5 replies to this topic

[murpia]

Anyone familiar with the concept behind this BMW X6 rear differential?



It's supposed to be able to transfer torque side to side independent of drive torque (including under braking). I can't make out from the drawing what the epicyclics on each output shaft are connecting via the clutch pack. Seems to me that you'd want no clutch engagement to be equivalent to an open diff, to reduce wear and tear and for the vehicle to keep driving if the electronics fail. But the only way I can see this working is if the planet gears always rotate to transmit torque... unless there's something hidden in the drawing?

Regards, Ian

[rhm]

As far as I can see there's no limited slip function at all unless there's a shaft not shown on the cut-away that connects the planet-cage on each side together.

I assume the electric motors are to actuate the clutches? I wonder what the response time is like compared to an electro-hydraulic system.

[Ben]

Originally posted by murpia
Anyone familiar with the concept behind this BMW X6 rear differential?



It's supposed to be able to transfer torque side to side independent of drive torque (including under braking). I can't make out from the drawing what the epicyclics on each output shaft are connecting via the clutch pack. Seems to me that you'd want no clutch engagement to be equivalent to an open diff, to reduce wear and tear and for the vehicle to keep driving if the electronics fail. But the only way I can see this working is if the planet gears always rotate to transmit torque... unless there's something hidden in the drawing?

Regards, Ian

Without studying it in detail it sounds like the torque-steer diff that McLaren had a few years back and will be geared such that it would lock up without any slip in the clutches and not transmit any drive if they were fully open.

Ben

[phantom II]

An electronic diff is a wonderful thing to behold. It can optimize the torque to each wheel unlike a mechanical one. Porsches e unit is simply opened or closed. The F430 is more along the lines of a F1 diff with many electronic inputs. If the clutches slip to optimize torque, then the unit must be serviced regularly. Normally, the side shaft gear tries to get out of mesh when that particular shaft's torque is reduced or curtailed thereby applying pressure to the clutch pack which locks the side shaft to the ring gear carrier. Different ramp angle on the spider gears and multiple clutch packs will transmit the desired torque. A mechanical unit must be a compromised setting for a variety of situations unlike this unit.
I'm wondering if their isn't any torque multiplication taking place in those planetary gears if that is what they are and if it is used in conjunction with a type of Active Handling.
Normally, if the diff is open, the unloaded wheel can rotate up to twice the diff ratio. What function do the two gears between the clutch pack and the shaft gear perform? Do they over or under drive that particular shaft with hidden planetary gears?
Maybe the clutch locks the one set of these gears to the shaft and the electric motor rotates the other set either clockwise or anti clockwise. This would increase the life of the clutches. The clutch would be used in slow high torque turns and the motor would be used in fast high G turns where there is high transfer and little downforce as is the case with a road car.
I found in my high powered light roadster that I was getting too much torque to the outside wheel in high speed turns so I would have to reduce throttle to prevent power oversteer. When the clutches began to wear out and the diff began to open, I could use more power and go faster. Opening the diff more, reduced the life of the clutches though. The BMW diff also looks heavy.

Originally posted by Ben


Without studying it in detail it sounds like the torque-steer diff that McLaren had a few years back and will be geared such that it would lock up without any slip in the clutches and not transmit any drive if they were fully open.

Ben

[cheapracer]

Looks like a complicated way out of paying Mr Torsen (Gleason) his rightful dues to me.

[murpia]

I found a link suggesting that the planetary gear sets have a 10% overspeed compared to the differential cage. Therefore those clutches that are actuated by the DC motors & ball ramps can transmit extra torque from the differential cage to the halfshaft as long as the halfshaft does not overspeed the differential by more than 10%. The diagram above must be incorrect as those planetary sets don't look 10% different to me, hence my initial confusion...

Clutch / motor failure or 'normal' driving = open diff with no torque in the planetaries (OK for wear)

I must say I like the idea of this kind of active diff, would be interesting if one of these found it's way into a racecar with a decent control system, or even a passive system generating hydraulic pressure under roll conditions to activate the clutches...

Regards, Ian