Does AWD work the same in reverse?

[gandalf1]

This might sound like a foolish question, but there seems to be a difference in my B9's AWD performance depending on whether I'm moving forward or backward.

The test data: we've had a fair amount of snow here, and, eager to give that world-reknown AWD a test, I did the following: we have a fairly steep hill in front of our house at about 20 degrees, and there is a good strip of snow about 16" deep and several feet wide. So I drove my B9 up onto the side snow and straddled the edge of the snow strip so that both passenger wheels were on snow and both drive wheels were on pavement, with the vehicle pointing downhill.

Then I tried to back up. Both passenger wheels spun in the snow, and the traction contro/ABS was fighting to redistribute power, but the vehicle did not move, despite having both driver-side wheels solidly on pavement. So I then put the vehicle in drive pulled out (I was headed downhill) and repeated the test, but this time with the vehicle point uphill (driver-side wheels in snow, passenger wheels on pavement), put it in drive and the vehicle had no problem at all going up the hill.

I tried this both with the vehicle cold and warm (had read here and there about mechanical diffs relying in viscous heating--don't know if applicable, but I tried). Vehicle has 4 new (< 5k) General HTS grabbers that are well rated for snow as an all-season.

This would suggest that the Subbie AWD is sensitive to Forward/Reverse, either through some asymmetry in the drive train (e.g., clutch has a preferred rotation direction), or the computer/ABS system sees a wheel turning backward differently that turning forward for its decision making.

I've searched, but the few articles that I have found seem to suggest the AWD shoudl have no bias in forward vs reverse, i.e., working identically in both directions, althoug with no discussion as to why.

We have some more snow due tonight. I'll try some more tests and see if I discover anything new and/or am able to repeat the above tests.

Thoughts welcome, mostly to satisfy my curiosity.

Thanks All,

-Jon

[PollieKrismis]

Hi,

I find your test really interesting. Though I cannot comment on the reverse issue I do have something that I also could not explain on my B9.

I took it on a 4x4 (offroad) course to see where I would get stuck. (This is of course ignoring the approach angle issue which would stop you from getting to the first obstacle.)

Where I did get unstuck and where I thought I would not was the opposing holes. I made it though about two, but the she just kept on wheel spinning. So right front and left rear is right up the wheel arches with the car swinging/tilting diagonally with the opposing wheels in the air.

I would have thought that it would get the power to the correct wheels but instead it sent it to the two wheels without traction.

The second obstacle was similar but this time I bottomed out. So the car was lying on it's belly. The holes were not as deep or wide as before but now with the chassis on the ground the wheels that did have traction did not have enough grip to inch the car forward for the body to lift off again. I had the stock tyres which of course did not help either.

So the second one I can explain as it makes sense, but the first one I really thought the AWD system would be clever enough to send the traction to the wheels that were not in the air, even if they were on opposing sides.

Anyway, Sunday I'm doing the sand part of the course and hope to do much better there.

Regards

Paul