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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > All-Wheel-Drive (Xi / xDrive) Talk > Handbrake(Turn) and xDrive logic



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      12-05-2022, 08:12 PM   #1
k90
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Handbrake(Turn) and xDrive logic

Hello xDrive pros!


I'm new to this particular flavour of AWD. It goes like stink on snow and ice - Legit!
Altough it's a bit mysterious as it's based on countless routines performed by microprocessor(s).

It appears that xDrive stays fully engaged through attempted manouvering using handbrake.

Tried to align the car to garage door by habit by locking rears, but instead I felt fronts digging into snow and the car just stopped right at the spot!
- Nannys off
- Clutch pressed - 1st gear

In mechanical AWD systems there is some resistance and vibrations in center diff when locking rears but not like this.

Can you confirm xDrive stays engaged when there is no drive to wheels?

Also drum brakes didn't release after that heavy braking by driving forward but I needed to reverse a bit and drum brakes popped open. Is there any fix for this issue? I just did first handbrake adjustment for this car as the thread became visible 3-4mm inside drums. Do I need new pads?

And if there is options to make xDrive to disengage momentarily in example when:
ignition on &
nannys off &
speed is below certain threshold &
no drive to the wheels &
throttle pedal 0% &
handbrake is used
or some combination of those?


Misc questions!

What happens when ignition is switched off? Does xdrive actuator release the multiplate, leave it where it was or bind it to 100%?

Can xDrive be towed 1 axle on road? Which axle? Ignition on/off? Any special buttons or tricks? Just only flatbed? If you had to?
It will burn multiplate to crisp?

Is there rotation speed requirement for front shaft or rear shaft in order to lubricate and cool all the bearings, gears and plates inside transfer case? Does it work in reverse direction too?

I reversed a driveway that went up a slight grade. Rears spinned at top(and the fronts didn't push me where I wanted to go) when I tried to do a reverse turn to road. What happend? It went momentarily RWD!

How quickly does xDrive actuator cover the full range from 0%-100% or 100%-0% torque to front? I know the marketing says xDrive reacts in milliseconds or 10milliseconds, but for sure it just the frequency of program loop updating state of the system. The actuator itself must be much slower. Any idea of the maximum no-load RPM of the actuator motor?

Can you overheat & destroy the multiplate of stock car by any driving style etc. ? Is multiplate a service item like in some haldex cars you might need to do the plates sometimes from what I have gathered. Yeah I know xDrive has all the issues, but is plate wear one of them?

Last edited by k90; 12-05-2022 at 08:16 PM.. Reason: +the +from
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      12-06-2022, 05:18 PM   #2
Brian86
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I can answer some questions.

Ignition off: TC clutch disengaged

Towing: Both axles off the ground all the time. (unless you remove a driveshaft)
Even disengaged there is still some 'drag' in the TC clutch. With one side at zero, and the other at wheel speed the clutch gets smoked. Short distances in and out of the garage are safe, but no road speed.

Driveline lubrication: It'll be fine, it all just works.

X-drive reverse: The software is likely programed to disengage the front axle in reverse depending on wheel speed and stability. BMW's overdrive the front axle by a few % so the potential is there for the car to become very unstable or do strange things in reverse.

Parking brake: Probably needs adjusted, better shoes. The hand brake on e9x is very fickle. I have to leave mine much looser than I like otherwise it vibrates when engaged at speed.

Handbrake slides: I don't really know. I would imagine in low traction conditions the computer keeps the TC engaged even off throttle. A little torque to the overdriven front will help keep the car straight. When locking the rears that creates a very large differential between the front/rear plates in the TC so I could see it binding and locking the fronts as well.

TC durability: They are extremely durable. My first one had 280k miles and worked perfectly when removed. Swapped it out due to a failed front output yoke due to the previous owners neglected maintenance.


