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Thread: Anti-dive effect on weight transfer when braking

  1. #1

    Anti-dive effect on weight transfer when braking

    Hello ppl there. I'm Lee from South korea. I have a question about the effect of anti-dive on weight transfer.

    My suspension team will design anti-dive geometry to reduce the amount of dive.
    But, we didn't decide specific angle of wishbone in side view. Currently, we designed 30% anti-dive.
    My question is that does anti-dive geometry help to reduce weight transfer to the front when braking?

    I look forward to you guys' advice. Thank you.

  2. #2
    shawnBaek89
    No anti dive doesn't reduce the weight transfer to the front when braking.
    simply refer to the weight transfer equation:
    Load Transfer = W*(ax/g)*(h/l), so the weight transfer is function of wheel base, CG height,Braking Force.
    what anti dive does is changing the load going through the springs which will affect the car pitch.

    Regards
    Last edited by Ahmad Rezq; 10-29-2014 at 02:05 PM.

  3. #3
    "My question is that does anti-dive geometry help to reduce weight transfer to the front when braking?"

    Well…NO but maybe yes…... it depends....

    More anti whatever (squat, dive or lift) = more load into suspension elements ("geometric weight transfer") and less in spring and damper and ARB (= "elastic weigh transfer"). In steady state the weight transfer at the tire is the same.

    The problem is that there no such thing as steady state: maybe under lateral acceleration (skip pad) and that is only when you look at your lateral acceleration in your data with a big filter. But try to have steady state, constant longitudinal acceleration in braking or in acceleration...

    In transient the weight transfer will effectively be dependent of your "anti" numbers for many reasons starting with the fact that you will be changing your suspended mass inertia around the pitch center.
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  4. #4
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    In the interest of a balanced discussion, I'd put it out there that its not valid to calculate the pitch inertia about the (kinematic) pitch centre. Reason being simply that the chassis doesn't rotate about this point for 2 reasons:
    1. Like the roll centre, the pitch cente is found by applying 4 bar link theory to the chassis and hubs. This is not valid because the contact patches are not pinned to the ground but they move longitudinally
    2. Kinematic pitch and roll centres assume "symmetric" contact patch forces (same front/rear or left/right). So the fact that the contact patch forces in acceleration and braking are MASSIVELY different front to rear which means that the kinematically defined centres wont predict the correct location of pitch/roll. Given that the M.O.I. changes by the SQUARE of the distance from the CG, any errors you introduce here are going to have big consequences in the results of your calculations.

    The classic proof of this is the front axle of a RWD car. You can have any anti raise (and therefore pitch centre location) that you want on the front axle but since there is no traction force acting on that axle, you will not see any change in pitch response because there is never any jacking force on that axle under traction.

    The other proof if that you can setup a car with the pitch centre at infinity so theoretically it has infinte pitch inertia and won't pitch. But if you solve such a system either by hand or in a multi-body solver you will see that it will in fact pitch.

    In my opinion, a more correct way to look at the problem (in the time domain) is to use the tried and trusted newtons second law:
    1. Assume a longitudinal acceleration loadcase
    2. Find the contact patch and wheel centre forces from the brake/drive ratio
    3. Use the suspension geometry to find the jacking force of the front and rear axles
    4. Find the elastic vertical forces based on the current pitch angle
    5. Sum all of these forces (from 3. and 4.) about the CG (using the M.O.I. at the CG) to find the pitch and vertical response of the chassis mass.
    5a. If you want to get really detailed you can add the unsprung mass inertial reaction forces to the sprung mass but these are pretty small potatos in the overall "what makes the body pitch" problem.

    In my opinion, the only time it is valid to calculate angular accelerations at any point OTHER than the CG is when there is only ONE kinematically constrained rotational degree of freedom in your system. I'm not talking about suspensions here but any mechanism. A chassis of a vehicle has rotational and vertical degrees of freedom, and the topology of the suspension systems means that its not valid to forget the vertical d.o.f.

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