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S13 VLSD shims for autocross

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  • S13 VLSD shims for autocross

    Hello,

    I have a s13 that I use as a daily driver and for autocross. I have a VLSD from a R32 in my car now. When I got the car the diff had 0.8mm and 1.4mm preload shims installed. The rear end wasn't predictable exiting corners, it seemed to spin the inside wheel then aggressivaly transfer the power to the outside wheel and oversteer. I tried changing the shims to 1.49mm and 1.49mm, and now the diff has both tires locked no matter what. I like how easy the car is to throttle steer with the tires locked, but feel like this is nowhere near optimal for carring speed through tight autocross corners. I have the following shims for this VLSD: 0.8, 1.0, 1.4 and two 1.49. Does anyone have any suggestions on what would be the best combination of these shims for an autocross application and why? I don't have the funding to go to a clutch or helical diff.

    Thanks

  • #2
    Don't bind up the gears in your differential to get biasing. Worst idea ever...
    '18 Chevrolet Volt - Electric fun hatch for DD duty!


    DefSport Koni Sleeve and Spring Perch Buy!!!
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    • #3
      The VLSD is where you can't see it and it is NON-adjustable.
      If you don't want inside wheel spin with a VLSD, drive differently and work on your suspension.

      What Def said.

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      • #4
        I assume that the 0.8 and 1.0 are around what came from the factory in that diff. What can I do to the suspension to get any signaficant amount more grip on the inside rear tire?

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        • #5
          What do you have done now?

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          • #6
            Originally posted by josh_s13 View Post
            I assume that the 0.8 and 1.0 are around what came from the factory in that diff. What can I do to the suspension to get any signaficant amount more grip on the inside rear tire?
            The shims are only in there to set backlash on the ring & pinion. To check your VLSD, jack up one rear wheel and turn it with your hand. It should present some resistance (like moving through honey) but still be able to turn.

            The best thing you can do (to your car ) for corner exit traction and stability is to adjust your suspension to keep the rear wheels on the ground. The first thing is that you can't run a big rear sway bar. Put the stock one back on or even go without a rear bar. Second is make sure your rear suspension allows the rear wheels to "droop" and stay in contact with the pavement when you're turning. You didn't say what kind of suspension you're running but many of the OTS coilover kits are way too short. If you're running pre-load on your rear springs, don't. Adjust them just tight enough that they don't rattle, so you have some compression/rebound movement.
            Don Johnson (really!)
            Just so you know.

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            • #7
              Sway bars are fine in the rear. They don't change things over the same roll stiffness achieved with springs alone. That said, our cars need quite a bit more front roll stiffness vs. Rear.
              '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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              • #8
                Originally posted by Def View Post
                Don't bind up the gears in your differential to get biasing. Worst idea ever...
                Originally posted by logr View Post
                The VLSD is where you can't see it and it is NON-adjustable...
                Originally posted by djsilver View Post
                The shims are only in there to set backlash on the ring & pinion.
                We're getting confused here...he is, in fact, shimming the viscous coupler.
                You can "shim" a VLSD.

                You take apart the "sealed unit", and change the shim thickness. It wears out the viscous unit faster, but people have reported good results form doing this.

                It's a lot of work (IMO) for only "meh" results (considering the increased wear)...but it works.

                ...

                Here's a copy-n-paste from Zilvia:

                Originally posted by Mtndrifter14
                By: Hunter Rodziczak

                Pry apart:

                The Almighty shim:

                Now take this and put it over the existing shim:
                ...Thread here: http://zilvia.net/f/s-chassis/200226...-j30-vlsd.html

                ...

                Anyway, just wanted to try and get this thread back on track (although the direction it's heading is probably a good one anyway - Suspension changes).

