Open diff operation with DSC

[dbruce]

Complaints abound about the lower end 5 series not having an LSD and being stuck with an Open Diff.

After doing a bit of research, the Open Diff DSC combination can work as well as some LSD applications.

By design, an Open Diff applies more torque to the wheel with less traction. In a vehicle with no DSC, this is bad since you can be stuck in a situation with one wheel spinning on ice, and the other sitting still on asphalt:

http://auto.howstuffworks.com/differential2.htm

With DSC, the equation changed significantly. Once a wheel spins, DSC applies braking to that wheel which causes the open diff to transfer torque to the the other wheel, since it has less grip. One might even say the open diff works better in combination with the electronic nanny's in an Open Diff setup....since a diff like a quaife or Torsen may fight the electronic nanny's.........but with the right software......I'm sure it's an improvement (M5 etc).

http://classes.engr.arizona.edu/engr498a/2006/fsaediff/resources/techart3.pdf

I used to think the Open Diff was a poor choice by BMW........but after some research......with DSC (ASC will not work in this manner)...the solution is sound and could easily rival an LSD car without the electronics.

[chromius]

ASC does exactly the same thing.

While it's a good theory, it really doesn't work well in practice. The DSC is too slow to react and the torque transfer is way too abrupt to make it anywhere near as stable and controlable as a true limited slip diff. Hence why the M5 has DSC as well as a true limited slip.

Drive an open diff car with DSC next to a car with a true limited slip and you will soon see there is no comparison to the real deal.

[NNY528I]

this is of course all academic as it really only matters when you are frying tires. LSCs are not a miracle solution to anything, they can actually be quite dangerous under certain conditions( IE fresh puddles in my dakota + turn= 180 degree powerslides= new undies) For a performance car there are some advantages(like when you are frying tires), but as is noted the M5 has DSC to help eliminate the inherent problems(like uncontrolled wheel spin in loose conditions)

[spaghettininja]

ASC does exactly the same thing.

While it's a good theory, it really doesn't work well in practice. The DSC is too slow to react and the torque transfer is way too abrupt to make it anywhere near as stable and controlable as a true limited slip diff. Hence why the M5 has DSC as well as a true limited slip.

Drive an open diff car with DSC next to a car with a true limited slip and you will soon see there is no comparison to the real deal.

What he said.

[dbruce]

ASC does exactly the same thing.

While it's a good theory, it really doesn't work well in practice. The DSC is too slow to react and the torque transfer is way too abrupt to make it anywhere near as stable and controlable as a true limited slip diff. Hence why the M5 has DSC as well as a true limited slip.

Drive an open diff car with DSC next to a car with a true limited slip and you will soon see there is no comparison to the real deal.


ASC does not do the same thing. ASC is traction control (engine power cut) while DSC is traction control and braking to each wheel independently (you can also cut the ASC part out of DSC by holding the the DSC button for 10 seconds).

DSC is anything but slow. Once braking is applied........torque transfer should be instantaneous and possibly faster then a standard LSD if done right.


like uncontrolled wheel spin in loose conditions



An Open Diff with DSC will control wheel spin. Is it better or worse then the average LSD?........kind of hard to test without having the same car with each setup. With DSC, your rear wheels will not just spin like crazy once DSC brakes the spinning wheel....torque will transfer to the other wheel.

Without DSC, an Open Diff car on ice will use only one wheel when there is limited traction and spin that wheel like crazy. With DSC, braking will allow torque transfer (which really won't matter in a 0 traction environment :D) and essentially emulate an LSD.

Why the M5 used an LSD probably has something to do with the extra power of the car.....but that's just a guess. If done right........an LSD or an Open Diff with DSC should work well for allowing torque transfer (I believe BMW's 4 wheel drive system is an Open diff on each axle with DSC performing torque transfer.....this is independent of the transfer case between axles).

I have installed a Torsen in another car.....but it had an Open Diff before with no electronic nanny's.......so it was a great improvement. With the ability to transfer torque from a spinning wheel....the installation of the Torsen would not have been needed.

