Curious re: Lorry and speed camera.

[atpbx]

Something wrong with it then.. The air in a truck holds the brakes off until you press the brake pedal. Some air pressure is released and the brakes start to come on. more you brake more air is released..

Trucks are not limited to 60mph. It will do whatever spped you let it down hill until you apply either the foot brake or exhaust brake.. Even on idle the brake compressor is working building pressure and at revs it should be able to produce more than enough even under heavy braking..

As I've already said:
"You can free wheel down hill to any speed mavity and the engine breaking will let you in a lorry (because they still rev as if you are doing 56mph and in top gear). "

Brake compressors are gear driven, the volume of air they supply is entirely dependant on engine speed.
At idle they wont supply enough air to keep the tanks charged as you know already if you make continued demands on the air reserves.

Thats why, even in the latest mercedes gearboxes which they say you can "free wheel" down hill to save fuel, you arent, it just automatically changes gear to use engine breaking to keep you at the set speed, because "free wheeling" in an HGV, is a spectacularly bad idea, because you are not running the compessor.

 

[James_200sx]

Dont they have a valve on the exhaust to create more engine braking as auto braking on very long hills could cook brakes.

 

[Scania]

Dont they have a valve on the exhaust to create more engine braking as auto braking on very long hills could cook brakes.

Yes and the reason you give - bar the "auto braking" is why, brake fade is bad with HGV's !

the trucks throttle limited to either 52, 54 or 56 mph however they are physically limited at 60mph, meaning they can only get to either 52,54 or 56mph under their own steam but can get up to 60mph on a downhill stretch, also 56 on your speedo will be about 50mph real speed.

Physically limited to 60?

I think you'll find that its limited to the steepness of the gradient its travelling down, and the size of the drivers balls!


It is very easy to let one run away from you, 28ton payload, steep hill, stupid driver can easily see it going way past 70....

The limiter will not apply the brakes, Scanias, for example, will display brake lights when you are using the engines "exhaust brake" even though the service (i.e. wheel) brakes are not being used.

As for the camera & plate, the camera should catch the plate from the tractor unit, as the plate is on the n/s rear mudguard, normally, they do get the trailer reg also.

Incidentally, with tall trailers (15' & above) I have found for some odd reason that they tend to set off Gatso's even if you are well within the limit.

I had a ticket a while back - computer generated I guess - showing photographs of my trucks rear and a speed of 43MPH, I was on a dual carriageway at the time that had a limit of 60 for cars & 50 for HGV's....

Weird.