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  • Front anti-dive

    What's everybody's thoughts on changing the front anti-dive of our suspension? I've seen it described on how to calculate it is to assume the lower links are like the lower arm of a SLA setup, and the upper strut mounting point you assume to have another point exactly 90 degrees perpendicular to the top mount.

    Looking at this pic:




    You'd get an upper arm that's pointed down towards the CG of the car (i.e. pointed up going forward), and pretty much a flat lower arm. I'm not sure the amount of anti-squat, but I'm thinking with our stiff springs we run to control the struts, we could reduce the anti-dive some by raising the tension rod rod end (lotta rods there).

    Thoughts? I generally think the braking performance is good on my car for a front heavy car, but it is skittery over bumps in a braking zone, which would be a symptom of maybe a little too much anti-dive for the suspension stiffness.
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  • #2
    It seems to me like raising the tension control rods up would cause screwy shifts in caster as the suspension travels. Depending on how exactly it's raised, suspension binding may negate the positive effects of changing the anti-dive. I would think switching to a SLA suspension would be better than relocating a tension rod performance wise, but far more expensive and labor intensive.

    With some data acquisition devices it shouldn't be too hard to see if it works. Were you thinking about relocating the front or rear of the tension control rod? Testing may be the easiest way to see if it will work.

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    • #3
      My suspension is all rod ends up front, no binding. I don't think the caster changes much with travel given how long the links are, but there would be an effect there.
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      • #4
        http://www.e30m3project.com/e30m3per...l_center_comp/

        Pictures of how the M3 E30 race cars has zeroed antidive due to a raised controllarm. Note that BMW's has it's "tension-rod" pointing backwards.

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        • #5
          I'm not sure so sure the E30 has no anti-dive in its front suspension. From looking at it kinematically, the top of the strut forms an upper arm 90 degrees from its relationship from the lower balljoint and upper strut bearing. On pretty much all strut cars I've seen that have positive caster, this gives you a virtual upper arm that slopes downward towards the rear of the car.

          Unless I'm completely missing something, but it makes sense kinematically when you draw out the points of rotation and consider the upper strut mount to be aft of the balljoint. Any force in line with the balljoint will create an upward moment - i.e. anti-dive.

          It's more likely the E30 M3 factory car had an adjustable rear mount to tweak the amount of anti-dive, but not run zero.


          Anybody coming up with any illustrations of anti-dive on a strut car? Everything I can find is about a SLA a-arm setup.
          '18 Chevrolet Volt - Electric fun hatch for DD duty!


          DefSport Koni Sleeve and Spring Perch Buy!!!
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          • #6
            I'll take some shots from my kinematics book in a bit for you. The 90* relation is to the strut axis, not the steering axis though. I think on the side view of our cars, they're pretty much in line though. Front view however...

            My vehicle dynamics book does mention that if the anti-feature is over 100%, then the control arms are taking the load transfer, where as if it's under 100%, the spring will begin to take more load (at 0% anti, the spring has all the load transfer). At first I was thinking you could try to reduce spring rate up front if it's too skittery, but after reading that, it would have been a wrong assumption on my part. I would figure out how much less anti-dive you want so you can back engineer the lower a-arm position to match the ideal SVIC.

            EDIT:





            The formula listed in my book states Anti-dive % is = (% front braking bias)*tan(phi)*(CG length/CG height)
            Last edited by Hijacker; 09-12-2013, 09:58 AM.

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            • #7
              Yea, I need to find some time to break into my Milliken and Milliken book to get a better handle on it. And are you sure the instant center lines extend from the strut axis itself and not the steering axis (which is really what I was referring to). How would the instant center lines even know how the strut is oriented? Versus the steering axis being the imaginary line between your upper strut bearing and lower balljoint, which is what I see the instant center line coming off 90 degrees to.

              Take for instance if you spaced the strut 1" forward on the spindle connection with a funky bracket, besides messing up the motion ratio, you'd still have the same steering axis, but according to that diagram you'd drastically change the anti-dive of the front suspension. Maybe it is that way, but I'm struggling to come up with an explanation for it in my memory of my dynamics class. The IC lines always seemed to extend out relative to the imaginary lines of motion between points of rotation (the steering axis in this case).
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              • #8
                I'm no mechanical engineer, but the strut axis makes sense. if you angle the strut in relation to the steering axis it puts less force though the spring and more side force through the strut contributing to anti-dive...
                Last edited by eage8; 09-12-2013, 11:11 AM.
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                • #9
                  What about computer modeling this? Is there any software that would allow this? I think that the way that it was done on the BMW is risky, if you've moved the pickup points and it doesn't work right you're almost stuck with it.

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                  • #10
                    Originally posted by Def View Post
                    Yea, I need to find some time to break into my Milliken and Milliken book to get a better handle on it. And are you sure the instant center lines extend from the strut axis itself and not the steering axis (which is really what I was referring to). How would the instant center lines even know how the strut is oriented? Versus the steering axis being the imaginary line between your upper strut bearing and lower balljoint, which is what I see the instant center line coming off 90 degrees to.

                    Take for instance if you spaced the strut 1" forward on the spindle connection with a funky bracket, besides messing up the motion ratio, you'd still have the same steering axis, but according to that diagram you'd drastically change the anti-dive of the front suspension. Maybe it is that way, but I'm struggling to come up with an explanation for it in my memory of my dynamics class. The IC lines always seemed to extend out relative to the imaginary lines of motion between points of rotation (the steering axis in this case).
                    I agree as I've always modeled the IC off the steering axis, but those images are from the Milliken & Milliken book. They say to take the IC off the strut axis itself, which makes some sense as the force line for the strut would be along its axis making the IC an orthogonal vector to the axis. It would make sense somce it would prevent the upper intersect line of the IC from moving since the upper mount doesn't move. If you base it off the steering axis, the upper line can move during travel.

                    That's the one problem with the M&M book. They give so much information you end up taking at face value. But it's still the most thorough treatment for the vast majority of suspension setups out there.

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                    • #11
                      Could you get the same effect by moving the strut spindle pick up point for or aft, to change the perpendicular angle on the strut tower?

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                      • #12
                        It'd make almost no difference due to how long that link is. Plus I need caster, so if anything it'll go back some, which will reduce anti-dive just a hair. But it's position is going to be more dictated by caster requirements than anything.
                        '18 Chevrolet Volt - Electric fun hatch for DD duty!


                        DefSport Koni Sleeve and Spring Perch Buy!!!
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