[ViolinARC]

ASC does not do the same thing. ASC is traction control (engine power cut) while DSC is traction control and braking to each wheel independently (you can also cut the ASC part out of DSC by holding the the DSC button for 10 seconds).

DSC is anything but slow. Once braking is applied........torque transfer should be instantaneous and possibly faster then a standard LSD if done right.





An Open Diff with DSC will control wheel spin. Is it better or worse then the average LSD?........kind of hard to test without having the same car with each setup. With DSC, your rear wheels will not just spin like crazy once DSC brakes the spinning wheel....torque will transfer to the other wheel.

Without DSC, an Open Diff car on ice will use only one wheel when there is limited traction and spin that wheel like crazy. With DSC, braking will allow torque transfer (which really won't matter in a 0 traction environment :D) and essentially emulate an LSD.

Why the M5 used an LSD probably has something to do with the extra power of the car.....but that's just a guess. If done right........an LSD or an Open Diff with DSC should work well for allowing torque transfer (I believe BMW's 4 wheel drive system is an Open diff on each axle with DSC performing torque transfer.....this is independent of the transfer case between axles).

I have installed a Torsen in another car.....but it had an Open Diff before with no electronic nanny's.......so it was a great improvement. With the ability to transfer torque from a spinning wheel....the installation of the Torsen would not have been needed.
+1 :drink1 :urtheman

[chromius]

ASC does not do the same thing. ASC is traction control (engine power cut) while DSC is traction control and braking to each wheel independently (you can also cut the ASC part out of DSC by holding the the DSC button for 10 seconds).

DSC is anything but slow. Once braking is applied........torque transfer should be instantaneous and possibly faster then a standard LSD if done right.


ASC DOES do the same thing. It applies braking to the drive wheels in exactly the same way that DSC does. DSC simply adds the front wheel independent braking, along with added sensors such as steering angle and yaw rate.

DSC is slower than an LSD, not slower in respect to transfering the torque after braking because that's simply a inherent property of an open diff. Where it is slower is in applying the brakes. The wheel has to slip first before the computer registers that it needs to apply the brakes. At which point it applies the brakes in a fashion that is neither smooth nor controlable except by the computer rather than the right foot.

[dbruce]

ASC DOES do the same thing. It applies braking to the drive wheels in exactly the same way that DSC does. DSC simply adds the front wheel independent braking, along with added sensors such as steering angle and yaw rate.

DSC is slower than an LSD, not slower in respect to transfering the torque after braking because that's simply a inherent property of an open diff. Where it is slower is in applying the brakes. The wheel has to slip first before the computer registers that it needs to apply the brakes. At which point it applies the brakes in a fashion that is neither smooth nor controlable except by the computer rather than the right foot.

While DSC and ASC aren't the same.......you are correct it allows for braking of the rear wheels in order transfer torque using the Open Diff (learn something new everyday):

http://www.usautoparts.net/bmw/technology/dsc.htm

The speed of the torque transfer all depends on how well DSC is designed and works. In case of a Torsen, wheels must also slip before torque is transferred. In the case of a clutch based diff......keeps thing equal to each wheel and avoids slipping......but then you have to deal with a clutch......so each is a trade off in one way or another.

If it was poor and wasn't fast enough, why is BMW using it in their AWD setups?

FYI: Audi is using the same design with Open diffs on the current Quattro revision:

http://en.wikipedia.org/wiki/Quattro_(all_wheel_drive_system)

And if Audi is using it in their AWD setups....what are we complaining about. :eek:

[chromius]

If it was poor and wasn't fast enough, why is BMW using it in their AWD setups?


Actually BMW used that in the older AWD systems. The new Xdrive system uses a series of clutches in the transfer case and diffs to transfer power where it needs it most, which allows for faster distribution of power than just DSC alone. They do however still use DSC to suppliment this for stability.

Just like on the new M-cars that use the active differential that also uses a series of clutches.

[dbruce]

Actually BMW used that in the older AWD systems. The new Xdrive system uses a series of clutches in the transfer case and diffs to transfer power where it needs it most, which allows for faster distribution of power than just DSC alone. They do however still use DSC to suppliment this for stability.

