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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 06-07-2011, 09:42 PM   #1
karatechop5000
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Transfer Case - Open or Limited Slip?

I am confused about how my xi driveline is configured mechanically (particularly the transfer case). Please discuss with me if you have the patience. Hopefully it will be a chance to resolve some conflicting information I have noticed in different threads.

The point of contention I hope to resolve is whether the xi transfer case is 'open' or 'limited slip'. There appears to be general agreement that the front and rear differentials are 'open' and are both the same ratio, and that the transfer case splits torque (not speed) between the front and rear propeller shafts.

I have seen pictures of the transfer case and can roughly summarize. The input (from transmission) turns a 'planetary' carrier. I can't even begin to describe a planetary gear set with words; but basically there are two outputs (one slow and one fast). The fast goes to the rear output and the slow one turns a small gear linked to the front output. The front (weaker output) is your basic slow turn of a high gear. The rear is fast turn of lower gear (hill climber). Speed output is normally equal (front to rear).

No part of the planetary set is held except the ring to the case. No clutches or worm gears are present. I cannot identify any mechanism to limit 'slip'.

Once again, what's at issue is whether the transfer case is 'open' or 'limited slip'. It's not 'fixed' so don't even go there (finger waving). Bench testing at low speed indicates 'open'. I can no longer find that thread; so I apologize for not thanking the appropriate member.

We know from working on automatic transmissions that planetary gear sets can vary the speed of one output if another is 'held'. I'm thinking traction is the other 'hold'. The transfer case is open; but when there is traction at all four wheels, it splits torque.

I do not intend to discuss how traction control and DSC use wheel brakes to replace friction in the differentials or transfer case. That part of the discussion, I agree, has been beaten to death.

This has implications for people considering a mechanical LSD, and for those wishing to alter their rear tire diameter.
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Old 06-08-2011, 09:49 AM   #2
TiAgXi
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So you're saying it's NOT fixed? I have always thought the transfer case was "locked" to use terminology from 4x4's. A 4x4 has a transfer case which you can engage in order to go from RWD to 4WD. when the transfer case is engaged, it is locked and power is sent to the front and rear. On our cars, the transfer case is constantly engaged.

So, if I were to answer the question I'd say it was neither open nor limited slip.

???

Last edited by TiAgXi; 06-08-2011 at 09:50 AM.
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Old 06-08-2011, 10:31 AM   #3
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Originally Posted by karatechop5000 View Post
know from working on automatic transmissions that planetary gear sets can vary the speed of one output if another is 'held'. I'm thinking traction is the other 'hold'. The transfer case is open; but when there is traction at all four wheels, it splits torque.
Doesn't that, in essence, make it limited slip? I do, however, remember the member who did that test but I can't remember who it was either. I can at least say you're not going crazy on that one and that you didn't dream it one night.

This is a good discussion to have and I look forward to discussing it with all of you!
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Old 06-08-2011, 11:33 AM   #4
Mark M
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I am the one that posted a while back that did the bench tests.

I have a used transfer case in my attic if you want to buy it cheap for tinkering or disecting. The mounting lobe that contains the bushing was cracked off, however the case is still fine and intact. Perhaps a good welder with experience in aluminum could even weld it all back together.

PM me if you are interested in it. I'm in PA.
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Old 06-08-2011, 03:53 PM   #5
karatechop5000
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Originally Posted by TiAgXi View Post
So you're saying it's NOT fixed? I have always thought the transfer case was "locked" to use terminology from 4x4's. A 4x4 has a transfer case which you can engage in order to go from RWD to 4WD. when the transfer case is engaged, it is locked and power is sent to the front and rear. On our cars, the transfer case is constantly engaged.

So, if I were to answer the question I'd say it was neither open nor limited slip.

???
'Constantly engaged' - yes. The gears do not shift to disconnect front or rear. The difference is they are constantly engaged to a planetary set which allows slip (my theory).

Thank you Mark. At this point, I trust your results. The TC is not 'fixed'. As you reported, turn the input while holding front or rear output and the other output turns. I cited your results primarily for the benefit of those who believe the TC is 'fixed'.

