Limited slip differential efficiency?
 

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?

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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.

Ahh...er...I understand how each works and can make plenty of assumptions, but I think it would be better for an expert to try ans answer your question.

For the most part, I don't see them throwing away efficiency, beyond the additional rotating mass, except for when they're engaged...and 99% of the time they aren't. There may be a slight efficiency hit for the viscous, but I doubt it would be noticeable.

As to the helical....yes, it needs traction to engage, otherwise there's not enough torque to spread the gears/clutches/whathaveyou apart and cause them to lock up. If you're on ice or with a wheel off the ground, you aren't going to be able to apply any torque because said wheel will spin freely.

The point of them is on the street or track, you will almost always have some traction, so they still do their job. Even if one breaks loose, it still takes torque to keep spinning a tire on pavement, and chances are the diff will have already locked up before you do.

Sometimes ice and streets are synonymous here. :D

Traction control is probably better for the average joe driving in winter.

Makes sense. I know you can add a LSD to many manual transmissions though, but haven't the faintest idea about how one would retrofit traction control.

I see no reason for an LSD to decrease fuel-efficiency, but a handful of extra pieces in the differential might increase internal frictions. Well, nowadays that most cars are fitted with ABS brakes (even in my country it's now mandatory), it's easier for automakers to resort to traction control.

Even vicious wouldn't have an efficieny hit, at least not enough to notice. It's not that much weight, or rotational mass. Maybe a pound difference, but again, I would guess not even enough to notice. Also vicious is absolutely trash. I don't think anyone uses that technology anymore. Most Miatas (90-93) that had it, no longer had it beyond about 50k miles. Just a bad design.

It's still used at least in trucks, but the modern crossover SUVs mostly replaced it for the ASR.

I can't imagine a FE reduction from an lsd. They only engage under moderate to heavy torque situations.

I had one in my cts and I can attest, it is very beneficial when paired with traction control. In rwd vehicles it can cause understeer when cornering and accelerating(NOT eco friendly :) )

Well, I'm sure it may actually benefit the efficiency in slippy terrain conditions, such as driving uphill in a rainy morning in Porto Alegre...