Just like on the new M-cars that use the active differential that also uses a series of clutches.

I was under the impression, from all I've read on the new Xdrive, that the new clutch setup is specific to the transfer diff. Standard open diffs are used for front and rear axles.

The previous version used a fix transfer of torque (essentially 60 rear....40 front) for the transfer diff and open diffs for front and rear. In any case, if it was slow and crap................it's used by a lot of manufacturers.

[chromius]

I was under the impression, from all I've read on the new Xdrive, that the new clutch setup is specific to the transfer diff. Standard open diffs are used for front and rear axles.

The previous version used a fix transfer of torque (essentially 60 rear....40 front) for the transfer diff and open diffs for front and rear. In any case, if it was slow and crap................it's used by a lot of manufacturers.

That's possible, maybe it was just the transfer case, but if they are not on the front and rear now, I'm pretty sure on the latest models they are going to be implementing the active diffs. I read that in an article about the X6.

Here's a link to the new commer from Audi as well going to an active diff.
http://www.auto-innovations.com/actualite_en/13.html

Edit:
Just found this video which is pretty good as showing the reaction speed.
At around 3:30 in the video where the touring trys to climb the icy hill take a look at how long it takes for the dsc to brake the wheel.
http://youtube.com/watch?v=TaVrQT5vpII
either way it's a cool video:)

[dbruce]

That's possible, maybe it was just the transfer case, but if they are not on the front and rear now, I'm pretty sure on the latest models they are going to be implementing the active diffs. I read that in an article about the X6.

Here's a link to the new commer from Audi as well going to an active diff.
http://www.auto-innovations.com/actualite_en/13.html

Edit:
Just found this video which is pretty good as showing the reaction speed.
At around 3:30 in the video where the touring trys to climb the icy hill take a look at how long it takes for the dsc to brake the wheel.
http://youtube.com/watch?v=TaVrQT5vpII
either way it's a cool video:)

Your probably looking 2009 for the Dynamic Performance Control:

http://www.hotbimmer.net/forum/showthread.php?t=24427

In any case, on our E39's.....the new advanced electronic multi clutch based diffs are irrelevant....since we will never see one on our cars. The current technology on both BMW and Audi's are electronic nanny's controlling torque using an OPEN diff.

Complaining about having an Open diff seems silly.....since the electronics allow the diff to simulate a a gear based LSD (like a Torsen)

[chromius]

I'm not complaining, I'm just saying it's not a replacement for a proper LSD. (Yet)

[dbruce]

I'm not complaining, I'm just saying it's not a replacement for a proper LSD. (Yet)

Forgot one other thing on this discussion, the M5 diff is clutch based. Clutch based diffs will keep the rear wheels moving at the same speed when a wheel is slipping......which makes them very easy to deal with when using DSC (shouldn't need special software etc).

A clutch based diff is great for high power applications and is probably what everyone would want in a diff.....except they wear out after about 100k and have to be rebuilt. I've read some posts on this board with people mentioning that DSC is not compatible with the LSD in an M5 (if moved to a 528i or 540).....but if it's clutch based.....shouldn't make a difference.

As I think was mentioned.....if you plan on dropping your clutch at high RPM.....that should be the only time the M5 LSD will make a noticeable difference. For basic traction.....the current system should be more then adequate.

My point when starting this was that the current setup and design is more then adequate for 90+% of what any driver will get into.

