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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > All-Wheel-Drive (Xi / xDrive) Talk > Attn: If Interested In 100% Rwd



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      11-02-2011, 07:42 AM   #23
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Originally Posted by Doyle View Post
Have you done any research into the TCU and how it communicates with the ECU? Have you had any experience with the logic of the ATC 300? Have you ever coded/tuned an x-drive car? Or are you just spouting off about something that you have based entirely around assumptions? I'm sure you are fully aware of the Haldex controller and how it can "rewrite" how the transfer case reacts in VW/Audi cars...

RWD is for performance and AWD is for traction? I'm sure the Porsche Turbo, Nissan GT-R, Audi R8, most Lambo's, Evo's and STi's would prove otherwise. Can't have it all? I'd point you to the GT-R's Nordschleife wet lap record.

Terry himself has said that it could be very possible. The point of this is to get more attention on this capability, not to shit on every idea and stagnate the community. Take your negativity back to the Raceland downpipe threads.
What does the tcu have to do with the dme (ecu)?

Tcu is telematic control unit or something like that. It handles bluetooth and bmw assist. What does that have to do with the dme?

Edit: nevermind. I realized it can also be transfer case or torque converter. Just never seen it abbreviated as TCU.

Last edited by fdriller9; 11-02-2011 at 08:08 AM.
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      11-02-2011, 07:45 AM   #24
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Originally Posted by CrimsonIvy35i View Post
I read somewhere. You can pull a fuse, and it disactivates the FWD. 100% RWD. Just by pulling a fuse.
Yea its fuse 26 on n54s and 20 on n52s.
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      11-02-2011, 08:13 AM   #25
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The TCU is the transfer box control unit. From what I understand it receives the diagnostics for all speed/safety sensors from the ECU and processes the input into the ideal torque split:

http://bmwfans.info/parts/catalog/E9..._transfer_box/

I think the torque split discussion is just semantics. Also, note that the ATC300 is gear driven, not chain driven like the SUVs (due to packaging issues in the transmission tunnel, I believe).

Danny: that was a good read, and similar to what I believe some members were attempting on their own. I have no idea where they stand with that progress, but intercepting and resisting the signal to the transfer case (thus keeping it open 100%) would be the simplest solution as it does not mess with the coding/logic. It's been awhile since my EE class, but that looks easy enough. I'm not sure any codes/errors would be thrown up, since the signals that the ECU and TCU read would still be within safety range.

Some other option that I have been thinking about are (in order of increasing difficulty):

1) Isuzu solution
2) Remove the front driveshaft. Problem: wheel speed sensors, dsc, and subsequently the front diff might get a little funny still thinking that they have drive, but can't actually direct the torque.
3) The TC goes into "limp mode" at a certain temp. Fool that individual sensor, and code clear with JB4/BT tool. Problem: That may pop other sensors/limp modes, though. You also lose any safety features on a $2000+ transfer case.
4) The TC opens up at parking lot speeds and above a certain "highway" threshold (I can't remember if BMW said 70mph or 100mph). Allow the JB4/tune to fool the TCU by sending an increased/dummy speed signal to the TCU. The benefit of doing this gives you complete control over when the TC acts like normal and when it opens up. At a drag strip? Send the TC a speed signal of 30mph, and use it as launch control. At the road course? Send the TC a speed signal of 0mph and have quasi-RWD. Winter time? Send the TC a speed signal of 100mph. Problem: this gets into coding/tuning and I'm not sure how problematic that could be. However, it may be the least "invasive".

Also, conveniently enough, one of the other BMW sites linked this tool to override the stock DSC and ABS. It looks extremely promising:

http://www.racelogic.co.uk/index.php...action-control

Last edited by Doyle; 11-02-2011 at 08:43 AM.
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      11-02-2011, 08:55 AM   #26
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Quote:
Originally Posted by ajsalida View Post
Danny you are right, stand alone is the wrong word. Foundational would be better, it cannot be turned off just like, and it is intergral with, ABS. BTW the e-diff (DBC) is also on 2WD cars. Very curious to see what can be done here.

As far as the %TQ that can go F/R, we are into marketing semantics here. If the rear cannot be disengaged, and the front can be, then the most the front gets is 50%. In the same manner an ordinary diff, when welded, is sending at most 50% TQ to each wheel.

In xDrive, if the rear driveshaft cannot be disengaged from the tranny, then the least amount of TQ it can receive is 50%. Convoluted scenarios involving what tire/axle has zero traction don't count as part of the machinery.

