Torsen "Limited Slip"

[boris]

After being howled down over my criticism of the so called (Torsen) limited slip diff. and moving on to a clutch type locking diff, how is yours performing in the snow? [:(]

Now you know how mine felt in the wet. [:O]

 

[Goof]

 I’ve replied to your other Torsen thread.
They’re useless in snow & ice - but I wouldn’t change it.

[boris]

Useless in the wet too! One wheel spinning with all the power is really nbg.

[rogerzilla]

 Are they that bad in the snow?  I thought Torsens only allowed one wheel to spin up if it was off the ground, i.e. no resistance at all.

[boris]

I get one wheel spinning in the wet all the time if I don’t take care. I suspect that it would spin one wheel in the wet up to 100 mph in 4th gear (if we were allowed to do that kind of speed).

 I tried the handbrake trick and I certainly found it easier to get a grip on my icy driveway this morning.

Also in here … http://en.wikipedia.org/wiki/Torsen

 

[HK_Phooey]

I get one wheel spinning in the wet all the time if I don’t take care. I suspect that it would spin one wheel in the wet up to 100 mph in 4th gear (if we were allowed to do that kind of speed).

Boris, what tyres do you have on the car? When I first got my car it had cheap hedge finders on the back, I experience power oversteer at 85mph on a curved stretch of autobahn, which was ‘interesting’!

Saying that the Goodyear F1’s I currently have fitted struggled on frozen compacted snow, with one wheel just spinning up uselessly. I’ve put a 32kg battery from work in the boot, which seems to give the car enough grip to make progress.

[RostokMcSpoons]

This is bizarre for me. I’ve just bought a Mk2 1.8S (which will have an LSD). I had no problem driving along my snowy road tonight, even though it’s very slippery. So that’s fine.


The bizarre thing is my old '92 Eunos 1.6 got stuck on a road today… and the problem was that one wheel was spinning and the other wasn’t moving at all.
Does that mean my Eunos has a LSD? I have to say if that’s cast-iron evidence that it has, I’ve certainly enjoyed the handling of the car in the wet and snow And today has been the only time it’s got stuck, except the other big snowstorm a few years ago, but that was on a steep hill.

[rogerzilla]

 It will have a VLSD (viscous unit) and these lose their effect after about 60,000 miles.

[ast]

 

 It will have a VLSD (viscous unit) and these lose their effect after about 60,000 miles.

 
I’ve seen this stated many times; I thinks its largely apocryphal. Certainly, my vlsd seemed perfectly functional after 140k miles. Superb write-up by Lance Schall over on miataforum.com;

Viscous couplings are generally not well understood and the Miata unit
is no exception. First, it is thought to wear out. While, as Don points
out, there is no clutch plate contact, the usual notion is that the
heat cooks the silicone oil and destroys it. This may be true and if so
would be evident in demanding competitive applications. However, I
suspect the opposite is true. The Miata VLSD
doesn’t get hot enough during an autocross to work effectively. Which
brings us to the second point, the lock-up is lame anyway. Not if it’s
hot enough.



Historically, the technical papers that indicate the beginnings of understanding how VLSD work date to the late '80’s and early '90’s, precisely when Mazda came up with the VLSD in the Miata. And they were proud of it, sending their service technicians to a special course on the VLSD. I remember one dealer mechanic confiding in me, “They were so excited about the VLSD, I thought it would be on every Mazda car…”



Now, the alternating plates in the VLSD
are toothed wheels with a couple dozen radial tabs. The reason for this
is to eliminate warpage of the plates with temperature. Turns out there
was some serendipity to this design. It has long been known that VLSD
transmit more torque than estimated by shear forces alone, sometimes 10
or 20 times more torque! But sometimes NO more!(?) Engineers had been
muddling along with empirical test results until about the '90’s when
several scholarly attempts were made to quantify and predict
performance.



It turns out that the plate tabs twist and shift axially giving rise to
all sorts of interesting phenomenon with fluid phase changes, bulk
pressure, dissolved air, Coulomb friction, blah blah blah (see SAE
2004-01-0867 - Mohan). The short story is the VLSD
differential enters an operating phase Mohan calls “Self-induced Torque
Amplification” (STA). This is also described by engineers working with Nissan, Takemura and Niikura, although they call it “hump” (SAE 900558).



In any case, the VLSD has 3 phases. First is initial viscous dissipation where the oil heats and viscosity drops. The VLSD
transmits less torque. This is the operation you can measure with a
torque wrench as outlined in the Shop Manual and gives rise to the idea
that the VLSD is lame. As the fluid
heats, the transmitted torque levels and at last in the final phase, as
the fluid temperature exceeds 200 C, the torque graph shoots up, with a
very steep slope, to transmit 300-400 ft-lbs or more. The ability of
the VLSD to transmit this torque will continue as long as the high shear rates are present. To achieve this temperature, the VLSD
may have to be subjected to high shear rates for several minutes.
Clearly, this is not accomplished at an autocross or even, perhaps, on
the track. So, the poor reputation of the VLSD
for typical Miata performance applications is supported by the
scientific explanation. However, this continuous shearing may be
experienced in off-road center differentials and, indeed, Mohan worked
for New Venture Gear and viscous differential are still found on 4x4 vehicles here and there.



So, the Miata VLSD is probably not
weak or broken, just poorly matched to its application. You notice
Mazda dropped it and moved on to the Torsen…

 
The lifetime of the vlsd seems to be variable; from forever to 30 minutes, depending if the car is hammered or not. Most are driven normally, so the vlsd should be in good shape (though there were a spate of diff failures on very low |Roadsters; the diff housing getting cracked, an entirely different issue).