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Thread: Spring rate theory and application regarding our abnormal chassis.

  1. #76
    Join Date
    Jul 2012
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    Herndon, VA
    Posts
    625
    My Cars
    98 M3/4/5, RIP 528e
    Hi everyone, sorry to bump an old thread but it seemed to be the most relevant to my question, so I figured I would give it a try.

    I rallycross my 98 M3/4/5, and I need some advice about what to do with my spring rates when I remove both sway bars. The car is on JVAB offroad Bilstein-based shocks, and has/had stock sway bars. Front springs are 12" 250 lb, rears are 7" 350 lb (although I'm probably going to try and squeeze some 8" springs in there). I recently removed my FSB in an effort to get rid of understeer when on course (it helped), and I think I would like to also remove the rear to help me put down power and get better wheel articulation. I believe that I will need to raise my spring rates to compensate for the lack of sway bars, but I'm not an engineer and can't calculate how much I should change. If I had to guess, I would start with 300f/400r, but it's just a guess. Can anyone comment on how I should go about this? I can't exactly buy a bunch of different springs and try them all out, because that would be too expensive on my tight budget.

    Thanks in advance!

  2. #77
    Join Date
    Jul 2010
    Location
    Philadelphia, PA, USA
    Posts
    975
    My Cars
    2001 M Coupe, 2002 M3
    Quote Originally Posted by 95maxrider View Post
    Hi everyone, sorry to bump an old thread but it seemed to be the most relevant to my question, so I figured I would give it a try.
    I would think you would have better results posting either to the motorsports forum or the E36 forum- the rear suspension on the E36 sedan is completely different from the Z3.

  3. #78
    Join Date
    Jul 2012
    Location
    Herndon, VA
    Posts
    625
    My Cars
    98 M3/4/5, RIP 528e
    Quote Originally Posted by cleanerPA View Post
    I would think you would have better results posting either to the motorsports forum or the E36 forum- the rear suspension on the E36 sedan is completely different from the Z3.
    I did, no responses yet I'm not looking for exact numbers here, just general ideas from people who have played around with removing sway bars and spring rates.

  4. #79
    Join Date
    Dec 2007
    Location
    Great Dismal Swamp
    Posts
    16,026
    My Cars
    E36/7 E36/8x2 E46 F25
    One of the reason you are not getting much response is because what you are asking doesn't have a\ simple answer. Figure out the roll couple for the front axle. Figure out the roll couple for the rear axle. Determine what roll couple distribution you want for your off road endeavours (I gots no clue on this one). Do a bit of light math.


    /.randy

  5. #80
    Join Date
    Jul 2005
    Location
    Temecula Ca.
    Posts
    4,489
    My Cars
    99 M Coupe LS Swap
    I went through the same questions a while ago on FrankenCoupe. The rear bar is off, and I just can't get the level of front grip I really want. Part of my problem is that I refuse to run a splitter do to the hassle of trailer loads. I tried more bar, for sure that was not a surprise and lost a few seconds. I tried removing the front bar. It resulted in loss of feel and just a mushy feel that netted less than the stock bar. I raised the rear spring rate and that improved things a bit, including a little less front wheel lift. I still have too much. I've come to the conclusion I have personally reached the limit of the shock hardware relative to the car as is, and either plunk down 4 to 6k for better tubes or just stop chasing the holly grail and enjoy the ride as is. Given I now only go out 3 or 4 times a year, I've come tot he conclusion, there are other things to piss away my hard earned cash on. All this to say, you just have to experiment and test. dream/guess/try, repeat.
    Dan "PbFut" Rose

  6. #81
    Join Date
    Dec 2014
    Location
    San Francisco, CA
    Posts
    1,138
    My Cars
    2001 Z3 3.0 Coupe
    Quote Originally Posted by kornfeld View Post
    Bumping this again....I started with 400lb front and rear, and the rear was too soft. Went to 500lb rear. I'm still finding it too soft. I still feel like I'm close to bottoming out on longer whooomps on the highway, and I don't like it.

    Thoughts on 600 vs 700 lb rear springs? I'm tempted to go straight to 700lb springs....
    I can update my storyline....I ended up going straight to a 700lb spring, and am so much happier. The front and rear suspension now feel like they're well matched, as opposed to being stiff but not overly so in the front, and completely squishy in the back. I would strongly recommend not going lighter than 500lbs or so in the rear (and even that is too light for my taste), and this is coming from someone who has no desire to have a stiff or jarring ride.
    2001 Z3 3.0 Coupe--Sterling Gray/Sunroof Delete/5MT

  7. #82
    Join Date
    Feb 2006
    Location
    Tim @ BMW of Dallas
    Posts
    1,889
    My Cars
    98 Z3 2.8
    Quote Originally Posted by kornfeld View Post
    I can update my storyline....I ended up going straight to a 700lb spring, and am so much happier. The front and rear suspension now feel like they're well matched, as opposed to being stiff but not overly so in the front, and completely squishy in the back. I would strongly recommend not going lighter than 500lbs or so in the rear (and even that is too light for my taste), and this is coming from someone who has no desire to have a stiff or jarring ride.
    I also upped to 500F/700R on my roadster and I feel like it's absolutely perfect. I swapped my 440/650 set to the coupe and now it feels squishy.
    96 320i Touring
    98 Z3 2.8 Roadster
    01 PY M Coupe
    96 Z3 1.9 - DASC
    95 318ti Clubsport
    94 Miata M-Edition
    13 smart fortwo



  8. #83
    Join Date
    Jul 2012
    Location
    Herndon, VA
    Posts
    625
    My Cars
    98 M3/4/5, RIP 528e
    Quote Originally Posted by kornfeld View Post
    I can update my storyline....I ended up going straight to a 700lb spring, and am so much happier. The front and rear suspension now feel like they're well matched, as opposed to being stiff but not overly so in the front, and completely squishy in the back. I would strongly recommend not going lighter than 500lbs or so in the rear (and even that is too light for my taste), and this is coming from someone who has no desire to have a stiff or jarring ride.
    Is this all referring to track driving, or street driving?

    I'm pretty sure I was bottoming out hard in the rear at the rally-x this past weekend, so I'm thinking I need stiffer and taller springs back there. If I'm going to remove the RSB, then all the more so.

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