You do not work for BMW marketing by any chance do you?

edit: said another way the rear driveshaft can never rotate slower than the front, and that means at least one rear wheel is rotating at the same rate as the fronts, always.
I'm not trying to bust your balls here, but an open diff always provides equal torque to both outputs, a welded diff (or a limited slip diff) can send unequal torque (a locked jeep with a wheel in the air, an AWD sports car with spinning rear tires). Or, imagine a spool being driven around a corner. Outside wheel is trying to overrun the carrier but can't, the torque to it is against the direction of travel, inside wheel is trying to underrun the carrier, torque is with the direction of travel. (tension builds in the tires until one or the other slips, which releases the tension).

Remember also that because the front is the steered axle, it's wheel speed is greater than or equal to the rear axle (dependent on steering angle, and many RWD based AWD units use a tiny fixed overdrive % to the front, meaning they're not being locked progressively to unity, but towards a slight front overrun). The transfer clutch is coupling the front and the rear axle together, not dividing transmission output torque.
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      11-02-2011, 08:59 AM   #27
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also, I asked in the RWD thread but never saw a response: Has it been verified that the parking brake does not disable the transfer clutch? most AWD systems open any transfer coupling or electronic slip limiting devices when the brakes are applied in order to not interfere with ABS braking, and this usually also applies to the parking brake.

Has anyone taking the xi to gravel or snow and pulled the e-brake up just enough to close the contact for the brake lights to activate, and then tested for absence of front drive?
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      11-02-2011, 09:31 AM   #28
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I know this is a bit of a trollpost, but although I find the idea of having temporary RWD entertaining to do some donuts in a parking lot, I find the whole concept preposterous - considering we have better traction than RWD people, we can put more consistent power down without worrying about traction loss, and we can run circles around RWD guys on wet tracks, where are the benefits? And if there are so many, why did we end up with AWD cars in the first place? I love it when it starts raining on the track - you see all the Corvettes and Mustangs panic and lay off the gas, while you happily pass them all. Heh.
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      11-02-2011, 10:00 AM   #29
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I get what you are saying in regard to torque distribution. I think the disagreement comes from how "available torque" is defined. When you say "The transfer clutch is coupling the front and the rear axle together, not dividing transmission output torque" do you mean that the maximum the front driveshaft can turn is the same speed as the rear driveshaft, plus overrun through gearing?

Not sure if you've seen this post from AJ, as I think it explains his point the best: http://www.e90post.com/forums/showpo...2&postcount=25

With regard to the parking brake, I can give it a go this afternoon. However, since the default setting for parking lot speeds is 100% RWD, I wonder how accurate the results would be... That would be a quick solution, though!
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      11-02-2011, 10:11 AM   #30
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[quote=Pseudo Nim;10726783]Entire post.QUOTE]

No worries. I don't think it is a troll post since you came in asking a question instead of being baselessly declarative!

It goes much further than "donuts in a parking lot". I agree that on wet tracks the AWD is great. I can fearlessly drive my car 12 months in basically all conditions. That is why I bought an 335xi, after all!. However, BMW wouldn't let me take the car on a test drive to Summit Point or VIR.

However, driving where there are quick transitions, I have found that the x-drive system is very intrusive. During tight corner exits, my rear wheels will slightly break loose, and then drive will rapidly switch to the front, which then slams the car into understeer, which in turn pushes power to the rear etc... The ability to steer both ends of the car in RWD mode on dry pavement would be great. Additionally, there might be a way to use the AWD system as a "launch control". This will give us the traction advantages of AWD, but above a certain speed/grip threshold, we can gain back some of the driveline losses.

If nothing else, it gives the owner the choice as to what they want. As an American I'm all about freedom of choice, LOL.
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      11-02-2011, 10:38 AM   #31
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Pseudo Nim, people seem to complain that BMW's management of the AWD system is moderating slip excursions too aggressively. People want to be able to use the throttle to get the car to turn, like they can with RWD.

BMW seems to think a slight understeer/near neutral attitude is ideal, and is pretty strongly enforcing it on the chassis with the AWD.

Of course, everything that makes an oversteery awd system fun in the dry makes it harder to manage in the wet/slippery stuff. Most car companies are looking to simplify the operation of the cars, especially four wheel drive, because many users either do not want to or can not handle the complication of making these drivetrain decisions.

It seems that most people don't want to deal with a tarmac/gravel/snow button, a la the evo, and they probably want less scrutable systems like the subaru DCCD with Auto/+/- and the 6 levels of manual lockup even less.

