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Old 03-12-2016, 04:28 PM   #1
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disc and drum brakes

I often hear people discussing hydraulic vs. pneumatic brakes. is the actual stopping mechanism still either disc or drum?
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Old 03-12-2016, 06:35 PM   #2
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Air brakes ARE drum-type brakes, but with an S-cam that actually moves the shoes outward to contact the drum surface. Likewise the hydraulic brakes, but they use hydraulic pressure to do the work.

Bear in mind that liquids are, by their very nature, incapable of being compressed to any measurable degree. Air brakes, OTOH, have about 150 lbs of air pressure behind them, much more than any hydraulic brake system.
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Old 03-12-2016, 09:07 PM   #3
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Typical brake fluid pressure on a drum type system is in the 600-900 psi range, when tested with a pressure gauge. The brake fluid doesn't compress (unless it contains air.)
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Old 03-12-2016, 10:39 PM   #4
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I have heard of 4 bus fires that happened because of hydraulic fluid from brakes. It makes me a little hesitant to get a bus with fluid brakes.
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Old 03-12-2016, 10:47 PM   #5
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Most every car on earth has hydraulic brakes.

Air brakes "fail safe," meaning that if the system loses pressure you stop right now. When a hydraulic system fails you lose braking completely.
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Old 03-13-2016, 07:07 AM   #6
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Actually air brakes come in several different flavors.

Most are drum brakes with 'S'-cams to operate them.

But there are more and more buses and trucks equipped with air disc brakes that use similar air cans to operate them. Air disc brakes have many of the same advantages as the disc brakes used on light duty trucks and most new cars. The big disadvantage at this point is it usually costs more for one rotor than it costs to do a brake job on a complete axle with conventional 'S'-cam air brakes. Bendix Spicer Foundation Brake - Air Disc Brakes

On some air brake equipped buses and trucks instead of 'S'-cams to operate them they used wedges. An air can was attached to a wedge that was used to separate the brake shoes that applied the brakes. It was a good idea in theory that never worked as well as an 'S'-cam.

Prior to about 1975 all hydraulic brake equipped buses used basically the same drum brakes as had been used for more than 50 years. The only real difference was the brakes were much wider than they had been years before.

IHC started to phase in disc brakes as an option in the mid- to late-70's on all hydraulic brake equipped buses (I owned a 1978 IHC conventional chassis bus that had drum brakes on the front axle and disc brakes on the rear axle). By the early '80's disc brakes became standard on both the front and rear axles of all IHC hydraulically braked buses.

Initially hydraulic disc brakes were a whole lot more expensive to service but now that they have been in service for quite a few years the price is about the same to service a bus with disc brakes as well as drum brakes. At the same time the drum brakes were being phased out electric over hydraboost systems were being phased in over vacuum hydroboost systems.

So in answer to your original question, yes brakes are still either disc or drum. The big difference is air vs. hydraulic.
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