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School Me On LCAs

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  • School Me On LCAs

    Some friends and I are planning on making our own LCAs, but we've run into some issues with our designs and I want to run them by you guys. We are trying to use a stock ball joint at the end with sphericals for the tension rod (both ends) and subframe connection, basically mimic the PBM LCAs but with easily replaceable rod ends and an oem ball joint. We got into a discussion about rod ends in bending and how to design an LCA with no REIB, but even after looking at PBM's design we don't know how to design something that doesn't involve REIB. Here's a very rough free body diagram of the first issue we came across:



    Say you brake really hard. The red arrow is the braking force. Since the tension rod is pinned (green), the LCA will rotate around that point. Since the pivot point isn't in the center the forces won't be equal, but there will still be an opposite force on the spherical (yellow). Wouldn't this force put the rod end in bending?

    Exhibit 2:



    Now you're going over a bump, cornering hard, etc. The red arrow shows this force. The gray arrow is the sway bar which is pushing down on the LCA. The moment arm isn't very big, but since it's not directly in the center of the LCA it's going to put a rotational force on the LCA (green), and the spherical (also green) will allow it to spin. Now since the LCA is rotating, the tension rod spherical is going to bind instantly. It also has force pointing up since the suspension is compressing...isn't this rod end going to be in bending?

    Are we thinking everything through way too hard? One of us is an engineer so he's flailing about yield strength and doing the math to make sure it's going to be fine, whereas the other two are designers and we don't really understand/care and would just rather build something and test it. Can anyone give us some insight as to if PBM has a poor design, or if what we're worried about doesn't really matter. Thanks!

  • #2
    That's likely a rod eye at the end of the tension rods. It still isn't a good design because it does put that rod eye in bending due to sway bar loads. It does also put the inner rod end in bending thanks to the pivot there vs. 2 bolts on the stock tension rod.


    There's a reason they used 2 bolts on the stock deign. Not a surprise PBM missed that basic fact.
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    • #3
      We came to the same conclusion about the stock 2 bolt tension rod. What if the sway bar mounts directly in the center of the LCA? Is there anything else that could introduce rotational force in the LCA? How much force does the sway bar actually put out?

      Our original tension rod design was swage rod with sphericals on both ends. A 2 bolt design would be more difficult to manufacture.

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      • #4
        It might be in bending, but if a 3/4" rod eye were used and sandwiched between tabs like PBM's, would the bending be enough to cause concern?

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        • #5
          Originally posted by Chris28 View Post
          We came to the same conclusion about the stock 2 bolt tension rod. What if the sway bar mounts directly in the center of the LCA? Is there anything else that could introduce rotational force in the LCA? How much force does the sway bar actually put out?

          Our original tension rod design was swage rod with sphericals on both ends. A 2 bolt design would be more difficult to manufacture.
          Figure the wheel rate of a sway bar and back calculate. From what I remember most of our front sway bars are in the 250-450 lbs/in range. Now that's for one end, so multiple that by two for an inch of travel on one side (both sides move).

          Then you can calculate the moment by knowing how far off center you're putting the sway bar loads.


          I ripped a 1/8" steel single shear sway bar mount off my LCA in short order, so I'd say the loading is not inconsequential. I'd also have to have a fatigue failure in the thread root of a LCA rod end. That would not be fun...

          Keep in mind the geometry of a thread makes it MUCH more prone to fatigue failure than a smooth geometry of the same dimensions, so it's not as easy as looking at the diameter and saying it's all good. Maybe it is... maybe it isn't. Maybe it will be for a long long time, maybe it'll be a cause for concern in a year.
          '18 Chevrolet Volt - Electric fun hatch for DD duty!


          DefSport Koni Sleeve and Spring Perch Buy!!!
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          • #6
            Thoughts on this design?

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            • #7
              Here are the LCA's Bill fabbed for the 240RS Maxi.





              NASA Time Trials TT3


              BTW I work for Garrett

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              • #8
                So it seems like if the sway bar mount is directly in the center of the LCA you can "pin" the tension rod with a spherical with no ill effects.

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                • #9
                  Originally posted by kewilso3 View Post
                  Thoughts on this design?

                  "I want that on my car" (but that is not the kind of thought you were after i guess )

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                  • #10
                    Figure the wheel rate of a sway bar and back calculate. From what I remember most of our front sway bars are in the 250-450 lbs/in range. Now that's for one end, so multiple that by two for an inch of travel on one side (both sides move).
                    Where did this info come from? I came up with a much lower approximation of swaybar wheel rates. The sway bar mount is so far inboard on the lca that the wheel has a lot of leverage on it. Due to the motion ratio, mechanical advantage of the wheel, the diameter of the bar would have to be quite high to acheive those wheel rates... The center torsion section of the oem style s13 sway bar is short in length, and there are a lot of bends that make the lever arm hard to calclulate the rate, so maybe I'm wrong.

                    My sway bar motion ratio is .45:1, & thats just just the ratio calculated from the ball joint to inner pickup vs the inner pickup to the sway bar mount. If you measure from the center point of the tire,(which imo, is a better way to measure motion ratio when trying to calculate wheel rates) or the hub face it's even greater.

                    The motion ratio has to be squared to account for the mechanical advantage. .45:1 squared comes out to .20:1. The diameter of a 37.5" length nascar bar to achieve a 200lb wheel rate, with the length of the sway bar arms that are standard on s13 chassis, would have to be atleast 1-3/8" or 35mm's.
                    http://www.schroedersteering.com/RateChtPDFs/78x43.pdf

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                    • #11
                      That's the rough range of actual sway bar rates. Has nothing to do with wheel rates at this point.
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                      DefSport Koni Sleeve and Spring Perch Buy!!!
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                      • #12


                        This is the Engineered to Slide "Drift Ute"

                        http://engineeredtoslide.com/category/ets-drift-ute/

                        The builder has self taught all his fab skills and while the workmanship is nice his designs are not always the best engineered solution.

                        Since it is a drift specific car the swaybar was spaced to gain maximum wheel clearance and consequently sacrificing swaybar motion ratio.

                        From what I can see as a internet keyboard observer his many of his designs are compromised by packaging ease and esthetic at the sacrifice of optimal mechanical function.

                        To be fair the 240rs Maxi and even OEM has a similar swaybar motion ratio but better alignment on the endlinks. Packaging is a bitch but championships are one on little details like this.
                        Last edited by Umai Kakudo; 01-09-2013, 01:39 PM.
                        The Nerd shall inherit the podium for knowledge is power.

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                        • #13
                          Ended up giving in a modifying my stock ones. Extended 1" and boxed them in, way easier and cheaper than making them from scratch haha.

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                          • #14
                            Get rid of that nasty stock swaybar bushing deal. Just use some rod ends with a double shear mount.
                            '18 Chevrolet Volt - Electric fun hatch for DD duty!


                            DefSport Koni Sleeve and Spring Perch Buy!!!
                            http://www.nissanroadracing.com/showthread.php?t=5902

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                            • #15
                              BTW - what'd you do for misalignment bushings on the rod end?
                              '18 Chevrolet Volt - Electric fun hatch for DD duty!


                              DefSport Koni Sleeve and Spring Perch Buy!!!
                              http://www.nissanroadracing.com/showthread.php?t=5902

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