205 suspension basedata
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  1. #1
    Tadpole
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    Apr 2004
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    Wiseman's Ferry, NSW
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    Icon5 205 suspension basedata

    Wotcha,

    I've a partially modified '89 205 (S2 1.9 gti) that has a front/rear suspension balance problem from hell. The back hasn't been touched (apart from shocks) but the front has an increased spring rate and roll bar (19mm). I'd like to restore the original weight transfer behaviours but don't know the original numbers for roll + spring stiffness.

    I'm wondering if anyone out there knows what the rates/diameters are for a stock S2 205gti suspension (can't find this anywhere on the net - apart from the torsion bar - thanks PeterT). I can then make some reasoned guesses about how stiff to make the rear...then restore balance to the force.

    The stock torsion bar is 19mm. A bit on floppy side.

    Any takers the front bar?
    A spring rate on the front would be hoopy.
    How about the rear sway bar?

    I've been hitting the books on suspension mods and will post a summary of calculations on the options available once I have the basedata.

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    Cheers

    Tony

  2. #2
    Tadpole
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    Icon3 ...is the correct answer?

    Hi again - don't you hate it when this happens...

    Just found 205 central again - it quotes a few numbers for the 1.6 and rallye which might be the ones I'm after:

    19 mm torsion bar
    17mm front sway bar
    19mm rear sway bar

    Now for the springs...


    T
    --

    "Aaaaarrgghhhhhhhh..."

    205 GTi 1.9

  3. #3
    1000+ Posts PeterT's Avatar
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    Quote Originally Posted by Aarrghh
    Hi again - don't you hate it when this happens...

    Just found 205 central again - it quotes a few numbers for the 1.6 and rallye which might be the ones I'm after:

    19 mm torsion bar
    17mm front sway bar
    19mm rear sway bar

    Now for the springs...


    T
    wire diam. 12.80mm
    5.5 turns
    OD 158mm

    about 200psi from memory

    '92 205 Mi16
    '90 Mi16x4

  4. #4
    Fellow Frogger! winnie's Avatar
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    Default

    Quote Originally Posted by PeterT
    wire diam. 12.80mm
    5.5 turns
    OD 158mm

    about 200psi from memory
    p/i
    If you want to restore it to original, could you not just fit original components?

  5. #5
    1000+ Posts PeterT's Avatar
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    Default

    Quote Originally Posted by winnie
    p/i
    If you want to restore it to original, could you not just fit original components?
    That is the std. Gti spring. Do you want some?

    '92 205 Mi16
    '90 Mi16x4

  6. #6
    Tadpole
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    Wiseman's Ferry, NSW
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    Icon1 Excellent

    Thanks again PeterT - you're a font of knowledge

    So - in theory I can get the spring rate from the following calculation (converting to inches as I go)...

    coil diameter = overall diameter - wire diameter
    = 158 - 12.80 = 145.2mm
    Active coils = free coils + 1/2 = 5.5 - 2 + 1/2 = 4
    Assume G is 12,000,000 (Hmmmm)

    K = ( (12.80/25.4)^4 * 12,000,000 ) / (8 * 4 * (145.2/25.4)^3)
    = 129.5 lbs per inch

    Hmmm - does this sound right?

    Doesn't work too well with 200


    I've had another thought - can find any numbers on weight distribution f/r...

    any ideas?


    As to fitting the originals back in - the front is very very firm and pleasing so I'm going to uprate the back to get close to a match. I'll probably have to bugger about with roll stiffness at the front after changing the rear but I'm hoping to eliminate the displeasing pitching problem by getting the torsion bars to match the existing front springs. The rear seems to be the most irritating and expensive to change so in theory that should have been done first - but we work with what we've got, hey?


    Cheers
    T
    --

    "Aaaaarrgghhhhhhhh..."

    205 GTi 1.9

  7. #7
    Tadpole
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    Icon3 Some results

    Hi

    At the risk of going blind I've been reading books again

    I've worked out the change in natural frequency and period for the differing size torsion bars. Given a constant weight and mechanical advantage (i.e. geometry) for the rear suspension you can get the new frequencies via the following equation:

    new freq = sq. root( (old freq)^2 * (new diameter)^4 / (old diam)^4 )

    period = 1 / freq

    So for bars from 20mm up to 24mm the percentage changes (from stock 19mm) are:

    20mm: freq = 111%, period = 90%
    21mm: freq = 122%, period = 82%
    22mm: freq = 134%, period = 75%
    23mm: freq = 147%, period = 68%
    24mm: freq = 160%, period = 63%

    Finding out the original frequency for the rear (+ front) requires calculating the sprung and unsprung weight for each wheel and making some direct measurements of suspension travel under known weight increases. Since I haven't got a scale this will have to wait a bit.

    For anyone wondering why the **** I'm doing this - the balance of the frequency of the front suspension to the freq. of the rear suspension controls whether the car pitches (fore/aft rocking) while travelling over bumps. The idea is to set the front suspension to have a slightly lower frequency than the back so that when the rear wheels travel over the bump both suspensions come back down onto the road at the same time. The factor is the time the bump takes to travel the wheelbase - which means you can only tune the suspension for one speed. At the moment my car pitches all the time (unless I control it with the throttle) because the front frequency is modified and much higher than the back. I'm going to measure the front frequency then pick a new bar for the rear to match the frequencies again.

    Cheers

    T
    --

    "Aaaaarrgghhhhhhhh..."

    205 GTi 1.9

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