[ashamans]

i have 255 rears and the traction is not adequate, at many times downshifting or simply flooring it in any gear will cause me to go sidewaze for a second untill dsc kicks in(and its like a second)

i have great tires and get great traction, but my stock toruqe is just too great

and for those who plan on dropping their clutch at high rpm also plan on dropping their clutch right out of their car

[dbruce]

i have 255 rears and the traction is not adequate, at many times downshifting or simply flooring it in any gear will cause me to go sidewaze for a second untill dsc kicks in(and its like a second)

i have great tires and get great traction, but my stock toruqe is just too great

and for those who plan on dropping their clutch at high rpm also plan on dropping their clutch right out of their car

Having used a gear based diff in the past, Torsen, the effect is the same and the rear end can begin to slide if too much torque is applied (and the car I installed it on only had 140 HP). One wheel will slip which then transfers torque to the other wheel (opposite effect from an Open Diff). It was more then controllable in either case, and I've experienced a similar effect on the E39. The one big disadvantage with the Torsen was the fact that if one wheel loses all traction.....it converts to an open diff till traction is regained........sucks when your stuck in a snow bank. A Quaife diff should be similar.

With a clutch based diff..........none of the above will happen and if you really want the best launches it should be the best overall. Then again.....is it worth the smack to install it...........not in my book.

I've had an E36 M3 with a clutch based diff......and it was an extremely stable car and the diff was rarely noticed due to how a clutch based diff works. Is the tradeoff BMW used to decide to use electronics over a standard LSD worth it?..........seems to be considering the cost savings using the DSC system which would have been on the car anyway.

Performance has been mentioned as a negative.....but one of the best AWD system used this exact setup on both axles (Audi).

Software programming specified how long before DSC kicks in. For me personally, after having driven tons of cars with no DSC or ASC or anything.....nanny's adjusting my driving immediately is a negative.....I like the slip etc before the system kicks in and they include it on purpose. Each manufacturer uses a different program that controls when and how DSC engages (or whatever each manufacturer calls it). All BMW would have to do is adjust the time before the system turns on to remove the 1 second delay.....but then there is a trade off with some other aspect of how the car performs (kind of lame if your BMW never spun a tire).

[chromius]

except they wear out after about 100k and have to be rebuilt.

That really depends on how hard you are on it. There is almost zero wear during regular driving (aside from the preload), My last car the clutch based LSD was still in very good shape at 210,000miles.


As I think was mentioned.....if you plan on dropping your clutch at high RPM.....that should be the only time the M5 LSD will make a noticeable difference.

The LSD also makes big differences in cornering. Preventing inside wheel spin and allowing controlable power oversteer, as well as powering out of corners. Even the feel in corners is greatly improved.

And even on the 540, punching it in 1st (sometimes 2nd) while you're driving at a constant speed will brake traction without dumping the clutch.


My point when starting this was that the current setup and design is more then adequate for 90+% of what any driver will get into.

That I agree with.:)

[boomshakalaka]

just weld your open closed :buttrock

[dbruce]

I just did some testing today in the 6+ inches of snow that fell around my area. I have Blizzaks on the car.

I tried all 3 modes....DSC on, DSC off, and Hold the button 10+ seconds.

DSC On - Car was controllable....but had a tendancy to cut power too much....which made it annoying if you wanted to move quickly. I was able keep up with or surpass AWD/4WD cars with standard tires and could probably easily outhandle and brake them. From a starting stop.....engine cutoff made acceleration annoying

DSC OFF - My favorite Mode - With the system off, engine cutoff and wheel braking for stability was gone. I was able to slam on the gas in a few inches of packed snow without any stability problems. This leads me to believe the slip control on the rear axle is still enabled even with DSC disabled. My past experience with an Open Diff would have one wheel spinning away and the rear end swing out to that tire. I enjoyed driving in this mode the most and felt I had the most control......as long as I knew what to expect. I am used to driving cars without the nanny's....so this shouldn't come as a surpise.

10 second hold - Couldn't tell a difference between it and DSC on. The engine cutoff still came on when I slammed the gas. Control and annoyance was similar to the ON mode. Roadfly posters have tagged it "competition" mode since it may initiate engine cutoff etc later. Couldn't tell since I was on on ice and snow.

So........IMO....torque transfer on the rear axle seems to be enabled no matter what you do. I slammed on the gas on ice/slush to confirm this with the system off........both tires will spin all day long which wouldn't happen with an Open Diff by itself.

