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E46 Xi Forum
The E46 XI was produced from 01-05 in sedan and touring body styles. Powered by either a 2.5L inline 6 in the 325xi or a 3.0L inline 6 330xi. Discuss all thing about BMW AWD E46 'Xi' here.

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Old 06-10-2011, 03:04 PM   #41
GeoffS
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Originally Posted by SamDoe1 View Post
Wait, so the final determination is that the the transfer case is essentially an open differential?
No. An open diff would transfer power to the axle with least resistance. The transfer case doesn't seem to care, and will send power to both regardless.
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Old 06-10-2011, 03:10 PM   #42
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No. An open diff would transfer power to the axle with least resistance. The transfer case doesn't seem to care, and will send power to both regardless.
Uh oh. I fear we do not have a consensus.
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Old 06-10-2011, 03:17 PM   #43
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I covered most of this in my post here: http://forum.e46fanatics.com/showpos...0&postcount=38

As to the mechanics of HOW the transfer case does its thing, since I'm not a mechanical engineer, that's beyond my scope. However, BMW's documentation says that it effectively behaves as a pseudo locked/rear biased center differential, keeping an even 32-front, 68-rear no matter what.
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Old 06-10-2011, 03:30 PM   #44
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Originally Posted by GeoffS View Post
I covered most of this in my post here: http://forum.e46fanatics.com/showpos...0&postcount=38

As to the mechanics of HOW the transfer case does its thing, since I'm not a mechanical engineer, that's beyond my scope. However, BMW's documentation says that it effectively behaves as a pseudo locked/rear biased center differential, keeping an even 32-front, 68-rear no matter what.
And that's what I thought as well but my opinion was shot down. I don't agree with the statement that it is open and that the traction is what provides the "hold" on the planetary gear set. But, based on what testing has been done, it does function as an open diff since holding one output shaft does not hold the other. Based on BMW's documentation, I would agree with you but practical testing has proven otherwise. In addition, there's no way a planetary gear set would work without holding one of the components stationary...

I'm confused between the documentation and the practical testing and I am a mechanical engineer...
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Old 06-10-2011, 03:35 PM   #45
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Originally Posted by GeoffS View Post
I covered most of this in my post here: http://forum.e46fanatics.com/showpos...0&postcount=38

As to the mechanics of HOW the transfer case does its thing, since I'm not a mechanical engineer, that's beyond my scope. However, BMW's documentation says that it effectively behaves as a pseudo locked/rear biased center differential, keeping an even 32-front, 68-rear no matter what.
I'm confused again. Mark's video shows that one output can be stopped without stopping the other one. That means it's not locked, which perhaps is why BMW calls it "pseudo" locked. If it were locked, you wouldn't be able to stop any of the outputs from spinning if the input was spinning.

An open differential doesn't prevent power from going to one wheel when the other wheel is stopped. that behavior is identical to what the transfer case appears to do. A locked differential requires power to go to both wheels. a LSD allows the wheels to spin at different speeds but only up to a point...one wheel cannot completely stop without stopping both.

Seems to me that the transfer case is open but with whatever gearing is needed to allow torque and power to be unevenly distributed.

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Old 06-10-2011, 04:20 PM   #46
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I'm confused again. Mark's video shows that one output can be stopped without stopping the other one. That means it's not locked, which perhaps is why BMW calls it "pseudo" locked. If it were locked, you wouldn't be able to stop any of the outputs from spinning if the input was spinning.

An open differential doesn't prevent power from going to one wheel when the other wheel is stopped. that behavior is identical to what the transfer case appears to do. A locked differential requires power to go to both wheels. a LSD allows the wheels to spin at different speeds but only up to a point...one wheel cannot completely stop without stopping both.

Seems to me that the transfer case is open but with whatever gearing is needed to allow torque and power to be unevenly distributed.

So if it's "pseudo locked", that in essence means it's limited slip in that it will allow for a speed variance up to a certain point. HOWEVER, BMW's documentation specifically states that output speeds to the front and rear drive shafts are the same (1:1). If this truely was an open differential it would not allow the rears to grip when the fronts have no traction and vice versa. That really makes the whole AWD concept rather pointless without relying on the DSC to make it work (which I don't think is right). I agree that this is quite a pickle here and I'm not sure where to go short of either finding one and taking it apart or somehow getting a CAD model of this thing...

With my car, I can assure you that if I turn the DSC off it does not activate the ABS to brake individual wheels at all. I would be able to feel and hear that happen. I don't think this car is capable of applying progressive brake force and to any wheel without using the ABS.

With all of that said, I am truely at a loss to explain anything so I will chalk it up to magical leprechauns.
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Old 06-10-2011, 04:28 PM   #47
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So if it's "pseudo locked", that in essence means it's limited slip in that it will allow for a speed variance up to a certain point. HOWEVER, BMW's documentation specifically states that output speeds to the front and rear drive shafts are the same (1:1). If this truely was an open differential it would not allow the rears to grip when the fronts have no traction and vice versa. That really makes the whole AWD concept rather pointless without relying on the DSC to make it work (which I don't think is right). I agree that this is quite a pickle here and I'm not sure where to go short of either finding one and taking it apart or somehow getting a CAD model of this thing...

