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xDrive (Deutsch, English, Русский) |
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| Last changed: 2024/01/01 00:00 / History | Edit |
bmw's xdrive all wheel drive system (Edit)Proactive automatic all wheel drive system. The rear wheels are powered at all times. Torque can be transferred to the front wheels via electronically controlled multi plate clutch that is located in the transfer case. XDrive does not have a center differential. Most of the time, the clutch is partially locked and power is transferred to both axles in proportion 40/60 front to rear. The power distribution can be altered to continuously variable levels (from 50/50 to 0/100). For example, when driving at high speed, or when parking, the clutch is disengaged and all power goes to the rear wheels. XDrive is linked to DSC (Dynamic Stability Control) - when the vehicle understeers{f}, the clutch is disengaged thus transferring more torque to the rear wheels (10/90citation needed torque split). When oversteering is detected, the clutch locks fully transferring more power to the front wheels (50/50 torque split). DSC (Dynamic Stability Control) also brakes wheels individually to regain traction and to help cornering. XDrive intervenes before the car becomes unstable and is unnoticeable by the driver. Figure: bmw 5 series xdrive  Figure: bmw xdrive transfer case 
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| Last changed: 2024/01/01 00:00 / History | Edit |
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Hi,few questions:
1.In Xdrive,if rear axle is permanently connected to the transmission,then how it can transfer 100% torque to the front axle?
2.If driving straight with constant speed with equal traction on all 4 wheels,how torque is split?Won't it be 50-50 split of torque in that case?
thanks
1. "simply" due to the fact that in the case of spinning rear axle there is almost 0 torque going to it. in order to transfer torque, you need to have traction.
2. since there is no center diff, for driving situations where no wheel slip is likely (say highway speed, straight line, etc), only the rear axle is powered
Are there any viscous couplings in the xdrive system?
Is this system better than the T31 xtrail (lastest version).
And does tyre wear need to be balanced?
And what about when DSC is off ? Power distribution and other things ?
sounds crappy.. needs to be taken care of - clutch + it is reactive: it acts after the wheelspin has already occurred. torsen based 4wd is proactive - it acts BEFORE wheelspin occurs + there's not much to wear out! 
This system doesn't have to be reactive. There can be initial pre-load on the coupling (easily changeable via software) and that can be variable depending on steering angle. (more if the front wheels are straight)
Seeing as BMW (and almost everyone) use throttle-by-wire, if the software detects that you have 'floored' the throttle, the clamp load can go to maximum, resulting in no rear wheelspin-maybe both tires on the same side, then traction control will brake those two.
Under extreme conditions, when you operating outside the torque bias ration, this can be better than a torsen differential.
I could not understand how HDC works and where is it located in the vehicle? (hill descent control) this is on a automatic transmission.
Hi Hassan. You can find HDC description on the BMW web site
HDC on BMW Web site
Also see youtube video:
DEAR SIR
AT WHAT VEHCILE SPEED X-DRIVE STRAT ACTIVE
Hi,
I am sure the x-drive four wheel drive is active right from 0 kph.
http://www.youtube.com/watch?v=dGWsogAlZLE
