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X5 E70 (2007 - 2013)
E70 BMW X5 produced between 2007 and 2013. Discuss the E70 X5 with other BMW owners here.

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  #1  
Old 11-10-2015, 06:05 AM
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Doug Huffman Doug Huffman is offline
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What xDrive component enables 60/40 split?

Yesterday I found this BMW training description of XDrive for an E83 and ATC-500. It also mentions the 60/40 split but without showing a power path schematic causing such a split.

http://share.qclt.com/bmw%E8%B5%84%E...3%20xDrive.pdf

Will some one explain for me, please?
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  #2  
Old 11-10-2015, 09:55 AM
Flying Ace Flying Ace is offline
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I've never seen such technical document myself and I'm interested in this too. It's got to be some type of electronic LSD in the center diff that creates this torque split. Can a pure vicous coupling unit default to a 60/40? I don' think it can.

The only article I've read on xdrive center diff was from Automobile Magazine that stated the xdrive in the 335xi computer controlled unit:

"A computer-controlled transfer-case clutch sets the share of torque dispatched to the 335xi's front wheels."

http://www.automobilemag.com/reviews...008_bmw_335xi/
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Last edited by Flying Ace; 11-10-2015 at 09:59 AM.
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  #3  
Old 11-10-2015, 10:46 AM
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From the document -
Quote:
In normal driving, the clutch is operated with minimum slip so that a permanent four-wheel drive with a driving-torque distribution of 40% on the front axle and 60% on the rear axle is available.
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  #4  
Old 11-10-2015, 11:29 AM
Flying Ace Flying Ace is offline
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one thing for sure, the BMW version of the center LSD is far less sophisticated than those installed in high performance Subuaru Sti and Mitsubishi Evo cars. Part of the reason is likely the intention of the vehicle itself as the STi and Evo are designed to have a more aggressive characteristics that allow quicker slips of the clutch pack in both the center and front/rear diff units b/c they are designed to be more "track/rally" ready. More impressively, Subie and Mitsubishi designed the center diff to be controlled by the driver to allow either more rear bias performance (STi) or to slow down the slip speed of the center diff in slick conditions (Evo).

BMW xdrive and the B7 RS4 AWD systems are more designed around "safety" and conservate slip "speeds" to behave more like the traditional viscous coupling and purely mechanical units or had electronic controls programmed to allow a conservate slip of the clutch discs and split of torque/power.

I don't know enough about the Nissan GTR center diff unit. It'll be interesting to read something about its unit and how sophisticated it is.
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Last edited by Flying Ace; 11-10-2015 at 11:35 AM.
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  #5  
Old 11-10-2015, 11:59 AM
ninja_zx11 ninja_zx11 is offline
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Quote:
Originally Posted by Doug Huffman View Post
From the document -
So then it means clutch is always slipping just enough to accommodate 60-40 torque split even while driving straight.
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  #6  
Old 11-10-2015, 12:06 PM
u3b3rg33k u3b3rg33k is offline
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Originally Posted by ninja_zx11 View Post
So then it means clutch is always slipping just enough to accommodate 60-40 torque split even while driving straight.
No. the clutch only slips when there's a difference between front and rear axle speed. continuous slippage would destroy the clutch in short order, which is why it is so important to have the same overall front and rear tire diameters.
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  #7  
Old 11-10-2015, 12:08 PM
edwardmagichand edwardmagichand is offline
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Probably the size(diameter) of the clutch.. ?
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  #8  
Old 11-10-2015, 12:13 PM
ninja_zx11 ninja_zx11 is offline
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Quote:
Originally Posted by u3b3rg33k View Post
No. the clutch only slips when there's a difference between front and rear axle speed. continuous slippage would destroy the clutch in short order, which is why it is so important to have the same overall front and rear tire diameters.
True but if the clutch is not slipping then vehicle would get 50-50 while driving straight.

Interesting question by doug..
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Old 11-10-2015, 12:27 PM
DjD-X5 DjD-X5 is offline
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Saw these the other day...



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  #10  
Old 11-10-2015, 12:38 PM
Flying Ace Flying Ace is offline
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Quote:
Originally Posted by ninja_zx11 View Post
So then it means clutch is always slipping just enough to accommodate 60-40 torque split even while driving straight.
not necessarily. Perhaps in simple systems. But there's ways to electronically control the clutch slip's delay in the slip and speed.
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  #11  
Old 11-10-2015, 12:38 PM
u3b3rg33k u3b3rg33k is offline
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Quote:
Originally Posted by ninja_zx11 View Post
True but if the clutch is not slipping then vehicle would get 50-50 while driving straight.

