sep 10, 2014
  13. zedonist

    zedonist Guest

    All tosh mate, you've wrote a lot of words and not read any of them. And ousting a video is the same as posting a photo, how do you come the conclusion from that the car is too powerful for the mount, what data and test information supports your theories? None is the answer, please post the data to substantiate your claims for the assumptions above
     
  14. vintage42

    vintage42 Dedicated Member
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    The problem of the weak differential mount and trunk floor first appeared with the powerful M Six. Here are consumer complaints for the model years 1998, 1999 and 2000.
    http://www.aboutautomobile.com/Compl...Roadster/Frame
    http://www.aboutautomobile.com/Compl...Roadster/Frame
    http://www.aboutautomobile.com/Compl...Roadster/Frame
    As years and miles accumulated, the problem eventually began to appear in some smaller Sixes, except for those with automatics. Fours were rarely affected.

    In 2005 the following thread on the problem was started in BimmerForums:
    http://forums.bimmerforums.com/forum/showthread.php?402183-The-definitive-subframe-strategy-thread

    In that thread, Post # 901 in 2013 offers a concise summary:
    http://forums.bimmerforums.com/foru...frame-strategy-thread&p=26163106#post26163106

    What I have said in this thread is the collected and accepted wisdom of the Bimmerfest and Bimmerforum forums, on a subject that has been discussed there for many years. I think enough information has now been put into this thread by both of us for readers to draw their own conclusions.
     
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  15. Brian H

    Brian H Zorg Guru (V)
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    oooh interesting reading, for what it is worth my 1.9 had 3 failed spot welds, had the spot welds plug welded by BMW and they failed again within a year! Another BMW repair but this time seam welded end of problem.
     
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  16. Brian H

    Brian H Zorg Guru (V)
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    Some more food for though :)

    It may not be proven with calculations or testing but has stood the test of time as have similar repair methods that have not had testing and calculations etc., what did all the calculations and testing do for BMW? This issue is not only restricted to the E36/7,E36/8s models E46s have a similar issue.


    IMO having just changed out the rear beam bushes, there is a lot of lateral movement in the rear beam, the older bushes on my car had 20 - 30mm movement on each side, this movement will allow the whole rear beam to move on the car, with this movement in mind it must effect the diff hanger bracket? I also believe that the rear anti roll bar hanger bracket location plays a part in finally ripping the weakened boot floor apart.

    Agree it is not a six pot only problem, but do not agree that there is not a design issue.
     
    #16 Brian H, Sep 10, 2014
    Last edited: Sep 10, 2014
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  17. zedonist

    zedonist Guest

    Brian, you raise some valid points, i don't disagree that the RF option has stood the test of time, just redoing the spot welds will stand the test of time, my issue (not an issue really, because i would not fit it) is that people keep pushing the RF as a better option and more robust, and its not, RF is a car enthusiast not an automotive structural engineer, he looked at the fall out of his torn up car and decided he would weld in something that in his opinion (and thats the cruncher) would be better.

    He did not look at the structural calculations and the loading figures, the lateral G etc etc the list goes on to include material grades, strengths, thicknesses etc, and he welded in some profile cut plates. Now if you could present all the figures in the specification of the BMW design and overlay the RF data as a comparison and the data was better, i would agree that his option is better.

    What nobody has done when this problem manifests itself (except BMW) is get to the root cause of the failure, they all quote the massive torque and power of the //M and the 6 pot engines, when in fact it has occurred on all versions. Power and torque does not come into it, the rear carrier is held in place by two high tensile bolts of large diameter through the bushes, the carrier is then secured to the body by two plates, it cannot move very far and it cannot twist to the extent to rip the floor the diff is connected to the carrier by 4 bolts, the rear bush on the diff is a damper. The body on all versions shares the same suspension geometry and travel, lowered versions actually travel less through the cycle. If you take this into account the only force that is applied to the boot floor and diff bracket is weight. If you then understand the root cause of the failures is actually poorly supplied product from a supplier and poor process control on welding then the weight is the key factor, because as welds pop the applied force per surface area increases until the majority of welds pop and the floor fails.

    If you take this into account the correct course of action is to re-weld the floor as per specification, it will then be to the design and there are 100's of 1000's of zeds that have not failed, they just don't post on a forum that their boot floor is fine, because there is no point, people on post on a forum when they have an issue. Now the next question i deduce from this is that people say well others have had the spot welds repaired and they have failed. I would say i agree that they will, and i will also say that someone with a RF kit fitted will fail also due to welding. The reason is because welding is classed as a special process, as such you cannot guarantee any weld defect free, as it is down to the skill and experience and process that each individual welder applies, now if robots cannot always get it right on a production line, what chance does your mate, the guy in the indie or the stealer have of getting every weld right, now the risk increases by going to seam welding.

    Now if i ever experience this I will be going to get it done in line with the BMW repair procedure, the reason is because it will use approved materials and hopefully be done by a qualified welder (coded), i wont be welding in place some angle iron.

    How many RF users have informed their insurers of this modification, because it is a modification, it holds no homolgation? i guess none, and if you did you would find they would ask for a structural engineers report, and then you would only get a pass if the installer followed a correct code of welding practice, used specified materials, which included NDT testing of the final weld.

    I do not want to influence anyone, but i think people should have all the facts, and then they can make their own decision on what to do.
     
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  18. hard top

    hard top Zorg Expert (I)
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    Bloody good food for thought @zedonist reading through that......:beer
     
  19. Brian H

    Brian H Zorg Guru (V)
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    @zedonist, did BMW ever get to the root cause? Some of the extensive repairs BMW did under warranty and FOC as good will gestures which involved removing the whole boot floor and replacing with a new updated one have been known to fail for a second time, this to me points to the fact that they did not know the full root cause. The problem spanned over many years and was not put down to one specific production run if you have an early or late car you could have this issue.

    Question: what stress if any does the rear ARB put on the boot cross member?
     
  20. zedonist

    zedonist Guest

    Yes Brian they I have read a number of notices marked process issue and against various models and years, but never a recall notice raised.

    ARB is resisting the stress when in cornering, doing nothing when at rest, stronger ARB's reduce the movement further. If you took an ARB to its max, I.e welded the body to the axle (go cart) there would be no movement and no stress, but you would spend a lot of time going sideways.

    I think the worry for me with these cars is that they getting old now and corrosion at these points will be the biggest cause of failure through fatigue
     
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