Why don't all vehicles have air brakes?


#1

I drive big trucks in the course of my work and most of them have air brakes, they have 4 wheel drum air brakes. Some of the trucks have 4 wheel disc hydraulic brakes and they take more effort and distance to stop than the air brake equipped vehicle. Now I know that despite what many believe, that the big part of why the air brake equipped trucks stop better is because they have drum brakes which have more surface area than disc brakes and more friction area= better brakes.

Why do cars only have hydraulic brakes? Wouldn’t they stop quicker with air brakes? Most cars have 4 wheel disc brakes despite being problematic and prone to warping, and they have reached maximum performance in regards to stopping distance, so isn’t air brakes the next logical improvement?

Here are some benefits to air brakes.

-No fluid to house or cool, simplified brake maintenance

-If you get air brakes too hot (or any brakes for that matter) they will “fade” and your vehicle will no longer stop. Air brakes are not fade proof, BUT can dissipate heat a lot better than hydraulic brakes

-You can use drums and still have better stopping distance than disc brakes without the warping pulsating discs.

Drawbacks

-Disadvantage: Air compressor (bigger load on engine) bigger possibility to freeze up on you (winter time + humidity), but I have never had air brakes freeze on me.

-People say the pedal feel is poor, but alot of hydraulic brakes have poor pedal feel.

The advantage of air brakes is quite simple. Air brakes are a proven technology that will apply more pressure that hydraulic brakes and will stop a heavy vehicle far quicker with a foolproof system. One leak in hydraulics and it’s game over. Air brakes are almost foolproof. The parking brakes are kept off by air pressure. Lose air and the brakes slam on. Apply the foot valve (pedal_ and air will apply the wheel brakes like nothing hydraulic can do. Love 'em. Millions of semi’s can’t be wrong


#2

Here we go again, disc vs. drum, with air added on top. Since modern disc brakes can lock up all wheels (without ABS), so no chance to make them stop better. I imagine the costs are the reason, plenty of braking engineers around to price out an air brake system on a car, I would think.

And ‘good old’ drum brakes on cars were, in most cases, poor compared to discs. But we’ve had this discussion…as you well know…
http://community.cartalk.com/discussion/2292415/the-underappreciated-drum-brake/p1


#3

@texases‌

So basically airbrakes and their advantages only come into play on heavy vehicles, what about big f-350s?

I will admit that overall disc brakes are better, but they sure do like to warp these days. I am so tired of warped discs. I was in one of our smaller bucket trucks today and it has hydraulic disc brakes and when you hit the brakes, no surprise, pulsation!!!

My Prius has disc brakes and they are getting corrosion because they do not apply enough I guess since alot of the time the generator is doing the braking.

If they could make a modern disc brake system that would not allow rotors to warp under normal driving conditions and wouldn’t have rotors that rot away I might think differently about disc brakes.


#4

Coupling a semi trailer to the tractor using hydraulic brakes would really be a problem. The lines would have to be bled each time a trailer was coupled to the tractor. Air brakes make it easier to connect the tractor to the trailer, just as air brakes allow different numbers of railroad cars to be coupled together without problems.
An automobile or a straight truck doesn’t have this problem. I suppose one reason for the hydraulic brakes is that in the hydraulic system, when you press the brake pedal, you force the brakes to be applied, while with air brakes, when you operate the foot pedal, you release the air and the brakes are applied. As you stated, if the air pressure is lost, the brakes are immediately applied. This may be a problem for a non-commercial driver. Having a compressor for the air brakes is probably more expensive than having a master cylinder to operate the hydraulic brakes. I have wondered, like you, if air brakes would be feasible on automobiles.
I will concede, however, that hydraulic brakes beat mechanical brakes–I’ve driven a 1938 Ford with mechanical brakes. On the railroad, the brakeman’s job became unnecessary due to air brakes. Before the air brakes, the brakeman went from car to car applying the brakes. Air brakes eliminated a job, although trains had to carry the brake man long after Westinghouse perfected the air brake. A second job that was eliminated with the air brakes on the railroad was the flagman. When the train had to stop, the flagman jumped off and ran back the tracks to warn following trains to stop. When I rode passenger trains back in the early 1960s, there was a flagman who ran to the back window of the last car and waved a warning flag when the train stopped. The advent of air brakes on the railroad cost jobs.


