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Subject: That Cheapie LSD Thingy in Turbo From: Dg50 <dg50@daimlerchrysler.com> Date: 10 Apr 2000 11:47:08 Message-ID: <#48> [Next] [Prev] [Top] [Post Reply] [Bibliography]

Someone asked on Friday's Digest about that cheapie LSD Doohicky that was featured in Turbo Magazine this month. When I first saw it, I thought "Hrmph, more snake oil" and skipped the article, but now that someone has expressed interest in it, I thought I'd go back and examine it in detail, and see how it works. Before I report my findings, here's a little differential theory. This will be _really_ basic for some, but bear with me. :) There is only one power shaft coming out of the engine, but all cars have at least 2 drive wheels, so the power transmission must be split so as to drive both wheels. Normally, all you need to do something like this is a simple ring and pinion gear set. 2 gears, case closed.

This works really really well in a straight line, but problems arrive when you want to drive the car around a corner. The inside wheel tracks a physically shorter distance than the outside wheel, as the radius the inside wheel inscribes during the turn is smaller. This means that in order to turn the car in a given radius, the inside wheel must turn slower, and the outside wheel faster, in order to maintain a given input shaft speed. But if the drive wheels and drive shaft are all connected by a simple ring and pinion, something has got to give. The drive shaft wants to keep the wheels turning at the same speed; the road surface wants the wheels to turn at different speeds. Normally, the inside wheel breaks loose and spins, and the car "hops". You can see this effect with 4WD trucks that allow you to lock the diffs. So instead of splitting the power via a simple ring and pinion on a common shaft, we use a gearing arrangement called a "differential", so called because it allows the output shafts to turn at different speeds. With a differential, you can transmit power to two wheels even when they are rotating at different speeds, like they do when going around a corner. Now a differential has two special characteristics. The first is that the two output shafts are no longer directly connected, so that they can rotate at different speeds - one of which can be "zero speed". The second is that the output shaft that is turning faster gets the higher portion of the input shaft torque. Put these two characteristics together, and you get an unhappy side-effect: if one wheel has traction, and the other does not, then all the torque goes to the wheel that has _no traction_. You could have 1000ft/lbs of torque, and 12" race slicks, but with an open diff and one of those slicks on an ice patch, that car ain't movin'. The wheel on the ice patch is just going to spin. The differential gearing is too tough to draw in ASCII, but this illustrates the important point:

No connection here So what you need is a way to connect the two output shafts in such a way that there can be differential rotation, but that tries to resist the rotation up to a point. But whatever the doohicky that connects the two sides has to be able to slip a little or you're back to the solid-shaft wheel-hop problem - thus the name "limited slip". There are many, many ways of doing this. The AWD rear LSD uses a viscous coupling that works much the same way as an automatic transmission torque converter. The case of the LSD is attached to one output shaft, the impeller is attached to the other, and any time there is differential rotation, the viscous fluid in the casing resists the slip in direct proportion to the amount of slip - the more one wheel spins, the closer the coupling gets to locking up. Simple and elegant.

Casing Gear |----| Gear |\ ||-|:| /| ===output shaft===| |==| |:|=| |==output shaft=== |/ ||-|:| \| |-^--| | Impeller

The more traditional LSD uses mechanical clutch packs that work much the same way as the engine clutch with the pedal partially applied. The "flywheel" is attached to one output shaft, the "pressure plate" to the other, a series of friction surfaces are placed in-between, and a preload spring adjusts how hard the assembly clamps together. This also works pretty well, but unlike the viscous unit, they eventually wear out and need servicing. No ASCII art - you should have the idea by now. :) There are others - the Torsen, which uses a tricky worm-gear arrangement, the cam-and-pawl, the Detroit Locker, and a couple of others. Anyway, back to this little doohicky from the magazine. What this is is a pair of spring-loaded metal blocks with some strategic holes drilled in them. The unit comes pre-compressed, with a little latch you remove once the unit is installed. The idea is that it fits into the open space between the two output shaft gears, you pop the latches, and the spring forces these two metal blocks against the output shaft gears. Presto! You now have a clutch-pack LSD, with the "clutches" being the metal blocks rubbing against the gears themselves.

Gear Gear |\ |-----|\ /\ |-----| /| ===output shaft===| ||Block| Spring\|Block|| |==output shaft=== |/ |-----| \/ |-----| \|

There are a series of problems I see here: The most damming one is that there's no friction surface - the blocks ride right up on the little machined flat spots on the gears, direct metal-to-metal contact. Ignore the fact that these gears were never intended to be used as friction surfaces. Ignore the fact that they had to use a MONSTER spring to get the kind of clamping pressure required to get any sort of resistance. Ignore that the axle bearings were probably never designed to take that kind of side load. Ignore that removing the damn thing is probably next to impossible once the spring is released. Ignore the heat that it would probably impart to the diff fluid. Instead, imagine for a second a 5500lb engine clutch, installed with no friction disk between the pressure plate and flywheel. About how long do you figure it would last before the two metal surfaces are so polished that they stop gripping each other? Not long I'd bet. Secondly, this thingy looks like it's custom machined to fit the Honda diff ~ so it probably won't fit a DSM anyway. Thirdly... well, stop ignoring all the stuff I mentioned above. :) Overall, this looks like a BAD IDEA, especially when Quaife - who make real race car parts for real race cars - sells a real, race-proven Torsen diff for what amounts to not a whole lot of money when you consider what it is you're getting. My advice? Put this thing on the same shelf with the Tornado intake fan, the Nology capacitor plug wires, and the Unorthodox "you don't need a torsional dampener" underdrive crank pulley. Get the Quaife. DG