Press 4WD under HEAVY load, or not needed?

[jaapv]

On a slippery slope it is even worse than off-road. The braked wheel will grip after the car moves forward a few Centimetres and the brake will not release quickly enough to prevent speed loss, and create a spinning wheel on the other side, so the effect will cumulate.
It is not specific for the PHEV. Two-wheel drive cars with Traction Control will do the same.

 

[maby]

On a slippery slope it is even worse than off-road. The braked wheel will grip after the car moves forward a few Centimetres and the brake will not release quickly enough to prevent speed loss, and create a spinning wheel on the other side, so the effect will cumulate.
It is not specific for the PHEV. Two-wheel drive cars with Traction Control will do the same.

Certainly so and also applies to other "softroaders" too - my original Landrover had no diff locks on the axles and it was amazing the difference between that and the later models that we owned with permanent 4WD and locks on all three diffs.

 

[anko]

The problem with this approach is that the cross-axle loss of traction in serious off-road conditions is a rather transient thing. You lose traction on one wheel, the electronics recognises it and locks the brakes on it - the car moves forwards and both wheels gain traction, but the previously spinning wheel now has the brakes engaged - how does the car recognise that it has gained traction again? The locked wheel is now in contact with the ground and not turning - it's fighting the wheel that is driving the car (which probably does not have very good traction itself) - and there is no easy way to detect that it is now time to release the brake...

How do Anti Lock Brakes solve it? When the car releases the brake on a wheel without grip, how does it know it is time to apply the brake again? Somehow it does not decide that all four wheels have lost traction and release all four brakes at the same time. If it can handle such a critical situation, somehow I don't see why it could not at least try to get itself out of a stuck, but far less dangerous situation.

Just apply the brakes on the spinning wheels briefly, until the other wheels start turning. And then release again .... If the other wheels start spinning slower, lock again. And so on. Of course it won't beat a lock diff, but still. ....

 

[anko]

On a slippery slope it is even worse than off-road.

I don't mean to use TC all the rime. I was just suggesting to turn it back on briefly to get out of a cross-axle stuck situation.

 

[maby]

The problem with this approach is that the cross-axle loss of traction in serious off-road conditions is a rather transient thing. You lose traction on one wheel, the electronics recognises it and locks the brakes on it - the car moves forwards and both wheels gain traction, but the previously spinning wheel now has the brakes engaged - how does the car recognise that it has gained traction again? The locked wheel is now in contact with the ground and not turning - it's fighting the wheel that is driving the car (which probably does not have very good traction itself) - and there is no easy way to detect that it is now time to release the brake...

How do Anti Lock Brakes solve it? When the car releases the brake on a wheel without grip, how does it know it is time to apply the brake again? Somehow it does not decide that all four wheels have lost traction and release all four brakes at the same time. If it can handle such a critical situation, somehow I don't see why it could not at least try to get itself out of a stuck, but far less dangerous situation.

Just apply the brakes on the spinning wheels briefly, until the other wheels start turning. And then release again .... If the other wheels start spinning slower, lock again. And so on. Of course it won't beat a lock diff, but still. ....

My limited experience of anti-lock braking - not gained on the PHEV - is that it pulses the brakes on and off rapidly. It's really intended for very infrequent emergency situations and is focussed on bringing the car to a halt as soon as safely possible. The diff locking mechanisms on serious 4WD vehicles targeted at keeping the vehicle moving, not stopping it - and are actively working almost all the time in real off-road use. The fact that manufacturers of 4WD ranges like Landrover and Toyota continue to fit lockable and limited slip diffs which are more expensive than standard diffs leads me to believe that it is not feasible to get an equivalent effect by playing with the brakes.

 

[jaapv]

On a slippery slope it is even worse than off-road.

I don't mean to use TC all the rime. I was just suggesting to turn it back on briefly to get out of a cross-axle stuck situation.

Possibly. I never tried.

 

[anko]

The problem with this approach is that the cross-axle loss of traction in serious off-road conditions is a rather transient thing. You lose traction on one wheel, the electronics recognises it and locks the brakes on it - the car moves forwards and both wheels gain traction, but the previously spinning wheel now has the brakes engaged - how does the car recognise that it has gained traction again? The locked wheel is now in contact with the ground and not turning - it's fighting the wheel that is driving the car (which probably does not have very good traction itself) - and there is no easy way to detect that it is now time to release the brake...

How do Anti Lock Brakes solve it? When the car releases the brake on a wheel without grip, how does it know it is time to apply the brake again? Somehow it does not decide that all four wheels have lost traction and release all four brakes at the same time. If it can handle such a critical situation, somehow I don't see why it could not at least try to get itself out of a stuck, but far less dangerous situation.

Just apply the brakes on the spinning wheels briefly, until the other wheels start turning. And then release again .... If the other wheels start spinning slower, lock again. And so on. Of course it won't beat a lock diff, but still. ....

My limited experience of anti-lock braking - not gained on the PHEV - is that it pulses the brakes on and off rapidly. It's really intended for very infrequent emergency situations and is focussed on bringing the car to a halt as soon as safely possible. The diff locking mechanisms on serious 4WD vehicles targeted at keeping the vehicle moving, not stopping it - and are actively working almost all the time in real off-road use. The fact that manufacturers of 4WD ranges like Landrover and Toyota continue to fit lockable and limited slip diffs which are more expensive than standard diffs leads me to believe that it is not feasible to get an equivalent effect by playing with the brakes.

