can any of you knowledgable backroomers help with a 'discussion' that has started on another messageboard I frequent. You may have to excuse the simplicity of the arguement as it is a BMW forum....
The discussion revolves around how LSD actually works and I'll remove the names to avoid any embarrasment (I'll let you know which one I am after you've ruled in my favour!)
Mr A:
Hello.
Having just bought a 325i se, i can say that i am very very happy with it( having come from a series 1 rst) - my only problem with it is that it is very tail happy in the dry but very scary in the wet. ive found out that this is due to it not having a limited slip diff.
Mr B:
A Limited Slip Diff will make the back end go out more!
LSD's allow both rear wheels do spin at the same speed instead of the wheel with the least grip spinning and the other not spinning. It was a sort of early traction control device, the side effect is making the car go sideways easier.
Mr A:
limited slipp diff means limited slip i.e less slide!!!
1 wheel actually spins quicker to compensate!!
Mr B:
Limited Slipp Diff means limited slip ie between the two wheels, both wheels recieve the same power as oppose to a normal diff where the wheel with the least grip spins up. If you where to go round a roundabout with a normal diff and hit the power the inside(right hand) wheel would spin and the other wouldn't, however in a car with an LSD, both rear wheels would spin causing the back to go out a lot easier than a car with a standard diff would.
Mr C:
I think you're wrong there Mr B - what Mr A has said is true, LSD actually allows the wheel that has the most grip to have more power (apply more torque), taking it away from the spinning wheel, giving the car more traction and therefore less slide.
For example, if you hit ice with one wheel, it would spin and the LSD would provide less power to the spinning wheel, transferring it to the wheel with more grip, allowing you to control the car better.
What would be the point of putting such a device on a sports car if it made cornering at high speeds less controlable?
try this link....
www.howstuffworks.com/differential6.htm
Mr B:
What i've been told by various rear-drive veterans is that the LSD is only put on sporty models ie 325i Sports, Sapphire Cossies, Corrola GT-Twincams etc. It is what is required to do proper donuts an go sideways in most conditions.
With a non-lsd car when that inside wheel starts spinning you cannot propel the car round the corner any faster. With an LSD equpped car the outside wheel still drives the car round the corner allowing faster cornering "on the ragged edge" it is a finer line between cornering fast and sliding with an lsd therefore it is suited to sporty drivers that can handle it.
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i take it youre mr b ?
certainly b has the closest to the better understanding
Limited slip or power lock as the USA calls it does indeed limit the difference in speed of the two driven wheels on one axle, which allows better driving in slippy conditions
- whether this will always make back end more prone to sliding out depends on setup etc
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Cars need a differential because when they go round a corner the inside wheel follows a shorter path than the outer wheel. Therefore they have to be able to turn at different speeds and the differential allows this.
The down side of this is that if the level of grip is different between the wheels, eg one on dry tarmac, one in mud, the maximum amount of drive which can be applied is that which will cause the wheel with the least grip, in this case the one in the mud, to slip.
When that happens virtually all the drive to the wheel with grip is lost.
This could be counteracted by a having a simple crown wheel and pinion arrangement or locked diff. In this case both wheels must turn at the same speed but the downside of this is it makes for difficult cornering in normal driving situations and places extra stress on components. Eg in a rwd car with a locked diff you can get mega understeer.
Therefore limited slip diffs have been used with the idea that a certain amount of slippage is allowed, but the intention is that a significant amount of power can still get through to the wheel with the best grip.
The traditional type of slippy diff is the clutch type. A series of friction plates or clutches are pressed together by spring pressure. The amount of pressure is determined by the degree of torque applied when you build the diff, ie more torque=more pressure=more friction. The purpose of these clutches is to prevent one wheel turning faster than the other, ie establish a locked diff situation. In order for one wheel to turn faster than the other then it must overcome the friction of the clutches. So in the situation with one wheel on tarmac one in mud, you can still supply enough power to the wheel on the tarmac equivelant to the amount of power it would take to cause the clutches to slip so you can still drive forward but with reduced power.
You can also have a viscous coupling. Here the ?locking? mechanism is a special silicone fluid which sits between two sets of impellors, each set connected to one drive shaft. When one shaft starts to turn faster than the other then the fluid alters its properties to prevent shear effects and thus begins to transfer more power to the other drive shaft. Personally I don?t like this as it needs one wheel to slip before any torque is transferred to the skidding wheel.
The torsen diff is a very clever design, which again relies on one wheel to slip before it acts. When it detects slippage its gears lock and the torque split is determined by the set up of the diff. So if you had a diff set up of say 4:1, however much torque you could apply to the wheel in the mud, the diff would automatically supply 4 times that amount of torque to the wheel on the tarmac. So if one wheel were on ice for example, whilst you could get four times the power through to the other wheel it still would not be much. With the clutch diff you could still apply power equivalent to the pre-torque value, and it does not need one wheel to slip before it works which is the case with both viscous and Torsen diffs.
Personally I prefer the clutch type even though they need more maintenance but thats just me.
You also get situations with one wheel having more grip than the other in loads of situations, hard cornering is one with the inside wheel having less grip due to weight transfer.
The downside of slippy diffs is the stability factor mostly seen in very slippery conditions or if someone is driving like a total knob without the skill to back it up. With a normal diff, as soon as one wheel spins, drive to the other is almost lost, therefore all the tyre on that wheel has to do is provide lateral grip.
With a slippy diff whilst the driver is less likely to lose traction because of the action of the diff, very often especially with poor driver ability or if very slippery conditions exist both wheels lose traction more or less together and then of course lateral stability is also lost.
Moral understand what sort of diff you have, how it works and therefore what it can and cannot do. Then apply that knowledge accordingly.
