Tech: LSD function

Discussion in 'Tech, Tips, Newb Info' started by FuJi K, Dec 9, 2006.

  1. FuJi K
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    FuJi K Well-Known Member

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    humm...I just learned how the LSD works mechanically. Posting this for a young learner, maybe too technical for him.

    HERE I'm explaining how the KAAZ clutch type works.

    Explaination
    A differential has spider gears(pinions gears) and side gears so left and right tires can spin at different speeds. W/o it, the tires would be fighting against each other trying to spin the same speed because the INSIDE tire wants to spin slower and the OUTSIDE tire wants to spin faster. Inside tire has smaller circumference than the outside tire when you're turning.

    [​IMG]
    Here you have the LSD unit. Lets look at the side gears. The axles go into here, and they are connected to the hubs, wheels. The pinion gears are meshed with the side gears, thus they are connected and move together and spin around, which is why it's called a differential. The pinion gears slide onto the pinion cross shafts. The pressure ring closes over the pinion gears when put together and the pinion cross shafts (the 4 of them) come up and are FLUSH with the pressure ring where you see the little NOTCH that's been cut on the pressure ring.
    Looks like this when the side gears, pinion gears, and pressure rings are put together:
    [​IMG]

    NOW the clutch (RS) plate B are connected to the side gears by cutout on the inner part of the plate/disc. Plate B will be spinning the speed the axles are spinning because it's connected to the side gear, which the alxes are conneted to.
    [​IMG]
    The clutch (RS) plate A are connected to the differential case. The Plate A's sandwitch the Plate B's when put togehter.
    [​IMG]

    The assembled Pressure ring is ALSO connected to the differential case.
    [​IMG]

    NOW This is where the LSD comes into play. When you apply power according to gas pedal, the differential case spins and the clutch plate A and pressure ring turns together. The PINION CROSS SHAFT, which the pinions gears and side gears are meshed with will get turned by the pressure ring being that it's assembled with both pressure rings clamping onto the pinion cross shaft. The pinion cross shaft, pinions gears and side gears are what the PRESSURE RING is trying to spin.

    [​IMG]
    The triangular cutout on the pressure ring where it clamps onto the pinion cross shaft is cut this way for a reason. The pinion cross shaft will try to push away the pressure rings because of this triangular cutout. When the pinions cross shaft pushes the pressure ring away from each other, the pressure ring sandwitches the clutch plates A&B together.

    This sandwitching keeps the sidegear spinning with the speed of the differential case because the plates on the side gear and the diff casing are clamped together by the pressure ring. This pressure ring can side left to right in the differential casing if you're wondering, but it's just a little movement about 1mm. The more plates in there the tighter the space, and the initial tight space will make the LSD have some initial LSD effect to start out with even without applying force by throttle.

    I hope I got this right.

    EDIT: Here's a vid IN PERSON.
     
  2. TSTRBOY2004
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    TSTRBOY2004 Well-Known Member

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    I dont know about any of you guys but those who think that an 02/03 wrx comes with an lsd must have some bad info!?!?? ( I post htat cause there were people saying they did) cause mine didnt look anything like that... it had the normal planetry gears with no plates... where as my Quaife has all the clutch plates...
     
  3. FuJi K
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    FuJi K Well-Known Member

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    The '02+ WRXs have a VLSD. The sealed units are on the side gears. The VLSD unit doesn't look like a LSD at all on the '00-'01 2.5RS and the '02+ WRX.
     
  4. project/driven
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    project/driven Well-Known Member

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    I think the 02/03 is a viscous coupling. It is a different assembly, and not a true lsd as it will spin only one wheel in some cases...I think...
     
  5. Shibbs
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    Shibbs The Daywalker

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    Does anyone know about the 07s? Does mine have LSD? And what is the difference in the viscous units?
     
  6. carl
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    carl Well-Known Member

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    now let's go drifting
     
  7. Vector
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    Vector Rally Organizer

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    The 02 has an VLSD in the center diff, a VLSD in the rear, and the front is open.

    VLSDs use a special fluid that thickens when it gets hot, so as one side spins faster, the friction causes the fluid to heat up and then it starts sending more power to the other side. They are very smooth, but the fluid does deteriorate with use, and they don't tranfer the power to the traction side as well as mechanical LSDs do.

