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Front suspension geometry upgrade

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  • Front suspension geometry upgrade

    I've been thinking about upgrades to the front suspension for an SM or Prepared car I'll never build. I've thought the R14 build from a while back was cool, but that would put the car in Prepared with all the other non-streetable mods.

    To stay in SM you have to keep the same mounting points, but free outside of that. There have been some improvements in front strut design, most notably, the new s550 mustang. It now uses a dual ball joint that pushes the lower steering axis pivot point out to an imaginary point inside the brake rotor. Good write up here.
    Swapping a Mustang knuckle could be done with some work except the mustang has a front steering rack. Any ideas of other cars to borrow from? It is probably more feasible to get the GK tech front knuckles and add a second ball joint mount and connect a customized tension rod to that point.
    The main concern is if the two lower, inner points are closer together than on the mustang, then it wouldn't have much resistance to longitudinal force (braking). As they approach parallel, the stiffness gets lower and lower. I have no idea how close this would be to being problematic.
    I would imagine GK Tech has the suspension modeling and FEA capabilities to design something and produce it. Here's their s-chassis grip knuckle.

  • #2
    Take a look at the Ford Focus Revoknuckle or the GM Hiperstrut. They separate the strut angle from the steering angle to get it from typical strut territory of 15-18 degrees, to something more like a SLA setup, with 7-10 degrees of SAI.
    Don Johnson (really!)
    Just so you know.


    • #3
      I've seen that, I was wondering about it but didn't dig into it. I have only seen it on FWD cars, is there a reason for that or just coincidence, or is it used on RWD somewhere? Would that work for RWD just fine? I would imagine there are som accommodations for FWD that could be dropped.


      • #4
        The stock front lower balljoint in an S-chassis sees all longitudinal braking force not supported by the strut, so why would you be worried about the loading into two balljoints that are effectively sharing the load?

        IMO, I'd just mod a GKTech knuckle or cut up a stock one if you want to do something crazy like this. They're steel, so you can easily weld on them, and the steering and braking attachments being in the "right spot" gets you most the way there. You just need to fab a new tension rod connection and the tension rod of course. It's going to be pretty curved to miss the front tire, so it won't be a super easy fab job there, but totally doable IMO.

        That said, how much benefit do you really feel there is by tweaking the instant center of the outer lower pivot on a strut suspension? You will get a bit more camber gain in compression I think, but you you'd have to really model it with the stock pivots to make sure it'd behave like you think it will.

        Sounds like a lot of effort before doing extensive suspension modelling to make sure you'll get what you think you'll get.
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        • #5
          I'm wondering how much benefit there would be. I'm not sure myself and was hoping that be part of the discussion but didn't explicitly state that. I guess I should have, sorry. I don't see it being huge, but I do remember some discussion on the new Mustang that with the IRS upgrade, they realized the front sucked and upgraded it a notable amount so it would keep up. Seems like it could be worthwhile and not terribly difficult, but as I stated up front, I'm not building an SM car so this will probably never happen either way.

          Modeling the system is certainly necessary, and that's a capability I don't have. I've seen a few online modeling systems but only with traditional MacStrut setups. I'd guess the free systems aren't capable of this and you'd need something more powerful/expensive.

          One thing I realized, if you push the pivot point out without moving the outer ball joint a similar amount, you will increase the ackerman of the system as well. Moving the tie rod end out then seems necessary. GK tech has a Drift knuckle which has that so it may be a better starting point.

          As for the loading issue, it isn't the ball joints I'm worried about. Just conceptually, as you move the inner pivot points closer together, the tension and compression in the arms for a given longitudinal load gets higher and higher. At a certain point, the arms are parallel and have zero resistance to longitudinal load and would just fold over into a parallelogram (obviously not a realistic issue, just for illustrative purposes). Before you hit that point, even a minor longitudinal load would require a massive tensile force in the tension rods and compression in the lower control arm. It shouldn't be an issues for the arms, but the structure around the pivot points may be overly stressed. I don't know where we're at on that scale, but just eyeballing some of the other systems, it looks like the pivot point spread on the 240 is smaller than the Mustang and others.

          Maybe I'll just feed the idea to GK Tech and let them investigate. I would imagine they have a front suspension model already built that they can tweak easily enough.


          • #6
            Just thought this would be more interesting with pictures.


            • #7
              A similar "virtual pivot point" system is used on the rear upper arms on the S chassis. The traction rod and the camber arm act like an A arm with a pivot point located in the brake disc. Which makes for a strangely reversed KPI on the back.


              • #8
                I have a friend that built a Factory Five Cobra Replica with parts from an SN95 Mustang. The FFC used an SLA front setup, but retained the funky Mustang SAI of 17-18 degrees. We didn’t observe any bizarre issues with it, but when he installed a kit that corrected the SAI to 7-8 degrees to match his caster, front grip at turn-in and in cornering was noticeably improved. I believe FFC eventually made the same change on the kit from the factory.

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                Don Johnson (really!)
                Just so you know.