Is there any kind of cockpit-adjustable limited slip differential available on the aftermarket?
Something, depending on the driving conditions, can beadjusted by the driver via a knob or dial. Something that clamps down on the friction disks/cones with pneumatic, hydraulic, or electrical servomotor pressure.
Something adjustable from anything from full open differential to fully locked, and anything in between?
It’s not exactly what you’re asking, but the Toyota Tacoma TRD series allows you to completely lock the rear diff from the cockpit. It’s not adjustable in between, either locked or normal.
(I think it locks all 4 together in 4wd, but I’m not sure.)
I have never heard of such a thing.
Why would you want one?
An open differential is best for low speed tight turns…y’know, maneuvering in and out of a parking spot and such.
Limited slip is good for high speed maneuvers…autocross.
A locked diff or spool is good for straight line acceleration, such as found in drag racing.
Locked diffs are also good for serious 4X4ing.
Now, I don’t expect a drag racer to go off-road and vice versa, but one component could provide a wide range of performance in different applications.
I can’t imagine any situation in which you would want to lock a limited-slip diff, except possibly competitive drifting or something…
Such a thing exists in the ARB air locker. As far as I know though, this simply changes a fully open diff to a fully locked diff. Either/Or, nothing in between.
As for why you would want one, it has the advantages of an open differential and the advantages of a fully locked differential (at different times, of course). From what I’ve read about them, locked diffs are great in rocks and mud, and anywhere where one wheel may come off the ground. They suck on normal dry roads causing the tires to wear on turns and on ice where they can cause both wheels to slip, resulting in no lateral traction.
If you don’t drive off road, you’ll never need one. If you do, you might need one, depending on the conditions.
Here you go. Second down on the list.
Or there’s Detroit Lockers. Instead of the normal guts to a diff, imagine a rachet driving each wheel. So straight ahead both wheels are driving together. In a turn, power goes to the slower inner wheel; the rachet mechanism allows the outer wheel to free-wheel faster. Wiki, almost at the page bottom
It works quite well on surfaces where a wheel is likely to break completly loose, such as deep mud, snow, or uneven terrain to the point that one wheel is off the ground.
Limited slip diffs have some preload, but if one wheel has NO traction, they can only apply the preload amount of torque to the other wheel, which isn’t much. (I think the spec on my Ford 10.25 was 35-70 lb-ft) If the loose wheel has SOME traction, the pressure angle of the spider gears applies axial force to to the clutches, and the wheel with traction gets some multiple of the loose-wheel’s torque plus the preload.
That preload is supplied by springs of some type: coil springs, leaf springs, or Belleville washers in the designs I am familiar with. It should be possible in some designs to replace those springs with one or more pneumatic cylinders, fed via rotating seals, and controlling the air pressure create what the OP is asking about.
As far as I know, no such thing has been done. I recall a “popular” magazine article many years ago that had a differential with a fluid with a magnetically controlled viscosity creating such an effect. I think this may have been employed as the center diff in a European variable torque split AWD uber sports car.
The ARB air locker (mentioned up thread) mechanism is very close to this. If the locking splines in this differential were replaced by a friction clutch, you’d be there. Straying a bit, the ARB is interesting, as the mechanism directly locks only one side of the axle to the differential case. (case =/= housing) Since the other side is coupled to the locked side via the spider gears, this suffices to fully lock both sides.
Dag Otto’s point regarding locking diff’s on ice is not to be taken lightly. Even limited slip diffs can cause both drive wheels to slide at the same time, and it is very difficult to control…enough so to make the ARB look like a good deal to me.
The detroit lockers are a very good and popular traction solution for off-roading, and acceptable for a utility vehicle on pavement. They have the undesireable trait of driving only the inside wheel when cornering. Since this wheel is unweighted by cornering forces, they tend to spin that inside wheel a lot if the driver has a heavy foot.
Finally, no discussion of enhanced traction axles would be complete without mentioning the the Gleason Torsen design. These exploit the asymetry of a worm drive. * These essentially lock both driven wheels to the input, yet still allow one wheel to back drive the other with little friction when cornering like on open diff.
These (Torsen) were marketed for retrofitting at one time, but were expensive, and largly unappreciated. The last I knew they were only available to OEMs. I believe they are used on HUMVEES and the origional Hummers, and again as a center diff on some AWD uber sportster I can’t recall. The Wikipedia indicates they have been more extinsivly applied since I quit keeping track.
*A worm can drive a gear with only modest friction loss, but the same setup experiences huge friction losses when the gear tries to drive the worm.
Being able to lock a diff in deep snow/ice conditions where one wheel has lost traction is very, very helpful. My new Pathfinder uses it’s anti-lock brakes to compensate for this. If one wheel starts spinning, it will apply the anti-locks to it to transfer more power to the other side. So far, it seems to work pretty well.
Which is a simpler more elegant solution than having a switch on the dash. If you have a switch (open, limited, locked), sooner or later Mr. Murphy is going to riding with you and the switch will be in the wrong position. leave it in locked on ice, and it could be a dangerous and expensive mistake.
Gleasons are only OE now? Back in the 1980s I know they were availble for the off-road aftermarket. See what happens when you lose track of a company for 20 years or so. 
Thanks - about as good an answer as I could have hoped for.
FTR, the Wiki page is missing a few cars here and there.
UK-only Saab 9000 CS “Carlson” edition, Porsche 968, Audi S2 and RS2, and probably heaps more.