I never belived the TC was fixed because the front and rear wheels on a road car absolutely need to be able to turn at different speeds. Also, if the TC was 'fixed', it would not be able to split torque.

Sam, thanks for joining the discussion. I wonder if Kubica will have anything to say. I guess if the front has steeper gears, there is more resistance to front slipping; but I think the term 'limited slip' has a more narrow definition. Specifically, if front or rear does slip the TC would have to stop it.
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Old 06-08-2011, 04:30 PM   #6
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Here's a cool educational packet.

Need to educate myself on how planetary gears work but I dont see how either the planetary gear or the sun gear can be stopped if the planet carrier is the input. Have a hard time seeing how this works in my mind.

If one output can be stopped without having any effect on the other than it seems it would be open. Torsen, viscous coupling or any other "limited slip" would prevent one weel from coming to a complete stop. That's how diffs work at least.

But that makes no sense to me.
Attached Files
File Type: pdf e46.pdf (1.19 MB, 325 views)

Last edited by TiAgXi; 06-08-2011 at 04:51 PM.
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Old 06-08-2011, 05:39 PM   #7
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Originally Posted by TiAgXi View Post
Here's a cool educational packet.

Need to educate myself on how planetary gears work but I dont see how either the planetary gear or the sun gear can be stopped if the planet carrier is the input. Have a hard time seeing how this works in my mind.

If one output can be stopped without having any effect on the other than it seems it would be open. Torsen, viscous coupling or any other "limited slip" would prevent one weel from coming to a complete stop. That's how diffs work at least.

But that makes no sense to me.
First of all, thanks for posting that I've never seen it. I can explain what you are seeing and seem to be confused about.

First of all, the principal of a planetary gear set is that you do not drive the planet gears. You can only drive either the ring gear or the sun gear (mainly cause there are usually more than one planet gear...). The way this transfer case seems to be layed out is that the planetary gear set is only for driving the rear axle. The front is driven by a normal gear (not planetary). This is how they get the split of torque along with matching the output shaft speeds. A planetary gear set is much more capable of increasing torque than a normal gear so that's how the rear gets more torque than the front while maintaining a 1:1 speed.

Secondly, this seems to work like this. The transmission's output shaft is linked to a transfer gear. This is phase one of getting power to the wheels. the transfer gear is the one that drives both the front and rear output shafts. The front output shaft, as I said before, seems to be driven by a normal gear. Nothing fancy here. The rear is a little different. The sun gear on the planetary set is meshed to the transfer gear, which then transfers power to the planet gears for changing the gear ratio, and then is transfered to the ring gear which then transfers power to the front axle by means of an output flange and a driveshaft.

Now where I get confused is this. This device seems to be a constantly meshed gear system without any play (hypothetically). How is this an open differential? From an engineering standpoint, that's not possible unless I am missing something. It would seem to me like this is a locked or fixed system. Granted that the description of the transfer case in that document is fairly vague and just gives a very basic overview of how it works.
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Old 06-08-2011, 05:43 PM   #8
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Originally Posted by karatechop5000 View Post
'Constantly engaged' - yes. The gears do not shift to disconnect front or rear. The difference is they are constantly engaged to a planetary set which allows slip (my theory).
It can't "slip". A planetary gear set is a constantly meshed system, it has no room to slip without adding a clutch or another type of device to disengage or vary power input.

Quote:
Originally Posted by karatechop5000 View Post
I never belived the TC was fixed because the front and rear wheels on a road car absolutely need to be able to turn at different speeds. Also, if the TC was 'fixed', it would not be able to split torque.
Sure it would, why wouldn't it? That's what gearing is all about! I think the variation between wheel speeds front and rear is managed entirely by the front and rear differentials but that's just my theory at this point.
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Old 06-08-2011, 06:15 PM   #9
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The part that makes no sense to me is Mark's experience that he could stop one of the outputs from moving. How on earth would you get the wheels moving if one of the output shafts is permitted to stop moving when resistance is applied!?

Mark, are you sure that the TC you were testing wasn't broken?