I even drove my car over some pretty high drifts etc that a plow moved. A standard Open Diff could easily get in trouble with this once a wheel loses traction. I had no issues with the DSC on or off leading me to believe slip control is always on for the rear axle. The only way to tell the difference would be to pull the plug on the ABS system completely and give it a run (not something I am interested in doing due to safety).

I only tested DSC.....since that's what I have.

[pushundersteer]

I can't get involved in the debate over what works how - I just don't know enough about it. However, I can tell you that - at least on my car - DSC is absolutely useless as a launch assistant. As soon as one rear spins, feul cut and brakes kick in, and you may as well have stalled. It is agonizingly slow to give feul back, and by the time it does, your revs are down well out of the power band. The same thing on corner exit - what would amount to 6-12" of kick-out with DSC off turns into a stutter that can easily send the rear end slapping back the other direction if you have dialed in the appropriate countersteer.

For me, it's very simple - get in, twist key, seatbelt on, DSC off. Every time, regardless of weather, destination, or mood. Once you learn car control, any form (up to the most recent and advanced versions) of traction control is slower, more dangerous, and less predictable than either an open diff, clutch-pack style LSD, or a torsen LSD.

I personally don't like the way clutch-pack based systems act on entry or at apex. They are nice off the line, but if dragging is your priority, follow the advice earlier in the post and have your peg-leg welded or buy a spool. Differentials are pretty rare in purpose-built drag cars. If you want the best handling - in and out of the corners - torsen rules. If you just want better hookup and won't be at ten tenths on the track - clutch packs are fine.

[dbruce]

For me, it's very simple - get in, twist key, seatbelt on, DSC off. Every time, regardless of weather, destination, or mood. Once you learn car control, any form (up to the most recent and advanced versions) of traction control is slower, more dangerous, and less predictable than either an open diff, clutch-pack style LSD, or a torsen LSD.

As I posted above.....no matter what what setting you are in on the DSC or ASC (I think)....the rear axle ABS system will still transfer torque between wheels when one slips. There can be no other explanation.....since I literally full throttles all over the place on snow and ice......and never had a Single wheel spin. My past experience with an Open Diff did not happen with this car in any way.......so the electronics are the only explanation (even though they were off).

If you really want a techical writeup of how it works:

http://classes.engr.arizona.edu/engr498a/2006/fsaediff/resources/techart3.pdf

[chromius]

As I posted above.....no matter what what setting you are in on the DSC or ASC (I think)....the rear axle ABS system will still transfer torque between wheels when one slips. There can be no other explanation.....since I literally full throttles all over the place on snow and ice......and never had a Single wheel spin. My past experience with an Open Diff did not happen with this car in any way.......so the electronics are the only explanation (even though they were off).

If you really want a techical writeup of how it works:

http://classes.engr.arizona.edu/engr498a/2006/fsaediff/resources/techart3.pdf

Nope....It's completely off when the DSC is off. Unless 528's are different then 540's, but I highly doubt that. I would suggest trying with the dsc off and put one wheel on the snow and the other on some dry pavement, then post the results. No offense, but what you may have "felt" isn't really a great reason to definitely say the EBD is still operational with DSC off. I can guarantee you that is not the case with 540's.

My best guess is that there is something wrong with your engine if you can't get wheel spin at full throttle and DSC off.:stickoutt

[dbruce]

Nope....It's completely off when the DSC is off. Unless 528's are different then 540's, but I highly doubt that. I would suggest trying with the dsc off and put one wheel on the snow and the other on some dry pavement, then post the results. No offense, but what you may have "felt" isn't really a great reason to definitely say the EBD is still operational with DSC off. I can guarantee you that is not the case with 540's.

My best guess is that there is something wrong with your engine if you can't get wheel spin at full throttle and DSC off.:stickoutt

I could get both rear wheels to spin constantly.......I just couldn't get one to spin. Having had a bunch of Open Diff cars, this just makes no sense on a low traction surface with the electronics off. The rear end was completely controllable. I also have a poster on Bimmerfest that said he runs into similar issues with a 540.....and it doesn't act like other Open Diff cars, with DSC disabled. It should be noted that when I did the same gas slams with DSC on, engine power was cut immediately.

