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  • Spring Rate Question

    I'm looking at an ///M right now with GC C/O's...The owner is running equal spring rates in both front and rear...The rates are 400lbs.

    Now obviously, this can be easily remedied with a set of springs, but i'm wondering why someone would run equal spring rates? Is this problematic, or is there some type of benefit?

    Thanks,
    -Caine

  • #2
    Secondly, what spring rates would be a decent street/track setup?

    Something along the lines of a 375/425, or a 375/450? F/R respectively.

    Thanks

    Comment


    • #3
      it's been almost 3 years since I installed my GC, but I think I'm running 400 front and 600 rear, or vise versa. from talking to people at GC it is important to have a higher spring weight in the rear, this seamed odd to me becuase there is no engine in the back, but I also don't ask NASA why they build rockets like that (hmmm, maybe I should:idea: )

      since you have 400lbs springs for sure on the car, why not just change out the rears? my setup is stiff for the street (annoying if daily driven, but fine for the everyother day outing)

      Karl Kraus --Education is a crutch with which the foolish attack the wise to prove that
      they are not idiots.

      Comment


      • #4
        "yaw" is rotatin along the longitudinal axis

        whay you are refering to is pitch, either up or down.

        Karl Kraus --Education is a crutch with which the foolish attack the wise to prove that
        they are not idiots.

        Comment


        • #5
          The reason for the higher spring rate in the back is the location of the spring. In the rear, the spring is inboard on the trailing arm and needs a much higher spring rate to translate to a more normal spring rate at the wheel. If the springs were in true coilover fashion, you would use a very different f/r bias for spring rates.

          /// M3NTAL Kev

          Comment


          • #6
            So the next question is:

            Keep the 400lb in the front and go with a 450lb rear? Or go with a little lighter front (~375) and a 425/450 rear?

            Thanks for the help guys

            BTW: The ///M is an '88, with just shy of 100k...Motor was rebuilt 35k miles ago..I'm not too sure about the car, because of the rebuild, but already having C/O's is very enticing..

            Comment


            • #7
              Depending of street conditions, I would probably go with 275-300/400 or 400/600 based on you only changing one pair of springs.

              If you're willing to change both sets you have some options.

              /// M3NTAL Kev

              Comment


              • #8
                that's right kevin, thanks for reminding me:idea: I had a great conversation with the guys at ground control at tech fest west a few months back. I remember something he also said was you should maintain a 200lb difference between front and rear. they have found that to be the best for bimmers. Don't know if this is track only info or not, but that number does stick in my head

                Karl Kraus --Education is a crutch with which the foolish attack the wise to prove that
                they are not idiots.

                Comment


                • #9
                  Stick with the 400 ft-lb in front and somewhere around 600 rear if you dont mind a slightly bumpy ride. If you wanted a smooth ride of a Caddy them you are barking up the wrong tree anyway.

                  I have 350-500 in mine and love it! I also have the GC Koni shocks too. They added a huge improvement as well. I would give them a serious look if you dont already have em installed. Either way, call GC and ask them what they think. They have always been really helpful.

                  I think that the springs are around $50 each corner so its not a real big deal cost wise to get a new set. An advantage to only changing the rears is that they can be done with little hastle. You only need to jack up the car, turn the gland nuts all the way down, and remove the spring. OK you may need to remove the sway bar links too, but I have had luck going without this step. If you change the fronts, you need to remove the whole strut assembly to get them out.

                  Comment


                  • #10
                    Thanks a bunch for the help guys..

                    I'm pretty much only accustomed to e36's, and wasn't quite sure about rates for the e30 m3, but it's not much different..

                    I'll consider 400/600...I'd like to try and find someone with those rates in particular and take a ride...Because right now in my e36, I have bilstein/H&R sports...That equates to about a 185/375 (WEAK)

                    Thanks again

                    Comment


                    • #11
                      On the rear spring rates...it's a motion ratio thing. Leverage and geometry and such. Or were you looking out the window that day in Math class.....

                      Ever cut something with a scissor and notice that it's easier if you get the thing further into the v of the scissor? (see "A" below) A crude attempt at an ascii pic:

                      xxxxxxxxxxxx = The various X's form the cutting end of a scissor!!!
                      .x A....B
                      ...x
                      ......x
                      ........x

                      If you do this, your hand force is multiplied by the more favorable leverage at "A". If you start a cut near tip (B) of the scissor, you have to squeeze much harder. The wheel rate would be analogous to how hard you squeeze. Keep a constant squeeze (same corner weight on a car), and the force at "A" will be higher than for "B" so in the car world a higher spring rate would be needed. If you think about it, as the M3's trailing arm rises toward the frame rail the upper end of the spring pushes on, it's very much like a scissor action. What you really care about is how stiff the spring rate winds up at the tire. If you ran true coilovers out back, with the spring around the damper, you can get the same effective wheel rate by using a softer spring than one mounted in the stock position.

                      Since there is less leverage on the front strut acting on it's spring, the effective wheel rate is much closer to that of the spring.

                      BTW stiffer springs do not push down on the ground harder nthan stock...else with firmer springs our cars would leave the earth...

                      Stan
                      Last edited by Stan; 08-25-2003, 09:19 AM.

                      Comment


                      • #12
                        BTW, I have a set of 600lbs" rear springs for GC if you're interested...

                        600lbs"
                        2.25" ID
                        5" length

                        Let me know...

                        Kev

                        /// M3NTAL Kev

                        Comment


                        • #13


                          When I ordered my GC street bolt on kit, I said I wanted something primarily for the street but with occasional track use and they recommened 375/475 springs (linear I believe). Why would they recommend this when everybody else seems to have at least a 200lb difference f/r?
                          "It is needless to say that self-propelling vehicles, like other machines, will never do as much for one who does not understand them as for one who does."

                          Comment


                          • #14
                            Originally posted by AlpineRunner


                            When I ordered my GC street bolt on kit, I said I wanted something primarily for the street but with occasional track use and they recommened 375/475 springs (linear I believe). Why would they recommend this when everybody else seems to have at least a 200lb difference f/r?
                            I think the aim is to have a ratio of roughly 66-70% of the rear on the front . So a 400/600 or 325/475 or 350/500 would all be good.

                            /// M3NTAL Kev

                            Comment


                            • #15
                              Yep,
                              Kevin nailed that one.
                              The factory race teams found the optional ratio to be a between 68% and 74% front to rear if I remember correctly. It isn't so much that 200lb/in difference is optimal, but that at 400/600, 200lb/in is the difference in a 66% ratio. I currently have 450s in the front and 550s in the rear (for other reasons), and the car is a little tight. The appropriate 650lb springs would help alleviate this.
                              And like Stan said, the higher spring rate in the rear is due to the leverage of the trailing arm as opposed to the direct action of the strut in the front.
                              For the street I would run softer than 400-600, especially if you are daily driving in Dallas. I live in Fort Worth and when I had 450/650lb springs on the car it was a bit much for our roads everyday. Not terrible, but not smooth. The adjustable konis make a big difference in ride quality.

                              HTH,

                              Will
                              Speed, as Iíve said many times before, makes you cleverer.

                              -Jeremy Clarkson

                              Comment

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