Why Do Semi-Trucks Use Air Brakes?

čas přidán 24. 09. 2023
Why Do Semi-Trucks Use Air Brakes?
Are you curious about why semi-trucks use air brakes instead of the traditional hydraulic brakes?
In this video, we take a deep dive into the different types of brake systems and explain the benefits of air brakes.
We compare hydraulic brakes and air brakes and their pros and cons. You'll learn how each system works, their reliability, and the fail-safe features of air brakes.
Discover why air brakes are the preferred choice for heavy vehicles like semi-trucks and why most car manufacturers stick to hydraulic brakes.
Learn about the extra costs of air brakes and how they can be used on trailers too. Don't miss out on this informative video and hit the subscribe button for more interesting content like this. "
▬▬▬ The video ▬▬▬
00:00 Start
How do hydraulic
brakes work?
01:51 How do air
brakes work?
02:54 Why Semi trucks
use Air brakes?
04:16 Conclusion
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▬▬▬ End ▬▬▬
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  • Watch: Why European And American Truck Cabs Are So Different? here cs-tv.org/tv/video-y6MijXQycT4.html

  • CORRECTION: air brakes absolutely do fade from over heating, just like hydraulic brakes. It’s much more common of an issue with air brakes in fact, since they are more powerful they press the pads harder and heat up faster, they have to absorb a lot more energy to stop a heavy truck. If used too much the brake drum can expand enough from heat to make the S cam unable to push the pads far enough. Most run away truck situations are caused by the driver relying on the brakes too much on a downhill, instead of engine braking, and overheating the brakes to the point of failure. As mentioned hydraulic brakes fade too from over heating, but this is mostly seen with sports cars on race tracks. It’s why a lot of race cars and super cars have exotic ceramic material brake rotors that can handle higher temperatures

    • I think this applies to cars too, but i dot think if they use air brakes. We have escape lanes in the event a vehicle's brakes fail but that is only in downhill. I have been told that you, in most cases, use the same gear when going up- and downhill, but that is only in manual vehicles. Do automatics have manual gears but what will they do with the clutch?2

    • I'm not sure this person actually did real research before writing this video

    • @Annabel Holland I just got my Class A CDL, so all of this is fresh in my mind. At lest according to the Pennsylvania CDL manual, truck drivers should go down a hill in a gear 1 LOWER than the gear they would use to climb the hill. Depending on the grade and length of the hill and the weight of the truck, you can calculate a “safe speed” for going down the hill, some big hills even have signs poster that say what the safe speed is for different weights. You coast until you hit your safe speed, then you brake until you are 5mph under, let off until you hit it, brake again, ect… until you are at the bottom, this on/off braking gives the brakes time to cool down during each cycle. As for automatic trucks, I’ve driven a few, they automatically down shift and engine brake when you take your foot of the gas pedal, so when you are cruising you always keep your foot on the pedal a little, or it starts engining braking like you would driving a manual.

    • @Random Guy i was talking about cars. idk anything about trucks as i do not drive and have a license for that. i would say though that the steeper or narrower or bendy the road gets, the lower the gear you need. for the same reason, do not coast in neutral going downhill unless at an extremely slow speed (

    • ​@Annabel Holland Read your owners manual for transmission shifting options. It can vary between make and models.

  • Another big perk of air brakes too is that if they leak a little bit, they might still work for some time. Even the tiniest leak in a hydraulic system will render that circuit inop and will quickly deplete the fluid reservoir, which can knock out all circuits. A small air leak may only cause the compressor to work more.

    • Leaking is actually an expectation, to the point where there is an acceptable amount of leaking before it’s considered unsafe.

    • @Jacob Leyva I thought I recalled as much but I wasn't 100% so didn't want to make that claim. I haven't driven large vehicles in over 10 years so I'm a bit rusty on the facts. Thanks for confirming!

    • @Jacob Leyva technically any amount of leaking is considered safe since it will just activate the spring brakes and stop the vehicle from moving at all.

    • @Jacob Leyva according to European law: A leaky brake system (even if it is a super small minor leak) is deemed having no brakes et all!

    • Your only allowed a very small amount of air leakage to be allowed to drive that rig down the road. I'm thinking about 4 psi in a minute.

  • Air brakes are used because of the trailer. The trailer has to have the ability to decouple from the tractor. The connection for fluid transfer would potentially have a leak. With a liquid being the fluid to move the brakes, you might lose all your fluid. The air compressor replenishes the air which is the fluid used for air brakes. Air leaks are bad but leaking hydraulic fluid would be more dangerous.

    • I agree. But also, even a small air bubble in the hydraulic system would render it unusable. There is no practical way to make it work unless the driver bleeds the hydraulic brake system of air every freaking time a trailer is hooked up. Air brakes are the best solution.

