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Any advantages of using a cabrio 21mm swaybar

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  • Any advantages of using a cabrio 21mm swaybar

    Any advantages of using a cabrio 21mm swaybar upfront on the M3? vs. the 19mm stock one. I have one laying around so that's why I'm asking. I don't want to upset the balance of the car as I would leave the stock 14.5mm rear one. What do you guys think? Should I just leave it alone?

  • #2
    It will make the front end stiffer. Go for it and see how you like it since you already have it. I believe it will make the car understeer more.



    • #3
      It'll make the car understeer badly - very badly. 19 to 21 is an approximate increase in stiffness of 21^4/19^4 (149%), so 50% stiffer than standard - and the front bar makes up about 2/3 of the front roll stiffness as standard. A stock eqpt E30 has a front roll stiffness distribution of about 60%, so is inherently understeery (in steady state conditions) - putting the 21mm bar on pushes this to 67%. By my calcs, you will need an 18mm rear bar to regain the balance - H+R do one but will only sell as akit with their 22mm FR, I believe.

      I'd keep it and try to find an 18mm rear bar if you can - or you could get one made if you know a machine shop with a CNC bender. I appreciate that's a stretch though!



      • #4
        Personally, i would give it a go, IMHO the M3 suffers from roll induced understeer in standard trim, this bar could well get rid of that and make it better as a result.
        Just look at the std spring rate ratio front to rear - and then compare the spring ratio to all of the after market set ups.

        When I had my H&R bars (with H&R springs) i played around with all different setting combinations to see what happened. Soft on the rear and 'middle' on the front worked best. With full hard on the front and soft on the back it actually oversteered MORE.... this must be due to increased grip on the front end.


        • #5
          Can't argue with that experience - my advice is based on theory, where increasing the load difference on an axle pair of tyres (eg by stiffening an ARB) can only ever reduce available grip. However, the devil here may be in the details - can I ask what springs (H+R standard are 180/380lb/in AFAIK, but coilover springs could be anything), tyres (R-rated? Same F/R?) and track(s) you were running on when you found this?

          I ask the latter only as I've only ever found changes like this to work the 'opposite' way to convention when large degrees of asymmetry are involved. For example, if you have a track with mostly long right handers, say, you could find a stiffer front bar reduces understeer on a tight left hander as the less heavily loaded inside wheel has started to 'go-off' due to being hammered around the rest of the track. The outside wheel has lots to give as a result and can potentially provide more grip on its own, with a high vertical load on it. If the tracks you have run on are similar to this, you may have observed this phenomenon - possibly!


          • #6
            It was with the 180/380 combo. When I went to the H&R coilovers, the H&R bars IMHO destroyed the balance no matter what i did, really tail happy, almost felt as if the bars and the springs were 'fighting each other'. went back to std bars and it was fine, but a little soft when pushing hard. Tyres were Yoko R's and Falkens.

            We put these H&R bars on another car, with 250lb front springs and the 380h&r rears, best was with both bars on soft. This was really nice, very controllable.


            • #7
              Just realised, this sounds familiar - is that you Dave?

              Interesting stuff, this puts it way out of the sweet spot according to my thinking, but if it works it's worth looking into. Might need to call Faulkner for another set of specials - I could be heading down the wrong path with a 150/430 split. Time will tell!


              • #8

                150/430!!! If you have 430 on the rear I really do think something around 250 would MUCH better. Andy should have Uwe's 250 spec springs on file, get a set.


                • #9
                  interestring topic, I think i can relate to this.

                  My setup is

                  HR Coilovers not sure of spring rates but the standard/normal rates supplied not custom or anything
                  HR Antiroll bars

                  On type 5 alloys with 225 35 profile 17 with HR 15mm spacers (et 5)the car handled awesome, perfect turn in with LOADS of grip and balance. handled like a go kart. Took it to a Silverstone in UK and was really great to drive and performed well.

                  I have know reverted to standard 15" bbs and kept the same setup and the result is awful. no balance at all, no grip and just slides, tyre scrubs on fast corners even at 25/30mph fighting for understeer then oversteer.Scary stuff.

                  I was also advised from one of my good friend who knows his stuff to revert back to standard bars with the suspension setup i have. And as Dave mentioned it i think i will.

                  Cant wait to try it out.
                  sorry if off topic but hope it helps. I would next concentrate on spring rates you have/getting and use the Anti roll bars accordingly.


                  • #10
                    A lot of the reason for snappy oversteer in my set up i put down to poor grip at the rear from increase rear camber due to the lowering, and when combined with the bars just made it worse. I wanted to try turning the powerflex bushes upside down to move the subframe up, this moves the subframe up approx 10mm. This alters the curve of the trailing arms. It has the same effect of raising the suspension by the same 10mm. Eitherway you get less camber. But i never got that far as I stripped the car of anything bolted to it, to start again from scratch.........

                    M3215. I was using 16's on road tryes, 15's on R's. on R's with 15's with the H&R coilovers it was great.
                    Last edited by UK215M3; 05-21-2008, 01:14 PM.