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  • ACS shock length

    Does anyone know if I can fit standard springs with the ACS shocks ? I'm going to change the springs back to OEM but was hoping I could just swap them over rather than have to bother fitting the OEM shocks which will take ages. I think the ACS shocks would be better anyway but was wondering about the length of them ?
    Ex 2001 S54
    New cars:
    Lotus Carlton
    350Z

  • #2
    They'll be fine and would probably make a good compromise over standard suspension; most of how the suspension works/feeels is down to the dampers anyway. They have more than enough free travel to cope with the standard springs IMO. I think the ACS shocks are Bilstein Sprint dampers branded ACS/BMW which are slightly shorter than stock ones but they will be fine IMO.

    Comment


    • #3
      The ACS dampers are basically Bilstein Sprint (i.e. shortened stroke) that are customed valved for ACS. So using them with the OEM springs is feasible, though they might arguably be a touch 'stiff' and over-damped for the OEM spring rates. However given that a lot of people feel the standard setup is under-damped, that might be a 'good' thing.

      However I question the apparent ease of just swapping the springs versus the dampers as well. At the back since they are separate then I could see that just changing the springs is potentially easier than changing the dampers (unless you have changed to the Rogue Engineering 'upside-down' mounts). However be careful about pushing the trailing arm too low to get the spring out as you can damage the CV joints on the driveshafts. Hence it is probably best to use a spring compressor or disconnect the drive shafts. Will probably also need to do the same to install the OEM springs which are longer.

      At the front however the spring and damper are a combined strut, so you will have to compress spring and undo it all to change springs in which case it is as easy to put OEM dampers back. It is as easy/difficult to change dampers as springs at the front as they are part of the same combined strut.

      I would not recommend running mixed OEM and ACS dampers front and back whatever the springs. You can potentially make the change easier if you have the OEM front dampers and springs available, by building them up as a complete strut unit. (Would need to have a set of rubber spring seats, top hat, washers and upper mounts for each side). Then the swap of complete units is very easy as it is just unbolt old and bolt in new. However if you do this I would also change rear dampers to OEM and if you haven't done it already fit the Rogue Engineering RSMs so that future rear damper changes are a 5 minute job.

      Also remember if you change the front springs you will probably need to get the front toe checked.

      Comment


      • #4
        The question that needs to be asked is why are you wanting to ditch the ACS springs?

        Whilst I was in dispute with KW about the crappy rear dampers it sells with the KW V2 suspension, I ran with KW springs (the same springs for both V2 and V3) and OEM dampers for a few months and KW V2 at the front and this was fine and a considerable improvement on KW V2 rear dampers.

        From this I deduce that since the spring rate of the KW springs is at a fixed rate, then this spring rate must be within the operating range of the OEM dampers. It's the adjustment of the flow through the damper valves which ultimately determines the ride and handling characteristics. With KW V3 I can make the ride either rock hard or sedan squishy soft without changing the springs, just by adjusting the flow through the valves.

        In my thinking on all this, it's the springs which essentially determine the ride height in a neutral loading position and the dampers that do the work; The maximum length of the damper limits the vertical opening of the suspension and the spring in full compression determines the limit to the vertical closing of the suspension. If the suspension is correct for the car then operating range will be such that the suspension rarely troubles the bump stops.

        Therefore with a fixed damping rate of your ACS dampers both in bump/compression (spring closing) and rebound (spring opening) then I would expect that they will work fine with your OEM springs, although the limiting factor might be the stroke length of the damper with a slightly longer spring than the ACS springs, which could limit suspension travel. The only way to find out is: try it.
        /// Exdos ///
        "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

        Comment


        • #5
          I am also curious as to why you want to change the springs back to OEM. Do you find the ride too firm with the ACS suspension? Was it on it when you got the car or did you make the change and are now regretting it?

          Comment


          • #6
            I think MGR wants to gain some ride height because the car keeps grounding on speedbumps/driveway? (there was another thread about it a few weeks/months back).

            Originally posted by exdos View Post
            In my thinking on all this, it's the springs which essentially determine the ride height in a neutral loading position and the dampers that do the work;
            The spring should be weighted properly for the job and it is the spring that should be what absorbs the impact force; the damper (in theory) is only there to control the rebound. Otherwise on that logic, you coud fit the same sprint to any car regardless of weight. Obviously with clever adjustable dampers extra bump can be dialed in to compensate for softer springs, but this only works dynamically and up to a point; if you loaded the car up with springs that are too soft the ride height will sink regardless of the dampers fitted and the car would also settle on the spring under longer corners.

            IMO, if you go too stiff on the dampers the car can start to skip but as you say you find and plenty of people find that the KW works well, but like Lee said in another thread, it might sometimes be better to run a stiffer spring (at least on track).
            Last edited by c_w; 22-01-2008, 10:21 PM.

            Comment


            • #7
              The reason for changing back to OEM is only to do with ride height, I use rural roads daily and do have ground clearance issues. I think I have about 2cm clearnace at the front so under hard breaking almost nothing and it does occasionally catch on things if the obstacle is small enough to fit between the wheels, it is more noticable with a passenger too.

