Transfer Case stress - wheel diameter/tire pressure/wear

[fmugur]

I measured the length from the ground to top of the rim and I got 62cm (35psi) on the rear and 61.5cm (32) on the front. I reduced the rear to match the front and I got close to 32psi. Is this too low for the rear tires 315/35? It feels very smooth comparing with 35psi.

[Turbo_Bimmer]

Originally Posted by fmugur

I measured the length from the ground to top of the rim and I got 62cm (35psi) on the rear and 61.5cm (32) on the front. I reduced the rear to match the front and I got close to 32psi. Is this too low for the rear tires 315/35? It feels very smooth comparing with 35psi.

With your measurements, you are within 1% of difference. If you reduce the rear pressure to better match the front in height, the transfer case will see less wear over years, but the handling may be affected (understeer/oversteer?) and also maybe the tire wear pattern. I would keep an eye on those.

[bigwave2255]

i run my staggered 20 inch wheels at 40 psi

i also struggle to understand how the x drive transfer case clutches can survive as unless they are locked, or 50% 50% split front and back then the clutch is slipping, and stands to reason a slipping clutch has a finite life expectancy, if the clutch pac wasn't slipping we would get axle wind up when cornering, so when they talk about the x drive being able to deliver torque to there its needed then the clutch has to be slipping.

the way around this is as in most transfer cases you have a centre diff that is open/unlocked for highway usage and lockable for off road where axle wind up is not a problem due to loose surfaces and wheels in the air heehehehe

it seems to my limited understanding that the x drive while good is slowly destroying its self as we drive along.

i would love to be wrong about this

[Turbo_Bimmer]

Originally Posted by bigwave2255

i run my staggered 20 inch wheels at 40 psi

i also struggle to understand how the x drive transfer case clutches can survive as unless they are locked, or 50% 50% split front and back then the clutch is slipping, and stands to reason a slipping clutch has a finite life expectancy, if the clutch pac wasn't slipping we would get axle wind up when cornering, so when they talk about the x drive being able to deliver torque to there its needed then the clutch has to be slipping.

the way around this is as in most transfer cases you have a centre diff that is open/unlocked for highway usage and lockable for off road where axle wind up is not a problem due to loose surfaces and wheels in the air heehehehe

it seems to my limited understanding that the x drive while good is slowly destroying its self as we drive along.

i would love to be wrong about this

The X-drive clutches don't slip under normal driving conditions (straight line, properly matched/inflated tires). The torque split 62%-38% is related to the mechanical arrangement inside the TC. Even if torque is not sent equally F-R, the rotation speed is the same.
There is splippage in the clutches when you turn (to avoid binding and understeer), or when you go on ice (cluthes will slip for some milliseconds then lock harder to stop spin).
So, in theory, yes the X-drive clutches destroy themselves slowly, but in reality, the wear is very minimal, according that nothing goes wrong like towing the X5 with the front in the air with the wheels blocked and the rear rolling on the ground, or doing hundreds of miles with deflated tires etc.
An AWD vehicle is more sensitive to have the same rolling radius at 4 corners to save the TC.

It's not only the X-drive, even the pre-facelift ones will eventually destroy themselves if they are abuses. There are no clutches inside to wear, but the ...(I forgot the name in english) but the guizmo that has the planetary gears in it is not designed to spin continiously, and if forced to, will wear prematuraly.

[Gurjit]

correct me if im wrong

but doesnt the new xdrive system use open x-case and open diffs
there are no clutches being used when driving normaly
the actuator in the xcase only works when the cpu tells it to split power?

[Turbo_Bimmer]

Originally Posted by Gurjit

correct me if im wrong

but doesnt the new xdrive system use open x-case and open diffs
there are no clutches being used when driving normaly
the actuator in the xcase only works when the cpu tells it to split power?

The clutches in the X-drive don't work like the clutches in a rear diff for example. Instead of being applied with a predetermined load (springs), it is applied using an electric motor and controlled from a computer.
The X-drive doesn't have a differential in it. The rear driveshaft is always driven and the clutches under normal driving are engaged with a moderate pressure to transmit torque to the front wheel and will engage more if slipping is detected between the front axle and rear axle.
From what I read, the only time the clutches will completly disengage is when you drive at high speed (over 100mph from memory, correct me if I'm wrong), then the X-drive will be RWD.

[JCL]

Originally Posted by Turbo_Bimmer

From what I read, the only time the clutches will completly disengage is when you drive at high speed (over 100mph from memory, correct me if I'm wrong), then the X-drive will be RWD.

I believe the clutches are also open during parking, to prevent axle wind up.

[Turbo_Bimmer]

Originally Posted by JCL

I believe the clutches are also open during parking, to prevent axle wind up.

That may be possible as well.
There is an interresting device that BMW uses for demontration that I would like to put my hand on, and can show torque applied and percentage of locking of the clutches. (see picture).
Unfortunatly, Inpa and GT1 don't seem to provide a way to show those information.