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Thread: roll centre, roll axis, bump steer, practical tuning

  1. #1
    Aerial Superpony Domestic Engineer SeptemberSquall's Avatar
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    Default roll centre, roll axis, bump steer, practical tuning

    hello

    i'm too daft to work this out on my own, have been reading phun and smith and need some practical clarification

    leaf spring rear live axle
    macpherson strut front end

    using a leaf spring rear axle i am understanding that roll centre is given by the point at which a vertical plane intersecting the axle centreline in turn intersects a line intersecting the centre of the front and rear leaf shackles, correct?

    so as the car is lowered by whatever means (blocks, reset springs, whatever) the roll centre height change will be equal to the change in ride height, as both leaf spring shackle points which detirmine the roll centre are brought closer to the ground by an identical amount.

    at the front, lowering the car will cause the axis of the lower control arms to change, as the inner (fulcrum?) (maybe not the right term) changes position in relation to the outer side ball joint. this will push the roll centre down substantially.

    by adjusting the angle of the lower control arm the roll centre can be adjusted, this can be achieved by sticking blocks in between the steering arm and the strut tube

    when resetting the roll centre, do you aim to restore the factory roll axis inclination? OR do you instead try to set the roll axis inclination to something specific? what are the effects and advantages of moving it one way or the other, either closer to or further away from the horizontal/parallel to ground? i think lowering the roll centre on one end relative to the other (increasing roll axis inclination towards that end) will decrease some component of weight transfer at that end and promote grip? at the expense of grip at the other end? or am i wrong entirely?


    then bump steer

    in principle, after setting the lower control arms to deliver the intended roll centre characteristics, is my assumption that steering and lower control arms close to parallel will generally deliver the smallest amount of bump steer? becuase they are differing lengths naturally they will toe on bump travel. it seems as though whether the steering arm pivot points are above or below the lower control arm (relative to ground) will affect whether the wheel toes in or out for any given bump (droop) travel?

    so in principle, after acheiving the ride height we desire, is the process to reset the roll axis inclination to intended spec through the use of appropriately sized roll centre adjusters then change the position of the steering arms to achieve a parallelogram with the lower control arms to minimise bump steer characteristics?
    Last edited by SeptemberSquall; 28-07-2008 at 12:22 AM.
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  2. #2
    Aerial Superpony Domestic Engineer SeptemberSquall's Avatar
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    Default Re: roll centre, roll axis, bump steer, practical tuning

    on another note, the front roll centre for a macpherson strut suspension has been explained to me to be detirmined like so:



    so what happens when the lower arm angle changes so the plane at 90deg perpendicular to the strut axis intersecting the strut top to chassis mount never intersects with the plane along its axis? like so? how is instant centre detirmined? ie. in the below sketch x<y

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  3. #3
    Junior Member Backyard Mechanic
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    Default Re: roll centre, roll axis, bump steer, practical tuning

    Do you have access to a suspension program? I'd like to have a play with one simply because so many characteristics are affected by one component alteration.

  4. #4
    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: roll centre, roll axis, bump steer, practical tuning

    the roll centre is the intersection of the planes of the LCA's, but the instant centre is infinite...
    gotta remember that when you roll, the LCA's on either side do different things..

    perhaps also important to think about is the distance between the centre of mass, and the roll centre, as that moment arm will affect how much the car actually rolls..

    ie, lowering a car can induce more body roll (which is why RCA's are so effective)
    another reason RCA's help is that they gte back closer to stock setup, where you have camber gain during bump...

    at the rear, the addition of panhard rod or watts link for lateral location will allow you to specify the height of the rear roll centre.

    how different are the lengths of the LCA and tie rods? toe in during compression is more stable than toeout.

    what do you mean roll axis inclination? height?
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  5. #5
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    Default Re: roll centre, roll axis, bump steer, practical tuning

    Quote Originally Posted by oldcorollas
    what do you mean roll axis inclination? height?
    Or do you mean the angle from the front roll centre to the rear roll centre?
    Quote Originally Posted by oldcorollas
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  6. #6
    Junior Member Backyard Mechanic
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    Default Re: roll centre, roll axis, bump steer, practical tuning

    I believe that not only should the roll centre and COG be closer rather than farther, but also that the relationship between the two during suspension movement should be as close to linear as possible, so that the car may remain predictable when entering/exiting corners etc.
    Naturally as OC mentioned, minimising bump steer (changes in toe) is also important for predictability.
    As opposed to the front, consensus is that the rear should default understeer under cornering forces.

  7. #7
    Aerial Superpony Domestic Engineer SeptemberSquall's Avatar
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    Default Re: roll centre, roll axis, bump steer, practical tuning

    Quote Originally Posted by Plonka
    Or do you mean the angle from the front roll centre to the rear roll centre?
    this is what i mean
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    the oceans will rise
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  8. #8
    Junior Member Automotive Encyclopaedia
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    Default Re: roll centre, roll axis, bump steer, practical tuning

    roll center at the back is where your springs join you rear end and as oc said watts link or panhard will be the new roll center '
    if you put in a adjustable pan hard that can move up or down at both ends this will be come the new roll center like i showed you on my race car
    ps moving roll centers up and down at the back .down softens rear springs up hardens spring rates and you should always be higher than the front roll center for car to handle

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