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I was having a discussion today, which led to the question

why do heavy trucks still use air brakes?

To my knowledge, it has been used for at least 40 years (I remember that as a kid), and apparently (I was told today but I haven't gotten around to verify it) they are still widely used.

From what I remember, one of the things I was cautioned was that if you repeatedly pressed on the air brakes, then after a while the air buffer would empty and it would take time to fill up (thus losing braking capacity).

Anyway, I wanted to know what are the benefits and disadvantages of air brakes compared to other technologies, e.g. hydraulic lines, or electrical system, (even KERS systems for more modern electrical vehicles).

UPDATE: From the answers I understand that the main issue is reliability and "technical debt". I want to push a bit further and understand, what's stopping air brakes from being used in other vehicles. E.g. is it cost, performance, inability to accompany AC/DC drum rhythm?

NMech
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3 Answers3

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Biggest advantage is that when connecting to trailers, there are no bleeding issues.

Imagine having to connect hydraulic pipes and remove the air bubbles...

As for the amount of air - the compressor and receiving tanks are designed for normal use. However if you wish to accompany AC/DC with the air brakes you will run out.

Disc brakes already exist in an airbrake version for trucks - came out over 10 years ago IIRC.

But one big difference is that if you lose hydraulics on a car braking system then you lose stopping power (except for handbrake etc) but with air brakes, they lock on as the air pressure is used to hold the brake shoes out of contact.

This explains some of those brake marks seen on motorways, as the trailer brakes lock on at 60mph when a pipe or connector fails, with the tractor unit still pulling.

Community
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Solar Mike
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Several characteristics led to the adoption of air brakes. The primary one is reliability. A leak on a hydraulic brake system means the brakes don't work. A leak on air brake system means the compressor needs to run more. This is made more important by the need to apply brakes on a trailer, which needs to be connected to the cab easily.

The next is that even if a better alternative were found, there is already a millions-strong fleet of trailers that would require either retrofitting or having dual systems on power units. The only driver of this would be regulatory (a government mandate), and there is not a perceived safety benefit for such a change.

Tiger Guy
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"Power" brakes, like the ones you have in your car, use air pressure to assist your pedal effort. In passenger cars the air pressure is actually negative: it relies on intake manifold vacuum. That vacuum is limited to about -15 PSIG: once all the air is drawn from a container, the difference between that pressure and the sea level atmosphere is all you can get. Air brakes use compressors to get unlimited (in principle) pressure which is needed in heavy truck brakes.

Brake geometry, disc/drum, has nothing to do with it.

The vacuum reservoir (I hate using that term to describe a continer of emptiness.) can be depleted by pumping the brakes while the engine is idling and that will increase the pedal effort - just like in air brakes.

stretch
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