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S13 rear suspension geometry

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  • S13 rear suspension geometry

    I've read numerous vague mumblings about how to set the rear adjustable arms. So, I started playing with lengths yesterday on my S13. I did find some possible solutions, but more questions are arising. The goal is to get the most forward grip I can within reason.

    There are 2 debates here: traction arm usage and anti-squat.

    Traction Arm:
    I've got the 3 common rear adjustable arms, with the traction arm staying the stock length (ruca slightly longer, toe slightly shorter). Now I'm looking at the angle of the mounting tabs and spindle mounts and they don't point the same direction as the arm. With the arm lengths like this the spindle is rotated forward. This is probably causing a ton of bind on the non-spherical end, which is one reason I think I can't put power down. So I extended the traction rod 1/4" and shorten the ruca 1/8" to keep the same camber. Now the arms are at a better angle with respect to their mounts and there's less bind.

    So the question is, did rotating the spindle back do anything geometry-wise (besides change the angle of the toe arm, which I don't care about)? This lessens bind, but why do I hear all the drifters say that shortening the traction arm gets you better exit grip/speed? Are there anti-squat considerations at the spindle that are unrelated to the pickup points on the subframe? If I shortened that arm, that would increase the bind, but a set of sphericals on the spindle side would fix that. What is the correct line of thought for the most traction?

    Leading into anti-squat, S13's have bucket fulls. I've got a few pics of s13 and S14 subframes and the difference in anti-squat based on their pickup points is huge. So what can I do to reduce it? From what I can tell, most of the difference is in the location of the front lower a-arm mount. But you can't just move that mount because the rear-most mount will have to rotate to keep the axis of rotation in line. Major PITA. Or, I heard of tilting the subframe to rotate the instant center down, which sounds like it would work. Thoughts? I'm initially considering spacing the front subframe mounts down 1/2". Does anyone have any rough idea or calcs/sims of the S13 anti-squat percentage and/or IC location?


    This is basically the only place that could discuss this subject. I feel like if I post this question/topic anywhere else the responses will be filled with "sick drift", "WAT", or both. Please chime in!
    She's built like a Steakhouse, but she handles like a Bistro.

  • #2
    I would think something like on FC rx7 rear subframes with droplinks to lower the front arm.

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    • #3
      I've been thinking much about this lately. S15 subframes seem impossible to find, and the changes from the S14>S15 subframes make me think that maybe they made a few errors on the S14.

      I really wish someone had a bumpsteer gauge and would plot out a few curves with varying arm lengths.
      '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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      • #4
        I roughed out a bumpsteer curve a long time ago by clamping a pencil to my rotor and setting some cardboard up against it while jacking the wheel up. Nothing scientific, but it gave you a line to look at. Finding: S13 has a ton of toe in under compression. Visible amounts just by watching the assembly move. That was with a stock length traction rod and a ride height set for 1 finger gap.

        I thought that S14 and S15 subframes were the same...?
        She's built like a Steakhouse, but she handles like a Bistro.

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        • #5
          Hmmm... I can't find any definitive info on it, but I thought there were quite a few changes from S14 to S15 rear subframes. That's why the holy grail for "grip driving" is the S15 subframe... or at least that's what I hear from the kids these days.
          '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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          • #6
            I'll post some pics of S13 vs S14 subframe geometry later on.

            By the way, I tried shortening the traction arm like the drifters say is best. Welcome to bind city. It was binding so hard I could barely turn the adjuster with a wrench. Yeah, I'm pretty sure that's not how I want the rear suspension to work.
            She's built like a Steakhouse, but she handles like a Bistro.

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            • #7
              Originally posted by Epstein View Post
              I'll post some pics of S13 vs S14 subframe geometry later on.

              By the way, I tried shortening the traction arm like the drifters say is best. Welcome to bind city. It was binding so hard I could barely turn the adjuster with a wrench. Yeah, I'm pretty sure that's not how I want the rear suspension to work.
              Binding is the only way to fly
              Between rides...unless you count a WD21...