An "open" TC seems like an impossibility without more.
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Old 06-08-2011, 06:56 PM   #10
jerryscott
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the transfer case in the BMW xi is a full time engagement, the only thing that can be limited slip is your front and rear diffs
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Old 06-08-2011, 09:06 PM   #11
karatechop5000
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The carrier does hold all three planets which turn the sun and ring together (the whole unit spinning together but no movement within the set). If you stop the sun, the planets rotate around the sun and spin causing the ring to spin. If you stop the ring, the planets rotate, spin, and cause the sun to spin.

See my shabby drawing:
http://i1228.photobucket.com/albums/...00/planets.png

This would mean I was wrong about the ring being joined to the case. I'm sure the planet carrier is the input. Yes it turns all three planets together. Picture two triangles sandwiched together with gear axles separating them at all three points.

As far as a 'fixed' unit being unable to split torque, I am pretty sure that if you are mechanically unable to vary output speeds, you can't split torque. That's like saying "I'm not going to push you very hard; but you ARE going to move."

Also... now that I think of it... why bother having a planetary set at all if not to allow slip? Why not just have input turn both outputs? I know you will be tempted to say "because it makes the torque split"; but again, you don't have torque split if you don't permit speed difference. That is the way gears work. Low gear is slow and strong. High gear is fast and weak.

Last edited by karatechop5000; 06-08-2011 at 09:23 PM. Reason: Addition
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Old 06-08-2011, 09:39 PM   #12
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Originally Posted by SamDoe1 View Post
Secondly, this seems to work like this. The transmission's output shaft is linked to a transfer gear. This is phase one of getting power to the wheels. the transfer gear is the one that drives both the front and rear output shafts. The front output shaft, as I said before, seems to be driven by a normal gear. Nothing fancy here. The rear is a little different. The sun gear on the planetary set is meshed to the transfer gear, which then transfers power to the planet gears for changing the gear ratio, and then is transfered to the ring gear which then transfers power to the front axle by means of an output flange and a driveshaft.
You got it wrong. The output from the transmission goes to the planetary carrier and drives the three planetary gears. the sun gear connects to the transfer gear then to the front output. the ring goes to the rear.
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Old 06-08-2011, 10:08 PM   #13
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I just took a short 'hack' video with my iphone showing the operation when spinning the input shaft and holding one or the other output shafts. If you hold the rear output, the front continues to move. Vice versa. This transfer case is an operational unit and was working fine when removed.

Here is the short video taken in my attic at 11pm.

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Old 06-08-2011, 10:15 PM   #14
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Quote:
Originally Posted by karatechop5000 View Post
The carrier does hold all three planets which turn the sun and ring together (the whole unit spinning together but no movement within the set). If you stop the sun, the planets rotate around the sun and spin causing the ring to spin. If you stop the ring, the planets rotate, spin, and cause the sun to spin.

See my shabby drawing:
http://i1228.photobucket.com/albums/...00/planets.png

This would mean I was wrong about the ring being joined to the case. I'm sure the planet carrier is the input. Yes it turns all three planets together. Picture two triangles sandwiched together with gear axles separating them at all three points.

As far as a 'fixed' unit being unable to split torque, I am pretty sure that if you are mechanically unable to vary output speeds, you can't split torque. That's like saying "I'm not going to push you very hard; but you ARE going to move."

Also... now that I think of it... why bother having a planetary set at all if not to allow slip? Why not just have input turn both outputs? I know you will be tempted to say "because it makes the torque split"; but again, you don't have torque split if you don't permit speed difference. That is the way gears work. Low gear is slow and strong. High gear is fast and weak.
Ok I misread what it said initially and you are right. It appears that the input to the transfer case is through the planetary carrier to spin the planet gears (all of them at once spin around the center axis of the sun gear like well...planets). The sun gear then transmits torque to the transfer gear and subsequently the front axle output gear. The transfer gear is there to both change the gearing as well as making sure the drive shafts turn in the same direction, it's an idler gear. The ring gear, however, is not held fixed as it must rotate in order to power the rear drive shaft. I am confused as to how this works since you'd need to hold at least one part of this system stationary for it to work...never seen a one in/two out planetary gear set before.

