Tech: Retrofitting a Supra Limited Slip into an 8 Inch Rear Axle
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| http://www.4x4wire.com/toyota/tech/supra_lsd/ |
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By: Frank Falcone. 10/2002
Editor's Note: The following was taken from the 4x4Wire Trailtalk Forums. After running a welded carrier on his daily driven 4Runner, Frank started looking for an inexpensive alternative that would offer increased traction over an open differential without the handling quirks of the welded carrier.
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| The assembled carrier in the third member. |
The 1986.5-1992 MKIII Supras were equipped with an 8" differential housed in an independent rear suspension (IRS). Most turbocharged Supra models of this generation should have the limited slip differential (LSD), the naturally aspirated Supras only got the LSD with the Sport Package.
The Supra rear third member isn't a “drop-in” affair because it is laid out a differently (IRS vs. straight axle). The carriers, however, are transplantable. The Supra carrier uses a four pinion design, just like the rear axle in V6 equipped pickups. The Supra carrier will only retrofit into the V6 third member, not the more common 4 cylinder third member. Although I did not do this, it would be a good idea to replace the carrier bearings at this time too, since you have everything apart.
The carrier setup is in every way identical to any other with one exception, you install one of the bearings from your old carrier onto the loaded side (opposite the ring gear teeth) of the carrier. On the other side, just leave the original Supra bearing, which is smaller and allows more clearance for the preload adjuster. Use both of the 8" outer races from your existing third member, as the pickup axle races are narrower than the Supra races.
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| Clearanced race to fit the carrier. |
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| Untouched, mini-truck race. |
It should be noted that it is difficult to get many threads on the unloaded side. Given the narrower Supra bearing, having someone take off .030" or so from the outter race with a lathe could remedy this, as well as the issue described below. This may or may not need to be done depending on the tolerances of your differential. In my case it necessary to make a little more room in the axle housing opening in order to squeeze the third member back in. Basically, just grind conservatively where the protruding adjuster hits. You don't need to grind much, .010-.020 inch tops.
As the last step, each axle shaft needs to be shortened by 1/8 inch. If equipped with a bench mounted grinder, as illustrated in the photo, it is possible to take each one down in about 5 minutes a piece. Just put the axle against the wheel and turn one of the lug nuts.
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| Grinding the axle shafts down. |
Compared to the welded rear differential in my 4Runner previously, this thing drives like a dream. It almost seems weird to just turn a corner, no big production, no chirping/bucking, just turning. Now if I get on the throttle, the inner tire will start to make some noise but not in that squeely open diff way. You can tell the limited slip is working.
I recently disassembled my rear third member and decided to further investigate the inner workings of the Supra limited slip. In the center is this spring. It pushes the outside gears (that attach to the axles) against the inside of the carrier with the clutches inbetween. If there is slippage, an axle spins faster or slower than the carrier and friction transfers torque.
It should be noted that because of the way spider and ring gears are shaped they not only try to turn one another, but they also push themselves away from one another because of the tooth tapper. That means that as torque increases, the spider gears start to spread and push harder against the clutches. So it seems that these clutch type LSD's are also "torque sensing" in that they do apply more friction and therefore torque transfer as loads increase. Operating under this theory, brake biasing should work with the Supra LSD and provide some torque even if a wheel is off the ground. This is not true for other limited slip differentials such as the Truetrac.
I also shimmed the differential for more preload. This proved to be very easy. Just pull it apart and identify where the spring is and where the washer goes. I drilled out a washer to shim my shpring to 0.25", but there is enough room to shim further. Perhaps as far as 0.4".
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