[Ecky]

I've been reading about transmissions a lot lately, and I realize I don't fully understand limited slip differentials.

My reading has lead me to believe there are a few types, at least that apply to FWD vehicles:

-Viscous
-Clutch-type - Either electric or mechanical
-Helical

My understanding is that the viscous types behave much like torque converters in automatic transmissions. Clutched seem relatively straightforward. Helical I can vaguely conceptualize.

What isn't clear to me is what sort of effect each of these would have on efficiency, and what their disadvantages are.

Clutched, my reading suggests that they're wear parts and require both frequent oil changes and occasional service. Also, when engaged, there's a 1:1 between wheels and would greatly increase wear on tires. I can see this being useful off-road, but it sounds very inappropriate for road cars.

I could see viscous torque converters eating power, but I'm unsure if this applies all the time, or just when there's a difference in wheel speed? Can someone tell me more about these?

As far as helical, my reading suggests that if one wheel is free-spinning, it won't send power to the other wheel - both must have some traction. Is this correct? What would be the point of a helical LSD in that case? Also, do the extra frictional losses apply all the time, or only when there's a difference in wheel speed?

~

What got me thinking about these was parking on icy slopes this last winter. I can think of a few times I had to have my neighbor pull me out, because I couldn't get traction to both wheels. Perhaps in the future when we have a motor for each wheel, this won't be an issue, but for now I'm interested in educating myself on the various engineering solutions to this.