#5

Well Rick,I too am a lover of airbrakes(no brake fluid to boil a minor leak will not leave you stranded and the list goes on,but the simplicity and ease of maintainence make the disc brake a winner and has time and time again proven to be more then adequate for autos,now if we had other pnuematic systems on cars,I’m sure we probaly could look for air systems to scale down.We could concievably have an air powered climate control system and other things as well,but what we have is more then good enough(the heavy drums on trucks are marvelous)but my crystal ball sez if any advancement occurs it would be some kind of electromechanical system probaly.You did know in Europe that there are actually electric retarders to enhance the braking systems in lieu of the good ol Jake Brake in some applications? Anyway one advantage of the the good ol hydraulic disc brake is that a klutz like me can reliably repair them-so keep your eyes open,anything better will eventually take the market-Kevin


#6

Just cause I do not know, do air brakes lock up when they loose pressure? Are air brakes a little more maintenance due to moisture building up in the system?


#7

"Are air brakes a little more maintenance due to moisture building up in the system?"
You may have something there. I got on an intercity bus once that had been sitting outside and the air brakes wouldn’t release due to a frozen air line. I think the driver thawed the line with a cigarette lighter and we were underway in a few minutes. Also, I was on a passenger train that ground to a stop in subzero weather due to a frozen air line. I don’t know what the train crew did, but we got rolling again within 5 minutes.


#8

@Barkydog‌

Yes an air brake system uses air pressure to release the brakes. When you release the parking brake you are filling the system with air. When you press the brake pedal you are actually decreasing the air pressure going to the brake chambers on the wheels. If you lose air pressure the parking brakes automatically apply, the problem is if you have a catastrophic leak you will stop right NOW weather you want to or not.

The parking brakes on air brake equipped trucks are the very same brake shoes as the service brakes, they are just applied in a different manner. When you apply the parking brake you hear a large discharge of air so basically when you pull the parking brake knob on a truck you are dumping all of the air out of the brake chambers.

The trucks I drive at work automatically purge the moisture from the system so no added maintenance there.

Airbrakes can be considered a bit touchy, once you get used to them you tend to prefer them.


#9

The school bus that services the route that goes past our house has air brakes. The bus is on a Ford chassis and also has a diesel engine with an automatic transmission. The buses I rode to school in the late 1940s and early 1950s were a Wayne body on a 1939 GMC chassis and later a Superior body on a 1946 Chevrolet chassis. Both buses had hydraulic brakes with no power assist. The drivers probably had to use both feet on the brake pedal to stop the bus. One driver of a different route than I rode purchased a 1951 Superior body on an International chassis and it did have vacuum assisted hydraulic brakes. There was a vacuum gauge on the dashboard. I rode this bus for field trips. I would guess that the air brakes on today’s school buses add a margin of safety over the hydraulic brakes. None of the buses I rode in those days had power steering as do today’s buses.


#10

One practical disadvantage to air brakes I haven’t seen mentioned is delay in action.

Pressure in a compressible fluid like air travels at the speed of sound in that medium. In air, at typical temperatures, that’s ~720 MPH. If the truck’s going 72 MPH, there’s a delay of 10% of the truck length (actually, call it 15% because the line snakes around a bit) before the rear brakes begin to respond. Not a deal-breaker, but something to keep in mind.

Actually, I’d get a kick out of air brakes in my pickup. In addition to brakes, you’d have a tank for an air chuck, and an enterprising sort could see how to combine this tech with TPMS for a run-flat setup. Also, the “BRRRR-ZIP!” of an air starter would be really, really cool. :smiley:


#11

I dunno about a couple million cars with air brakes when its 20 below in Minnesota. Frozen lines would be a perpetual problem with the cars at a dead stop with the brakes on until they thawed out.


#12

A properly designed and manufactured hydraulic brake system is an excellent system for cars. I see absolutely no advantage to air brakes in a car. And, frankly, chronic problems with disc warpage are generally due to improper maintenance, like “turning” discs rather than replacing them to try to save a buck, not properly cleaning the rotors when installing them, not properly installing pads, or skipping the dab of antisqueal grease.

I believe the reason we read about problems with hydraulic brakes and never about air brakes is simply because this is a car forum and cars use hydraulic brakes. If cars had air brakes, we’d be reading about air brake problems and probably wondering why cars don’t use hydraulic brakes.

There’s no question in my mind that there are cars out there with underdesigned brakes. But I’m sure that if those cars had air brakes they’d be poorly designed in some regard in order to save a few bucks. IMHO the problem isn’t the technology itself.

