Open diffs, locked diffs, limited slip etc
Discussion
After all these years I am going to have to admit that I am not sure how these all work and what all the different ones are. Would someone explain to me in simple terms 
?
My 1998 BMW 323i leaves two black marks from the rear tyres when I wheel spin in a straight line yet my 2000 Mercury Grand Marquis just leaves one black line from the rear passenger side tyre when I do the same.
What does that say about my two different diffs? Is my big V8 a single rear wheel drive car?
?My 1998 BMW 323i leaves two black marks from the rear tyres when I wheel spin in a straight line yet my 2000 Mercury Grand Marquis just leaves one black line from the rear passenger side tyre when I do the same.
What does that say about my two different diffs? Is my big V8 a single rear wheel drive car?
Have a read/watch of this: http://auto.howstuffworks.com/differential.htm
Your Mercury has an open diff - it can't control the split of torque between the wheels, so as soon as one loses traction, that wheel will start to spin, all the torque going to the diff will be directed to that wheel.
The BMW acts as if it has a limited slip diff - it can control the split of torque, so when one wheel starts to lose traction, the diff forces torque to go to both wheels so that it doesn't all get wasted.
I say "acts as if" not "has" as I don't think E46 323is came with LSDs.
Cars with open diffs don't always end up spinning one wheel, it is perfectly possible for them to spin both rear wheels as long as they both have the same amount of grip. Does the Mercury have different rear tyres? Are they at different pressures? Was the (private
) road you were doing this on damp on one side but not the other?
Your Mercury has an open diff - it can't control the split of torque between the wheels, so as soon as one loses traction, that wheel will start to spin, all the torque going to the diff will be directed to that wheel.
The BMW acts as if it has a limited slip diff - it can control the split of torque, so when one wheel starts to lose traction, the diff forces torque to go to both wheels so that it doesn't all get wasted.
I say "acts as if" not "has" as I don't think E46 323is came with LSDs.
Cars with open diffs don't always end up spinning one wheel, it is perfectly possible for them to spin both rear wheels as long as they both have the same amount of grip. Does the Mercury have different rear tyres? Are they at different pressures? Was the (private
) road you were doing this on damp on one side but not the other? Edited by rsox87 on Thursday 15th January 16:39
rsox87 said:
Your Mercury has an open diff - it can't control the split of torque between the wheels, so as soon as one loses traction, that wheel will start to spin, all the torque going to the diff will be directed to that wheel.
Your car doesn't become one wheel drive, but both wheels deliver the same "drive", so if one has limited grip, the other cannot use the addition grip it may have.
In practice most open diffs will spin one wheel. When one wheel starts to spin, its "grip" is reduced, reducing the torque it can transmit. Therefore the spinning wheel limits the torque to the non-spinning wheel, so there is insufficient torque to break traction. (assuming a fully open diff, without too much internal frcition)
Edited by balls-out on Thursday 15th January 16:54
rsox87 said:
Have a read/watch of this: http://auto.howstuffworks.com/differential.htm
Your Mercury has an open diff - it can't control the split of torque between the wheels, so as soon as one loses traction, that wheel will start to spin, all the torque going to the diff will be directed to that wheel.
The BMW acts as if it has a limited slip diff - it can control the split of torque, so when one wheel starts to lose traction, the diff forces torque to go to both wheels so that it doesn't all get wasted.
I say "acts as if" not "has" as I don't think E46 323is came with LSDs.
Cars with open diffs don't always end up spinning one wheel, it is perfectly possible for them to spin both rear wheels as long as they both have the same amount of grip. Does the Mercury have different rear tyres? Are they at different pressures? Was the (private ) road you were doing this on damp on one side but not the other?
My Beemer is even older, an E36. The Mercury has the same tyres and always spins the passenger side. Someone on the Yank forum called the Crown Vics/Mercurys and Lincolns 'Peg Legs' Your Mercury has an open diff - it can't control the split of torque between the wheels, so as soon as one loses traction, that wheel will start to spin, all the torque going to the diff will be directed to that wheel.
The BMW acts as if it has a limited slip diff - it can control the split of torque, so when one wheel starts to lose traction, the diff forces torque to go to both wheels so that it doesn't all get wasted.
I say "acts as if" not "has" as I don't think E46 323is came with LSDs.
Cars with open diffs don't always end up spinning one wheel, it is perfectly possible for them to spin both rear wheels as long as they both have the same amount of grip. Does the Mercury have different rear tyres? Are they at different pressures? Was the (private
) road you were doing this on damp on one side but not the other? Edited by rsox87 on Thursday 15th January 16:39
croyde said:
My Beemer is even older, an E36. The Mercury has the same tyres and always spins the passenger side. Someone on the Yank forum called the Crown Vics/Mercurys and Lincolns 'Peg Legs'
Does the Mercury have a live axle? If so then it will definitely light up light up the same wheel. The drive shalf will be trying to 'twist' the diff and therefore entire axle (like the Mercury?). On a car with a diff mounted to the body(Like the BMW), this has no effect, but when the diff is on the live axle, its the difference in 'weight' on the two wheels that has to resist this torque.Edited by balls-out on Thursday 15th January 17:08
Sounds like the BMW has a slippy diff and the yank tank has an open one. It's very rare to get a 323i with a LSD as standard though, I think it was part of the winter pack iirc.
Welded diffs are fun if you don't mind eating tyres, looking like a plank when maneuvering slowly or going sideways 90% of the time.
