Wing mirrors on cars

[NY]

An interesting way of looking at it. But you're absolutely right: if the gearbox remained cool enough when driving up the hill (overcoming friction and air resistance and increasing the car's potential energy) then less energy will be dissipated in it coming down: friction and air resistance will be about the same, but some energy is being dissipated in the brakes and not in the transmission.

I presume that the oil pumps that circulate the hot oil to oil coolers in the transmission work even when the gearbox is in overrun, with wheels driving the engine which is running with no fuel input - it's not like the problem with 2-stroke engines where the engine got bugger-all oil on overrun because there is little fuel (and oil) entering the crankcase when your foot is on the brake rather than accelerator.

I presume 2-stroke cars, which had a freewheel to prevent overrun from driving the engine fast in a low gear, needed higher-spec brakes because even when going down a long steep hill (such as the one near Lynton/Lymouth in Devon) there is *only* the brakes and no engine braking to dissipate the potential energy lost as the car goes down hill.

[James Wilkinson Sword]

So, I'm going down a gentle hill and wish to slow slightly and touch my brakes, then the auto drops a gear and I find myself slowing even more than I wanted. No thanks. The car cannot possibly in any way whatsoever know if I want to use engine braking. That's why there's a lever.

I've done it ONCE in my entire life. Engine braking is rarely needed. Use the brakes, that's what they're for.

[James Wilkinson Sword]

And if you actually only wanted to slow slightly?

[NY]

I agree. It needs to be a clever algorithm to be able to second-guess when it should and should not change down to give extra engine braking. Manual control in that situation is better, as long as it's easy to change down without jerking.

In order of preference (most preferable first) I slow down by:

- lifting off the throttle and letting friction/air resistance slow me a little

- footbrake

- supplementing the footbrake with changing down (on a long steep 1:5 or steeper hill)

I'm never sure whether changing down on a steep hill is still necessary (as warning signs still say - eg

and (further down) also this one or whether it became less important as drum brakes gave way to disc brakes.

That second hill

is a nasty one. I was once coming up and as I approached a hairpin bend, having hooted as I approached it, as I tend to do on blind bends on steep hills, I saw a cyclist who was coming down shoot across the road in front of me and land in a heap on the grass verge: presumably his brakes weren't strong enough to keep his speed down low enough for him to negotiate the bend and he bottled out. As you can see, cyclists *are* warned :-) Doing a hill-start on a 1:3 hill after I'd stopped to check he was OK was "interesting" ;-) The Tour de Yorkshire bike race went down there the other year and they took it at a speed that I wouldn't dare do in a car, never mind a bike, even if I knew that nothing would be coming the other way.

[James Wilkinson Sword]

The current system (well the one on a 1998 VW Golf) works perfectly well. Under normal driving conditions, it's always in exactly the right gear. The harder I press the gas, the more power I get, it drops gears as required and the torque convertor smooths the change so I hardly notice it. And it can change fast enough that if I want to overtake, I put my foot to the floor and immediately get full power from the engine in the lowest gear possible. If I want to engine brake (which I've done only once in 21 years of driving, in the French Alps), it's simply a matter of moving the lever down a notch or two. And since it's an intelligent gear system, if I want maximum engine braking I can select 1st, knowing it won't actually use 1st, but the lowest possible without overrevving the engine.

However I have driven my neighbour's 2004 Rover 75. Its auto box is abysmal, it's like having a learner driver change the gears for you. It feels just like you have a manual box and an incompetant drunken clumsy oaf is changing them for you. Perhaps they've never heard of torque convertors.

I don't use the gears unless I'm going to br braking for a considerable time (as in I'm in a mountainous area). Why bother? Brakes can take you from full speed to zero without overheating.

Try just using the brakes. If they smell, you need the gears.

I can't really tell from the images on Google, but it doesn't look that steep. I assume the sign is for lorry drivers.

[Bob Eager]

Sometimes they come a real cropper:

(summary; A driver got far more than he bargained for when he cut up a tractor on a busy road. During the manoeuvre, the impatient driver collided with the sewage outlet on the tractor's trailer. Bizarrely, the car then became attached to the trailer and was dragged along as the vehicle started to be filled with pig manure.)