So, as with many other aspects of the car, it's up to the enthusiast to fend for him/herself.
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      11-02-2011, 10:50 AM   #32
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I see - thanks for the comments, Doyle and DannyZRC. Now that you mention it - it's true, it's pretty bloody hard to control the car with the throttle, even on ice - let alone on dry pavement. That said, I found that generally speaking, even with a tremendous amount of mods, the car holds so much traction in the dry that it would be pretty difficult to do it in the first place... but then again, you probably have a point in that as soon as traction starts going it shifts torque to the front to prevent "accidental" (in its view) oversteer. So fair enough - I guess that's what the major complaint is.

Enh. Way I see it, the solution isn't to go RWD, but to have a more adaptable AWD, as someone said, like in the GTR.... or have something much more simplistic, like in an Impreza. Since neither's an option, I guess we'll just have to ... "suffer", if it can so be said. Heheh.
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      11-02-2011, 11:13 AM   #33
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Oh, I've def broken all 4 tires loose on a sweeping left. That was with JB4 map 1 and 245 square re01r's. The torque on this car is just absurd.

Absolutely, the idea system would be a new "standalone". That would take some serious backing/coin to devise. Right now, that may not be feasible, but in the future, I think it will be. Alpina rewrote the x-drive for the B7x and there are rumors that the new m3/5 might be available with AWD. Also, check the link I posted a few hours ago. That essentially hijacks the stability control systems and lets you tune how much slip angle you want at each axle before safety systems cut in. Potentially that, plus a rear LSD may work...

Great looking Shiba, by the way. I had one growing up!
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      11-02-2011, 11:18 AM   #34
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I know one of the aftermarket AWD controllers for the Nissan ATTESSA E-TS in the GT-R Skylines works by intercepting the gyro and accelerometer signals and tweaking them.

remember, the xdrive in the 3 series is basically identical in hardware layout to the GT-R everyone mentions, the difference in behavior comes from how the transfer clutch lockup duty cycle is managed.
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      11-02-2011, 12:14 PM   #35
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Danny-

How time/labor intensive would that be to develop? I believe you are talking about the Field's ETS controller? That solution would be superb, if not elegant.
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      11-02-2011, 12:36 PM   #36
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Quote:
Originally Posted by DannyZRC View Post
I'm not trying to bust your balls here, but an open diff always provides equal torque to both outputs, a welded diff (or a limited slip diff) can send unequal torque (a locked jeep with a wheel in the air, an AWD sports car with spinning rear tires). Or, imagine a spool being driven around a corner. Outside wheel is trying to overrun the carrier but can't, the torque to it is against the direction of travel, inside wheel is trying to underrun the carrier, torque is with the direction of travel. (tension builds in the tires until one or the other slips, which releases the tension).

Remember also that because the front is the steered axle, it's wheel speed is greater than or equal to the rear axle (dependent on steering angle, and many RWD based AWD units use a tiny fixed overdrive % to the front, meaning they're not being locked progressively to unity, but towards a slight front overrun). The transfer clutch is coupling the front and the rear axle together, not dividing transmission output torque.
Not sure what you are trying to do, but an open diff does not always send 50% TQ to each wheel. In fact it never does exactly when you'd want it to. Consider when one wheel is on ice/sand and one has traction. It spins the wheel with the least traction. This is why we want LSD, to avoid the fact that open diffs send all the TQ in these situations to the one wheel with no grip. Exactly the opposite of what one would want. BMW's e-dynamic brake system (fake LSD) mitigates this by braking the spinning wheel while under power, thus sending TQ to the opposite wheel that has traction. Your argument makes no sense to me, though I may just not understand what you're trying to say.

I raised the issue of a welded diff only to illustrate how the BMW transfer case functions when locked up. Both drive shafts rotate same speed. If you wish to interpret that as under certain conditions it is transferring 100% TQ to the front wheels, by all means do so. BMW marketing does and it is somewhat disingenuous, attempting to hoodwink folks into thinking there is a center diff in place when there is not.

My layman's opinion of the BMW xDrive AWD system is that it is adequate. Not great, definitely not bad but adequate. They went cheap, with two open diffs and one TC case (not a diff). But they added it to a brilliant RWD platform, with good electronics, and that makes up for a lot.

Very interested in what you have to say about modding this system.
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      11-02-2011, 01:05 PM   #37
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Quote:
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Great looking Shiba, by the way. I had one growing up!

