Please explain Limited Slip Diffs to me

[menousername]

LSDs on rear wheel drive cars

What do they do / offer than standard diffs dont, on normal every day driving, eg normal driving and the occasional thrashing as opposed to track day visits which I dont really do

Purely in terms of the drive, handling etc, what does the LSD do?

Do you think they are worth a mark-up on price on an otherwise good handling car? Would they help re-sale?

thanks!

[Caulkhead]

Simply, the torque going to an axle will take the line of least resistance, ie the wheel with least grip. An LSD reacts to this slip and makes sure some torque also goes to the grippier wheel.

[TheHeretic]

An LSD is always welcome, although not necessary. It all depends how much extra.

[menousername]

thanks!

what would I experience though - what difference would I notice as an average driver

whats the interplay with traction control too? Wouldnt traction control kick in on that senario you describe? How does a lsd change the interaction with traction control

[falkster]

Caulkhead said:

Simply, the torque going to an axle will take the line of least resistance, ie the wheel with least grip. An LSD reacts to this slip and makes sure some torque also goes to the grippier wheel.

Is there an award for explaining it well in the least amount of words?

[XplusYplusZ]

If you go round a corner hard, the inside rear wheel becomes unloaded, in extreme cases, it might be completely in the air.
Without a limited slip diff, when you put power down, all the power is transferred to the wheel which spins most freely. Result is that you get no power through the corner.
LSDs force power to both wheels, regardless of load which allows more speed through the corner.

[V88Dicky]

With an LSD and a V8 under the hood, one can make big number 11s on deserted highways....

[Ozzie Osmond]

But remember why the diff was fitted in the first place - so that the wheels could freely turn at different speeds on the same axle. By definition, an LSD limits their ability to do that and means the power applied will be trying to push the car in a straight line rather than round the corner. It can also make a car more twitchy because of an increased tendency for both wheels to break away at the same time. There are many different types of LSD, each with its own characteristics. Also the degree of "slip" permitted can vary.

LSD is a mixed blessing and its suitability will depend on exactly how you are using the car. If a car was having traction issues I'd start but making sure it had the best possible tyres rather than fretting about LSD.

PS LSD or a locking diff is obviously a good idea if you're going offroad. Or drifting. Or quite likely both at the same time!

[menousername]

that was a good explanation yes

how is having a LSD any different to standard traction control?

in my old e46, when I went round particular left hand corners, I would feel the lack of grip but the traction control would kick in and it would feel like the traction control was actually killing the power to the wheel that needed it most - power would just drop off

[paranoid airbag]

^ that's it, traction control is killing power to both wheels when it senses one has lost grip. An LSD is moving power from one wheel to the other.

[matt-ITR]

Traction control stops engine power electronically.
LSD diverts power mechanically

[Effortless]

menousername said:

LSDs on rear wheel drive cars

What do they do / offer than standard diffs dont, on normal every day driving, eg normal driving and the occasional thrashing as opposed to track day visits which I dont really do

Purely in terms of the drive, handling etc, what does the LSD do?

Do you think they are worth a mark-up on price on an otherwise good handling car? Would they help re-sale?

thanks!

Although I am by no means an expert, I'll try to answer your points in the order that you make them.

Not just on RWD cars. On front wheel drive, and four wheel drive cars. For example, most Renaultsport front drive cars have an LSD of some description, normally Torsens or ATB (torque sensing & automatic torque biasing). Some Subaru's have, not one, not two, but 3 LSD's ;-)

What they do is try to improve traction on the driven axle or axles when wheels might spin due to too much power for the conditions, or conditions of heavy weight transfer like cornering at speed.

As Caulkhead has described, torque going through an open diff to two driven wheels will take the line of least resistance. For example, in the case of a car cornering hard, weight transfer to the outside will mean that the wheels on the inside of each axle will have very different loadings to the ones on the outside of the axles. If the driver tries to accelerate hard in this condition, torque will take the line of least resistance, and will go through the wheel with least grip, the inside wheel, and will spin off any power. Therefore, until the weight equalises, the driver is unable to accelerate.