Yes, these cars are absolutely brilliant in the snow
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      12-07-2022, 10:25 AM   #3
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You cannot transfer 100% of torque to the front. Rear is always driven, just as in RWD. Front can only be "attached" aditionally, so it can be 50% most.
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      12-07-2022, 05:36 PM   #4
k90
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Quote:
Originally Posted by Saico View Post
You cannot transfer 100% of torque to the front. Rear is always driven, just as in RWD. Front can only be "attached" aditionally, so it can be 50% most.
You would think so but..

Quote:
Originally Posted by Brian86 View Post
X-drive reverse: The software is likely programed to disengage the front axle in reverse depending on wheel speed and stability. BMW's overdrive the front axle by a few % so the potential is there for the car to become very unstable or do strange things in reverse.
This results in various percentages of torque to front axle up to 100%.

If center is locked all the way by xDrive system then fronts will rotate faster than rears inevitably.
In low-slip driving condition that would actually result in situation where rears are dragging? If rears are dragging then for certain 100% front torque.

And even without ratio differences of front/rear if you set rears on rollers torque to rear is for practical purposes 0%. That means 100% front torque with one exception: All 4 wheels on rollers and then torque % is undefined as it would require division by zero.


Static state torque table for xDrive:

100% to rear by command....Possible
100% to front by command...Impossible*
100% actual torq. to rear......Possible
100% actual torq. to front....Possible**
0% to rear by command.......Impossible*
0% to front by command......Possible
0% actual torq. to rear.........Possible**
0% actual torq. to front........Possible

Static state torque table for Haldex:
Same than xDrive but swap rear<->front(excluding one or two car models that have mid engine + haldex).

Static state torque table for center locked 4x4(traditional / truck / offroad4x4):
100% to rear by command....Impossible***
100% to front by command...Impossible***
100% actual torq. to rear......Possible
100% actual torq. to front....Possible
0% to rear by command.......Impossible***
0% to front by command......Impossible***
0% actual torq. to rear.........Possible
0% actual torq. to front........Possible

Static state torque for Torsen diff & Open Diff:
0% and 100% torque impossible.
No by command torque % to any axle.
Open diff has fixed torque split.
Torsen diff has biased torque split and can transfer torque between front-rear within boundaries of torque biasing ratio of the diff.
Torsen example: TBR 3:1 and 50/50 ratio results in 75% maximum to any axle. Whatever absolute torque that 25% axle has will be "amplified" 3x to another axle.

Static state torque for Electric AWD:
0%-100% torque actual or by command to any axle possible, but might limit the total absolute torque. Usually the rear motor has the beans.

*: as rears are driven always. 100% to front by command would require no drive to rears.
**: ie. rears on rollers
***: as rears and fronts are driven always and 100% locked. There is no by command torque % to any axle.

Last edited by k90; 12-07-2022 at 07:00 PM.. Reason: awd autism kicks in
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      12-08-2022, 07:22 AM   #5
Saico
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I've had my xdrive transfer box on the bench disassembled and assembled 4 times myself in the last few weeks and I can tell you that rear drive is FIXED, just as in the RWD car. There is no option to disengage rear drive, so saying it might be 0% engine torque in usual road conditions = not possible. What you're saying might have to do with rollers and applying load only on front rollers with rears spinning freely - then yes ACTUAL torque will go to the front BUT the same amount of torque is also initially applied to rear whells and makes them spin.

Last edited by Saico; 12-08-2022 at 07:30 AM..
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      12-08-2022, 08:32 AM   #6
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Quote:
Originally Posted by Saico View Post
I've had my xdrive transfer box on the bench disassembled and assembled 4 times myself in the last few weeks and I can tell you that rear drive is FIXED, just as in the RWD car. There is no option to disengage rear drive, so saying it might be 0% engine torque in usual road conditions = not possible. What you're saying might have to do with rollers and applying load only on front rollers with rears spinning freely - then yes ACTUAL torque will go to the front BUT the same amount of torque is also initially applied to rear whells and makes them spin.
That's why I defined it as "Static state" so change in rotation rate is not a factor or rotation at all to say.
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