I was not looking for / thinking about a full time alternative for a locking diff. Just wondering why the brakes could not help you out of a situation where you have lost grip on diagonal wheels and got stuck completely.

 

[maby]

...

I was not looking for / thinking about a full time alternative for a locking diff. Just wondering why the brakes could not help you out of a situation where you have lost grip on diagonal wheels and got stuck completely.

And I guess that turning the VSC back on might help to get you moving again - but I don't think I'm going to test it! Anyone else want to volunteer their PHEV for a run round an off-road track? I'll be happy to give it a go. Actually, the last time I went seriously off-road was a competition where we were taking a fully equipped LR Defender round a track blindfold with a partner providing instructions over a two way radio into a headset - that was particularly scary! (I didn't win!)

 

[Neverfuel]

Don't forget that ASC also includes yaw control which speeds the wheels up as well as having ABS which slow the appropriate wheel down. So there is more to the ASC button than just stopping the wheels - hence why the ASC can either help or hinder a cross axle manoeuvre and should be employed/disabled according to the particular conditions. There is not a single answer that can solve all situations. Having said that, I have owned land rovers / land cruisers for the last 15 years and this car is just as capable in the same conditions - even though the tyres are just road tyres.

 

[jaapv]

Well, I stupidly drove onto a patch of blue ice in a parking lot a few weeks ago, slightly inclined too. Braking was completely impossible, it came to a halt against the snow edge, but the car reversed out without a problem. If any traction, it cannot have been on more than one wheel. ASC + 4WD lock on.

 

[asterix550]

I guess this video shows that cross axle spinning wheel doesn't take you anywhere. Even though I don't know if ASC was on.
If you guys understand the comments (I guess it's polish) to the video might get the answer there.
https://youtu.be/uxhdFGmbk8k

 

[Claymore]

I bought the car despite seeing this...I reluctantly concluded that the car could do better but the driver was an idiot.

(The video does say that traction control was off.)

I hope I was right otherwise it's depressing.

JimB

 

[gwatpe]

The main problem with the PHEV is the restricted travel movement of the suspension. It is usually possible to find a driving line that keeps all wheels in traction. Inertia is a useful tool for those spots where a wheel may come off the ground.

I found that even in 4WD lock, that lack of torque to a pair of non spinning wheels will still prevent the PHEV from climbing a steep grade.

Turning traction control OFF seems to allow the PHEV to keep the power available, even when wheels are spinning a bit, and the car will climb, rather than slowly coming to a stop.

 

[asterix550]

The main problem with the PHEV is the restricted travel movement of the suspension.

That's the price you pay for a better suspension dinamic stability behavior.

I found that even in 4WD lock, that lack of torque to a pair of non spinning wheels will still prevent the PHEV from climbing a steep grade.

What do you mean for the lack of torque? Not enough power to the axle or no power at all because the brakes stop the wheels movement?

Turning traction control OFF seems to allow the PHEV to keep the power available, even when wheels are spinning a bit, and the car will climb, rather than slowly coming to a stop.

I doubt that because like you can see in the video above if the wheels are spinning cross-axle even with ASC off the car won't do it unless you use momentum to help the engines efforts

 

[maby]

The video doesn't surprise me and I would never have tried to take a PHEV up there. I don't think any fiddling with the various controls would have made much difference to the final result.

He probably would have made more progress if he had gone further to the right on the track - I don't think that that particular cross-axle was unavoidable. However, the low ground clearance (by 4WD standards) and low suspension movement might have forced him to follow the path that he did - it is not possible to be sure based on that video.

Bottom line? The PHEV is a softroader. That track is definitely at the upper limit of its off-road capabilities. We don't know how experienced the driver was - I've seen inexperienced drivers getting into trouble on relatively benign ground in proper off-roaders too! I'm out of practice myself - both the PHEV and the Landcruiser belong to my wife and she will not let me take them seriously off-road - it seems that I've developed a reputation for breaking cars!

 

[anko]

He probably would have made more progress if he had gone further to the right on the track - I don't think that that particular cross-axle was unavoidable. However, the low ground clearance (by 4WD standards) and low suspension movement might have forced him to follow the path that he did - it is not possible to be sure based on that video.

I am pretty sure he did this on purpose to demonstrate the (un)ability of the car or himself to master that situation.

 

[jaapv]

Indeed, taken at a little more speed that bump would not have stopped the car, momentum would have carried it through. The same with deep sand, I can bog down a Defender -easily-, by failing to keep the car moving.

 

[anko]

I know it is not a phev, but it seems this guys does know which buttons to push ...

http://youtu.be/AI9lMky5lUU

 

[ian4x4]

Surely with all the clever electronics the PHEV should be braking spinning wheels like this.

The Freelander shown has no locking differentials, just 4 wheel drive and electronics.

https://www.youtube.com/watch?v=T2EDn-5aEBE

 

[asterix550]

I know it is not a phev, but it seems this guys does know which buttons to push ...

http://youtu.be/AI9lMky5lUU

That's something i'd really like you to clarify. Does this 4wd system work the same way of the one's on the PHEV. Does the PHEV have the same capability to get out of trouble as the diesel??