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Thread: Suspension 'Bounce'

  1. #1
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    Suspension 'Bounce'

    I could do with some advice on how best to dial out the 'bounce' when accelerating out of corners.
    It is evident on all but the fastest corners, such as Paddock Hill.
    If you try and push through it develops into understeer.

    I'm guessing it's a damping issue manifesting itself as the front lifts on acceleration, the rebound damping pushes against this, but maybe not quickly enough so the front rises, gets pushed down into compression by the damper, which in turn converts to rebound, so we get an oscillation until the steering is straight.

    If so is the best solution to increase the rebound damping on the front coilovers?
    If not please advise!

    This video illustrates what I'm talking about, first at normal pace, then with the corners in slo-mo

    https://youtu.be/uWC1JDslZHI?t=63
    Last edited by banhama; 23rd August 2017 at 18:41.

  2. #2
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    Looks like yours tyres are bouncing. Tyres are basically undamped springs, if your spring rates are too high or your dampers are too stiff or your tyre pressures are wrong you can get that.

    So what tyres, what pressures, what spring rates, what dampers, what arbs, what damper settings do you have?

  3. #3
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    Hi John,

    The arbs are 22mm adjustable. Came down from 27mm as they overwhelmed the springs and gave no feel of weight transfer. It is quite possible that 24mm (the size I haven't got) would be ideal.

    Tyres in this instance were ex WTCC Yokohama slicks 235 620 17.

    I think the rest of the information is here below, I would really appreciate your thoughts

    The tyres are slicks, and we've experimented a fair bit a Brands. Due to the clockwise track the pressures start cold at NSF 22, OSF 23, NSR 23, OSR 25. When the car comes in the the pressures are 30-31 psi at all four corners. It takes 2 or 3 laps to get up to full pressure, but the video is a lot later than that, so I would assume 30 psi all round.

    The coilovers are 3-way adjustable: Fast Compression (Kerbs), Slow Compression, and Rebound
    Springs are 11kg/mm front and 7kg/mm rear
    None of these settings are full hard, that tends to make the car horrendous under braking and launch over kerbs.
    The Nitrons 'measure' the settings by the number of 'clicks off full hard'.
    Just to confuse things each of the above have a different number of clicks in the range!

    Currently we have:

    REBOUND: Front -12, Rear -14, range 22
    SLOW COMPRESSION: Front -16, rear -16, range 26
    FAST COMPRESSION: Front -12, rear -12, range 16

    Just this discussion has made me look up the factory settings, which were developed by RCM on a spec-c test car:
    Rebound -10 all round, Slow Comp -8 all round, Fast Comp-8 all round
    It looks like I have gone a little soft!
    Perhaps I should begin again at factory settings for the next outing?

  4. #4
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    That's not a tyre size I recognise as a WTCC yokohama, you sure about that? What wheel size are you using?

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    Sorry John I'm mixing up my Dokos and my Yunlops! Should be 240 610 R17
    Wheels are 17" x 9"

    You will know that 17" ex BTCC and WTCC will be at least 2 years old by now, but the ones that have been stored correctly are still decent once they've been woken up.

    I'm hoping that damper setting adjustment may decrease the bounce, and look forward to your thoughts on that.

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    I would do the dampers first, then try higher pressures, try them at 35 and then 40 hot as an experiment. make sure you are running enough camber so the tyre sidewall isn't tucking under.

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    Many thanks, I will revert to the RCM damper settings for the next race and see what happens.

    Interesting about the tyre 'tucking under' I hadn't thought of that.
    Front camber should be -2.8 and rear -2.0 but haven't checked for a few races.
    I have a photo of the car at Druids that might indicate this happening, I've just got to figure out how to upload it without photobucket.

    Would higher tyre pressures also help prevent 'tucking under'?

  8. #8
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    if the sidewall isn't stiff enough for the load it would help

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    Have you seen/heard Dales bounce out of clearways?

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    I've had a look at the video of the 48.9 second lap, and there is definitely a similar thing at Clearways, and maybe Surtees, although not quite as pronounced.

    https://youtu.be/F8iUnH5hFqo?t=176


    Either he has a similar issue or maybe at Clearways it's enhanced by the fact that the track drops away as you accelerate. Although if that was the cause you'd expect it to be worse at Paddock Hill.

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    Problem not solved, but lucked into a better damper set-up in the process!

    I re-set the dampers to RCM/Nitrons default settings for qualifying, but this induced understeer when powering out, most pronounced at Graham Hill and Clearways.

    Decided to set the dampers somewhere between the default settings and my previous ones for race 1.

    REBOUND: Front -10 Rear -11
    SLOW COMPRESSION: Front -10 Rear -10
    FAST COMPRESSION: Front -8 Rear -8

    The car handled really well, producing my best lap time to date, 52.357, and more importantly a string of consistent 52's as I managed to catch the lead group having fallen back at the start.
    You could almost throw the car into paddock hill bend and it would give you a warning but it would still stick rather than throw you straight off. Very confidence inspiring as I could push harder with a good chance of still driving the car home!

    As Race 1 went so well I kept things the same for Race 2.
    The 'bounce' on acceleration is still there but less pronounced. Next time I will experiment with tyre pressures.

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