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  • #46
    Originally posted by exdos
    c_w,

    I am aware that the prop shaft and drive shafts allow for play. But you've still not told me how the butt strut does it's work if you think my explanation is wrong?
    Originally posted by exdos
    Tying the two subframe bolts together with a Strongstrut butt strut will considerably reduce any tendency of the subframe to move on the bolts. Fitting a body-brace to the butt strut will take this a step further and will prevent all movement of the suspension bolts, thus keeping the subframe bolts absolutely rigid. The purpose of a body-brace on an MC would not be to stiffen the bodyshell (and I doubt if it can do this either on a Z3 roadster either), but instead, it is to do what I've described above.
    The butt struts job is to make the subframe location more "positive"; like the strut brace at the front, it's benefits are for handling by more positive suspension location. It doesn't alter the movement of the subframe other than holding the location (pins/studs) it attaches to more positively.

    When I fitted some modified subframe bushes I noticed how flexible the "pins" are so bracing them is a good idea. The body brace you've drawn would probably brace them further as the butt strut mainly braces side to side so the handling may be slightly tighter (but I would say its OTT for what you'd probably notice driving the car), but I can't see how it can help the diff out.

    If you fitted all these braces and used standard bushes you're still going to get the rather large amount of subframe movement due to the pliable standard bushes, so I would say that if you wanted to limit subframe movement you need to look at the bushes since the bracing isn't physically attaching to the subframe, but the body.

    Looking through some american forums they have tried various subframe bushes, also packing them out above and below the subframe mounting points so it has no vertical or twisting movement but you get a lot of harshness from this. But again I'd say this is a handling mod.

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    • #47
      Originally posted by c_w

      It doesn't alter the movement of the subframe other than holding the location (pins/studs) it attaches to more positively.
      Maybe you just don't understand what I write :wink: but that's precisely what Im saying.

      Originally posted by c_w
      When I fitted some modified subframe bushes I noticed how flexible the "pins" are so bracing them is a good idea.
      See my comment above.

      Originally posted by c_w
      The body brace you've drawn would probably brace them further as the butt strut mainly braces side to side so the handling may be slightly more tighter, but I can't see how it can help the diff out.
      You've already explained this to yourself in your first comment above :!:

      By holding the location (pins/studs) it attaches to more positively (your words) you are creating fixed triangulation of the differential to the two ends of the subframe so that there is little or no movement of this fixed triangular relationship, as it really is intended to be. The moment that 2 points of the triangle (the siubframe pins) start to move there will be stress placed on the only fixed point of the triangular relationship, namely the differential on it's fixed mounts. It's interesting to note that BMW have produced the Z4 with a triangulated 2 part butt strut which keeps the subframe pins rigidly positioned, so I think my explanation must be correct.


      Originally posted by c_w
      If you fitted all these braces and used standard bushes you're still going to get the rather large amount of subframe movement due to the pliable standard bushes, so I would say that if you wanted to limit subframe movement you need to look at the bushes since the bracing isn't physically attaching to the subframe, but the body.

      Looking through some american forums they have tried various subframe bushes, also packing them out above and below the subframe mounting points so it has no vertical or twisting movement but you get a lot of harshness from this. But again I'd say this is a handling mod.
      The purpose of stiffer bushing around the subframe pins or packing out the OEM bushes is, in truth, an attempt to keep the subframe bolts permanently vertically fixed. Obviously when this is done, the bushes can no longer allow any "give" for the subframe, and as you say, there will be harshness of ride. Therefore, if you put all the pieces of this particular "jigsaw" together, you will realise that fitting a butt strut and body brace actaully keeps the subframe pins in a fixed position, thus allowing the OEM rubber bushes to allow a degree of movement of the subframe and the rear suspension, as intended. Do you get it yet :?:
      /// Exdos ///
      "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

      Comment


      • #48
        The moment that 2 points of the triangle (the siubframe pins) start to move there will be stress placed on the only fixed point of the triangular relationship, namely the differential on it's fixed mounts..
        Why would it do that?? The diff is seperate to the subframe and suspension, as said previously it's only connection is through the driveshafts.

        What you're trying to propose [to prevent the diff fracturing mounts] doesn't make sense and would be a waste of time IMO.