Interesting question by doug..
still no. the clutch only engages the front axle. the rear axle otherwise receives 100% of available torque all the time.

if you're driving straight and the clutch is disengaged, 100% tq to the rear, with all 4 wheels turning the same speed. engage the clutch 100%, and now you are transmitting torque to all four wheels, but how much is going where? that is a function of which wheel has the most grip, not just how locked up the drivetrain is.
- if you floor it, does 50% of the torque get put down by the front wheels? not necessarily.

if the clutch is locked 100%, all four wheels turning the same speed, and both the rear wheels are on ice, 100% of torque is now going to the front axle.
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Quote:
Originally Posted by MeNoo View Post
Although I have nothing scientific to say, I can confidently say that it works.
Quote:
Originally Posted by Burning2nd View Post
If you want to use the fuhrers snowmobile you gotta have das tires

Last edited by u3b3rg33k; 11-10-2015 at 12:40 PM.
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  #12  
Old 11-10-2015, 12:40 PM
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Quote:
Originally Posted by ninja_zx11 View Post
True but if the clutch is not slipping then vehicle would get 50-50 while driving straight.

Interesting question by doug..
If clutch doesn't slip, the center diff operates as if it's "locked". For fulltime AWD systems like the Xdrive and most passenger AWD cars and SUVs, essentially, the clutch is constantly slipping due to road condition or the simply when you're turning your steering wheel.

This is a pretty good general reference on how diffs work, then add the complex case of electronically controlled diffs:
http://auto.howstuffworks.com/differential.htm
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Last edited by Flying Ace; 11-10-2015 at 12:44 PM.
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  #13  
Old 11-10-2015, 12:41 PM
ninja_zx11 ninja_zx11 is offline
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Nice find DjD

It says in the first video at 1.49 that most of the time clutch remains partially locked to output split torque.So it means its always slipping just enough to give 60-40 or 55-45 split.
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  #14  
Old 11-10-2015, 12:43 PM
u3b3rg33k u3b3rg33k is offline
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Guys, the clutch has no mechanism to dispose of heat created by slipping. if it slipped constantly, it wouldn't last 100k miles, it would probably last 100 miles or less. it only slips when there is a F:R axle speed difference.

just because it's not slipping doesn't mean it's not splitting torque.
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Quote:
Originally Posted by MeNoo View Post
Although I have nothing scientific to say, I can confidently say that it works.
Quote:
Originally Posted by Burning2nd View Post
If you want to use the fuhrers snowmobile you gotta have das tires

Last edited by u3b3rg33k; 11-10-2015 at 12:45 PM.
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  #15  
Old 11-10-2015, 12:46 PM
ninja_zx11 ninja_zx11 is offline
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Quote:
Originally Posted by u3b3rg33k View Post
Guys, the clutch has no mechanism to dispose of heat created by slipping. if it slipped constantly, it wouldn't last 100k miles, it would probably last 100 miles or less. it only slips when there is a F:R axle speed difference.

just because it's not slipping doesn't mean it's not splitting torque.
i Agree with you too
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  #16  
Old 11-10-2015, 12:51 PM
Flying Ace Flying Ace is offline
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Quote:
Originally Posted by u3b3rg33k View Post
still no. the clutch only engages the front axle. the rear axle otherwise receives 100% of available torque all the time.

if you're driving straight and the clutch is disengaged, 100% tq to the rear, with all 4 wheels turning the same speed. engage the clutch 100%, and now you are transmitting torque to all four wheels, but how much is going where? that is a function of which wheel has the most grip, not just how locked up the drivetrain is.
- if you floor it, does 50% of the torque get put down by the front wheels? not necessarily.
What you're discribing just one type of center diff, and which I assume is NOT how BMW designed it based on the article mentioned above. In fact, I believe, Ferrari and Porsche's AWD behaves more like this, an almost complete rear bias center diff that distributes 50-50 when needed. Another version I've seen are Acura's center diff for their AWD RL and TL cars which puts 100% of tq to front wheels, and when wheel-slip is detected, distributes torque 50-50.

Then there are more complex systems like STi and Evos which defaults to 50-50 but can resdistribute tq more extermely when needed.
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Last edited by Flying Ace; 11-10-2015 at 12:54 PM.
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  #17  
Old 11-10-2015, 02:06 PM
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The conditions that I can see and understand are;

Clutch open, no power/torque to front axle, 100% power/torque to rear axle.

Clutch shut no slip, 100% power/torque available to front axle and rear axle as a function of each axle's traction force and total axle DSC braking force.

Clutch shut slipping, variable power/torque available to the front axle, less than 100% power/torque available to the rear axle (less the power/torque forward and power lost to friction slipping heating in the clutch).