    The clutch type is explained above. Very effective in pretty much all situations. Depending on the pre-load they can make low-speed manuvers a PITA, and they are clunky and noisy.

    A Quaife or Torsen differential is a torque sensing diff. Great for high traction surfaces, they work by sending a ratio of the torque to the side with grip vs. the side with no grip. They are very smooth. Because the torque distribution is a ratio, if the surface is low friction (ice, really loose gravel or sand), the amount of power sent to the wheel with more traction can be a very small amount. If one wheel is off the ground, the other may not get enough torque to do anything at all.
     
  8. TSTRBOY2004
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    TSTRBOY2004 Well-Known Member

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    hmmmm well i never saw any gearing, plates or anything other than planetery gears on my stock differential.. the Quaife LSD is real smooth but it definitely puts power down on both wheels... i take off turning right at a traffic light with some moisture on the road and can you saay rear wheel drive.... yep the thing just slides up the road then fishtails back... its fun...
     
  9. FuJi K
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    FuJi K Well-Known Member

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    They didn't have it on the 98-99 2.5RS. Then they put it on the '00-01 2.5RS. Then later they DROPPED it from the 2.5RS and put it on the WRX because the WRX was now the higher trim model. They kepted it on the WRX since '02. Yours will have it on the '07. The STi's have the clutch type as well and offers an upgraded pressure ring to replace the stock's 45* notch to 60* for more pressure on the discs.
     
  10. Shane86
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    Shane86 Well-Known Member

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    Good writeup fuji...
    the other 2 models of LSD are Viscous and Helical
    a Viscous LSD works along similar principals as a clutch type... but it actually uses a liquid. as the liquid heats up it prevents the drive axle gears from spinning at different speeds. a viscous is less aggressive than a clutch or helical type, as the wheels have to be spinning at a different speed longer to make the fluid thick enough to grab the whole assembly and force it to turn at a matched speed.

    a helical type uses gears and magic to force power to the wheel to that has the most grip.... i wish i could explain it better, and i can... but it's a lot of typing for 1am... ask me in person some time.

    The WRX has a viscous limited slip. it's not real aggressive, but it's definitely not an open diff. it's such a simple setup (mostly relying on the heating of the diff fluid to work) that it looks VERY similar to a regular open diff. if i recall right the main difference is in the carrier... they've added some springs that add pressure to the gears as the heat goes up... combine that with the thicker lube at that temp and you have a limited slip. not real aggressive, but it'll keep both wheels turning.

    Nathan, your new quaife is a helical type, and a very nice one at that. their great diffs as the offer very aggressive grab without the added maintenance and harshness of a clutch type lsd.
     
  11. Ryan
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    Ryan Sled drifting master

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    Bringing back the dead. I searched on this because I was wondering if my wrx had a limited slip diff or not. Maybe we could move this to the noob/tech area? Great write-up. It's funny that something I always wanted in my jeep comes stock on my wrx :)
     
  12. Shibbs
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    Shibbs The Daywalker

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    Yes, Ryan, you do have limited slip in the center and rear. Viscous tho. But its still LSD.

    We're all space cadets.
     
  13. mlgez
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    mlgez Well-Known Member

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    Not a bad idea. I will work on in.
     
  14. FuJi K
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    FuJi K Well-Known Member

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    VIDEO LINK at bottem of first post.
     
  15. webcrawlr
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    webcrawlr Well-Known Member

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    If we're talking rear diffs that's changed now. The 07 STI has a Torsen diff in the rear.

    Other then that it's been a helical limited-slip up front since 05 and some type of mechanical/clutch mix in the center since 06.
     
  16. FuJi K
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    FuJi K Well-Known Member

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    ewwwwWW, torson in the rear? I perfer torsen/helical in the front, but clutch in the rear.
    Maybe less maintenance, which is why they went with torsen?
     
  17. piddster
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    piddster Lone Wolf

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    The VLSD on my workbench is apparently from a '99 Impreza and is a 3.90.

    If I ever upgrade from a rear VLSD it'd be a clutch typw, like a KAAZ or Cusco. Since my tranny os from an '05, it has the helical front diff. Its nice on pavement, but pretty worthless in the winter.
     

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