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Well it just so happens I replaced my rear tires a few minutes ago and rotated the rears to the front, and I distinctly remember no such freewheel action like it was an old Buick.
I think this may be semantics if that's the right word... I believe only the rear wheels are propelled or 'driven'.
I won't pretend to understand how the transfer case works. I think the link posted by another member above (link) explains it. I looked it over a while ago but I have a bad memory.
Thanks for the feedback... keep it coming.
Here is a little info I have been able to dig up on the torque biasing center dif configuration. It uses what is referred to as an epicyclic planetary gear differential. to create the basic torque split between the front and rear drives. then in addition Porsche added the variable pressure clutch pack to enable the computer to control relative angular velocities between the front and rear drive shafts. this clutch pack is what enables the system to effectively control the torque split from 0/100 to 100/0 based on wheel slippage.
the Epicyclic Dif can actually be seen in the cut away from FCP Euro site posed above. but his interpretation of the system in that post is wholly inaccurate.
on youtube also hack open a Touareg/Cayenne T-case and you can see the Epicyclic Dif being extracted at about the 7:45 mark in their video.
I also stumbled across a Lancia Delta Integrale manual that does a reasonable job of explaining how the system works. In the Lancia they utilize a Ferguson Coupling (VC, Viscous Coupling) to limit the relative angular velocities of the front/rear drive, but the concept is the same as the clutch pack with the added benefit of the clutch pack having the ability to be computer controlled as opposed to the passive control of the VC.
In the end, what does this all mean? By mechanical design the transfer case of the 955/957 is full time AWD. There is no option in the stock mechanical form to convert them to 2WD. the electronic system is used to vary the amount of lock up force limiting the SPEED differential between the front and rear drive. disconnecting the electrics from the transfer case will result in the fixed mechanical front rear torque bias with no regard to drive shaft speed difference.
So what is the practical difference between AWD with a 0/100% power distribution to the rear and RWD?
All I care about is I have no clunk and the pig drives exactly like RWD.
So what is the practical difference between AWD with a 0/100% power distribution to the rear and RWD?
All I care about is I have no clunk and the pig drives exactly like RWD.
the Practical difference is that it can't be achieved with your transfer case.
Not practical, but in theory: to achieve 2WD that you will need to eliminate all torque carrying capability of the front drive line. i.e. remove the front drive shaft. BUT then you would need the system to fully lock the clutch pack to eliminate the speed differential between the front and rear t-case outputs, so the system would need to be tricked into thinking that you had front wheel slip even though there was no power being sent to the front wheels.
It would be nice if you could engage the mechanical center dif lock on its own, but unfortunately there isn't an independent input between the center dif lock and the low range selection. if you don't plan on going very fast, the easy solution would be to engage low range, then center dif lock and remove the front drive shaft. Then you would truly have 2wd ( and the benefit of being in 2 Low would enable you to do those smokey burnouts everyone's been asking for videos of).
In reality if you are able to drive with the electrics disconnected and get no abnormal noise, then you are fine sending power to all 4 wheels in the mechanically designed 38/62 split. You are still in AWD but the clutch pack limited slip is taken out of the equation.
My understanding is that you are are able to drive with the electrics disconnected and have no abnormal noises? If this is the case, I would just continue driving that way in AWD with the 38/62 split and no shaft speed differential limiting.(no clutch pack engagement)
With no noise when the electrics are disconnected it seems like the abnormal noise must be arising from something other than the basic high range AWD mechanicals. either the stepper motor is partially engaging low range or the clutch pack is being loaded and chattering, of the clutch pack thrust bearing is bad, or the system migrating toward low range and clipping the low range gear.
Have you tried low range to see if you get the same noise in low?
How about Low and Dif Locked?
I managed to stumble across that was very informative and helps describe the stepper motor action and how it effects the High/Low and clutch engagement.
Oh, ok, I guess a 38/62 split would make sense and then my understanding was wrong. Yeah with 450 HP I shouldn't need soapy water or snow to fishtail (I'll make sure tomorrow).
The source of the slight clunk while moving slowly (on and off the gas) in traffic was already determined to be a stretched chain by another expert. I don't see the link in this thread but I'll share if I find it.
The chain was undersized in the earliest models.
I'm beyond happy with my present fix and the bonus that comes with it. At least I still see it as a bonus.
Oh, wait, here is the link. It's in russian and not https so you may get a warning from your browser. http://transfer-case.com/
Oh, wait, here is the link. It's in russian and not https so you may get a warning from your browser. http://transfer-case.com/
thanks for the link, I was really excited to see an exploded view with individual parts listed, but then realized they were for the 958 T-case. Once I wen to the NV235 for out 955-957 it only showed a single cutaway drawing and no part numbers (rather disappointing). still a good site for reference though.
This has been a fun exercise in digging in and learning how these t-cases work. thanks for this thread and motivating me to go through the process to understand the inner workings of these.
Sure, I'm going to stop being mentally lazy and have a look myself.
Good thread, too bad I can't edit the title.
Also, I must be losing my memory... I already tried to do burnouts way back when. The ~60/40 split explains this nicely. I thought the pig was just too heavy.
Here's one last thread I found that explains what you said pretty simply:
Without the electric override the centre diff is an open diff.. That means if the front wheels are in the air you loose drive to the rear as well. The torque split at 62/38 means if the front wheels are bogged and there is only 38Nm of resistance then you can only apply 62Nm of torque to the rear regardless of how much torque the engine is capable of because there is no torque without resistance.
The electric clutch is actually a brake, it resists the speed differential between the front and rear drive shafts. Very similar outcome to the way traction control applies the brakes to a spinning wheel to provide torque across the axle to the wheel with traction. In the centre diff scenario you can have none, a little braking or a lot and everywhere in between the ECU determines how much depending on it's programming.
The car has lots of sensors that let the cars ECU know what is happening as you drive and it processes that information and applies or reduces the braking between the shafts. It does this in accordance with the programming Porsche installed. I doubt Porsche would share the specifics of how that works but pretty much as garret says above to provide a well mannered, safe car.
03-20-2024, 09:16 PM
