Hills, snow and ice

tomstickland

darich wrote: »

Since when did a first class degree in physics make someone a good driver?
Last I heard you had to have lessons, sit a test, gain experience and perhaps even sit an advanced test.

And those with degrees can still be stupid and wrong.

Blah blah blah, no common sense, too clever for their own good, think they know everything. All the qualifications but no idea about real life.

Keep going with your lazy stereotypes. These guys are not like that; they're practial people who get things done and also happen to know a lot about maths and analytical thinking, ditching preconceptions and building arguments from first principles.

tomstickland

skiddlydiddly wrote: »

Is that a joke question?If you aren't in gear you aren't in full control of the vehicle.See what a traffic offficer or driving instructor says about coasting.IN fact see what the highway code says about it too.I used to do no handers on my bike down big hills when I was a kid and didn't come off, it didn't mean I was in control, it means I was lucky.

Driving down an icy hall at walking speed with all wheels under braking is not coasting.

As you mention engineers it has some relevance to your brake temperature and fluid boiling.There is only a small amount of flui in the calipers, a few tablespoons......

I did some testing a while back where I went from 80mph indicated to about 10mph and did that a few times. The second session was not noticeably worse than the first, the third had some loss of braking.

Energy to dissipate per unit mass would be v squared over 2.
Time to dissipate it tens of seconds.
80mph is approx 36m/s
10mph is approx 5 m/s

Energy dissipated per unit mass of vehicle is: 630 J/Kg

Descending a distance of 133 metres in 1 km then potential engergy dissipated per unit mass is gh where g is acc due to gravity, ie: 9.81 m/s2

Energy per unit mass dissipated is roughly twice as much as one 80 to 10mph stop, at 1300 J/Kg

Braking from 80 to 10 takes, say, 20s. Descending hill takes several minutes. Call it 5 minutes and it's 15 times longer.

Rate of heat generation is about 7 times less for the descent compared with one braking cycle of 80 to 10mph.

Finally, have you ever seen the steep gradient signs on the road which tell you to use a low gear?They don't don't tell you to put the car in neutral and freewheel for a very good reason.

Without ice, yes, fine. There's a surfeit of grip available at the driven wheels.

With a lot of ice there then a two wheel drive vehicle can only generate half the total retardation force compared with all four wheels doing something.

In the dry I probably use too much engine braking; I see it as a challenge to control the speed mainly with gear selection rather than braking. I'm well aware of how it works. As I said a long while back, I'm talking about a descent down an icy hill. Conditions where some people refused to go any further because they said it was too dangerous.

optimus_primera

Driving in a low gear is to help you keep control of the vehicle from going to fast in the first place, not to slow you down.

tomstickland

Yes, and if that requires more force than the front wheels alone can generate then using the brakes will manage it when the front wheels alone wont.

darich

tomstickland wrote: »

Blah blah blah, no common sense, too clever for their own good, think they know everything. All the qualifications but no idea about real life.

Keep going with your lazy stereotypes. These guys are not like that; they're practial people who get things done and also happen to know a lot about maths and analytical thinking, ditching preconceptions and building arguments from first principles.

Of course they're not - it suits your argument that they're not.

Whether they really are or not will never be proven because we don't know them.
They may well be maths gurus, great analytical thinkers and be masters of ditching preconceived ideas and building arguments from first principles......but like I said, that never made anyone a good driver.

tomstickland

But the allegation was that these people clearly had no practical abilities because they had degrees. An argument that implied by association that therefore they were clueless with practical matters.

Considering that one of them spent a year doing plastering and plumbing, a lot of them restore vehicles. I spent 6 months doing landscape gardening, bricklaying, paths, patios, I do all my own vehicle maintenance, and plumbing, electrics, and block work. The boring old "I knew someone who said that they were clever but they couldn't even boil and egg" argument.

So far what reasoning has been presented as to why two wheel engine braking will provide better control down a steep hill in the ice apart from that it's the correct practice in good grip conditions?

The point that I was making was that amongst a group of people who are generally pretty good all rounders there was a lot of discussion of the complexities of this.

Furthermore, my points about engine braking plus applying the brakes might well show that when braking then being in gear actually doesn't make any difference, in which case the best practice is to be in gear and brake and be ready to go into neutral if a slide develops.

darich

tomstickland wrote: »

But the allegation was that these people clearly had no practical abilities because they had degrees. An argument that implied by association that therefore they were clueless with practical matters.

No, the allegation was that having all the brains in the world doesn't automatically make you a good driver.

tomstickland wrote: »

So far what reasoning has been presented as to why two wheel engine braking will provide better control down a steep hill in the ice apart from that it's the correct practice in good grip conditions?

The highway code advises that keeping the car in a low gear helps to prevent speed building up and therefore reduce the chance of losing control of the car. By selecting neutral, you're removing a method of control - a method of braking and therefore increasing the chance of losing control and skidding.

tomstickland wrote: »

Furthermore, my points about engine braking plus applying the brakes might well show that when braking then being in gear actually doesn't make any difference, in which case the best practice is to be in gear and brake and be ready to go into neutral if a slide develops.

If you genuinely believe that being in neutral on a steep hill gives you more control then you're entitled to think that. But it's not the advice in the highway code, it's not taught in lessons and it's probably not taught for the advanced driving test. That just might indicate that perhaps it's not the best method.

From a physics point of view, no one can argue that 4 wheels give better stopping/reducing power than simple engine braking. That's not debated. But what must be remembered is that when the wheels are connected to the engine in gear, then they're turning. When you select neutral and coast, then with very low friction between tyre and road, it's VERY easy to lock up - keeping speed down ABS deactivates at a few mph so at 5mph you may find you have no ABS - and locked wheels.
However with the engine connected, the wheels are turning, keeping the speed down (as do the brakes when coasting) but the big difference is that in gear the wheels will never lock meaning you always have steering and maintain control of the car.

mikey72

What do they say about the fact if you are using the engine braking, you can never lock the non driven wheels, whereas if you are coasting, in neutral, and brake, you can easily lock back wheels?

Inactive

tomstickland wrote: »

Considering that one of them spent a year doing plastering and plumbing, a lot of them restore vehicles. I spent 6 months doing landscape gardening, bricklaying, paths, patios, I do all my own vehicle maintenance, and plumbing, electrics, and block work. The boring old "I knew someone who said that they were clever but they couldn't even boil and egg" argument.

I seem to recall seeing a post on here with photographic evidence of your plumbing skills, not a pretty sight.

tomstickland

Inactive wrote: »

I seem to recall seeing a post on here with photographic evidence of your plumbing skills, not a pretty sight.

I've done a lot of plumbing without any problems, but I did make a serious mistake when emptying an immersion heater.

(I drained it by letting water seep out round the immersion heater thread because the drain system was blocked with limescale. I didn't realise how short the thread was....gave it an extra tweak to speed it up and the whole thing came out, followed by 200l of water).

I know a plumber and he told me that he's done it and most other plumbers he knows have had a similar disaster at least once.

I made a mistake. I'm happy to admit to that. The argument was that people with degrees don't know how to do anything and I'm showing that that's not always the case.

Last week I received a report written by a Dr (PhD) and it had spelling mistakes in it. Therefore everyone with a PhD is worse at spelling than me (using this thread's version of logic).