Wing mirrors on cars

[James Wilkinson Sword]

I assume he only does it long enough to get the revs up, which won't cause that much heat.

[rbowman]

Nice illustration. I like the 60% slopes. That's when you are hauling your ass up by grabbing trees and other vegetation holds while pausing every 20 feet to gasp.

[rbowman]

Therein lies the problem with big trucks. The diesels redline at 1800 rpm and you're normally working the range between 1000 and 1800. If your brakes start to fade and say your at around 1700 in 7th, you're not going to be able to get into 6th. At that point you clench the cheeks of your ass and get ready for Mr. Toad's Wild Ride.

I talked a guy down a hill where he had smoked his brakes. I knew the road, which dropped down into a valley and immediately climbed up on the other side. After the last bad curve I told him to let it go. It was all straight road from there and even if he went by Camp Verde at 100 mph he'd be slowed down halfway up the hill on the other side. Then he could stop at the top and change his underwear.

Coming off the Donner Pass going towards San Francisco there are even signs telling you where to try to keep it down and where there are flats coming up where you can coast down.

[rbowman]

If you miss a downshift, you've pretty much got to come to a complete stop and try again. Usually it isn't a problem getting underway but you're in a very low gear and can't get up enough speed to shift up a gear so you can look forward to a 8 mph ascent. Very embarrassing.

[rbowman]

The problem in the US is many of the eastern drivers have only ever encountered basically flat roads unless they've spent time in West Virginia. Even there, the grades can be very steep but they are short. Five or seven miles of downhill is something they have never dealt with.

[James Wilkinson Sword]

There's no way I'd park a car as expensive as

on that slope. Looks the same slope as a hill near where I grew up. Me and my friend used to cycle down it, hoping nobody was driving along the main road at the bottom. There was no way in hell a bicycle could have stopped at the give way (yield) line.

[James Wilkinson Sword]

So what you're saying is trucks ain't built properly.

[James Wilkinson Sword]

Why would anyone blame a big heavy lorry going slowly up a tremendously steep hill?

[James Wilkinson Sword]

That's their problem. We have a similar thing here. Apparently it's no longer allowed to put a right arrow in a lane at a roundabout incase Europeans (who drive on the other side) end up going the wrong way round a roundabout. I pointed out to my local roads dept that they were making cars get in the wrong lane and all barge through on the other side, but they lacked the intelligence to understand.

[rbowman]

No, I'm saying trucks should not be driven by incompetents. A vehicle that may weigh up to 80,000 pounds requires a little more forethought than your family sedan.

That said a Jacobs Brake is a very effective system:

However, it is a device many eastern trucking firms don't opt for since they typically don't operate in mountainous terrain -- until they get a load going to California and find there are these things called mountains that you hit around Denver.

[rbowman]

I am sure if you were following a lorry creeping along at 23 mph you would have quite a bit to say about it.

[rbowman]

I'm assuming he hopped out for the photo op and then went on his way. It was not as steep a hill but a friend parked his car on a hill one winter evening. We both got out and were about to walk away when we realize the car was slowly sliding downhill. Fortunately we managed to push the front end enough sideways to have a front tire wedge against the curb.

The college campus was built on the same hill so we got our exercise trudging up and down it between classes.

It was a classic metaphor for town and gown; the peons from the city below could climb the marble staircase to the glories of education available above their mundane existence.

The staircase merely put you at the bottom of the campus; there was much more to come. It was wonderful in the winter.

[James Wilkinson Sword]

My school bus had that. The driver claimed it was so he could slow down without operating the brake lights if there was a cop behind him. He then told me you had to be careful as when you let go it let off a cloud of smoke which some pigs knew to look for. It was a very old bus, and when the bus company was taken over and new drivers appeared, none of them knew how to double de clutch.

[James Wilkinson Sword]

I would if it was a car, but someone with a big or uneven load I have no problem with.

[James Wilkinson Sword]

That was quick thinking.

[NY]

I hadn't realised that big diesel engines redline at such a slow speed. I'd thought that a big truck diesel was like a car diesel (except slightly bigger bore on each cylinder and many more of them), and so would be able to run comfortably at up to 3500-4000 rpm - my car redlines at about 5000 and I think I've only hit that limit once or twice when accelerating hard out of a junction and have wanted to stay in the same gear until I'd got up to the speed of the car coming up behind me without having the time penalty of changing up half way through.

If truck engines redline at 2000 or less, then I can see why they need many more gears (possibly as a 6- or 8-ratio box with another gearbox "in series" with that) to cover all the speeds from zero to maybe 60 mph while still being able to pull 40 tonnes and cope with gradients as steep as the 1:4 /

25% that is marked as the maximum gradient on Sutton Bank, the hill I referred to earlier.

I presume the reason that big engines redline at such a slow speed is the greater mass of the pistons and therefore the bigger forces on the connecting rods and crankshaft, rather than because of incomplete combustion at higher speeds.

[NY]

I wouldn't blame the lorry, I'd blame the highway authority for permitting lorries to use that route if they had a power:weight ratio that limited their speed that severely. Cars towing caravans are banned from the hill, but 40-tonne HGVs are allowed to use it; which has a worse power:weight ratio?

The problem is greater than that. It's more like a lorry going at 5-10 mph up that hill: difficult to find a gear that is suitable for driving a car that slowly up a steep hill.

[James Wilkinson Sword]

I'm not familiar with lorries, but 50 seater buses (which weigh about 10 tonnes) rev around 1000-2000. I think from memory there's a green marker from 1000-1500 where you're meant to keep it, but I think it can go a lot higher, maybe 3000, for engine braking perhaps?

[James Wilkinson Sword]

Things shouldn't be banned unless there's a very convenient alternative route for them. It won't kill you to wait for goodness sake.

A car will go happily at any speed up a steep hill. I'd probably use 1st for going 5mph. 10mph, 1st or 2nd would do depending how steep. Also, probably easy to overtake the lorry.

[NY]

I remember seeing a video of an early diesel train where the rev counter in the cab was shown and that had a redline (except it wasn't red because it was black and white film!) at around 2000 rpm. So in scaling up from 1.5 to

2 litres in a car engine to whatever capacity is used in HGV and train engines, a much more severe max rpm limit is imposed. This article quotes a capacity of about 15 litres and only a 6-cylinder engine, so relatively few but massive pistons: 138 mm bore and 178 mm stroke (compared with about 85 mm bore/stroke for a car).

I believe that marine diesel engines routinely run at a few hundred rpm - evidently they have very massive flywheels which prevent the engine stalling at around 800 rpm, as car diesels (and petrols) do.