Suspension Brakes 1st Gen Succeed where everyone/thing has failed...

Discussion in 'Tuning and Performance' started by Bimmer Lite, May 11, 2009.

  1. Ryephile

    Ryephile New Member

    Apr 9, 2009
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    Yes!
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    The short of it is the front motion ratio is 0.99:1 and the rear motion ratio is ~0.875:1. Motion ratio is the proportion the spring axis moves compared to the wheel hub.

    Here's an example to chew on:
    I run 391#/in front and 279#/in rear springs [7kg/mm front and 5kg/mm rear]
    This means my front wheel rate is 387 pounds per inch and my rear wheel rate is 244 pounds per inch. Let's make it easy and say my corner weights are 750/e front and 400/e rear, for a total [dry] vehicle weight of 2300 pounds, with 65% on the front end. I run zero pre-load on my springs, so that means I have 1.9" droop in front and 1.6" droop in rear. I don't have it front of me but there is a quick formula to calculate how even and level the car articulates over a bump in the road versus speed. To do this you need the rear end should have more relative spring resistance in comparison to its corner weights. I usually choose a speed of between 70 and 100MPH because this is a good average for most club circuit tracks. At this "balance" speed, the car will react most neutral in terms of bump disruption. Going slower than this balance speed invites understeer, and faster invites oversteer. The effect of a soft rear spring will be the car feels like it's dragging booty, whereas a stiff rear spring will feel like a bucking bull, kicking the rear end up. Thankfully there is a reasonable middle ground, and the dampers can help quench some of the spring rate preferences. Remember that spring rates determine yaw through corners and the dampers just help settle the car during transitions.

    The other big aspect is the swaybar. Because I choose to run soft wheel rates, I compensate with huge swaybars [mainly because my coilovers have limited total stroke]. I run a H-Sport Comp in front on "soft" and the same in back on full-stiff. I'm leaving out all the geometries for now. Running stiff swaybars means you have to run sticky tires, and my Hoosiers fit that bill. My car understeers significantly on street tires [even "street class" tires with 180 UTQG], but is perfectly neutral on Hoosiers. This means I can throttle steer during at-limit cornering to adjust yaw if I've messed up my line [lift to tighten radius and mat it to widen my line].

    Now, since Bimmer Lite is running a soft rear swaybar, it's clear that he can run a stiff rear spring to get a similar overall dynamic wheel rate [which accounts for both spring and swaybar]. The downside is he'll run more body roll [which may put instant centers outside preferred regions] and more damper travel, which means the car may be more upset by mid-corner bumps since he'll be closer to bottoming his bumpstops.

    I hope that makes sense to someone!
    Cheers,
    Ryan
     
  2. Rooster

    Rooster New Member

    May 22, 2009
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    Systems Engineer
    West Hollywood, CA
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    I just wanted to say that this thread rocks!!!
    :smilewinkgrin:
    Hopefully a great example of things to come over here at MA!
     
  3. UKCoopeR

    UKCoopeR Active Member

    May 21, 2009
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    I agree
    I <3 threads were you can actually learn things because people use numbers and equations (can you tell i am an engineer? lol)

    anyway don't let this thread die! i want to do a bit more research and learning so I can make the best call for me!
     
  4. cct1

    cct1 Well-Known Member
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    May 5, 2009
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    Yeah, but all this information without hidden agendas and constant baiting and flaming is going to take some getting used to....:D
     
  5. Bimmer Lite

    Bimmer Lite New Member

    May 5, 2009
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    English/Journalism teacher and writer
    NJ
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    It does rock - Ryephile - while the above information is totally above my head, I actually think I understand it. Thanks a lot for taking the time to write it all out.

    I called Jeff and shot Dr.Mike another PM to get some more insight. I decided on 7kg (around 400) front and 8kg (around 450) rear. I think this will give me the feel I like (some rotation) and not be surprisingly tail-happy with the 19mm on soft to start. But I guess it's all theory until I get it out there:cornut:.

    - Marc
     
  6. Steve

    Steve Administrator
    Staff Member Articles Moderator

    Apr 23, 2009
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    Marc - the good news is you're already zeroing in on your target before even turning a wrench :Thumbsup: , plus if you find the result doesn't quite fit your driving style/preference replacing a single pair of springs is usually both relatively simple and relatively cheap. And the silver lining if you find you need to make a change is that as soon as you make one spring pair replacement you've started to build your collection of springs on the shelf you can use for swaps if you want to fit the setup to a particular track, etc.
     
  7. Mike

    Mike New Member

    May 4, 2009
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    Yep, it all makes sense, no arguments here. I would point out for other folks that the body roll is coupled to the spring rates as well as to the anti-roll bar rates for any street car, and the amount of coupling is of course dependent on the roll centers, the CG location, and the suspension geometry. So, stiffer springs also lead to less roll, all else being equal (though of course the anti-roll bar is a much bigger knob to turn).

    What we have here is a difference in preference. I prefer stiffer body springs, and you prefer stiffer anti-rollbars. Both approaches can be made to work fabulously. This type of thing is what makes car setup so fascinating to me!
     
  8. Bimmer Lite

    Bimmer Lite New Member

    May 5, 2009
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    Yep - good points. In 1-2 years I'll be tracking the car much more regularly, and I'll have a much better handle on what will work best for my driving style. Thanks to the people on here for explaining the repercussions of particular setups.

    - Marc


     

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