So to sum up the verdict of this juror is:-
Mr B = probably the best understanding
Mr C = only correct in so far as applicable to Torsen diffs I would say
(admit pedant mode set = on level = nitpicker)
Mr A = Richard Head Esq (little hope I?m afraid )
PS tell Mr A to turn his front fogs off ;-)
Hope that helps.
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Bit off-topic, but the differential was invented by John Starley, maker of the first 'safety cycles' that carried the brand name Rover .. and we know where that led!
Starley overturned a tricycle when going round a tight bend at speed and he ended up down a ditch in a patch of nettles. He realised that the cause was the two rear wheels rotating at the same speed and so he sat down (not in the nettles!) to design a way to allow power to both wheels whilst letting one turn faster than the other.
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The original Daf's didn't have a diff.Each back wheel was driven at the same speed by it's own drive belt. Negotiating a bend or roundabout in slippery conditions could be a bit hair raising as the car just wanted to go straight on - and often did if the front tyres lost grip. Another disadvantage was the strain placed on the transmission. Trying to pull away on a lock often resulted in a burnt out clutch or snapped half shaft. Rear tyres scrubbed out in a few thousand miles.
As a matter of interest - or not - railway wheels have a sort of diff built into the profile of the 'tyre'. They're tapered and each wheel on a fixed axle has a variable diameter. On a bend the axle simply settles into the 'best' place. Doesn't work too well on tight bends though, hence all the screeching and squealing in a marshalling yard.
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I will certainly tell Mr A to turn off those fogs!!
Cheers guys!
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Hello!
All these "A"s and "B"s and theorizing can be confusing, so, practical experience....with RWD of course....
Without LSD - Too much welly in the corner, wheels start to break traction, inside wheel with less weight on it spins, torque is lost to both wheels, outside wheel grips and control maintained quite easily - but it's SLOW.
With LSB - too much welly, both wheels break traction, tail goes, and you are through the hedge backwards if you do not catch it. BUT IF YOU DO catch it, full torque to both wheels and you are out of the corner PDQ!
That's how a Frazer-Nash (and GN before it) worked!
Tomo
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It might also be useful to know that mechanical LSDs are torque activated, whereas viscous ones are (differential) speed activated. Most diffs fitted to road cars come with a slip factor in the range 25 to 40% - the lower the %age the more "digital" it is ie cuts in easier. The more digital it is, the harder it is to tame and more likely to "bite back".
When a friend and I ran a MkII Escort rally car the LSD was set up extremely tight, with only about 15% slip - you won't be surprised to learn that progress was often very sideways. Once used to it, rapid through-corner progress was easy and enabled us to challenge much "faster" machines.
Nowadays I drive a Rover 820 Vitesse Sport which has a Torsen diff. Of the Rovers with a Torsen this model has the most digital set up (and the closest ratio 'box). When overtaking on a heavily cambered and bumpy / wet road in second it's essential to keep both hands on the steering else you'll end up in the right hand hedge. I have found though that with much better tyres fitted (Goodyear F1s ilo Uniroyals) it is a lot less vicious. Personally I love it and find that it allows great acceleration in tricky conditions that would leave other fwd cars trailing. It's also great for take off - this is a heavy car and so needs the front wheels locking together for a fast take off - with 200PS it's easy to spin the front wheels. For this reason I find it hard to understand Clarksons critisism of the Focus RS for having the same traits - being lighter the Focus isn't so desperate for the traction at standstill, but like the Rover it will have tremendous traction when the loud pedal is activated on the move. Perhaps he doesn't like having to "fight" with the steering - I don't find it that intimidating and appreciate the input instead of the usual "dead" power steering.
I have no preference for viscous or mechanical LSDs, but given a free choice I'd sooner have RWD with an LSD (rather than without) and a fairly digital set up.
One major advantage of viscous LSDs over the clutch type is that there is no clutch to wear out, thus improved reliability from the customers point of view - clutch relacement on mechanical LSDs tends to be very expensive. Hence their popularity with OEMs. Oh, cheaper to fit too.
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How easy is it to install LSD on a FWD car without it?
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I would imagine it's a straight gearbox/diff unit swap. Escort RS Turbo (with LSD) vs. Escort XR3i (without) springs to mind.
Although there are very few FWD cars with enough power to -need- a LSD that don't already have one... Correct me if I'm wrong, but there tends to be a limit of about 160bhp on FWD cars without some form of traction control already fitted.
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Aye Dave that's correct. I already run a 150bhp Volvo 480turbo which is awaiting a race built/mod head/manifold set up. It will most likely be quite unwieldy after that. LSD is one of the things l'm considering. Hell it may already have it - l never considered checking, assuming it didn't.
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Just had a little rummage round Google, apparently a LSD was never fitted to the 480, but there was a TRACS traction control system offered as an option on all 480s, see if you've got a warning light! There was something mentioned about how the whole 480 Turbo engine and gearbox was designed to eliminate torque steer, and how it pulls clean and true like no other car today! Hmmm.
Hth,
Dave.
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Yeah apparently it has some gimmick that gives even drive to both wheels (sounds like fixed dif to me!) but having cranked the boost up torque steer appears to be something of a problem regardless.
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Oh that traction control thing just stops the wheels spinning when pulling away by applying the front brakes. Not exactly sophisticated!
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It was the very latest technology in 1989!
Seriously, I don't think they made the 1721 engine and gearbox with an LSD in any of its applications.
Whether you could convert one from another Volvo (or Renault?) application would be another matter entirely!
Just had a look on here:
www.quaife.co.uk/
They only have them for the 240, S60 and Amazon though.
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