Besides, why add another compressor to the load on the crankshaft if we don’t have to?


#13

@the_same_mountainbike I hear your point. One salient question I have to ask is why my trailblazer had brakes at 85k, now at 157k still have 15% to go and so many cars are a 34K brakes worn out? The technology is out there but planned obsolesence is out there also.


#14

Most trains still have brakemen (the exception being passenger trains and some unit trains, like coal trains, where there is no switching) . Freight cars still have brakes that can be clamped down mechanically. It used to be done with a wheel extending up from the roof with the brakeman walking from car to car on a plank roofwalk. Needless to say there were a lot of accidents so they outlawed rooftop brakewheels decades ago. They moved the brakewheels further down on the car sides and many were eventually replaced with a ratcheting handle. Being able to set brakes on selected cars is extremely valuable before descending a grade, especially setting brakes on cars near the tail end of the train. It means the engineer doesn’t have to use the air brakes as much, preserving pressure in the train line (the main break line down the length of the train) and it keeps the slack right along the length of the train instead of having the cars bunching up and unbunching a serious problem that can cause broken couplers and derailments. It’s also important that the engineer apply brakes as few times as possible because each time the brakes are used the pressure in the train line drops. If it gets too low you have no brakes, so before that happens you have to bring thr train to a halt, set brakes on some cars, then pump up the train line. That’s a slow process and good train crews avoid that pickle by setting brakes on enough cars to keep the train easy to control without havingbto apply the aor brakes too often. If a train runs out of pressure in the train line you can easily get a runaway train. Of course, stopping to set brakes manually and later release them is also slow, bit it’s part of the job. If any of this sounds like a good idea for passenger cars, please jump in.

Air brakes are essential where you have connections that can leak, but the brake pressure is tricky to manage and they have no real advantages over hydraulic brakes on a car, where hydraulic brakes work exceptionally well. Other than improvementa in disk and pad materials, I don’t see a lot of changes likely to happen in brakes in the next few decades. They’re a very mature system. Of course, with electric motors regenerative braking can take over a lot of the braking. One of the surprises NYC cab fleets had when trying out hybrids was how much they saved on brake jobs. In low speed start-and-stop driving (like in NYC) regenerative braking does almost all the work.


#15

@markm Thanks for a great post, since you are so familiar with trains can you tell me why the caboose is a thing of the past?


#16

Barky, I’m inclined to blame cost-cutting in the design phase rather than planned obsolescence. Either way, there are definitely cars out there with underdesigned brakes. My feeling is that they’d be underdesigned if they were air brakes as well.

As ignorant as this sounds, the possibility exists that there are simply brake designers (or perhaps their managers) out there that are brake-performance focused and others that are cost-reduction focused. All design teams, all managers, and definitely all committees, are not equally focused. Some cars have great brakes, some are terrible, just as some cars have great engines and others are terrible.


#17

Idon’t know about trains but I believe on trucks and buses, the air pressure keeps the brakes off. If pressure is lost, the brakes lock up. Pushing the brake pedal releases the pressure to allow the brakes to lock. That’s why you see those rubber marks on the highway where a semi trailer has lost air pressure and the trailer brakes lock up all the way to the shoulder. Another reason not to follow trucks too close.


#18

Air brakes use compressed air to apply the brakes in the same way that hydraulic brakes use hydraulic pressure. The difference is that the system is kept pressurized in air tanks when the pedal is not being applied, whereas in hydraulic brakes the system is only pressurized when you push the pedal.

The reason for the “lockups” on truck tires is that most of them use drum brakes of a “self energizing” design. Their fulcrum is placed such that when applied, the friction of the pads tends to pull the shoe into the drum assisting in the application. This design has the side effect of tending to occasionally lock up. The pull being applied from the frictional pad material can overcome the return springs in the actuators (which on trucks are the canisters attached by brackets to the underside of the axles).


#19

A lot of blackmarks are caused by physics and inattentive drivers as to the caboose’s demise,dead weight and dont the cars have compressor system built in the chassis with the locomotive used mainly for signaling?(good grief George Westinghouse)-Kevin


#20

There’s a lot of hardware on air brakes. Hydraulics are much simpler.

And years ago I drove several school buses, all were Blue Bird bodies on GMC chassis. They were just bare bones 6 cylinder trucks with non power steering or brakes which were hydraulic. But weighing 150 pounds I could stop them with no problem. Of course the top speed was under 60 mph. The 0 to 50 time was measured with a calender.