[James Wilkinson Sword]

If you cut someone up, you end up in front of them, so how did he end up connected to the trailer?

[Dave Plowman (News)]

If the oil pump in an epicyclic auto isn't running, you get no drive at all. As it's oil pressure from that which engages the various clutches therefore gears. No pressure and you're in neutral.

[Bob Eager]

I had a Humber Hawk with switch operated overdrive on third and fourth. That's exactly how it worked. I found this out because I had to dismantle the whole thing to replace the layshaft and first gear. This turned out to be a much bigger job as I had to construct various bits out of steel rod to hold stuff in place.

[rbowman]

That's fine if you live in flatland. I live in what is sometimes called the Rocky Mountains.

[rbowman]

I've blown a tube on my bicycle because of the heat generated by the rim brakes trying to maintain a reasonable downhill speed. My current bike has a disk brake which is much better.

I never had that fancy a bike but when the high end road bikes ran sewn tubular tires that were glued to the rim, there was always the danger of ungluing the whole mess on a downhill leg.Clincher rims were fine for me.

[rbowman]

[rbowman]

Sort of related but the rule of thumb for big trucks is to use the same gear going down as you used going up. In other words if you had to shift down to where you were going up the grade at 23 mph, it would be prudent to go down the other side at 23.

We ran the mountains so we had Jake brakes, but the flatland truckers without compression braking soon realized the wisdom.

[NY]

The first one, Sutton Bank, is signposted as maximum 1:4 / 25%. The second, Rosedale Chimney, is signposted 1:3 / 33%. Sutton Bank is on a trunk A road and is used by HGVs - there's a sign at the bottom (which doesn't seem to have been updated for several years) which says "174 closures due to HGVs", referring to the fact that it is notorious for HGVs getting stuck, either on the sharp hairpin bends or else running out of torque. I followed an animal transport lorry which made very good speed on the flat approach to the hill, but then got stuck several times on the hill, causing traffic behind to grind to a halt, possibly because having once stopped he found it very difficult to set off on the gradient. But he made it, though I was beginning to think I might need to turn round to avoid the blockage!

Rosedale Chimney is a real killer, though it's a tiny moorland road so it doesn't get any big vehicles - just tourists who panic at a road that seems to disappear before their eyes and which has nasty hairpins which are sharp and steep (they are probably the 1:3 bits of the hill). I once had to rescue the car in front: the driver had stalled and couldn't manage a hill start on the steep section. When he kept rolling backwards towards me, I thought for the sake of my car I'd better intervene!

I posted the photos mainly to make the point that very steep hill like these have "use low gear" signs. I'd assumed that they were aimed at all drivers, but maybe they are more for lorry drivers. There's a steep hill near the coast, and when I was little and we went to that village for seaside holidays, I can remember signs at the top and bottom "United Counties Bus Drivers: Engage first gear and retain until next sign". Sutton Bank has signs at the bottom "HCVs (*) Use crawler gear"; it used to say "Use crawler

*lane*" which implies that there is an extra lane for slow vehicles, with another for faster ones to overtake - if only!

(*) I can't get used to using the new terminology: Heavy *Cargo* Vehicles

[Dave Plowman (News)]

Now that is a good way to overheat the TC.

I'd suggest you get a more powerful vehicle so it isn't needed.

[James Wilkinson Sword]

That just irritates me. I'm tailgating some f****it who's driving far to slow, and he starts flashing his brakelights. I just drive closer. Eventually he gets really annoyed and slows right down, making it easy for me to overtake.

[James Wilkinson Sword]

I see.

[James Wilkinson Sword]

Assuming the % is calculated the same as in in the UK, 15% isn't much. I regularly see 20% and sometimes 25%.

[James Wilkinson Sword]

I wouldn't use any rule, I'd just drop gears until I got enough braking.

[James Wilkinson Sword]

When I were a lad my friend did that, but by doing a very long skid downhill on his back wheel. It exploded.