Sketchy towing

[4rephill]

Cat said:

944 Man said:

Cat said:

Depends if you're talking about the service brake or the park brake. The park brake will be applied if there is insufficient air pressure but it is is possible to prevent this by winding the brakes back. The service brake on the other hand will not work if there isn't enough air pressure.

Cat

I am confident that the stored air pressure holds the brake linings off, and that applying the pedal bleeds air pressure which allows the linings to come into contact with the brake drum.

When air brakes fail due to loss of air pressure, the brakes engage fully and the wheels lock. When the air reserve is depleted, the brakes engage fully and the wheels lock.

This is not a matter of opinion.

You may be confident but unfortunately you are wrong. HGV's have two brake systems, the service brake and the park brake. The 2 systems operate in the opposite way to each. The service brakes are held off by springs and require air pressure to apply the brakes, these are the brakes which operate when you press the brake pedal. The park brake on the other hand is held on by springs and requires air pressure to release the brake.

if you have no/insufficient air pressure the service brake will not work. The park brake would normally be applied when there is insufficient pressure but they can be wound back so that shoe/pad doesnt contact the drum/disc and the brake won't operate even if there is no air.

Cat

Watch any of the HGV rescue programmes on TV, and they all show that, if the engine on the broken down vehicle can't be run, or there's a fault in the vehicle's air system, the brakes lock on.

To tow the vehicle, they have to attach an air-line from the recovery vehicle, to the broken down vehicle's air system (they should all have an emergency air-line sockets, to connect an external air-line to them)

Naturally it is a lot quicker to attach an external airline, than to try to manually wind off the brakes at the road side.

And unless they are doing a reverse tow (towing the broken down vehicle from it's rear end, with the rear wheels raised, they have to remove the half-shafts from the driven wheels, to avoid destroying the broken down vehicle's gearbox.

[Cat]

4rephill said:

Watch any of the HGV rescue programmes on TV, and they all show that, if the engine on the broken down vehicle can't be run, or there's a fault in the vehicle's air system, the park brakes lock on.

FTFY

Cat

[944 Man]

I must have been in an inverse universe then, when my enormous HGV's air system failed and the brakes locked on. You silly man.

Edited by 944 Man on Sunday 25th July 22:24

[No ideas for a name]

Thanks to this thread I learned something new today!

https://britishtrucking.co.uk/hgv-air-braking-syst...

[Max5476]

Cat said:

4rephill said:

Watch any of the HGV rescue programmes on TV, and they all show that, if the engine on the broken down vehicle can't be run, or there's a fault in the vehicle's air system, the park brakes lock on.

FTFY

Cat

Complicated because it all happens within the same body, so not obvious externally what's going on, but yes it's the parking brake that locks on without air pressure, and the service brake is activated by increasing the air pressure.

[Cat]

944 Man said:

I must have been in an inverse universe then, when my enormous HGV's air system failed and the brakes locked on. You silly man.

I'm guessing comprehension isn't one of your strengths. I've clearly said throughout that there are 2 different brake systems on a vehicle, one of which requires air to apply the brakes and one of which requires air to keep the brakes off.

If you have driven "enormous HGV's" I am genuinely surprised and slightly concerned that you think stored air pressure keeps the linings off and that pressing the brake pedal releases the air pressure allowing the brakes to operate - this the exact opposite of what happens when you press the brake pedal.

Take the time to read the link posted by No ideas for a name and educate yourself on the differences in operation between service and park brakes, only a silly man would not do this .

Cat

[Southerner]

Easy enough to suss it out if you think about the last time you drove - use the footbrake, and the brakes apply silently with a hiss of exhausted air heard as you RELEASE the pedal (i.e. as the brakes release), because stored air has applied the brakes, and is then exhausted as you release them again having done its bit.

Apply the parking brake and the hiss is heard as you APPLY the brake; i.e. as you operate the lever. This being because the stored air which holds the parking brake off is then expelled, applying the brake.

Simples!

[geeks]

This may not be the thread for it and it may come across as a dumb question. Why do lorries/busses etc use pneumatic rather than hydraulic brakes?