        The purpose of stiffer bushing around the subframe pins or packing out the OEM bushes is, in truth, an attempt to keep the subframe bolts permanently vertically fixed. Obviously when this is done, the bushes can no longer allow any "give" for the subframe, and as you say, there will be harshness of ride. Therefore, if you put all the pieces of this particular "jigsaw" together, you will realise that fitting a butt strut and body brace actaully keeps the subframe pins in a fixed position, thus allowing the OEM rubber bushes to allow a degree of movement of the subframe and the rear suspension, as intended. Do you get it yet
        The pins are fixed in their repsective holes but there is slight flex which the butt strut should prevent (along with the standard bracing on to the sills). Packing out the bushes isn't to prevent something moving that doesn't move already (ie the pins), but to prevent the subframe moving up and down on the bushes so that the only part of the suspension that moves is the trailing arms. This is all for handling benefits. You can see just how much the subframe "floats" on the standard bushes if you rest and axle stand under it with the wieght of the car on it; the subframe sinks up a good few inches which significanly alters the location of the trailing arms which could perhaps happen under hard driving. The pins and bolts are all tight but the subframe moves on the bushes, irrespective of any bracing.

        Do you get it yet?

        Comment


        • #49
          Originally posted by c_w
          Why would it do that?? The diff is seperate to the subframe and suspension, as said previously it's only connection is through the driveshafts.
          Each time I get involved in a thread with you I vow to myself it will be the last time. You really do talk (type) in circles :? :(

          We both know that the diff is separate from the subframe. It's the subframe bolts attached to the bodyshell and the differential attached to the same bodyshell that creates the triangulation, at the bodyshell. IF the subframe bolts were naturally held vertically rigid, then the subframe would be free to move slightly (mainly vertically) on the bushes and there would be no problem with either handling or diff mount failure, and the traingulation would remain fixed. HOWEVER because the subframe bolts can '"wobble" inside the OEM subframe bushes, the subframe can "slew" around, thus distorting the triangulation. I suggest that BMW didn't allow for this in the Z3 design, but realised the design fault and thus corrected it by making a triangulated butt strut in the Z4 platform.

          Now, have you applied yourself to sorting out your suspension problem and stopped your butt strut scraping tarmac yet :?: :wink:
          /// Exdos ///
          "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

          Comment


          • #50
            LOL - It's the c_w and Exdos show! I think we're all more confused than ever now

            I do love these little technical chats though 8) Personaly I think Exdos is right and c_w is not wrong :wink:
            Black 02 reg S54 M Roadster

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            • #51
              Originally posted by spunkym
              LOL - It's the c_w and Exdos show! I think we're all more confused than ever now

              I do love these little technical chats though 8) Personaly I think Exdos is right and c_w is not wrong :wink:
              i have a headache! could someone give me a brief idea of everything above :wink:

              Comment


              • #52
                Originally posted by B3N
                Originally posted by spunkym
                LOL - It's the c_w and Exdos show! I think we're all more confused than ever now

                I do love these little technical chats though 8) Personaly I think Exdos is right and c_w is not wrong :wink:
                i have a headache! could someone give me a brief idea of everything above :wink:
                Here you go. :wink:

                http://www.z3mcoupe.com/forum2/viewt...er=asc&start=0
                2002 phoenix yellow. "V3RY M" lives on, but on another beast.

                Comment


                • #53
                  Originally posted by exdos
                  Originally posted by c_w
                  Why would it do that?? The diff is seperate to the subframe and suspension, as said previously it's only connection is through the driveshafts.
                  Each time I get involved in a thread with you I vow to myself it will be the last time. You really do talk (type) in circles :? :(
                  I don't type in circles I just repeat what I've said in the hope it's clearer :wink:

                  Comment


                  • #54
                    Originally posted by B3N
                    Originally posted by spunkym
                    LOL - It's the c_w and Exdos show! I think we're all more confused than ever now

                    I do love these little technical chats though 8) Personaly I think Exdos is right and c_w is not wrong :wink:
                    i have a headache! could someone give me a brief idea of everything above :wink:
                    Exdos believes that by further bracing of the subframe pins - which after fitting the butt strut will have minial movement anyway [and this is just for suspension/handling benefits anyway IMO] - it will somehow have a positive effect on reducing the risk of tearing diff mounts and the boot floor.

                    I don't agree because the diff is a completely seperate entity from the subframe. What it needs are heavier duty diff mounting brackings, a double ear diff cover and maybe some plating along the rails under the boot floor.

