Limited slip diffs: How they work

[gsxr]

This video shows the basics of limited-slip (vs standard / open differentials). Factory Mercedes limited-slip differentials used through the early 1990's are the clutch-pack type. The LSD unit shown in this video is not identical to the MB design, but overall it's a good basic description of how the clutch-pack design works. MB design is claimed 35% lock with new clutches, this gradually reduces to near zero lock once the clutch friction materials are worn out. Mercedes LSD's do not have the center spring shown in this video. The clutch pack compression is completely based on the forces generated by the spider gears:

https://www.youtube.com/watch?v=WeLm7wHvdxQ

[video=youtube;WeLm7wHvdxQ]https://www.youtube.com/watch?v=WeLm7wHvdxQ[/video]

 

[gsxr]

The Quaife and WaveTrac limited-slips do not have clutch packs, they are a variation of the Torsen principle, explained in the video below. The advantage is, no clutch packs to wear out. One disadvantage is there is no full locking function; if one wheel has zero grip, the diff cannot transmit any power to the opposite side. It also works the other way - if one wheel has extreme grip (i.e. hot racing slick on sticky track), that wheel may stop spinning and all engine power is transferred to the spinning wheel with less grip.

The Torsen, Quaife, and WaveTrac are technically all variations of ATB (Automatic Torque Biasing) differentials.

https://www.youtube.com/watch?v=JEiSTzK-A2A

[video=youtube;JEiSTzK-A2A]https://www.youtube.com/watch?v=JEiSTzK-A2A[/video]

Thanks to omegabenz for finding this!

 

[gsxr]

I found this post from another forum where the guy seems to know what he's talking about, or at least makes sense, explaining more about ATB's:

"A helical and "Torsen" differential are essentially the same type of differential (Automatic Torque Biasing). Helical ATB differentials are built differently than the original Torsen design, but perform in the same manner. Quaife sells helical ATB diffs.

ATB differentials behave more or less like open differentials, the diff does not resist differences in rotational speed (unlike conventional clutch-type or viscous limited slip units). Unlike open diffs, when there is a difference in resistance (one wheel on a slippery surface) the ATB differential seamlessly biases the torque applied to the wheel with more traction. It does this mechanically and automatically, but the system is very difficult to explain. I don't fully understand it.

An ATB can only bias torque up to its built-in bias ratio (typically 4:1 or 5:1). This means if one tire is on ice and the other on pavement it can only transmit 4x or 5x the torque going to the wheel on ice over to the other side. Not much.

If one wheel is in the air that wheel spins and the opposite side gets no torque. 5 x 0 = 0.

With an ATB you can apply the brakes when one wheel has little or no traction so that the differential may apply a multiple of the braking torque on the slipping side to the other side (which will overcome the brakes on that side).


With a viscous-coupled differential you are working with a conventional differential ("open" if you will) and you are rotationally tying both sides together with a special coupling. The coupling only applies significant torque to the opposing side during slip. Normal speed differentials impose little to no drag (turning corners and such). Drag (coupling force) increases approximately with the square of rotational speed. One wheel starts to spin and the other side gets dragged with it.

Hope this is relatively clear, more info on AWD systems can be found from links off of Jeff L's site, although the descriptions could stand to be more thorough.

-Chris Wood "​