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240sx DARS (Diagonal A-arm rear suspension)

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  • 240sx DARS (Diagonal A-arm rear suspension)

    I picked up an old Nissan Product Bulletin recently called the 1989 Nissan S13 series Model Introduction. Most of it was a basic description of the then new model line, with stuff like how the A/C controls work and how the EFI system works. The only section that was unique is a discussion of the rear suspension. They used the term DARS (diagonal A arm rear suspension) and gave a good explanation of what it was designed to do and how it works. This should be required reading before modifying the rear suspension. The pdf was too big to attach directly so I'm trying a link to google drive for the first time...,

    https://docs.google.com/open?id=0B4a...G56NEdRSFlYbTA
    Don Johnson (really!)
    Just so you know.

  • #2
    Very good read. Definetly the most information compiled in one place that I've ever digested about multilink rear suspension kinematics.

    It's interesting to me how the uncoupled upper control arms create an axis that is tilted the opposite direction of a conventional double wishbone or mac strut suspension. This puts the "Pivot Point" (Think front scrub raduis pivot point) of the rear tire very far inboard creating a very large lever, or moment on the rear suspension during accelerating, & braking.

    On my particular setup, I have zero compliance of the lower control arm. (custom lca's with rod ends...) The moment from the upper link axis location/pivot raduis only takes all slop in the bearings and flex of the links/subframe, or compliance and causes the wheels to toe out during braking.

    The upper control arms being uncoupled also creates lots of spindle roll. Not sure if there is any advantage to this design with zero compliance bushings.

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    • #3
      I'll have to read it a few more times myself before I'll think I know what it's saying..., With any play at all in the suspension you'd expect to get toe-in during acceleration and toe-out during braking. The toe-in part tends to stabilize the rear under acceleration but the braking toe-out would contribute to oversteer when trail-braking. You're right that with spindle-roll, caster would increase under braking and decrease under acceleration but I'm not sure what the effect is. I think the toe-link will control some of that. The graph that they provided for showing camber change under bump/droop doesn't have any numbers but it looks just like some of the graphs that Def and others have posted when adjusting the torque arm on a lowered car.

      The explanation is good enough that you can understand why the compliance is there in each piece and what happens when you change or eliminate the compliance. I know the cars running in SCCA IT classes go all spherical but I think they're mainly trying to eliminate the dynamic loss of camber from sticky tires and rubber bushings.
      Don Johnson (really!)
      Just so you know.

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      • #4
        Braking causes toe-in predominantly. The bushing deflection is used to mostly give toe-in with any sort of load or compression travel. As in, they're trying to design in understeer, which is a primary goal of every stock suspension in the world.

        Get rid of that bushing deflection and these cars feel amazingly better.
        '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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        • #5
          I wish they went into why they gave an outsloping kingpin axis inclination. But I suspect it was their one bit of engineering to make the car handle better that they passed the bean counters. Since it causes some small toe-out moments.
          '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
            Here's my theory on why they leaned the Kingpin angle out;

            The upper arms are short to allow for camber gain on compression but I suspect they put the virtual pivot farther outboard to give the virtual upper A-arm more leverage on the bushings so that the suspension could overcome the natural tendency to toe-out when braking, which increases the chances of oversteer. The actual amount of rear steering going on is very small so the Kingpin angle isn't as important as it is on the front wheels.
            Don Johnson (really!)
            Just so you know.

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            • #7
              link doesn't work for me

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              • #8
                It worked yesterday but it's not working now. I just checked again to make sure I made it public and I did. I tried posting the link again and this one isn't working either. (?) Never used Google drive before (replaces Google docs, which I've never used either). Any suggestions on a better place to upload it?

                https://docs.google.com/open?id=0B4a...G56NEdRSFlYbTA
                Don Johnson (really!)
                Just so you know.

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                • #9
                  Originally posted by float_6969 View Post
                  link doesn't work for me
                  It works for me. I suggest downloading a program called CutePDF writer and "print" the document through that program, which will allow you to capture and save it as a PDF on your computer. That is, if you want to save it for later.

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                  • #10
                    It wasn't working for a while last night but it's working again now. Must be a google drive issue.....,
                    Don Johnson (really!)
                    Just so you know.

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                    • #11
                      Works for me now. Still reading it, but there is some interesting info in there for sure. There is a lot more "steering" effect built into the rear suspension than I was aware of. It makes sense now that Nissan went the extra mile beyond the "passive" and created an active steering as well (HICAS).

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                      • #12
                        I realized today as i was removing the ES poly bushings from the rear and installing the Megan Racing hard rubber bushings that there is a part of the steering effect of the rear that is eliminated with poly bushings and heim joints. Part of that paper was talking about the RLCA "sliding" along it's axis during a turn to help control toe. This effect would be lost with all but a rubber bushing, right?

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                        • #13
                          That dynamic toe change makes the car understeer. That's not something I want when I'm trying to drive at the limit. That's why the rear of these cars feels so much better when you go all rod ends and sphericals.
                          '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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                          • #14
                            bah! I didn't meant to delete that. I was wanting an explanation of what is happening in the rear that makes the car understeer.

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                            • #15
                              Originally posted by float_6969 View Post
                              bah! I didn't meant to delete that. I was wanting an explanation of what is happening in the rear that makes the car understeer.
                              Please restate your question so that we can discuss it. Thanks!
                              Don Johnson (really!)
                              Just so you know.

                              Comment

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