Tell me about Limited Slip Differentials

[Yuriman]

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

Some others exist, but these are what I'm likely to see.

My understanding is that the viscous types behave much like torque converters in automatic transmissions. Clutched seem relatively straightforward- they're either on or off. Helical I can vaguely conceptualize, but don't really understand.

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? Just how much power (and therefore fuel) do they consume, and when?

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.

 

[HarryLui]

These helped me a bit.


https://www.youtube.com/watch?v=JEiSTzK-A2A
https://www.youtube.com/watch?v=PEdnH7_7_yc

 

[mindless1]

If it's stock on a vehicle it's appropriate for road use and has very little efficiency difference, but yes eventually the more common pressure/friction plate type may need the plates replaced, especially if you're hard on it (drive like a madman )

Frequent oil changes, depends on what you call frequent. Typically 60K or not 100K mi or more.

You might be overthinking it. If you have icy slopes you want 4WD or AWD. And snow rated all season tires or winter tires. Winter tires are more important for ice traction than snow.

 

[Yuriman]

Frequent oil changes, depends on what you call frequent. Typically 60K or not 100K mi or more.

Good to know, I was imagining more frequent. When they fail or wear out, do they start grinding?

You might be overthinking it. If you have icy slopes you want 4WD or AWD. And snow rated all season tires or winter tires. Winter tires are more important for ice traction than snow.

Yeah, I already run Nokians 4 months of the year - just put my summers back on. It's only a handful of times each year where I'm dealing with icy slopes where I can't get a good run at it (or ground clearance isn't a bigger factor), and I'm not too keen on the 10-15% economy hit AWD brings the other 99.9% of the time I'm driving, nevermind added complexity and more parts to wear out.

I was mostly curious about how limited slips work, and which of them would make any difference in a case where one wheel is basically free-spinning on ice.

 

[Greenman]

There are also locking differentials. I have one on my pickup, push the button and the differential locks up sending power to both wheels. Really bad to use on the street, works pretty good in mud (no ice here).

 

[RayH]

Clutch diffs on road cars are not usually 100% locking and are based on a combination of pre-load with springs and torque applied by the engine or under braking. They're typically more for traction during racing than getting unstuck and can be setup for different amounts of locking under acceleration vs braking.

On a side note, left foot braking can be used on an open diff car as a sort of poor man's traction control but it may be a problem on newer cars where throttle is cut when braking because of unintended acceleration fears. Some newer cars use braking on wheels with low traction as a form of traction control when they have open diffs. Braking on individual wheels is also done for torque vectoring and stability control.

 

[BurnItDwn]

So, who here is crazy enough to DD their RWD car with a welded diff?

 

[Rifter]

So, who here is crazy enough to DD their RWD car with a welded diff?

Not welded but i drove my 84 chev 3/4 ton for 3 years daily with a Spool in the back, all locked all the time. Was a blast to drive in the rain on corners. Im sure the 350-400hp didnt help either.

Amazing off road traction though.