The only real test, for me, is to pull the plug on the ABS module and then give it a try. I could have sworn when I was doing my gas slams I smelled brake pads, DSC off, also......but I might just have been imagining it.

[chromius]

A car with an open diff is still going to be controlable. In snow it's still going to drift around corners and "feel" like both wheels are doing the work. I've owned several BMW's in my day, some with open diffs and some with LSD's, and I can assure you that drifting in snow an open diff will feel similar to an LSD. I have a feeling that it's "in your head".

Also, an open diff still can very well spin both wheels, it's not uncommon. During the summer at times I can leave 2 paralell black marks on the pavement in the 540.

I think what you need to do is get a friend with a cam to video your car with DSC off, one wheel on pavement, one wheel on snow, and you will be able to see first hand exactly what your rear wheels are doing.

[dbruce]

A car with an open diff is still going to be controlable. In snow it's still going to drift around corners and "feel" like both wheels are doing the work. I've owned several BMW's in my day, some with open diffs and some with LSD's, and I can assure you that drifting in snow an open diff will feel similar to an LSD. I have a feeling that it's "in your head".

Also, an open diff still can very well spin both wheels, it's not uncommon. During the summer at times I can leave 2 paralell black marks on the pavement in the 540.

I think what you need to do is get a friend with a cam to video your car with DSC off, one wheel on pavement, one wheel on snow, and you will be able to see first hand exactly what your rear wheels are doing.

I'll screw around with it next time it snows (once every 3 months these days). It wasn't "kind" of controllable......the rear end seemed to adjust to wheel slippage and it was significantly better then I expected.....to the point I could keep my foot on the the floor without the rear end shifting to one side.

I'll look into screwing with it again....assuming we get more snow.

[chromius]

...to the point I could keep my foot on the the floor without the rear end shifting to one side.


That's exactly what an open diff will do on these cars. An LSD will shift the car, because an LSD has the tendency to want to rotate the car around it's axis.

This happens on LSD diffs for 2 reasons. One, because both wheels are spinning neither has lateral traction because it is broken by the spinning of the wheels. Two, because the torque split to the rear wheels is not equal, it's being biased to the side with the most traction, and therefore tends to push on one side and rotates the car.

The open diff on the other hand splits torque equally to both sides, and as a result always wants to keep the car straight.

So what you experienced is exactly what you should with an open diff.

[dbruce]

The open diff on the other hand splits torque equally to both sides, and as a result always wants to keep the car straight.

So what you experienced is exactly what you should with an open diff.

The above....is not true:

http://auto.howstuffworks.com/differential3.htm

http://www.houseofthud.com/differentials.htm

While the open diff always attempts to split torque....traction is how the torque is split. In the simplest example of ice and pavement, the below needs to be considered:


The problem is an open diff always tries to balance the torque. That's a hard statement to get a grasp on, but it means that if the spider gears are pushing on both drive gears and one of them offers lots of resistance (tire sitting on pavement) and the other side offers no resistance (up in the air, or sitting on a patch of ice), then it will find a happy balance where both sides are receiving almost no torque at all. All the rotational energy is guided to the side with the least resistance. In the end, that side spins very fast and the pressure on each drive gear is the same.. Almost no torque is needed to spin one wheel, and since the open diff always sends the same amount of torque to both output shafts, almost no torque is going to the other side as well. Anyone who's driven on snow or ice knows this trick.

In a situation in which you are driving, traction continuously changes on each wheel....and the "rotational energy" will always be guided to the tire with the least traction. For most drivers, if a driver slams the gas.....even less traction will be on that wheel and it will spin. My experience with an Open Diff on other cars made them interesting on snow and ice.....since the torque transfer made the rear end change in ways you don't always want. In one occasion I needed to be towed.

The BMW doesn't exhibit what I'm used to.....so think I will pull the plug on the ABS to make sure it's out of the equation.

[chromius]

I'm well aware how a diff works. My above statement still stands. Read again what you just posted, it is essentially what I already said.