                    Comment


                    • #55
                      Originally posted by c_w
                      I don't type in circles I just repeat what I've said in the hope it's clearer :wink:
                      Next thing we know you will be TYPING LOUDLY AND S L O W L Y in the vain hope that we might understand you better...
                      CLK55 AMG Cabriolet

                      ex Estoril Blue Z3M Coupe

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                      • #56
                        Originally posted by Brett
                        Originally posted by c_w
                        I don't type in circles I just repeat what I've said in the hope it's clearer :wink:
                        Next thing we know you will be TYPING LOUDLY AND S L O W L Y in the vain hope that we might understand you better...
                        Nope, no need to do that. Any repetition wasn't aimed at you anyway...

                        Comment


                        • #57
                          Originally posted by c_w
                          Originally posted by Brett
                          Originally posted by c_w
                          I don't type in circles I just repeat what I've said in the hope it's clearer :wink:
                          Next thing we know you will be TYPING LOUDLY AND S L O W L Y in the vain hope that we might understand you better...
                          Nope, no need to do that. Any repetition wasn't aimed at you anyway...
                          I was only kidding.... :roll:
                          CLK55 AMG Cabriolet

                          ex Estoril Blue Z3M Coupe

                          Comment


                          • #58
                            Originally posted by Brett
                            Originally posted by c_w
                            Originally posted by Brett
                            Originally posted by c_w
                            I don't type in circles I just repeat what I've said in the hope it's clearer :wink:
                            Next thing we know you will be TYPING LOUDLY AND S L O W L Y in the vain hope that we might understand you better...
                            Nope, no need to do that. Any repetition wasn't aimed at you anyway...
                            I was only kidding.... :roll:
                            You should have been clearer :P

                            Comment


                            • #59
                              Originally posted by c_w
                              Exdos believes that by further bracing of the subframe pins - which after fitting the butt strut will have minimal movement anyway - it will somehow have a positive effect on reducing the risk of tearing diff mounts and the boot floor.
                              I agree with that statement. A case of "belts and braces" where the butt strut is the "belt" and the body-brace is just to be sure that the butt strut doesn't move at all. Over-engineering to prevent failure due to BMW's under-engineering.

                              Originally posted by c_w
                              I don't agree because the diff is a completely seperate entity from the subframe. What it needs are heavier duty diff mounting brackings, a double ear diff cover and maybe some plating along the rails under the boot floor.
                              Whilst I agree that the differential and subframe are completely different entities, the point that c_w just doesn't seem willing to accept is that there is actually a triangular relationship between the mounting point of the differential and the two subframe mounting bolts on the body shell and that movement of the subframe can distort the triangle made on sheet metal of the bodyshell, which after the passage of time may cause metal fatigue and tearing of the sheet metal of the differential mountings and/or other pars of the body shell in proximity of the points of attachment on the bodyshell itself.

                              I do not dispute that the welding of additional parts to the bodyshell to strengthen the obvious weakness is the best solution, as per the fix devised by Randy Forbes, which I've already referred to before in this thread. All I am saying is that the fitting of a butt strut and possibly a body brace as well, should go a long way to preventing the problem of diff mounts tearing and ripping of the boot floor, because fitting these bolt-on parts preserves and strengthens the triangular relationship between the differential and the subframe bolts.

                              I rest my case. :wink:
                              /// Exdos ///
                              "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

                              Comment


                              • #60
                                I'm sure i've discussed this before on this forum but this is my view on the whole subframe diff tear situation.

                                To really understand this i think you need to have removed the subframe (to which the diff is attached) in order to understand what is going on under the car.

                                The very reason i replaced my subframe bushes was to try and reduce the potential of any diff tears and improve handling, and whilst undertaking this it's easy to see why such an occurance might happen.

                                The use of butt strutt in my opinion will not reduce the lateral twist of the subframe itself, what it does is reduce the twisting of the cars tub, which in itself is qutie stiff on the coupe.
                                This will obviously reduce the abount of twist placed on the subframe but trust me when i say this, the subframe on these cars is very flexible so it will be twisting long before the tub begins to twist and thus place pressure on the subframe.

                                When i removed the old rubber bushes it was at this stage that i realised that it didn't take a lot of my strength to cause the subframe to twist and flex (imagine what 300 hp does)

                                The original bushes by the way are a bu**er to remove, you do require the proper tools.

                                Anyway this twisting of the subframe is what i think places undue pressure on the single diff mount, if you can reduce the twisting of this then i think you will reduce the chances of diffs tearing from their mounts.

                                Also when the subframe twists so your wheel geometary will be out and reduce the handling ability of the car.
                                99vS50 Titan silver, currently on life support waiting for a miracle cure

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