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              • #8
                Hardly scientific, but you can see how steeply the lower a-arm on the S13 points upwards. The S14 pic below looks to have the a-arm nearly level with the ground (or put the IC somewhere in the floorpan)




                edit: thanks random S13 for the pic. Thanks GSRacer for the S14 pic.
                She's built like a Steakhouse, but she handles like a Bistro.

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                • #9
                  I was planning on re-locating the LCA mounting points on my spare subframe to eliminate anti-squat, but after really thinking about it I decided not to. The thing is if you change the path that the LCA moves in then you change every single concievable arc or path of movement in the rest of the rear suspension. That is just too complicated for me to tackle. To really do it right you need a computer model of the suspension to get an accurate idea of what all is going to change and whether or not those changes would be beneficial. I have concluded that the best reasonable solution is to get the SPL solid bushings with the tilt spacers and tilt the subframe to eliminate as much anti-squat as possible. I have the exact same questions on trac arm settings as you do. I shortened my trac arms 1/4" because aceinhole reccomended it, but he doesn't seem to fully understand exactly what it does either. It seems to increase camber gain and the rear of the car honestly seems to bite better off the corners, but without a real data acquisition system that could likely just be in my head.
                  function > form
                  1990 240sx fastback: IN PROGRESS

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                  • #10
                    the tilt spacers you are talking about are? The little pineapple things? You would then install them in the front to make the line more flat I'm assuming? Just curious. Cause when I first rebuilt the car I had whiteline pineapples in the rear (bushings shot) but they snapped out somehow... literally snapped out...
                    "hexa-dodecahedron-triple-threaded-super-eleventy-way-adjustment-spec" dampers. -Def

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                    • #11
                      Increasing the camber gain curve makes a little sense, and is the first explanation that I've heard about shortening the traction arms. I just don't see how you can set the geometry so far away from stock and then move it through it's arc with 4 different length arms pulling on the spindle at different rates. With spherical bearings, there isn't even any give like a rubber bushing.

                      Does anyone have any clue how big the SPL shims are for tilting the subframe? I absolutely hate pineapples, but I'll probably try to tilt mine with them. I'd just like to get an idea of the % change.
                      She's built like a Steakhouse, but she handles like a Bistro.

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                      • #12
                        Originally posted by Epstein View Post
                        Increasing the camber gain curve makes a little sense,
                        Assuming nothing else changes, how does that improve the handling?

                        I'm curious, because I'm getting ready to focus on improving the rear suspension. However my focus may be slightly different then others. I'm happy with how the rear of the car handles in a turn (excluding lifting the inside tire) but I am not happy with how poorly it applies power to the ground.

                        So far, it's stock, but my current plans are the following.

                        - Put the stock swaybar back on, with fresh bushings/endlinks
                        - install the 300z rear uprights
                        - S15 HLSD
                        - Get some sort of adjustable RUCA to remove some camber.

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                        • #13
                          s15 subframe porn:

                          "hexa-dodecahedron-triple-threaded-super-eleventy-way-adjustment-spec" dampers. -Def

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                          • #14
                            Is that yours?? A side view pic and an overhead pic would be great!

                            Nice window in that block.
                            She's built like a Steakhouse, but she handles like a Bistro.

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                            • #15
                              I wish that was mine...

                              The guy selling it was asking 500$... + shipping probably 100ish... I spent 200$ locally on a non hicas s13 mint subframe already... if i knew before id spend that lil extra on this one.. still lots of damn money for piece of metal... but yeah...

                              I'm just not sure how i could mount the dif on this, as its like the s14 subframe with the bushings... im assuming i wouldnt be able to use my s13 pumpkin? And the offset bushings cost 200$ themselves as well.. its a pita to make work...

                              I already spent 200$ non hicas subframe and 120$ spl solid subframe bushings... would have to repurchase subframe 600ish, touge factory offset bushings 200ish, and something for differential as well...$$$ and assuming these come with LCAs then i just need all other arms (but i already got that sorted out, spl is sending me their arms, they are compatible s13/s14/s15 universally now anyways)

                              No idea where this guy is anymore though... he probably sold it by now to some lucky bastard.
                              "hexa-dodecahedron-triple-threaded-super-eleventy-way-adjustment-spec" dampers. -Def

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