Originally Posted by Tripod
It actually can disengage the front axle. There is a whole discussion on jeep renegade forum on that subject.
No, it can't, and the post you quoted from the renegade discussion backs me up on that. They don't know what they are talking about either, but the one accurate part is
However the GTK animation would seem to indicate it is 100% of available torque which may be close to 100% but not quite if your front wheels are free spinning.
^^^ That exactly. The front CV axles are always being driven, period. They do not disconnect. The PTU locks the rear driveshaft to the CV axles' assembly via dog clutch.
Lets keep the complexities of the RDM clutch out of the discussion for a moment and just assume the RDM variable clutch is at 100% lockup. In this condition, at least one rear CV axle is bound 1:1 to at least one front CV axle. I say "at least one" because there are open differentials front and back.
Leaving out the additional complexities of the open differentials and their side-to-side splits, lets assume left-to-right things are dead even on each axle but different front to back, so say front tires on glare ice and rear tires on tar. The PTU is locked and the RDM is at 100% lockup. In this situation, yes virtually all the usable
torque "goes to the rear" but the front wheels are still being driven
, they are just being driven nearly uselessly as they have no traction. So your thinking is close when you say "which I assume 10% is wasted on disengaged CV joints" its actually X percent wasted on front wheels that have no traction
. They (the front CV axles) are engaged and they cannot slip because of the 1:1 PTU, there is no differential between the front and rear axles so you can't slip the fronts unless the rear has also broken traction (still working under the assumption that the RDM variable clutch is at 100% lockup).
I can't really get more into this now though, other busy things to do today.
Edit: One last piece of 2 cents, though, in regards to the video:
As an engineer myself, I know that it is all too common for sales/marketing to mis-speak or mis-represent or mis-understand the product that I have built when they turn around and start selling or demo-ing it. It happens all the time and very, very easily. In this case, it seems to be a misunderstanding that stems from them saying "the torque" when they mean "the available
torque", and on top of that they are either accidentally or purposefully skimming over the finer details, probably in an effort to simplify the explanation for people (which is fine), but hurts the overall technical accuracy.
No matter that they suggest, it is impossible for this system to drive only the rear wheels and not the fronts. If the fronts have no traction and the rears do, then its kinda technically correct to say all usable/available torque is being used at the rear axle but the front is not disengaged, and as soon as the front tires DO have any sort of traction then that reduces what is being used at the rear axle.
I'm having a hard time boiling it down to something that is simple and easy to remember. How about "The rear axle can use up to
whatever input torque that the front wheels can't use (due to lack of traction)."
Easy enough until you start talking about the left-and-right splits from open diffs