                - Brian
                '96 240SX | KA-T - 320rwhp @ 12psi | ~2700lbs
                Koni 8611 & GC's - 600/450 | RPF1's - 17x9.5

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                • #9
                  Aren't there spider gears in the viscous coupler? I knew what he was talking about, but im pretty sure its just binding up gears in the Lsd itself to get more lockup.
                  '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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                  • #10
                    Originally posted by Def View Post
                    Aren't there spider gears in the viscous coupler? I knew what he was talking about, but im pretty sure its just binding up gears in the Lsd itself to get more lockup.
                    It's a vicous/fluid coupling...a certain about of heat or breakaway torque and it will spin both. Imagine a bunch of litlte plates with the fluid in it...once the fluid gets heated enough it sorta locks the alternating plates into one solid coupling, thus making both work. Exceed it and it will just smoke the fluid inside of it and eventually ruin it to the point where the fluid doesn't work anymore.
                    'Slicks on a car show me you care - broken axles show me you're trying'
                    [I]Nitrous Rental Cars - Turbo Festivas - Vehicular Lunacy[/I]
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                    • #11
                      What def is saying reguarding the spider gears is correct. The shim place a lot of preload on both the sealed viscous unit and the spider gear. Effectivly binding them.

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                      • #12
                        The coilovers are fortune auto. About -3 deg negative camber, about -2 deg neg camber rear. Some poly bushings. Last season i set the dampening in the mid-soft range of the damper according to what the manufacture recommended. I now have the damping on almost full soft in attempt to get more power to the ground. Stock front and rear sway bars.

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                        • #13
                          OK, I'm ready to stir the pot

                          Here's the diagram for the VLSD diff on page PD12;
                          http://www.240edge.com/manuals/91-94_240sx/pd.pdf

                          The VLSD does still have spider gears. The viscous coupling forms one side of the spider gear. One axle stub is fixed to the outer case of the VLSD coupling and the other stub is fixed to the inner case of the VLSD coupling. The "SHIM" selection guide is on page PD25 and gives a range of backlash, not pre-load. If you pre-load the spider gears, any change in LSD action you get is from grinding the spider gears against each other, not any change in the function of the VLSD coupling.

                          The VLSD coupling is filled from the factory with a viscous fluid. The function has already been explained. I have actually, really, for sure, experimented with one of these! When the units are filled from the factory, they drill two counter-bored holes to fill it, then they insert ball-bearings into the counter-bores and stake them in place. I removed the balls and tapped the two holes. I then flushed the old fluid out and replaced it with newer, thicker fluid. The VLSD was installed into a Z31 300zx running about 400WHP! It would work much better than a stock VLSD until it got hot. When it got hot it would lock solid and act like a spool. I did some research on this phenomena and the general theory is that when the interposing discs get hot enough, they expand, distort, and bind against each other. Our conclusion was that we couldn't make the VLSD any more effective.

                          Ultimately, if you're using a VLSD/HLSD/open diff, you need articulation/droop in the rear suspension and roll-stiffness in the front suspension, to keep the rear tires on the ground. If you're running a clutch/spool diff you can ignore this to some extent and drive on one wheel, at least until your power output overloads your outside rear wheel. Then you're back to the same place My buddy with the Z31 went around the world with LSD's, but came back to his shocks/swaybars/camber/toe/tire pressure settings to maximize corner exit.
                          Don Johnson (really!)
                          Just so you know.

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                          • #14
                            Excellent info. I will adjust the shims to pd-25. I believe that you and your wife at michelin this past year.

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                            • #15
                              Originally posted by CodyAce View Post
                              It's a vicous/fluid coupling...a certain about of heat or breakaway torque and it will spin both. Imagine a bunch of litlte plates with the fluid in it...once the fluid gets heated enough it sorta locks the alternating plates into one solid coupling, thus making both work. Exceed it and it will just smoke the fluid inside of it and eventually ruin it to the point where the fluid doesn't work anymore.
                              Yea, I know how a VLSD works. But there has to be a spider gear in there to transfer torque to each side.

                              You cannot preload the viscous coupler AFAIK, it's a sealed unit. The preload is affecting spider gear engagement. Thus, a ton more shim binds it up. Eventually you gnaw away at your spider gears and the "preload" goes away to some extent, but the differential is probably ruined forever at that point.
                              '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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