Not all 4 wheel drive systems are equal
 

I stumbled across this video and found some of the results surprising. no Volvos tested but still interesting.
Some of these systems leave a lot to be desired.

https://www.youtube.com/watch?v=t09ExAUgtyE

+ YouTube Video

Hmmm that's a bit of a propaganda video but does make some key points. All post 2003 AWD Volvo models are haldex based as it the Passat but Volvo do their own software so they may behave differently.

This is an interesting vid....

https://m.youtube.com/watch?v=tpp5tW71qYI

Early model XC90 doing its thang at 9mins, XC70 at 26 min and XC90 again straight after.

I can't see what these guys are trying to do?
If both front wheels are spinning on rollers then the 4wd will no engage so what are they proving?
Manual 4wd like a land-rover would always have drive to both axels so would drive over the rollers but what does it prove?

At a guess, I was hoping these systems would detect the rear wheels were not rotating and hence there was a problem.

The haldex system on Volvos is linked to traction control and if both front wheels are spinning but the rears are stationary it will send power to the rear. Also on the newer cars with the 5th gen system (new XC90, v90 etc) power is always sent to the rear when setting off from rest.

It gets tricky when one wheel on an axle is spinning as it will then rely on the traction control system to apply the brakes to the spinning wheel thus sending power to the stationary one.

Hmmm, that didn't seem to happen on the Volvo?
I looked at it and thought maybe the 4wd was happy if both front wheels were rotating at the same speed?
Maybe not, would be interesting to know for sure?

They seem to do okay when it's one front wheel on the ground but not so well when it's one rear wheel. I think this is because with just one wheel of the axle on the ground the car is relying on both the AWD system to send power to both axles and the traction control to brake the free spinning wheel. On an older car, the rear brakes are more likely to have weakened than the fronts?

The other thing I would add is there are not many real life situations where you would have zero traction on three wheels.

Interesting the older V70 in this second vid which has thermo viscous coupling based AWD does okay on the one rear wheel test.....

https://m.youtube.com/watch?v=CQPNH4XRbWA

I have and have had many 4wd cars over the years and the worse system by far is on the Honda CRV. In the scenario shown in the video, even after a 2014 [its a 2014 model year car built in 2013] software update, only done by request, it still isn't up to much.

The problem with it is that the clutch pack is designed to transmit no more than about a maximum of 30% of available maximum torque to the back wheels and will slip and quickly overheat if that figure is exceeded. In fact the propshaft, rear diff and rear half shafts are very lightly built to handle no more than that and the slipping wet clutch in the diff housing is designed as a safety limiter. I strongly suspect that they use the same components for petrol and diesel engines and the diesel's torque shows its limitations much more than the available petrol engines would.

Most owners will never notice, but I did very early on, when on a grassy hill the CRV failed to climb it. The Volvo can transmit 50% to the back and it has far more torque to start with. The Volvo climbed the very steep hill in the same spot with no problem. So has every other vehicle in the same ground conditions always.

I'm not condemning the Honda, because its very unlikely that many people would ever get into such a situation, but it can happen, for instance where front wheels are mounting a raised muddy verge and the rear are on tarmac. You would expect the rear wheels to push forward at least until they themselves encountered the muddy or slippery raised verge, but the Honda just can't transmit enough torque to push its own body weight using the back wheels alone.
One would expect that having paid for a 4wd car, that it would at least make a visible effort, but no. The 4wd system does work on it, but is best as a 4 wheel assist where all four wheels are on a near identically slippery surface, such as a snow covered road or a flat muddy field with a firm bottom.

The viscous coupling systems have limited slip diffs at back like the older 960's had so if one slips it will lock and drive both wheels

Animal farm moment
 

Found this thread and associated videos very interesting... Thank you.
Obviously some AWD setups are more equal than others, additionally, if some systems can just spin their wheels, it explains why a two wheel drive system with snow tyres can get better traction than an AWD without snow tyres in the white stuff.

Cool


Cheers

Bob