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Centre diff play and gearbox weight

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  • #16
    gmac78,

    Perhaps you did similar to what Steve Briance did, a full circle receiver bracket made from the original and a replacement poly snub of more volume? Where you happy with the direction it took you? I bet you now prefer your version over stock, the above listed changes are more of the same. I'm not certain if there is much difference between your's and my snubs, but in either case that each is just in contact such that it begins to work immediately I think is key.

    In any case, it ain't the snub alone, the whole list together is greater than the parts. Typically, as found on a 28 yr. old example, the motor / xmsn can with torque, lift and fall slightly which in my view, is not acceptable. The two central main engine mounts alone, while good at carrying the majority of the weight and resisting torque around the crank axis, are (purposely I'm sure) simply not positioned well to resist the torque which acts perpendicularly; around the front axle.

    Stock, this many decades on, the xmsn mounts and snub + bracket are not doing much of anything except being present to be struck; after the motor / xmsn has already pitched up or down. But they are in the right position to be more effective; if you choose to make them work. You can usually hear those strikes, feel them and perhaps particularly on a 5-spd., see the shift lever move during throttle induced torque reversals. They are always preceded by a short pause as the torque falls to and through zero. It is these pauses and strikes which make the car so unpleasant to drive. The reason this behaviour is so noticeably annoying is that the heavy eng / xmsn assy, with any backwards or forwards rotation, acts as a large flywheel and any small arc of its' travel (about the front axle) is also multiplied by the momentum of all the drive-train's rotating bits; and this drives all the mounts, each to the absolute limit of their available travel.

    Like a wandering river's water flow, torque will take the path of least resistance and find what is the easiest thing to move first, either the car; or mounts m, n, o, p, q, r, s, t, u, v, w, x, y or z (14 loose drive-train to chassis mounts).

    When set-up with the above listed kit, the 5 mounts together share in the suspending of the motor / xmsn more equally. In a similar fashion to that of your front suspension's 3 OEM mounting points per side, which can be made to more precisely hold steady the wheel / tire / brake disc / caliper & strut assy in a more fixed home position; the same (see below!) increased precision can be achieved with these 3 (the snub + xmsn mounts), an improved ability that they absolutely ought to have. Although the All-roads are frighteningly stiff and unyielding when examined in your hand, they deliver surprisingly plush quiet stillness in use. Apparently they are liquid-filled, clearly they are effective dampers. Often they are found on E-bay used and cheap.

    These listed changes have got to be (IMHO) one of the easiest and most cost-effective must do lists of things almost any of us can do to improve the everyday experience of driving our cars, period.

    Altogether the above changes offer 14 mount and bush locations (on a B3) where you can remove slack from the drive-train. If taken up, this removes the feeling that you are driving 15 rail-road cars along caterpillar fashion, as the characteristically loose inter-car couplings all rattle and hum in sequence during every acceleration and de-acceleration. Not to mention that the Arm-Stops are going to keep your new bushings "on-rails" steering feeling for a very long time. Unfortunately, even if you've just replaced them, if your A-arm bushes have been run un-aided (yup, just like top-mounts!), they are fated for an earlier than deserved demise. It is the extreme peak loads of braking and pot-holes that do them in.

    Finally, another benefit of all of the above (slack is now gone at all those locations) is that the throttle henceforth becomes a more timely, precise and useful tool for making small steering adjustments; mid-curve. A nice capability to have that you may never have experienced to the same degree previously.
    Last edited by Lago Blue; 20 February 2019, 17:21.

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    • #17
      I think it's worth revisiting my snub mount to see if I can decrease the amount of free movement within the cup arrangement.

      This is the one I fitted a few years back with a new stock rubber mount.

      https://www.ebay.co.uk/i/273194126629?chn=ps

      Off the top of my head I can't remember how snug the rubber mount fits inside the cup. I guess it's worth improving this arrangement further to see how it affects shunt.

      Wee job for today perhaps....

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      • #18

        While you are there, it may be worth examining the opposite end of the engine & transmission assembly see-saw that seeks to pivot around the front axle; and look at the rear two of the five mounts comprising the eng/xmsn cradle which must resist that motion.

