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  • #61
    Col has cleared something up here which I think both I and Exdos have missed!

    Originally posted by exdos

    The diff is NOT attached to the subframe; it's attached by brackets to the bodyshell. I haven't suggested otherwise either in words or in my diagrams.
    Exdos, I think we've both assumed that the diff isn't bolted to the suspension subframe? but it actually is, so yes reducing the movement of the subframe would be beneficial.

    BUT you're diagram and your reasoning is based on the diff not being bolted to it so I still think that is wrong. However you may unwittingly be on the right track by trying to reduce subframe movement! However IMO, in order for the bracing to be effective, the bushes need to be pretty much solid for the extra body bracing to be worthwhile. BUT maybe reducing the natural damping movement of the subframe may put more stress on the rear bracket?? The stress is still going to be at the mounting points so I would still say the best best is to strengthen the brackets (ideally with a double eared diff plate)

    The bolts no. "9" bolt to the two rear holes in the subframe.

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    • #62
      thats not an M coupe diagram!
      Ex 'V3RY M - MCoupe track monster'
      New toys

      Porsche widebody project

      Cayenne Diesel

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      • #63
        If you have ever compared the difference between ordinary rubber and polyurethane then you will know that there is a substantial difference in hardness, althougth i might add that fitting these doesn't make things too harsh as the suspenstion and shocks should be taking care of things.

        The only real solution to this problem is twin ears on the diff and seam wealing instead of spot.
        99vS50 Titan silver, currently on life support waiting for a miracle cure

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        • #64
          Originally posted by Lee
          thats not an M coupe diagram!
          Looks pretty spot on to me
          99vS50 Titan silver, currently on life support waiting for a miracle cure

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          • #65
            Originally posted by Lee
            thats not an M coupe diagram!
            What makes you think that? I got them off the www.realoem.com website (Z3/MCoupe/1999/RHD), they're usually pretty accurate I've found.

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            • #66
              Originally posted by Col
              If you have ever compared the difference between ordinary rubber and polyurethane then you will know that there is a substantial difference in hardness, althougth i might add that fitting these doesn't make things too harsh as the suspenstion and shocks should be taking care of things.

              The only real solution to this problem is twin ears on the diff and seam wealing instead of spot.
              tbh I'm not sure why I assumed the diff isn't mounted to the subfrrame because when I fitted stiffer subframe mounts I noticed slightly more transmission NVH. I also now remember looking at the front of the diff and tapping the metal where it bolts to and thinking that "tin" surely can't cope with this much power because it really doesn't look that substantial. I think strengthening this area would help too.

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              • #67
                I've dug my Bentley Z3 Manual out, and I should have done this earlier. I was wrong in stating that the differential is not connected to the subframe, because it is at the front. However the problem is even worse than I thought because the differential is also connected at the back to the bodyshell. :!: :!: :!: This means that there is still triangulation between the 2 mounting points of the diff on the subframe and one mounting point on the bodyshell. Therefore, any movement of the subframe will be transferred by the differential to place a greater strain on the diff mounting on the bodyshell. In other words, any movement of the subframe is fighting against the body shell where the differential itself is acting as the lever. All the more reason for fitting a butt strut and bodybrace, not just to improve handling, but also to keep the differential as restrained as possible in an almost fixed location.

                Here are some clips from the Bentley manual.




                /// Exdos ///
                "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

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                • #68
                  Hi all,

                  Interesting thread!

                  The difference in movement between subframe and diff and the resulting strain produced can only be reacted by the stiffness of the floor (and those spot welds).

                  In which case, were this a racing type of application, you'd be not only looking for a brace forward as already mentioned but some method of triangulating internally (a la full racing roll cage type method) between the subframe pickup and the diff mount. That would remove the stress from the boot floor but would almost certainly require additional strengthening of the rear diff mount struture as (as the previous link has shown) it looks pretty weak.

                  Just one other thing to look for when looking for the right MC...

                  <scribbles>
                  Cheers,

                  Ruaraidh

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                  • #69
                    The MC subframe/diff mounting arrangement is identical to my compact.

                    We studied all of this carefully, as obviously, we need more strength than a road car.

                    I'm not familiar with the exhaust routing on an MC, but the compact exhaust is oval shaped and passes under the subframe. Other than resiting the ideally placed fuel tank, the only solution for us was to pass the exhaust through the subframe. :shock:






                    The subframe had to be extensively strengthened to do this, a consequence of this is increased shock loading on the subframe bushes/mounts.

                    To cater for the extra strain on the subframe bushes and mounts, the mounts were strengthened, Poly bushes were used and the tank guards were bolted rigidly to the body on the front mounts and the subframe for the rear mounts.

                    As well as all this, the whole rear end was seam-welded and a stronger diff bush was used, as well as Poly bushes in the strengthened rear arms.



                    The result of all this work is a rear end set-up that's extremely rigid and strong, we were well satisfied with all this until it came to testing.

                    With only a few miles of testing the trailing arm bushes were absolutely destroyed, they were basically taking all the load as the only part that could 'give', a solid teflon bush can cope, but the end result is wholly inappropriate for a road car, the ride comfort is appalling and no way would anyone consider it acceptable for a road car.

                    Imo the standard rear end is a carefully calculated compromise.

                    There's a certain amount of 'give' in the subframe, its mountings/bushes, the diff mount, the trailing arms/bushes and the bodyshell, if you focus on strengthening individual aspects of its design, you're increasing the strain on other parts of the overall system and running the risk of causing failure elsewhere.

