Decending on ice

[mort]

I believe I have found a way to safely descend steep slopes in very Icy conditions without using the brakes.
I have found that the lack of a 4 wheel low is detrimental to descending in very icy conditions. The Vehicle moves too fast even in B5. The brakes have to be applied to slow the PHEV leading to temporary loss of control at speeds below the ABS threshold.
I tried it out this morning on a steep grade, by putting the PHEV into reverse and lightly applying pressure to the throttle the PHEV will descend at very slow speeds (sub 2 mph) without locking up the tires. I failed to monitor what re-gen was produced.
Any thoughts on this? Could it be harmful to the power-train?
Mort

 

[ChrisMiller]

Yes, on my driveway (15% gradient) the PHEV will descend slowly while in reverse, if I take both feet off the pedals. I only ever do this for a metre or two.

 

[mellobob]

You're going forwards with the transmission in reverse?

 

[anko]

You're going forwards with the transmission in reverse?

The transmission does not have a reverse. By putting the lever in R all yo do is instruct the computer to request negative torque from the motors. As if you are regening. At some point (most times) this may result in the motors spinning backwards. But it doesn't have to be.

 

[jaapv]

The whole thing about descending a slippery slope is not the way braking is applied to the wheels, but the distribution of the amount of braking.
As long as all wheels retain traction there is nothing to worry about. If the rear wheels lose traction by blocking you can still control the car. If the front wheels block, you will spin 180º and end end up front-to-end. If all four do so it will be an elegant walz.
This is caused by blocked wheels (or even turning against the movement) having less friction than wheels still turning.

It may well be that the reversing retarding force applied to the wheels is sometimes better distributed by reversing the electric motors.
BTW, did you use 4WD lock?

I the end, though, I think the ABS of the car will give better control than the reverse setting, especially as you can not even set it to reverse at speeds over (estimated) 10 kph. It will go to neutral.

 

[anko]

If the front wheels block, you will spin 180º and end end up front-to-end. If all four do so it will be an elegant walz.
This is caused by blocked wheels (or even turning against the movement) having less friction than wheels still turning..

I agree with the 'less friction' bit. But if the front wheels have less friction that the rear wheels, why would the car spin around?

 

[jaapv]

You're right, I'm the wrong way round . Rear wheels brake the most = rear block = traction loss rear = the rear overtakes the front = spin, the front wheels the other way around.
That is the reason that pulling the handbrake hard on a slippery road will spin the car.

 

[richr]

As an aside, couldn’t 4WD low ratio (ie lots of torque, low speed and high engine braking) can be replicated by B5 and a gentle right foot?

As the drive is pure electric at low speeds you can’t really think of the transmission in terms of mechanical gearbox concepts. There’s a huge amount of torque from the motors anyway (about double what my Pajero can manage), and you have control over the resistance to turning (like engine braking) via the regen system.

 

[jaapv]

That is the way I do it (in 4WD lock). However, we must realize that this is a heavy car, and descending it has no advantages over any 2wd car, as all cars have four-wheel braking.

It may well be that doing this the way the OP is doing this can cause disaster: if one of the wheel rotations exceeds the rotational speed limit in the computer for "R"-and -turning-the-wrong-way, the car will jump into "N", allowing it to run out of control.

 

[mort]

As an aside, couldn’t 4WD low ratio (ie lots of torque, low speed and high engine braking) can be replicated by B5 and a gentle right foot?

This is of course the preferred way, however I have found that when trailing a 4 wheel drive ICE with 4 low despite the following distance the PHEV is unable to match the extremely slow speed of the ICE (Toyota Land Cruiser) and as such more breaking power is needed than traction allows. This same situation put a friends Rav 4 over a mountain edge only to be arrested by a 6 foot sapling! The PHEV has better traction than the RAV 4 but still has lead me to white knuckles. When using this reverse scheme I cannot imagine exceeding 5 MPH.

 

[NightPHEVer]

Wouldn't the most useful addition you could hope for in this situaton be the use of winter tyres, to give you the maximum potential grip to start with? I would imagine the ABS would also respond better with the extra grip as well. Or do you already have winter tyres fitted? Downhill control on ice (and come to think of it uphill control too) is certainly one of the most apparent advantages of winter tyres in my experience - along with much less understeer into corners.