        If your xmsn mounts are stock, they will develop play, (if you've changed them before, you may have found the rubber torn apart which with stock mounts, always eventually occurs) which even before tearing, will allow the engine to rock about the front axle. Give the tail of the xmsn a good heave up and down to see if they allow any movement. If they do, the snub + receiver won't be able to resist that motion alone, no matter the arrangement. If there is clearance (it may be visible as a gap) that can be taken up between the rubber of the xmsn mounts and the large washers that bracket them, you've found 2 more of those 14 usual suspect locations. It may be that you can shim them to get a very slight negative clearance, as an interim measure.

        WRT total drive-train slack, you do understand that all of the ever-weakening (only if un-aided!) A-arm bushings are also just as much part of the drive-train shunt issue as the snub etc., correct ? You can lever onto them to see if they are movable fore-aft as well.
        Last edited by Lago Blue; 15 December 2018, 13:11.

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        • #19
          Fundamentally, the shunt problem is because old torsens are not pre-loaded and have inherent lash - you can feel this be twisting the prop next time you are under the car - every time you apply or remove torque, the lash is taken up - obviously this gets worse with wear. Replacing every mounting on the car and even buying a new diff won't cure it.

          Replacing all the bushes & mounts will help minimise it, but If you want rid altogether, you need to move to a more modern gearbox (0A3) with a later generation pre-load Torsen centre diff, or move to an aftermartket ECU and an e-throttle and use a damper on the throttle rate to gently introduce / remove torque from the driveline.
          Panthero Coupé quattro 20vt
          Indigo ABY coupé
          Imola B6 S4 Avant

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          • #20
            I'm thinking that's what needs to happen, every bush and mounting on the car has been replaced, however i haven't tried the A arm mod with the washers so kind of intigued what difference that would make. Checked the snub mount again and it's pretty darn snug in the cup arrangement so don't think theres too much to be gained there.

            What cars can the OA3 gearbox be found on?

            Cheers.

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            • #21
              B6, B7, C6.

              B7 RS4 is the one to go for, it has the 4.111 final drive matching the S2 diff. Ratios are shorter than the 01E and shift like warm butter compared.
              Panthero Coupé quattro 20vt
              Indigo ABY coupé
              Imola B6 S4 Avant

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              • #22
                I think the torsen diff is pretty much the same on 5 and 6 speed boxes , how ever if the car has been run with tyres with a different wear rate front to back , ie different rolling radiuses then the diff prematurely wears creating drive line shunt , forward and reverse regardless of how good your bushing and snub mount is , been there and it's just cost me nearly 3k to get it sorted

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                • #23
                  What I'm suggesting is:

                  a) Unashamedly really rather basic; and there's a good reason for that. What I want folks to understand is that the excellent functioning of these mounts and bushings, these connective tissues, are key to defining the character of the car; so if it is a better driving experience you seek, you ought not to ignore them, take them for granted, assume they are still good, etc. I believe that you will get a good deal of satisfaction from completing this list, as I have;

                  b) That fixing all the bits that are there, ostensibly fix the drive-train in place, is in my view just a compelling and logical first step to take in terms of time, cost and driving benefit; and would be complimentary to any changing-out of worn rotating bits. While completing this list does not also require replacing the Torsen to make a very noticeable difference, to not attend to these 14 suggested necessary, practical and affordable locations first would be to miss the low-hanging fruit;

                  c) Easily broken down into a series of small short simple jobs which while not dependent on each other, are cumulative in their effects; and deliver tactile driving benefits beyond shunt loss. Specifically, those would be in steering, braking and shifting; &

                  d) Just common-sense preventative maintenance that anyone with a stock or near stock car and modest means and abilities ought to pursue as an worthwhile end in itself. As an added benefit the Arm-Stops mod would appeal to someone who:
                  - recognizes at least either the cost or driving performance advantages of preserving their new A-arm bushings "as new" for a very long time;
                  - either prefers to stay with stock bushings or is not happy with poly bushes; &
                  - understands why such items really ought to be considered as fundamental first requirements when seeking to tune the suspension.

                  Finally, my point with the above is that stock or very near to it can be made significantly better than what it was when first delivered without too much difficulty or expense; and that you ought to give these items a good try. All that is required in most cases is little more than a suspension of disbelief.
                  Last edited by Lago Blue; 15 December 2018, 15:45.

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