What's the story with the really loud pneumatic noise coming from buses and trucks?

Pompous

I don't know if it's when they break or when they're changing gear, but a really loud noise comes from the side of a bus sometimes, like pressurised air being released. Does anyone know what exactly the mechanism is that causes this? Air brakes maybe?

It's really loud on some buses, and if you're unfortunate to be beside a bus when it happens it really hurts your eardrum and is probably doing some permanent hearing damage. I have considered putting an ear-bud in my right ear when commuting because of this.

Buchaill_Mor

Self levelling suspension on the buses that kneel for the wheel chair ramp I think.

Mycroft H

Excess air from the braking circuits of HGVs/busses etc.

Pompous

Buchaill_Mor wrote: »

Self levelling suspension on the buses that kneel for the wheel chair ramp I think.

So why does it happen when traffic is stopped and I'm passing the bus? Is the driver purposely hitting a switch to try blast me with it?

Call me paranoid, but I wouldn't put it past some of the Dublin Bus drivers. They have it out for cyclists.

Pompous

Mycroft H wrote: »

Excess air from the braking circuits of HGVs/busses etc.

Does that excess air get released at random intervals? It often happens when a bus is not moving at all.

Mycroft H

Pompous wrote: »

Does that excess air get released at random intervals? It often happens when a bus is not moving at all.

Yes as the compressor is belt driven (always on then) so when there is a head of pressure in the tank, the relief valve opens.

Buchaill_Mor

As Mr Holmes the elder says, the compressor for these systems is on, and it is just the release valve. In the old days of steam you would see a massive blast of steam from the pressure release valve of the engine, much like the pressure cooker my mum used to use.

Ole Rodrigo

The braking action releases air and is very audible. Actually, if you do get stuck in front of a bus its not unheard of for a driver to feather his brakes in annoyance at getting stuck behind YOU..so you're not entirely paranoid !

Just make sure you wave at him next time in gratitude for teaching you a lesson about road dynamics.

fondriest

The air brakes on HGV or bus are worked by air instead of fluid , the air pressure is built up by a compressor that is usually driven by a gear or cam . When the air pressure builds to about 7 bar the air pressure relief valve blows to release the excess pressure , can't really remember the full theory behind it because it's been 20 years since I worked on trucks but the more you use the brakes the less the relief valve should blow off .

Wishbone Ash

Buchaill_Mor wrote: »

Self levelling suspension on the buses that kneel....

Mycroft H wrote: »

Excess air from the braking circuits of HGVs/busses etc.

The doors are also activated by pressurised air.

Pompous wrote: »

Is the driver purposely hitting a switch to try blast me with it?....

The driver has no control over the air pressure release. The only way he could reduce the air pressure would be to continually open/close doors or raise/lower floor.

ror_74 wrote: »

... its not unheard of for a driver to feather his brakes in annoyance ..

Unless things have changed since I drove buses but when the brakes are 'feathered' it activates the exhaust brake not the air brakes.

fondriest wrote: »

...When the air pressure builds to about 7 bar the air pressure relief valve blows to release the excess pressure , ....... the more you use the brakes the less the relief valve should blow off .

When the kneeling floors came in I was reluctant to use them unless I really had to as they seemed to use so much air. If the air pressure drops below a certain level (caused by excessive door/floor movement) the bus will not move off until adequate pressure is reached again, Not pleasant with a load of angry passengers who don't understand!

07Lapierre

HGV brakes "default" setting is on. This means the air compressor must have enough pressure to release the brakes. ( this is for safety as the brakes will be applied if the compressor failed)

The pressure release valve will operate as soon as the max pressure is reached. The driver has no control over this.

m5ex9oqjawdg2i

Angry drivers releasing steam I guess.

radia

The thing I've noticed a lot at lights is bus drivers giving the very slightest little 'bip' on the horn, the absolute second the lights have turned green. It's not an aggressive 'BEEEP' or even a 'beep' - just the faintest little chirrup that might not even be audible to car drivers, just pedestrians and cyclists - but I hear it so often that it'd almost make you wonder whether there's some connection on buses between the horn and the clutch.

And they do it even when there's not a stream of pedestrians crossing in front of them or a dilatory driver at the front of the queue; it seems to be like a reflex.

OldBean

I think that noise is just the brakes releasing, not a beep from the driver.

Wishbone Ash

radia wrote: »

The thing I've noticed a lot at lights is bus drivers giving the very slightest little 'bip' on the horn, the absolute second the lights have turned green. It's not an aggressive 'BEEEP' or even a 'beep' - just the faintest little chirrup that might not even be audible to car drivers, just pedestrians and cyclists - but I hear it so often that it'd almost make you wonder whether there's some connection on buses between the horn and the clutch.

And they do it even when there's not a stream of pedestrians crossing in front of them or a dilatory driver at the front of the queue; it seems to be like a reflex.

As has been said it simply caused by releasing the parking brake lever. I used to hate it as a driver as I was conscious that cyclists thought I was beeping them.

radia

OldBean wrote: »

I think that noise is just the brakes releasing, not a beep from the driver.

Wishbone Ash wrote: »

As has been said it simply caused by releasing the parking brake lever. I used to hate it as a driver as I was conscious that cyclists thought I was beeping them.

Ah right - thanks. It does sound like a beep, though a gentle one.

the boss of me

Pompous wrote: »

So why does it happen when traffic is stopped and I'm passing the bus? Is the driver purposely hitting a switch to try blast me with it?

Call me paranoid, but I wouldn't put it past some of the Dublin Bus drivers. They have it out for cyclists.

Your paranoid...
If the bus is stationery it's simply the driver applying the handbrake. Nothing sinister about it.