The planetary gear set is there for the biggest reason of it takes up the least space for the most gearing change. Also, the document does say:

"The transmission ratio of the planetary gear
set provides a fixed torque transfer of 38:62
(front:Rear). The output speeds to the front
and rear axle are the same (1:1)."

This must have been accomplished by changing the speeds of the front axle output back up to where it should be to match the rear axle using the idler/transfer gear and the output gear.

Not knowing the specifications of the gears in this thing, I can't really tell how they accomplished this but it does seem that they somehow did.

Quote:
Originally Posted by hammerheart View Post
You got it wrong. The output from the transmission goes to the planetary carrier and drives the three planetary gears. the sun gear connects to the transfer gear then to the front output. the ring goes to the rear.
Yes you're right, I misread what it said initially. Sorry guys.

Last edited by SamDoe1; 06-08-2011 at 10:17 PM.
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Old 06-08-2011, 11:36 PM   #15
karatechop5000
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Originally Posted by SamDoe1 View Post
Also, the document does say:

"The transmission ratio of the planetary gear
set provides a fixed torque transfer of 38:62
(front:Rear). The output speeds to the front
and rear axle are the same (1:1)."
This is under normal conditions (when all the wheels have traction). Traction is the key. It's also the 'hold' or feedback pressure on the sun and ring gears. 'Fixed' does not mean that the output is absolutely fixed under any circumstances. It just means the ratio is none other than 38:62 given full traction and straight line travel.

The wording is confusing.

Thanks for the video Mark. Nobody can say your result is flawed by slow hand turning. There is no disagreement about the fact that the TC doesn't have any clutch, viscous coupling, or other friction componant which would behave differently at higher speed. Because the TC is all gear meshed, I think if it were bad, it would be catostrophically bad. You'd know.
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Old 06-08-2011, 11:46 PM   #16
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the output from the transmission goes to the planetary carrier and drives the three planetary gears. The sun gear connects to the transfer gear then to the front output. The ring goes to the rear.
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Old 06-09-2011, 12:07 AM   #17
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Need to educate myself on how planetary gears work but I dont see how either the planetary gear or the sun gear can be stopped if the planet carrier is the input. Have a hard time seeing how this works in my mind.
Planetary gears are a puzzle within a puzzle. One planetary gearset gives your old school auto transmissions 2 reduced gears, one direct 1:1 gear, and reverse. One planetary set with different hold combinations and other simple gears driven.

Our TC application is simple compared to that. I'm beginning to view our xi transfer cases sort of like a basic open diff which has a stronger side. Especially when it slips.

Last edited by karatechop5000; 06-09-2011 at 06:55 AM. Reason: bad code
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Old 06-09-2011, 08:30 AM   #18
TiAgXi
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Originally Posted by Mark M View Post
I just took a short 'hack' video with my iphone showing the operation when spinning the input shaft and holding one or the other output shafts. If you hold the rear output, the front continues to move. Vice versa. This transfer case is an operational unit and was working fine when removed.

Here is the short video taken in my attic at 11pm.

Thanks mark.
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Old 06-09-2011, 08:37 AM   #19
TiAgXi
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Originally Posted by karatechop5000 View Post
Our TC application is simple compared to that. I'm beginning to view our xi transfer cases sort of like a basic open diff which has a stronger side. Especially when it slips.
Comparing it to an open diff doesn't make sense to me. Think about what happens if one wheel on an axle comes off of the ground when that axle has nothing but an open diff: the wheel off the ground spins and the other wheel does nothing. So if the TC is open, then the same would be true for the front and rear axles....if one had no traction all the car's power would go to the axle without traction. My feeling from driving in the snow is that such is not the case.
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Old 06-09-2011, 09:03 AM   #20
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So, if I'm reading this correctly, the planetary gear set functions as a differential of sorts since it directly varies output shaft speeds given a level of torque split.

But, as TiAgXi said, my driving experience in the snow doesn't back up this claim at all. I can get one tire to spin on each axle (due to open diffs) without a problem when driving in deep snow or if I'm stuck. If it was truely an open diff, there would only be one tire spinning. I have yet to come across such a situation where there's only one tire spinning freely or where there's only one axle spinning freely.
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