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yep, that's one awesome dog.

quite proud of her

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      11-02-2011, 01:35 PM   #38
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Quote:
Originally Posted by ajsalida View Post
Not sure what you are trying to do, but an open diff does not always send 50% TQ to each wheel. In fact it never does exactly when you'd want it to. Consider when one wheel is on ice/sand and one has traction. It spins the wheel with the least traction. This is why we want LSD, to avoid the fact that open diffs send all the TQ in these situations to the one wheel with no grip. Exactly the opposite of what one would want. BMW's e-dynamic brake system (fake LSD) mitigates this by braking the spinning wheel while under power, thus sending TQ to the opposite wheel that has traction. Your argument makes no sense to me, though I may just not understand what you're trying to say.

I raised the issue of a welded diff only to illustrate how the BMW transfer case functions when locked up. Both drive shafts rotate same speed. If you wish to interpret that as under certain conditions it is transferring 100% TQ to the front wheels, by all means do so. BMW marketing does and it is somewhat disingenuous, attempting to hoodwink folks into thinking there is a center diff in place when there is not.

My layman's opinion of the BMW xDrive AWD system is that it is adequate. Not great, definitely not bad but adequate. They went cheap, with two open diffs and one TC case (not a diff). But they added it to a brilliant RWD platform, with good electronics, and that makes up for a lot.

Very interested in what you have to say about modding this system.
An open diff ALWAYS, ALWAYS, ALWAYS, gives equal TORQUE to both outputs. ALWAYS. Torque is one thing, Power is another.

An open diff is a path of least resistance for power, and an equal distributor for torque.

the example of the spinning wheel on ice, it takes very little torque to spin the wheel on the ice. Because the open diff ALWAYS, ALWAYS, ALWAYS provides equal torque to both outputs, the wheel that has traction is receiving the same torque as the wheel spinning on ice, which is very little. However, the wheel that is not spinning is receiving now power, while the wheel on ice is receiving lots of power.

When an eLSD brakes a spinning wheel, it is increasing the torque required to turn it, and thus also increasing the torque provided to the non-slipping wheel. This reduces the power flowing to the spinning wheel, and increases the power flowing to the gripping wheel.

Torque, Power, 2 different things.

There is nothing disingenuous about stating that a locked transfer clutch can transfer 100% of the torque if it can do so. Some transfer clutches don't have the torque capacity to do this, and will slip under such high loads, others won't. I don't know the torque capacity of the clutch used in the xdrive systems in question. Don't lose sight of the reason for the system in the first place, which is to get the power to the ground effectively. If the rear is not slipping, why drive the front? If the rear IS slipping, then the front can get up to 100% of the available power.

As to saying it doesn't have a center diff, this has both up and downsides. A center diff allows the front and rear to both receive torque constantly, irrespective of wheelspeed difference, but this also means that now instead of a diff spilling power on a wheel on an axle, you can have a whole axle slipping. an AWD system with 3 open diffs and no slip limiting would lose forward drive if ANY of the 4 wheels lost traction. xdrive can't have this happen, since the nature of the clutch coupling in the transfer case is to limit slip. Even without eLSD, xdrive with it's open diffs would need to lose traction on a minimum of one wheel on each axle for it to lose forward drive, if either axle has both wheels driving the car will have drive.
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      11-02-2011, 01:37 PM   #39
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Quote:
Originally Posted by Doyle View Post
Danny-

How time/labor intensive would that be to develop? I believe you are talking about the Field's ETS controller? That solution would be superb, if not elegant.
You have to reverse engineer the control laws for the AWD system, and then engineer your box to spoof them correctly to achieve the desired result, while not causing any untoward behavior in edge cases.

too many variables involved to know how much work it would be.
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      11-02-2011, 02:05 PM   #40
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Danny-

Makes sense to me.

I figured as much. Unfortunately, since the 335xi isn't considered a performance car, I doubt we'll get that much aftermarket support for it. I still believe that there is a grassroots or simple solution out there...
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      11-02-2011, 02:22 PM   #41
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DannyZRC - holy shit, that's a great writeup. Thanks for that.
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      11-02-2011, 02:23 PM   #42
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yep, that's one awesome dog.

quite proud of her

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most pain in the butt dog to own! its like having a new born baby, i have one now, just waiting for it to croak lmao!
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      11-02-2011, 02:28 PM   #43
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most pain in the butt dog to own! its like having a new born baby, i have one now, just waiting for it to croak lmao!
you have a what now - a newborn baby or a shiba?

totally true though. such a huge hassle dog. but love her to death. don't think I've been out once where she didn't get a compliment - and DON'T SHE KNOW IT.

women.
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      11-02-2011, 02:40 PM   #44
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Hahaha, they are more cats than dogs. I decided to try "real dogs" with my new family...American Bulldogs.



Now back on topic, lol!

Again, great, concise write-up Danny. I'll report back to see if the parking brake disengages AWD.
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