Its not so much about limiting the spin on the wheel without grip, its more about keeping the power going to the wheel with grip. A car without an LSD will spin the inside wheel as the load comes off in a corner and this takes the power away from the outer"driving" wheel. An LSD keeps as much power going to both wheels as possible without the driveshafts actully being welded solid i.e. Stockcar racers. Maybe.

 

LSDs mean you can do doughnuts

There's '3' stages of differential, Locked, Open and LSD

Locked: both wheels turning at the same rate. Great for traction except you won't be able to go round corners well! (bear in mind the wheels at each end of the axle turn a different amount when cornering). Locked diffs only really used on landrovers and suchlike and are usually turned off when on the road.

Open: Wheels at each end of axle can turn at different rates so you can go round corners. Traction can be lost as the power can only be delivered as much as the wheels with the least traction. For example, put an open diff'd car on the road with one wheel on sheet ice with zero grip and the other on tarmac then the car won't be going anywhere.

LSD - in between open and locked, i.e. a certain amount of 'slip' is allowed without losing all drive. This, in effect, means you can apply power and drive isn't lost (like with a locked diff) but you can still go round corners (like with open diff).

Apart from on the farm with your landy, LSDs are always better. The 'only' reason a car doesn't have one is due to cost (or in the case of the cayman, to stop it being faster than a 911! absurd really!) hence most performance cars worth their salt have LSDs. Many 'bigger' or expensive cars have them too. e.g. I think my 740 has one too.

To summarise

Locked - no power lost but can't corner well
Open - power lost (potentially all of it) but can go round corners
LSD - minimal power lost and can go round corners.

 

But if you turn in without power or on the brakes, then the LSD isn't doing anything.

Problem is, you can't go switch it off, so you can't try it with and without to see what it does.

As for the Elise, I think what they're saying is that without an LSD if you plant the throttle you'll spin just one wheel but the other wheel will still be giving you some lateral grip, so you won't get too much oversteer. With an LSD if you plant the throttle you'll spin both rear wheels, so no lateral grip, and round you go. I think that's why they're saying you need TC with an LSD.

 

That's the closest so far on this thread.

An LSD is of most use on-track, where getting as much power down as early as possible is desirable. It acts as a mechanical means of apportioning power where it's most needed when the two driven wheels have different levels of grip.

I don't believe it will have any real affect on turn-in (not rwd, anyway - fwd in the 'teg it was very effective), as both driven wheels will be rotating at the same speed.

Once INTO a corner, however, the inside wheel will want to spin-up if the same power is applied as the outside wheel, because the inside wheel has a shorter distance to travel. Therefore a method of applying more power to the outside wheel will prevent loss of grip on the inside wheel (and thus more lateral grip is available, hence higher cornering speed), and will give an asymmetric 'thrust' to the rear of the car, helping 'push' the car around the corner. So a good LSD increases cornering speeds through both grip and power.

HOWEVER. Because more power is available to the driven wheels, at the limit it will be easier for BOTH wheels to lose traction (without it, the inside one spins the excess power away unless you're being very rough with the throttle). If both wheels lose traction under power, you enter power-oversteer. It is the LSD (without TC) which is partly responsible for the S2000's snappy rear-end!

Sorry for the long post, hope this helps!

 

I wonder why they didn't put it in the elise as standard.

 

Ah,

Thanks havoc_uk. This seems to make a lot of sense. The turn in thing was bothering me a bit. I agree 100% that either RWD or the LSD (or the combo) certainly allows the back to come around quickly IN the corner, but I couldn't see how the LSD on a RWD would help with initial turn in.

The reason for needing TC is clear with the explanation that total loss of rear in grip will come along faster with an LSD than without, thus oversteer, which a TC will fix 4 you.

Case seems closed.

 

Probably a bit too hi-tech for the Norfolk folk who make them.

 

SMall point, but we don't have an actual LSD. We have a Torsen diff which is a torque sharing differential. So if one wheel is off the ground there is no drive to the other. But if one wheel starts to loose grip but still has some track X (where X is the setup ratio) times the torque goes to the other wheel.

http://auto.howstuffworks.com/differential10.htm

 

Sorry Aus, but a Torsen IS a Limited-Slip Differential. The other two types of LSD are viscous-coupling, and a clutch-plate style version (the oldest and simplest, fitted to old Esky's and Lotus Sunbeams).

The difference you're referring to is with a LOCKING differential, which is found on Land Rovers and their ilk. Can't see an S2000 cocking a rear-wheel, to be honest!!!

 

Yes, I beleive it is true to say that a Trosen is a genuine LSD - just another type.

I did manage to get one rear wheel off the ground once - it was one of those bloody deep drains that we have so many of between a minor road and a major one. Went over it diagonally so the front won't get scraped up as much (it will always get scraped up here, and I am stock ride height!) and managed to get one wheel off the ground in the rear. Car would not move until I pulled up the e-brake so that some torque would go to the wheel on the ground.