              As I have a set of OEM suspension and use as a daily drive then thought I would just change back.

              The ACS was fitted when I bought the car.

              I was just hoping to avoid having to change the rear shocks as I thought this would be difficult.
              Ex 2001 S54
              New cars:
              Lotus Carlton
              350Z

              Comment


              • #8
                Originally posted by c_w View Post
                The spring should be weighted properly for the job and it is the spring that should be what absorbs the impact force; the damper (in theory) is only there to control the rebound. Otherwise on that logic, you coud fit the same sprint to any car regardless of weight. Obviously with clever adjustable dampers extra bump can be dialed in to compensate for softer springs, but this only works dynamically and up to a point; if you loaded the car up with springs that are too soft the ride height will sink regardless of the dampers fitted and the car would also settle on the spring under longer corners.

                IMO, if you go too stiff on the dampers the car can start to skip but as you say you find and plenty of people find that the KW works well, but like Lee said in another thread, it might sometimes be better to run a stiffer spring (at least on track).
                The applicable physics on springs is Hooke's Law: The extension or compression of a spring is proportional to the load applied, provided the elastic limit is not exceeded. Therefore it stands to reason that the springs fitted to each different vehicle should be designed to remain within the elastic limits of the spring and proportional to the sprung mass which the springs support. Therefore, a heavier vehicle requires stiffer (higher rate) springs than a lightweight car to ensure that the springs operate within a useable range and within the elastic limits. If car suspension only had springs and no dampers then a single disturbance of the sprung mass would cause the springs to repeatedly oscillate until they finally came to rest. The purpose of the dampers is to limit a single disturbance to just one cycle of oscillation. The dampers control the bump/compression phase of oscillation and not just rebound.

                When Lee first fitted his KW suspension he told me which settings he had used on his dampers, these were way stiffer than I, Steve1 or Rags have settled at, and I think our cars would jump off the road (and racetrack) if we'd used Lee's settings. This leads me to think that KW supplied Lee with different springs to the rest of us. Likewise, Lee took a lot of trouble in making his car lose weight: in line with what I've written above, that weight reduction would have the effect of slightly increasing the relative spring rate on his MC.
                Last edited by exdos; 23-01-2008, 11:31 AM. Reason: spelling correction
                /// Exdos ///
                "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

                Comment


                • #9
                  You can raise the rear ride height by using different spring pads under the springs. They come in 5mm, 7.5mm, 10mm and 15mm thicknesses. I think the standard on most MCs is 7.5mm. The relevant part numbers are:

                  33 53 1 136 385 (5mm)
                  33 53 1 136 386 (7.5mm)
                  33 53 1 136 387 (10mm)
                  33 53 1 094 754 (14.5mm actually)

                  Due to the leverage ratio of the suspension arm, the actual gain in ride height is about 2x the difference in pad thickness (i.e. going from a 7.5 to a 10mm will gain 5mm in ride height). If you want to go even further you can cut the centre nipple out of one to stack it on top of another. (I did this on my old 3 series to gain some rear ride height after fitting lowering springs).

                  Comment


                  • #10
                    Originally posted by ZedVan View Post
                    If you want to go even further you can cut the centre nipple out of one to stack it on top of another. (I did this on my old 3 series to gain some rear ride height after fitting lowering springs).
                    That's what I've done with mine! Also using the thickest lower pad available.

                    Only problem with this is that it only raises the rear; the front is more difficult to raise like this (although I think diffrent thickness spring pads are available for the front but not as varying - and you can't stack them safely).

                    Comment


                    • #11
                      Interesting according to the BMW parts application, the lower 3mm spring pad at the front is the same part across a whole slew of vehicle ranges (E30 & E36 3-series, E34 5 series and E32 7 series). However the upper pad also 3mm is only used on the MCoupes, MRoadsters and E36 M3 Evos. But checking the possible upper spring pads for the 5 and 7 series reveals the following parts for 'rough road suspension'

                      31 33 1 128 522 (9mm E34 5 series)
                      31 33 1 135 589 (10mm E32 7 series)

                      Might be worth ordering one from a dealer to see if it would fit! (If either does, it is potentially a way of gaining a ~5mm front ride height gain).

                      Comment


                      • #12
                        Originally posted by ZedVan View Post
                        Interesting according to the BMW parts application, the lower 3mm spring pad at the front is the same part across a whole slew of vehicle ranges (E30 & E36 3-series, E34 5 series and E32 7 series). However the upper pad also 3mm is only used on the MCoupes, MRoadsters
                        I'm sure that can't be right as the spring diameter on the Z3M/M3 is smaller than the non-M E36s?

                        31332227901 is a 9mm upper pad for the M3/Z3M (902 3mm is fitted from the factory though I think)
                        Last edited by c_w; 24-01-2008, 09:52 AM.

                        Comment


                        • #13
                          Well we are both right (sort of!). The lower spring pad is only used on the M3 E36s, but is used across the board on the E30s and E34s as well as the majority of E32s.

                          Thanks for the part number for the 9mm upper spring pad.

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