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      02-18-2014, 05:52 PM   #1
GWA
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Multi plate clutch - seeking a mechanical genius

Does anyone know how the mechanics of the multiplate clutch can actually work?
1. The X3 does not have a central differential to enable the car to cope with the different speeds of axles on a bend. So what happens if the front axle receives power from a fully engaged multi plate clutch? In 4wd's without the centre diff, drivers are warned about transmission windup if turning on hard surfaces - "never do it" they say. 2. The standard setting is a split 40% front and 60% rear. Wouldn't a clutch trying to send part power to the front be constantly semi engaged and therefore slipping. Analogous to driving a manual car with the clutch semi engaged. An amateur might image this generates heat, wear and tear, and consumes power in the slipping activity. And an additional query is that even with a front axle partially engaged, on a turn, the car would be trying to even axle speeds and thus encourage slippage at one end, sending the driver into a skid.
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      02-18-2014, 06:22 PM   #2
philtrick123
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Any help?... http://www.awdwiki.com/en/xdrive/
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      02-18-2014, 07:39 PM   #3
EvilGenius
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My guess is that software "reacts" for this in favor of controlability, and allowing the clutch slip in the transfer case. That is also probably why that is a wet clutch: to more gracefully handle the slipping and subsequent heat. The computer knows your speed and steering angle, it probably works to decrease front drive coupling progressively as steering angle increases, unless DSC sees a need to intervene otherwise (understeer situation will transfer torque to the front wheels). In the end though, it isn't like feathering the drive clutch from a stop where you have a moving surface scrubbing against a non-moving one, this would be a comparatively minor amount of rotational deltas between front and back drives.

It would be my guess that we can pontificate ad infinitum about how the software does what it does based on our inputs given the driving dynamics at a given moment, but I'd imagine that is as close as we can ever get as I'm sure that is pretty close to the Holy Grail of BMW It's not mechanics anymore: it is all in the software. It is fun to try to guess though!
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      02-19-2014, 12:05 PM   #4
HighlandPete
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The system will only function across the breadth of working parameters with some slip in the clutch pack. We are right to assume the clutch pack is designed for a continuous duty cycle. If not it is a weak link. In normal use the clutch is partially engaged so in slip/drag mode. The wear rate of the clutch is continuously monitored, to keep the system's clutch torque rating in keeping with design intent.

Much of normal driving will have minimum slip anyway, as the clutch input and output shafts should be running the same/similar rotational speeds.

You can see why tires need to be in close tolerance, or there will be additional slip, possibly in conflict with the software settings. Hence why there have been transfer box issues with mis-matched tires in the E83 models.

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