Traction isn't how the torque is split, it is simply the limiting factor as to how much torque can be applied. Regardless of traction, torque is always equal on both sides of an open diff to a maximum of the available traction on either wheel.

So the reason the car wants to keep straight is because there is equal force acting on both sides of the car. Force is what moves an object, force in the terms of a rotational mass is called torque, since the torque is always split evenly in an open diff the car wants to move straight.

[Fiziks]

more info on this 10-second dsc thing... im not quite understanding.

[chromius]

more info on this 10-second dsc thing... im not quite understanding.

There is apparently a function that if you hold down the DSC button for 10 seconds the engine torque reduction of the engine gets disabled however the braking portion of the rear axle remains active, thus allowing the computer to act as a psudo limited slip.

However, this function does not work for myself as well as many others from all different model years. I have never seen an actual document anywhere from BMW or otherwise that outlines this procedure, therefore the only actual evidience that this function actually works is based on the internet and what people say they felt. So who knows.

The only change when I hold the button down for 10sec is that the DSC can no longer be disabled until the car is shut off and restarted. Therefore, my view is that this function is there as a safety feature in the event that something gets put in front of the button and holds it in by accident so that the system remains active.

[dbruce]

The only change when I hold the button down for 10sec is that the DSC can no longer be disabled until the car is shut off and restarted. Therefore, my view is that this function is there as a safety feature in the event that something gets put in front of the button and holds it in by accident so that the system remains active.

Same here from my test. It must do something.....and I doubt BMW created it as a child lock :cool .

[dbruce]

I'm well aware how a diff works. My above statement still stands. Read again what you just posted, it is essentially what I already said.

Traction isn't how the torque is split, it is simply the limiting factor as to how much torque can be applied. Regardless of traction, torque is always equal on both sides of an open diff to a maximum of the available traction on either wheel.

So the reason the car wants to keep straight is because there is equal force acting on both sides of the car. Force is what moves an object, force in the terms of a rotational mass is called torque, since the torque is always split evenly in an open diff the car wants to move straight.

Yes.....but it isn't always equal traction in the case of snow or rain. Once one wheel begins to spin......the traction on that wheel is reduced. The Open Diff will then apply more power to the spinning wheel...until essentially no torque is left on the non-spinning wheel.

One of the biggest annoyances with an Open Diff is having a wheel spin and suddenly hit a patch of asphalt etc. The wheel clunks when traction restores and power resumes on the non-spinning wheel and it's an unsettling feeling. None of this happens on my car.

Finally found a link that explains it well:

http://www.unofficialbmw.com/all/drivetrain/all_limited_slip_explained.html


Open Differentials and Snow
Differentials reduce tire wear and help a car turn more easily by allowing the rear wheels to travel at different speeds while turning corners. The inside wheel must slow down (smaller radius turn) while the outside wheel speeds up an equal amount (larger radius turn). To balance the drive torque at each wheel, more torque is applied to the outside wheel, speeding it up, while less torque is applied to the inside wheel, allowing it to slow down.

Open differentials always work well turning. They also apply power very evenly when both rear wheels have adequate traction. However, the big downside, is their torque balancing action when one wheel has much less traction, such as in ice and snow.

The torque applied to the wheel with the most traction can only equal the lesser traction wheel. Total applied torque for both wheels is only twice the traction of the worst wheel.

Open Diff Traction = 2 x Least Traction Wheel

With one wheel on ice and one on dry pavement, two times zero equals zero torque applied to the road. The differential keeps trying to balance the torque by applying more power to the spinning, lesser traction, wheel.

You can see why it is not good to spin tires in the snow. Once a tire starts spinning, more torque is applied which makes it spin more, further lessening traction. Any traction regained on the spinning wheel side is doubled because it is also applied to the other, non-spinning wheel.

So common sense says to increase traction any way possible to keep the tires from spinning. Snow tires are an obvious solution as is 150 lbs of sand in the trunk between the rear drive wheels. If you get stuck, sand can also be thrown under the spinning wheel (floor mats also work in a pinch).

Trick for Getting Unstuck When One Wheel Spins