Hills, snow and ice

tomstickland

cyclonebri1 wrote: »

However TS has got it seriousley wrong here in his method of trying to slow the car, a rear wheel drive car.

Can you elaborate on that please?
I drove over the brow of a hill in 2nd gear and didn't apply brakes or go into neutral. The rear broke away because the grip levels were too low. When that happened I realised that the road surface was more slippery than what I'd encountered on the roads previously.

The engine breaking in low gear on a steep decent will very quickly overcome that applied through the pedal

This is what I've been saying though - the engine braking effect might require forces that are larger than the grip available.

or if the wheels grip will screw the engine,:eek:, no degree in anything needed to figure that

If the wheels grip how will it screw up the engine?

tomstickland

skiddlydiddly wrote: »

That's assuming the rears are capable of giving the same amount of braking as the front, which they aren't.They don't receive as much pressure as fronts as the rear lifts up and the front squats down under braking, reducing rear grip and increasing front.More grip allows more braking effort.
On many cars the rear discs are also smaller with inferior calipers to the front, further reducing any potential braking they have.

If you drive as described in normal conditions, I don't understand why you'd switch to a style where you are in less control when the conditions worsen.
Driving down the hill in a lower gear, gives a much more predictable amount of grip than continually reacting to the gradient and ice, controlling your speed by braking.

Fronts do more work, yes.
So there won't be double the force available with 4 wheels braking.
Squatting is going to be minimal at low speeds.

Also good point somewhere earlier about risk of locking up.

cyclonebri1

tomstickland wrote: »

Can you elaborate on that please?
I drove over the brow of a hill in 2nd gear and didn't apply brakes or go into neutral. The rear broke away because the grip levels were too low. When that happened I realised that the road surface was more slippery than what I'd encountered on the roads previously.

And that is exactly what i was saying, quote;

I drove over the brow of a hill in 2nd gear and didn't apply brakes or go into neutral. The rear broke away because the grip levels were too low.

Noting to do with physics, just your underestimation of grip levels and driving tooooo fast for the conditions??

skiddlydiddly

tomstickland wrote: »

The wheels will lock up if the forces acting to pull the vehicle down the hill exceed the braking forces that the engine is trying to apply.

I came over the brow of the hill in gear and travelling at about 20mph, travelled down the hill and the rear of the car broke away.

No they won't, the engine will just speed up, giving plenty of warning where you can either apply the brakes or change up a gear.It would only lock up if you suddenly dropped it into too low a gear and there was a massive difference in wheel speed and gear speed.
The rear breaking away could have simply been down to the camber of the road adding another force onto the car that the tyres couldn't cope with due to the reduced grip.

skiddlydiddly

tomstickland wrote: »

The same forces are required to hold a steady speed regardless of the speed. Science allows anything to be compared.

It's not possible to feel the conditions until a response to an input is detected. Previous experience on the flat bits had shown that grip levels were low but not critically low. The hill first showed its low levels when the rear end went into the bank and stopping required using the bank as an aid. Getting out confirmed this.

To give your comparison some credability you'd have to replicate the conditions in and out of gear.Not compare travelling at 20mph in gear to travelling at walking pace in neutral.You are adding variables which skew any kind of "result".

You had a response to an input, you were steering the car and it responded by veering off the road.A fairly dramatic response.Your previous experience of grip levels being low on the flat bits should have maybe given you an inkling that when you add gravity into the equation ie the hill that grip levels would drop.

darich

No one denies that braking with four wheels provides more stopping power than engine braking with two.

But if the grip is genuinely as low as TS claims ie necessitating neutral and 2-3mph, then the additional braking power provided by the four wheel braking in neutral is irrelevant....because the wheels will simply lock, or activate the ABS.
When grip is that low, it requires experience of driving.....not a degree in physics.

mikey72

darich wrote: »

No one denies that braking with four wheels provides more stopping power than engine braking with two.

But if the grip is genuinely as low as TS claims ie necessitating neutral and 2-3mph, then the additional braking power provided by the four wheel braking in neutral is irrelevant....because the wheels will simply lock, or activate the ABS.
When grip is that low, it requires experience of driving.....not a degree in physics.

ABS won't activate at 2-3mph.

darich

mikey72 wrote: »

ABS won't activate at 2-3mph.

ok....but the point remains....so little friction below the tyres means it's irrelevant how much braking force you can apply

tomstickland

skiddlydiddly wrote: »

No they won't, the engine will just speed up, giving plenty of warning where you can either apply the brakes or change up a gear.It would only lock up if you suddenly dropped it into too low a gear and there was a massive difference in wheel speed and gear speed.
The rear breaking away could have simply been down to the camber of the road adding another force onto the car that the tyres couldn't cope with due to the reduced grip.

If the forces acting are larger than the grip available then the wheel will lose grip. I incorrectly said "lock up" when I meant "lose grip".

The engine can't speed up if there's not enough grip to apply the force needed to do that speeding up.

tomstickland

cyclonebri1 wrote: »

Noting to do with physics, just your underestimation of grip levels and driving tooooo fast for the conditions??

Physics doesn't stop if someone is travelling too quickly for the conditions. The point being, the conditions were variable. I negotiated a 90 degree bend at the top without any issues - this being the source material for the estimation of grip levels. The first I knew of the lower grip was the rear of the car sliding into the bank as it broke traction.

The forces needed to maintain a steady speed are the same regardless of the speed. So if I'd been travelling at 10mph down the hill the rear tyres would have been expected to generate the same forces to hold the car at a steady speed. That's ignoring tyre dynamics and steering of course.