I see no way to default to 60/40.
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Old 11-10-2015, 02:37 PM
Flying Ace Flying Ace is offline
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Quote:
Originally Posted by Doug Huffman View Post
The conditions that I can see and understand are;

Clutch open, no power/torque to front axle, 100% power/torque to rear axle.

Clutch shut no slip, 100% power/torque available to front axle and rear axle as a function of each axle's traction force and total axle DSC braking force.

Clutch shut slipping, variable power/torque available to the front axle, less than 100% power/torque available to the rear axle (less the power/torque forward and power lost to friction slipping heating in the clutch).

I see no way to default to 60/40.
and default is clutch open?
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Old 11-10-2015, 02:57 PM
edwardmagichand edwardmagichand is offline
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Because of the size(diameter) of the clutch, 100% engaged clutch can transfer 40% torque to front and 60% to rear.
Attached picture is actually center differential gear and how to distribute the torque, but basic idea should be the same.
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Name:	front_vs_rear_torque_distribution.png
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Old 11-10-2015, 03:30 PM
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Doug Huffman Doug Huffman is offline
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Quote:
Originally Posted by Flying Ace View Post
and default is clutch open?
Apparently not.
Quote:
Permanent four-wheel drive is only discontinued extensively or completely in three control situations:
- During very tight cornering with little engine torque in order to permit speed compensation between the front and rear axles (e.g. parking)
- At speeds in excess 180 km/h
- When the vehicle understeers markedly
And
Quote:
In normal driving, the clutch is operated with minimum slip so that a permanent four-wheel drive with a driving-torque distribution of 40% on the front axle and 60% on the rear axle is available.
... but there's that weasel word available in a vehicle that is designed to 50/50 weight distribution.
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Last edited by Doug Huffman; 11-10-2015 at 03:34 PM.
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  #21  
Old 11-10-2015, 03:54 PM
u3b3rg33k u3b3rg33k is offline
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Quote:
Originally Posted by Doug Huffman View Post
Apparently not.
And... but there's that weasel word available in a vehicle that is designed to 50/50 weight distribution.
YES - exactly. torque goes where power goes. if you floor it off the line, you reduce weight on the front tires adding weight to the rear tires - this puts more weight to the rear (even with a 50/50 static split). the back wheels do MOST of the work (because of the primary rear drive setup of the system), but whatever they can't put down will get transferred to the front via the clutch pack instead of resulting in wheelspin. on launch, this may yield an effective 80:20 rear bias. in the snow, it may be a true 50:50. with a wheel/axle on ice/gravel/sand, "it depends".

Also, to those who keep using the word diff(erential), please note the typical BMW system has no center diff, only a clutch pack. I'm not familiar enough with the torque vectoring cars to talk about those (e.g. x6M), but I believe they are a brake bias torque vectoring system.
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Quote:
Originally Posted by MeNoo View Post
Although I have nothing scientific to say, I can confidently say that it works.
Quote:
Originally Posted by Burning2nd View Post
If you want to use the fuhrers snowmobile you gotta have das tires

Last edited by u3b3rg33k; 11-10-2015 at 03:56 PM.
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  #22  
Old 11-11-2015, 02:13 PM
ninja_zx11 ninja_zx11 is offline
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Quote:
Originally Posted by u3b3rg33k View Post
YES - exactly. torque goes where power goes. if you floor it off the line, you reduce weight on the front tires adding weight to the rear tires - this puts more weight to the rear (even with a 50/50 static split). the back wheels do MOST of the work (because of the primary rear drive setup of the system), but whatever they can't put down will get transferred to the front via the clutch pack instead of resulting in wheelspin. on launch, this may yield an effective 80:20 rear bias. in the snow, it may be a true 50:50. with a wheel/axle on ice/gravel/sand, "it depends".

Also, to those who keep using the word diff(erential), please note the typical BMW system has no center diff, only a clutch pack. I'm not familiar enough with the torque vectoring cars to talk about those (e.g. x6M), but I believe they are a brake bias torque vectoring system.
Yes u3b3rg33k you are right and it makes sense.X5's AWD is a rear drive base system.No matter what there will be always 100% torque available to rear wheels and it will depend on the traction at real axle to use 100% torque or 0%.

Lets say the rear wheels are on ice sheet with absolutely no traction and front wheels are on a surface with full traction, then 100% of the torque(power) goes to front wheels as rear axle will be incapable of utilizing all available torque.Rear wheels wont stop rotating and will rotate at the same speed as front ones.

While driving straight ,even thou the transfer case clutch is partially locked there will be no slip as both axles are rotating at same speed.Not an expert but I assume this figure of 60:40 split is all depends on the torque demand on each axle (front or rear).
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