Similarly, even accelerating on a dry road in a straight line, if engine power overcomes traction, one wheel will spin off the power.

An LSD of some description will limit this.

So, in normal driving, an LSD will have little effect or impact on your driving. :-) If however, you habitually power out of wet roundabouts and find your inside wheel spinning all your power off all the time and your traction light blinking resolutely at you, an LSD will help you. :-)

This Wiki link might help... http://en.wikipedia.org/wiki/Limited-slip_differen...

And to complicate things, there are lots of different designs, and all have their pros and cons.

Viscous diffs, ATB's, TrueTraks, Detroit Lockers, Salisbury LSD, Electronic LSD's, they all do their own particular jobs!!

In terms of drive, handling, they can affect vehicle dynamics. They can, to differing extents, make the car more prone to understeer, although this will depend on the particular design of diff being used.

Are they worth it?? They are to me! I'm saving up to have Quaife ATB put in the back of my BMW. I find the DSC light flickers at me a lot when exiting roundabouts. :-)

HTH,
Effortless.

[matt-ITR]

Good animation/video that helps understand it here

[Welshbeef]

menousername said:

that was a good explanation yes

how is having a LSD any different to standard traction control?

in my old e46, when I went round particular left hand corners, I would feel the lack of grip but the traction control would kick in and it would feel like the traction control was actually killing the power to the wheel that needed it most - power would just drop off

Traction control reduces power once it detects slippage (simple terms) while a LSD you lose no power.

Say your going down a muddy field and try to go up an incline one half of the car on the slippery grass and mud as the other is on grippy Tarmac (and odd field).
If you didn't have a LSD be it fwd or rwd the driving wheel on the grass would spin up as a normal diff simply transfers the torque to the wheel of least resistance.
If you had LSD you would drive forwards both wheels rotating at the same rate


You can also have locked diffs which is for offload only or snow this if it was used on normal roads would cause issues when you come to a corner as clearly the inside wheel rotates less than the outside wheel.

[Ozzie Osmond]

menousername said:

how is having a LSD any different to standard traction control?

Some traction control systems just shut down engine power; others may apply partial braking instead or as well.

Many manufacturers now use the brakes as a form of LSD. I think I'm right in saying this includes both Lotus and Porsche. All you have to do is apply slight braking to the spinning wheel and everything behaves nicely. This is well within the capabilities of your ABS system and a modern computer.

[Effortless]

matt-ITR said:

Good animation/video that helps understand it here

Excellent vid. Thx for posting.

Effortless.

[cure]

I experience in my non-LSD RWD car (primarily in the wet) that I can only 'catch' the oversteer and not hold it there making it a more fluent event. I write this down to not having a LSD. Is this correct?

[hufggfg]

cure said:

I experience in my non-LSD RWD car (primarily in the wet) that I can only 'catch' the oversteer and not hold it there making it a more fluent event. I write this down to not having a LSD. Is this correct?

That is likely correct, not having an LSD will mean that as the inside wheel starts to spin, all the power will be spun up through that wheel, rather than both wheels spinning and resulting in power oversteer, aka drift.

[menousername]

[quote=Effortless]

So, in normal driving, an LSD will have little effect or impact on your driving. :-) If however, you habitually power out of wet roundabouts and find your inside wheel spinning all your power off all the time and your traction light blinking resolutely at you, an LSD will help you. :-)

quote]

this sums it up perfectly - im not an offroader, most times drive reasonably sensible, but often when stuck behind ditherers I like to power out around them, or out-break them on double-approach roundabouts on the inside lane and then power out the other side... all safe of course but just to get ahead when the opportunity is there

I do tend to pull out into main roads alot where I live and need to get the power down quick


ok two more questions pls guys

1. lsd performance in the snow? If its sending power to wheels thats a good thing surely?

2. modern hatches with 4wds of some sort,eg golf r32 or audi s3, I would assume they do not suffer the loss of traction that would require a lsd (I equally assume they dont have them already)

[OllieC]

you can write 11's on the tarmac, rather than 1's