Welded diffs are fun if you don't mind eating tyres, looking like a plank when maneuvering slowly or going sideways 90% of the time.
croyde said:
rsox87 said:
Have a read/watch of this: http://auto.howstuffworks.com/differential.htm
Your Mercury has an open diff - it can't control the split of torque between the wheels, so as soon as one loses traction, that wheel will start to spin, all the torque going to the diff will be directed to that wheel.
The BMW acts as if it has a limited slip diff - it can control the split of torque, so when one wheel starts to lose traction, the diff forces torque to go to both wheels so that it doesn't all get wasted.
I say "acts as if" not "has" as I don't think E46 323is came with LSDs.
Cars with open diffs don't always end up spinning one wheel, it is perfectly possible for them to spin both rear wheels as long as they both have the same amount of grip. Does the Mercury have different rear tyres? Are they at different pressures? Was the (private ) road you were doing this on damp on one side but not the other?
My Beemer is even older, an E36. The Mercury has the same tyres and always spins the passenger side. Someone on the Yank forum called the Crown Vics/Mercurys and Lincolns 'Peg Legs' Your Mercury has an open diff - it can't control the split of torque between the wheels, so as soon as one loses traction, that wheel will start to spin, all the torque going to the diff will be directed to that wheel.
The BMW acts as if it has a limited slip diff - it can control the split of torque, so when one wheel starts to lose traction, the diff forces torque to go to both wheels so that it doesn't all get wasted.
I say "acts as if" not "has" as I don't think E46 323is came with LSDs.
Cars with open diffs don't always end up spinning one wheel, it is perfectly possible for them to spin both rear wheels as long as they both have the same amount of grip. Does the Mercury have different rear tyres? Are they at different pressures? Was the (private
) road you were doing this on damp on one side but not the other? Edited by rsox87 on Thursday 15th January 16:39
By 1998 e36 bmw's didn't come with LSDs (apart from the M3 Evo) but I believe the traction control could simulate one by braking the spinning wheel - this, along with the stiffer suspension of the beemer - probably causes the smoking elevens!
Dannbodge said:
Sounds like the BMW has a slippy diff and the yank tank has an open one.
Not wanting to start an argument but why call a LSD a slippy diff? If it only allows a limited amount of slip then it is far from a slippy diff, that would be an open diff which allows an unlimited amount of slip.Just one of the phrases which annoys me.
jkh112 said:
Not wanting to start an argument but why call a LSD a slippy diff? If it only allows a limited amount of slip then it is far from a slippy diff, that would be an open diff which allows an unlimited amount of slip.
Just one of the phrases which annoys me.
Do you also get annoyed by the word inflammable ? Just one of the phrases which annoys me.
jkh112 said:
Not wanting to start an argument but why call a LSD a slippy diff? If it only allows a limited amount of slip then it is far from a slippy diff, that would be an open diff which allows an unlimited amount of slip.
Just one of the phrases which annoys me.
Because Clarkson said so, I believe. Just one of the phrases which annoys me.
It's a completely stupid term, even by his standards since it says exactly the opposite of what it actually does. An open diff is obviously the slippiest kind.
kambites said:
Because Clarkson said so, I believe.
It's a completely stupid term, even by his standards since it says exactly the opposite of what it actually does. An open diff is obviously the slippiest kind.
It's worse than that, actually. There is no such word as 'slippy' in the English language. It's a made up word by mongs and retards. People actually mean slippery.It's a completely stupid term, even by his standards since it says exactly the opposite of what it actually does. An open diff is obviously the slippiest kind.
Saying slippy diff is the same as saying bungletownbadgersniffler diff.
benjj said:
It's worse than that, actually. There is no such word as 'slippy' in the English language. It's a made up word by mongs and retards. People actually mean slippery.
If you really want to be pedantic, try looking up slippy in a dictionary, it's definitely in there and has been for a very long time.croyde said:
.....leaves two black marks from the rear tyres when I wheel spin in a straight line
The yanks call that a "two wheel peel" or "elevens". You and I don't need to know the mechanics, you just need to know that your new car can can do that! Now you just need to do a "brake stand" to make the most of the smoke screen!
One other thing to bear in mind about LSDs is that any tendency to avoid letting the wheels rotate at different speeds is going to cause the car to attempt to continue in a straight line; this is why LSDs are said to create understeer, especially in tight low-speed corners (ie those where the amount of slip required is greatest). Different methods of limiting the slip have this problem to greater or lesser degrees. Electronic diffs can avoid this problem entirely by controlling wheel speeds in different ways depending on steering angle.
kambites said:
One other thing to bear in mind about LSDs is that any tendency to avoid letting the wheels rotate at different speeds is going to cause the car to attempt to continue in a straight line; this is why LSDs are said to create understeer, especially in tight low-speed corners (ie those where the amount of slip required is greatest).
This problem is often greatly exaggerated, and only likely to exhibit itself on very tight hairpins. Having stated that, an LSD will still have a net benefit on almost every circuit or road, hence why all serious sports and racing cars have them.Olivera said:
This problem is often greatly exaggerated, and only likely to exhibit itself on very tight hairpins. Having stated that, an LSD will still have a net benefit on almost every circuit or road, hence why all serious sports and racing cars have them.
Indeed. Never had any problems like than with mine.....can't say I'd fancy driving a "locker/welded" around a sharp bend though!!
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