                    'All or nothing' imo, and 'all' isn't a suitable solution for an MC road car.
                    Simon.

                    www.bmwrallying.com

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                    • #70
                      Simon,

                      A very interesting post

                      Originally posted by M3 Compact
                      Imo the standard rear end is a carefully calculated compromise.
                      If there are cases of diff mount failure and the boot floor tearing apart, I'm not so sure the compromise was that carefully calculated. :wink:

                      Originally posted by M3 Compact
                      There's a certain amount of 'give' in the subframe, its mountings/bushes, the diff mount, the trailing arms/bushes and the bodyshell, if you focus on strengthening individual aspects of its design, you're increasing the strain on other parts of the overall system and running the risk of causing failure elsewhere.

                      'All or nothing' imo, and 'all' isn't a suitable solution for an MC road car.
                      IMHO, all you are doing when fitting a butt strut and body brace is ensuring that the subframe bolts are located in a fixed position in the vertical so that there is minimal horizontal movement of the subframe. As I see it, the advantage of this is two fold. Firstly, by keeping the subframe fixed in the horizontal plane, the differential should not act as a lever on the rear diff mount connected to the bodyshell thus causing minimal strain on the parts of attachment. Secondly, by keeping the subframe in a more or less fixed horizontal position you are preserving the resting suspension geometry at all times so that the car will handle better.

                      Otherwise I think you are correct and I would hesitate to put polyurethane bushes on the subframe and trailing arms, especially now that I'm running KW V3 set up somewhat harsher than OEM suspension. You can over-egg the pudding by seeking to uprate everything.
                      /// Exdos ///
                      "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

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                      • #71
                        Interesting points indeed. It does make me wonder somewhat if the lack of rigidty in the roadster actually takes some stress away from the subframe. As far as I'm aware there have been no reports of sub-frame failure on the roadsters.

                        If it is indeed all a compromise then I think we need to consider as many angles as possible before strengthening various parts of the underbody in order that we maintain suspension / chassis compliancy.
                        Black 02 reg S54 M Roadster

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                        • #72
                          Originally posted by exdos
                          Simon,

                          A very interesting post

                          Originally posted by M3 Compact
                          Imo the standard rear end is a carefully calculated compromise.
                          If there are cases of diff mount failure and the boot floor tearing apart, I'm not so sure the compromise was that carefully calculated. :wink:

                          Originally posted by M3 Compact
                          There's a certain amount of 'give' in the subframe, its mountings/bushes, the diff mount, the trailing arms/bushes and the bodyshell, if you focus on strengthening individual aspects of its design, you're increasing the strain on other parts of the overall system and running the risk of causing failure elsewhere.

                          'All or nothing' imo, and 'all' isn't a suitable solution for an MC road car.
                          I understand your point Exdos.

                          My point in a nutshell...

                          There's a fair amount of underisable action occuring at the rear end. :(

                          BMW have distributed the undesirable action fairly equally imo. Some diff carriers have failed, some diff rear mounting points, some bodyshell creeping/cracking. Is there a single, repetitive weak point?

                          If the 'Butt Brace' transfers enough of the 'undesirable action' far enough forward to dispel, it must weigh a bit?
                          Simon.

                          www.bmwrallying.com

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                          • #73
                            Originally posted by M3 Compact
                            If the 'Butt Brace' transfers enough of the 'undesirable action' far enough forward to dispel, it must weigh a bit?
                            The StrongStrut butt strut that I have is made of 2" x 0.5" billet aluminium and weighs in the order of about 4 lbs. I think the weight penalty is definitely worth it for the improvment in handling that it gives. My concern about the part is that it has a rectangular profile and therefore is unaerodynamic. It's my intention to enlist this part as a kind of under-car wing to create downforce.

                            The StrongStrut bodybrace is a heavy part though, because it's made of steel and weighs 24lbs. If it's sole purpose is to stop the butt strut moving fore and aft, then I think the part could be made of aluminium. I already have some aluminium "L" profile to make my own design body brace which will weigh about 8lbs in total.
                            /// Exdos ///
                            "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

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                            • #74
                              Like M3 Compact said, the movement of the subframe is by design; all of movement (except the very slightest of movement) is from the bushing in the subframe and suspension (the other tinest movement is from chassis which can be reduced by bracing it, eg. the butt strut etc). If the excess movement of the subframe IS the main cause then the stiffer subframe bushes will limit the movent but as M3Compact said again, this can put stress on other areas.

                              On my car I have standard bushes with "inserts" to fill the gaps in the bush, this seems to be a good half-way house between the two problems (subframe movement with standard bushes vs no subframe movement with solid bushes and problems elsewhere). NVH is slightly higher but the handling feels so much tighter because the subframe is moving around a lot less.

                              However, IMO the main problem is still the weak rear diff fixing and boot floor.

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                              • #75
                                If the problem of failed diff mounts and boot floor tears is actually caused by flexure of the metal of the subframe itself, then fitting a butt strut, or fitting uprated bushes, or packing out the OEM bushes will limit the movement of the subframe on the subframe bolts but will do nothing to stop the flexure of the metal of the subframe, if this is the root of the problem

                                So far, we've only discussed ways of tightening up the subframe mounts and strengthening the diff mounts and boot floor, perhaps instead, we should direct our attention to strengthening the subframe itself and allowing a more rigid subframe to move about in its OEM bushes without any attempt at restraint :?:
                                /// Exdos ///
                                "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

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