With my car, I can assure you that if I turn the DSC off it does not activate the ABS to brake individual wheels at all. I would be able to feel and hear that happen. I don't think this car is capable of applying progressive brake force and to any wheel without using the ABS.

With all of that said, I am truely at a loss to explain anything so I will chalk it up to magical leprechauns.
Read page 75 of your owners' manual or my previous post. It says that the diffs and transfer case use ADB-X to replicate a LSD. That means that the brakes are used to prevent the wheels without grip from spinning freely and robbing the other wheels of power.

If you could completely turn of DSC (and therefore ADB-X) then the way to test this would be to lift the front off the ground and see if you could drive the car off of the jack-stands. Someone should do that. I would but I'm very very busy

Mark's video demonstrates that the transfer case is neither locked, nor LSD. If it were locked, both outputs would spin when the input was engaged and you would not be able to stop or slow-down either one. If it were limited slip, you could slow one down, but you wouldn't be able to stop it so long as the input was engaged.

That only leaves "open" as the remaining possibility. One can come to a complete stop while the other continues to spin. This is precisely how an open diff works. Of course, the difference between a TC and a diff is that power is unevenly distributed.

There's my proof. Now someone disprove it.

Last edited by TiAgXi; 06-10-2011 at 04:41 PM.
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Old 06-10-2011, 04:33 PM   #48
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Read page 75 of your owners' manual or my previous post. It says that the diffs and transfer case use ADB-X to replicate a LSD. That means that the brakes are used to prevent the wheels without grip from spinning freely and robbing the other wheels of power.

If you could completely turn of DSC (and therefore ADB-X) then the way to test this would be to lift the front off the ground and see if you could drive the car off of the jack-stands. Someone should do that. I would but I'm very very busy
I know what the manual says and I know that the traction control uses the ABS system to brake the wheels to replicate a limited slip differential. I also know that I can feel when this is happening by both the noise and feel of the ABS activating and I'm sure you can too. I also know that when I push the button on the dash, it no longer runs the ABS at all. I can hold the throttle all the way down and it will just rev up and spin both axles if I'm on a snowy road without any braking anywhere. If you do the same thing, can you feel the ABS activate even if you have disabled the DSC?

Just saying based on my experience and my personal car...

Once winter comes around again we can really test this thing out. Rears on a sheet of ice and fronts on dry road. DSC off. Floor it and see what happens. If the center is truely an open diff we won't go anywhere.

Last edited by SamDoe1; 06-10-2011 at 04:34 PM.
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Old 06-10-2011, 04:42 PM   #49
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By the way, engineers can do amazing things with gears. My 4runner has a lockable torsen center diff. it's limited slip and lockable. never, however, is it open.
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Old 06-10-2011, 04:57 PM   #50
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By the way, engineers can do amazing things with gears. My 4runner has a lockable torsen center diff. it's limited slip and lockable. never, however, is it open.
Pardon my nerd moment for the day, but yes it's quite amazing what you can do with a handful of gears and a small space. It's possible to tow an F350 with a 10hp lawnmower motor if it's geared properly, although it won't be quick by any means.

My senior design project in school was to design a paddle shifted transmission that could both do the mentioned towing as well as race an off road course (SAE Baja for those who are familiar). The design of a manual transmission/transfer case is quite simple and pretty amazing all at the same time. Autotragic transmissions are a whole different animal though...
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Old 06-10-2011, 05:00 PM   #51
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I know what the manual says and I know that the traction control uses the ABS system to brake the wheels to replicate a limited slip differential. I also know that I can feel when this is happening by both the noise and feel of the ABS activating and I'm sure you can too. I also know that when I push the button on the dash, it no longer runs the ABS at all. I can hold the throttle all the way down and it will just rev up and spin both axles if I'm on a snowy road without any braking anywhere. If you do the same thing, can you feel the ABS activate even if you have disabled the DSC?

Just saying based on my experience and my personal car...

Once winter comes around again we can really test this thing out. Rears on a sheet of ice and fronts on dry road. DSC off. Floor it and see what happens. If the center is truely an open diff we won't go anywhere.
And therein lies the problem. Mark's video basically destroyed everything I thought I knew about our TC's.
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Old 06-10-2011, 11:13 PM   #52
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Once winter comes around again we can really test this thing out. Rears on a sheet of ice and fronts on dry road. DSC off. Floor it and see what happens. If the center is truely an open diff we won't go anywhere.
I think the rears will spin (ring gear not held very tightly at all). The fronts will have very little torque applied. Torque applied to the front will equal any amount of traction you can get from two spinning rear wheels plus internal friction in the rear diff and the TC (not much at all) reduced by the steeper gears in front. I think it would creep forward (if not on a hill or something). Any traction the rears can get as the car creeps forward will be transfered to the front. As the car moves, the rear traction will vary (more/less hold on the ring). These pulses to the front combined with increasing momentum as the car starts to move will bring the TC closer to it's normal torque distribution.