[Southerner]

geeks said:

This may not be the thread for it and it may come across as a dumb question. Why do lorries/busses etc use pneumatic rather than hydraulic brakes?

Presumably because air is a much simpler system to use, the air doesn't need to be stored within a sealed system and is required anyway for other systems incl. suspension, wipers and doors.

[EW109]

Some buses use hydraulic brakes. The original Routemaster is probably the best-known example.

[zarlak]

Occasionally the air brakes on trains can be ineffective, the Lac Megantic disaster was one such tragic event:

https://en.wikipedia.org/wiki/Lac-M%C3%A9gantic_ra...

[Southerner]

EW109 said:

Some buses use hydraulic brakes. The original Routemaster is probably the best-known example.

Indeed, and probably a good answer to the "why not" question; they require specialist kit to service and maintain them!

[Southerner]

zarlak said:

Occasionally the air brakes on trains can be ineffective, the Lac Megantic disaster was one such tragic event:

https://en.wikipedia.org/wiki/Lac-M%C3%A9gantic_ra...

In fairness, that wasn't a normal day at the office!

From Wiki:
"With all the locomotives shut down, the air compressor no longer supplied air to the air brake system. As air leaked from the brake system, the main air reservoirs were slowly depleted, gradually reducing the effectiveness of the locomotive air brakes."

Causes listed as:
"Neglect, defective locomotive, poor maintenance, driver error, flawed operating procedures, weak regulatory oversight, lack of safety redundancy."

Ouch!

[bigothunter]

EW109 said:

Some buses use hydraulic brakes. The original Routemaster is probably the best-known example.

Routemaster has power hydraulics where the driver modulates a valve in the same way as an air braking system. Line pressure relates to brake pedal deflection not pedal effort.

Car hydraulic brake systems amplify brake pedal effort using a vacuum servo (with a few exceptions such as Citroen DS). Some post WW2 full size buses still employed conventional hydraulics with engine driven vacuum pump.

Vacuum assistance can only develop 1.0 bar depression max, unlike much higher pressures in air and especially power hydraulics. High braking efforts on big trucks would demand huge components if relying on vacuum assistance rather than pressure.

Smaller trucks tend to utilise a hybrid system of air-over-hydraulic.

Anyway that's my limited understanding and interpretation. Can anyone more knowledgeable fill in the gaps please?

Edited by bigothunter on Monday 26th July 23:45

[bigothunter]

anonymous said:

[redacted]

I've driven an old German Ford Werke army truck on private ground, fitted with an early version of air brakes. Handbrake was by mechanical links and cables, not spring brakes. Without sufficient air pressure, you couldn't stop especially as the handbrake was ineffective. Bloody dangerous

[dhutch]

geeks said:

This may not be the thread for it and it may come across as a dumb question. Why do lorries/busses etc use pneumatic rather than hydraulic brakes?

One of the reasons is that whatever system you use needs to be able to disconnect everytime you unhitch the trailer.

Rigid body trucks and busses of the same sort of size, use the same brakes, because there are a lot more artics than busses and commonality is good.


Daniel

[bigothunter]

dhutch said:

geeks said:

This may not be the thread for it and it may come across as a dumb question. Why do lorries/busses etc use pneumatic rather than hydraulic brakes?

One of the reasons is that whatever system you use needs to be able to disconnect everytime you unhitch the trailer.

Rigid body trucks and busses of the same sort of size, use the same brakes, because there are a lot more artics than busses and commonality is good.

Scammell Scarab had hydraulic brakes with a mechanical coupling to the separate trailer brakes. I believes regulations overtook this rather crude arrangement.

[dhutch]

I mean it's all possible.

A lot of agricultural trailers have powered hydraulic braking with a quick release coupling.

Obviously plenty also have road trailer type air systems.


Daniel

[Spare tyre]

The Spruce Goose said:

Reminded me of this classic.

NSFW Naughty words.

Edited by The Spruce Goose on Friday 23 July 16:11

Amazing, truly amazing

[anonymous-user]

Spare tyre said:

Amazing, truly amazing

''That's fking mental... The have not strapped, they are just tied''

I wish he had followed him..

Edited by anonymous-user on Monday 9th August 00:18