Explain LSDs to me please?
 

Not the mind mending drug of course:D

I have a rough idea of how the lsd works but am confused by its effect on traction. Some say it improves it however some say that in the wet a rwd car with a lsd will let the back out more than if the same car didn't have a lsd.

Could anybody clarify this for me please?

Thanks

it transfer the power to the wheel that is getting power, its mainly for RWD as when one wheel cocks its leg the other wheel will realise it needs power so it will make sure that wheel has it :) If the car doesn't have LSD both wheels will have equal amount of power in tripod mode.

I have a 5 series and it has an fake lsd? I think its DTC where the car uses the computer to work it out?

Torsen LSD is best IMO. Like above and will give you better grip with minimal wheel spin hence more traction.

Now the Quattro audi later models have a nice trick LSD 60/40 split then split more power to whichever wheel is needed, have still yet to make it wheel spin.

Your not trying hard enough, I'm sure a full launch on road tyres would sort that out :D

Buy this months Evo magazine, there is an article explaining what LSD's do and the difference between the various types.

I HAVE. Done a full launch out of a corner a it just loaded all the power to the heavier side (t3 diff, from the b7 RS4) :D

lol

I thought you meant what I had on my old Smart Roadster
http://i37.photobucket.com/albums/e6...a/IMG_0983.jpg
http://i37.photobucket.com/albums/e6...a/IMG_0914.jpg

There is an overlay with traction/stability control that needs to be considered, and also whether the diff is electronically controlled.

In other words, you can have a mechanical setup that is often obscured by the electronics, more so on models that aren't performance orientated.

In two cars with all the systems disabled, otherwise identical apart from one having an open rear diff and the other a mechanical LSD, the car with the open diff will typically spin the unloaded inside rear wheel and paint a single line and not tend to oversteer so much, whereas the car with the LSD will transfer more torque to the outside rear wheel, paint 11s, oversteer more, but also develop more forward acceleration as the available grip from both rear tyres is utilised.

The enthusiast of average ability will usually want a powerful RWD or rear biased AWD car to have an LSD with electronics that allow it to come into play rather than immediately killing engine torque as it feels better, gives greater acceleration in many grip limited situations and without being scary can give a mild but amusing tail led stance.

2WD cars without LSDs should really be considered 1WD where it matters.

Thanks, it's a mechanical lsd that I'm talking about btw.

So in general a sporty powerful rwd car (with modern stability/traction controls) is better off with a lsd?

Thanks

Yes. A "torque vectoring" or electronically controlled diff can be even more handy as it can take account of throttle, gear, speed, steering angle, yaw rate etc. This can slacken off the diff at parking speeds to stop it binding and making little skids like the GTR does for example where the diff is too tight. Centre diff in the GTR is electronically controlled (quite like DCCD in a Subaru but more RWD bias), rear diff is LSD but dumb, front diff is open.

Thanks John.

I always thought that AWD is another way of saying LSD 4WD, im I right or wrong?

as John has pointed out, open diffs mean all drive will go to the wheel with least traction

so when push come to shove - an AWD car with three open diffs is one wheel drive

LSD is better....with a better driver. But not always.

The thing is with LSDs is they vary in "lock up" or slip ratio, so they can be very different in how they react, from one specification of car to another

For example: some LSDs can be quite gradual and allow a fair amount of slip before transmitting power to the other wheel. The Dana plated LSD on my Jag is like this...you have to be really aggressive to get it the tail-out, yet it has enough bite to allow both wheels to give plenty of traction when hoofing it, so its pretty easy to drive in varying conditions...it won't do doughnuts easily either. In contrast the opposite can happen; My Monaro, had a Hyrdratrac/ATB viscous LSD that was much less forgiving...Just pulling away gently from a junction would have the inside wheel squealing like a pig. It was a utter nightmare in the wet with less than half a tank of fuel; It always wanted to go sideways.

All AWD scoobs had a LSD on the rear, and generally speaking both the viscous and plated types are fairly benign to the point you'd struggle to tell it had one unless you are really hammering it. My NSX was pretty much the same in this respect unless ambient temps were below 8degrees with cold tyres (OEM spec Bridgestones simply don't work when its cold - wet or dry! ).

So it doesn't always mean perfect traction or better handling; I've driven 911s without a rear LSDs, some 1980's one, and they have plenty of traction in the wet; They simply didn't need one. A LSD would probably have made them lethal owing to their somewhat "unforgiving tail-happy at the limit" reputation.


The problem with eDiffs (or virtual LSDs) whereby the brakes are used to act like a LSD is that it is a reactive system as opposed to a active system. Its ability to react in proportion to the grip and load is fully dependent on programming, sensors and actuators. As well as what setting the driver has selected (which almost always is the default of..play it safe, reduce power, and allow some inner wheel slip). Its much more capable than any mechanical system, but more often than not its not set in teh correct mode for the conditions or for what the driver wants...

Case in point example...a modern run of the mill BMW trying to get up snow covered incline. They CAN do it....you just need to set it right and drive it right.

Thanks Ali, very good explanation.