At that point, magical leprechauns will be deployed, and the entire vehicle will float just a few inches off the ground.

Last edited by karatechop5000; 06-10-2011 at 11:14 PM. Reason: added an emoticon so I wouldn't seem snarky
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Old 06-10-2011, 11:25 PM   #53
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With my car, I can assure you that if I turn the DSC off it does not activate the ABS to brake individual wheels at all. I would be able to feel and hear that happen. I don't think this car is capable of applying progressive brake force and to any wheel without using the ABS.
The requisite brake force applied to the single wheel is not that much. It doesn't need to completely stop the spinning wheel. It just needs to shoot some force to the others. The other wheels are probably rolling; so its not like they are being hammered with torque at a dead stop. The force has two different buffers as it is transfered to both the other rear wheel and to the TC (front).

It's a soft hit on a soft target.
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Old 06-11-2011, 12:52 AM   #54
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the transfer case will NOT send power to the front wheel that has the least amount of traction
The front wheels will have better traction than the rear (with the engine weight over them). They won't have to do much work either. With both rears doing their best, it would be easy to pull the car forward. On ice, they might all spin (if DSC is off).

Your LSD rear will rely on the front less AND make more power available to the front (up to the torque split ratio). I doubt they would ever overspin the rears. You will still have the brake effected limited slip up front and redundant limited slip systems in the rear (plus the TC torque split will be more constant with the improved traction).
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Old 06-11-2011, 04:12 AM   #55
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That really makes the whole AWD concept rather pointless without relying on the DSC to make it work (which I don't think is right).
I think its easier to accept if you alter your perspective about the DSC. Try thinking of the similarities between what the brakes do and what tradional LSD mechanisms do (rather than how they are different). I know clutches aren't exactly brakes; but in terms of energy transfer, they are doing the same job. I think the DSC could be programmed to be as aggressive as the LSD; but BMW made some calculations about how they wanted it to behave. As I said before, sacrificing a bit of traction for the RWD feel and fun.

Every time you do a brake job, you are also effectively rebuilding 2 diffs and a transfer case. Nice.

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Old 06-11-2011, 11:17 AM   #56
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The requisite brake force applied to the single wheel is not that much. It doesn't need to completely stop the spinning wheel. It just needs to shoot some force to the others. The other wheels are probably rolling; so its not like they are being hammered with torque at a dead stop. The force has two different buffers as it is transfered to both the other rear wheel and to the TC (front).

It's a soft hit on a soft target.
Whatever the case might be, the system still utilizes the ABS in order to do its work. The ABS pulses are easy to hear and feel throughout the car and that simply doesn't happen in mine. I'm not sure why since the rest of you seem to say that the traction control is still active even when you push the button.

Then again, I am the guy with the freakish 36mpg 330xi...
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Old 06-11-2011, 11:18 AM   #57
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I think the rears will spin (ring gear not held very tightly at all). The fronts will have very little torque applied. Torque applied to the front will equal any amount of traction you can get from two spinning rear wheels plus internal friction in the rear diff and the TC (not much at all) reduced by the steeper gears in front. I think it would creep forward (if not on a hill or something). Any traction the rears can get as the car creeps forward will be transfered to the front. As the car moves, the rear traction will vary (more/less hold on the ring). These pulses to the front combined with increasing momentum as the car starts to move will bring the TC closer to it's normal torque distribution.

At that point, magical leprechauns will be deployed, and the entire vehicle will float just a few inches off the ground.
Once winter comes around we can hit this thread again and see what's what. Unless someone can reserve a hockey rink somewhere...
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Old 06-11-2011, 12:04 PM   #58
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I see this thread getting a gazillion reads yet, there are only a few contributors in the discussion. That would be an indicator the majority of us are curiously observing, with desperate hope someone will step up in an authoritative manor and reveal the definitive answer. Well, looks like that's never gonna happen. So, perhaps we should move on and simply catalog this as another mechanical mystery those clever Germans developed?

"Pseudo" differential used in their documentation? Okay, yeah, that's where I'm gettin' off this bus.
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Old 06-11-2011, 01:17 PM   #59
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If I jack up my front end, put on the ebrake, and put the car in drive; will that be a definative test? If the fronts spin, we have an open transfer case. Does anybody see a problem with that test?

Edit. DSC off.


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Old 06-11-2011, 04:43 PM   #60
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I see this thread getting a gazillion reads yet, there are only a few contributors in the discussion. That would be an indicator the majority of us are curiously observing, with desperate hope someone will step up in an authoritative manor and reveal the definitive answer. Well, looks like that's never gonna happen. So, perhaps we should move on and simply catalog this as another mechanical mystery those clever Germans developed?

"Pseudo" differential used in their documentation? Okay, yeah, that's where I'm gettin' off this bus.
The voice of reason. We are FANATICS! Anyway... as long as everyone remains civil, I'm in; but to be honest, I think I have my answer. Thanks everyone for your patience.
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