    • So that's why a true hydraulic braking system was never applied to any on-road vehicle. Even though applying a reversing flow of fluid to a vaned wheel assembly would stop the vehicle with no chance of overheating and never wear out, a leak would cause catastrophic system failure.

    • Air brakes are also much easier to amplify on the scale of a comercial vehicle. You just press in a control valve and the pressure will start building from an outside power source. With car brakes only a big vacuum powered booster is enough, but braking a large vehicle would be near impossible with a vacuum based booster.

    • but they're also used on trucks and buses without trailers.

    • @Richard Ruddock why not electric trailer brakes then?

  • WARNING! Before you work on any air brakes. There is a large heavy spring in the brake chambers under tremendous pressure and it will kill you, if disassembled wrong!

    • But im too lazy to cage the brake chamber😂

    • Who are seriously disassembling brake chambers? 😅 If one goes bad you swap out the whole unit

    • i am very well aware of it ...... that is why this video is wrong. that spring is the actual brake and if air or hydraulic pressure fails the brake engages.

    • That spring is what makes it a "fail-safe" system. It is normally engaged. It only releases when air is pressurised and holds the spring in place. You must release the spring tension in order to safely rebuild the whole chamber. It is much cheaper to rebuild the chamber than buy a new one, if said chamber is not rusted out of existence. There is another actuator that actually makes the system brakes using the pedal. This has a much weaker spring that only releases the brakes if pressure from the pedal is gone.

  • Another big advantage air brakes is that you can use that compressed air for other purposes. Obviously the brakes and trailer brakes. Compressed air is also used for the airbags on most trucks that act like springs. Some trailers also use airbag springs. The horn is obviously air powered. The seat springs/cushion is also air powered, makes the truck ride smoother cause the air the driver's seat floats on absorbs impacts. On older trucks they used compressed air to operate the windshield wipers. This was a stupid/trouble prone system, ie a slight leak makes the wiper malfunction. You can also use the air system to inflate a low tire.

    • My favorite accessory is the air gun you can install to clean the cab out.

    • @NY's Finest Didn't even think of that!! Here I am like a heathen buying $5 cans of compressed air used to clean computers to detail my interior.

    • ​@Aaron Smith Lots of truckers install an air gun.

    • Automatic transmissions use the air system to shift the transmission.

    • Here in Kenya we even do spray painting using a lorry's compressed air from the reserve tank😁😂

  • One other reason why hydraulic brakes are preferred in personal vehicles is their improved braking modulation and feel. Hydraulic fluid is incompressible, so when you engage them your foot feels the brake much more quickly and directly. Air brakes are slightly slower to respond and mushier feeling at the pedal, with a more on or off behavior. The other thing people are missing is that hydraulic brakes do fade in a way that air brakes don't, assuming moderately competent brake pads. The first type of brake fade you get in hydraulic systems is usually brake fluid fade, which is when the fluid gets so hot it begins to boil. When the brake fluid boils, it is no longer incompressible liquid, but partially compressible gas, which significantly reduces brake pedal feel and max braking force.

  • Air brakes on trucks are more accurately called spring brakes. There is a massive strong spring in each brake housing. It is applied by default. The air pressure is used to keep the brakes off, not on. When applying pressure on the pedal, the air pressure keeping the shoes off the drum is lessened, causing the spring to gain dominance. This is why you see the double skid marks veering off the road. Loss of air pressure causes the brakes to be applied full force. A technical detail Ton Clancy got wrong in one of his stories

    • Incorrect. The air pressure does keep the spring compressed this releasing the brakes. However, there is a second chamber that uses applied pressure to engage the brakes using the pedal. There is no lessening of pressure holding the springs compressed.

    • Nope. Spring brakes are part of the air brake setup. Spring brakes are another name for your parking brakes.

    • Um , Some brake pods DO NOT have a spring in them . Do you research .

    • @derek crymble the front brakes on a semi usually don’t have a spring in them but the drive axles and trailer axles all do (at least they all have since the late 1970s). There are two circuits on every semi, the service and the supply. Brakes with springs in them (maxis) are “fail closed” and will clamp down on the drum/rotor when there is no air because of the spring. The supply circuit pushes the spring open so the brakes release. The service circuit then can be used to push the brake closed again. They are two cylinders that act independently on the same piston. The front brakes use the supply circuit only so they stay open with no air. When you press the brake pedal, the service cylinders all pressurize to apply the brake. The supply cylinders’ pressures only change when you press or pull the parking brake knobs.

    • @Q.D. White I know . Been driving tractor trailer for more than 10 years . What I said is absolutely true . Your mom says "hi" .

  • Some 50 + years ago big trucks used the vacuum air brake system. This system was quite unreliable for quick stopping power. I know because I was involved in an accident that caused my truck to not get slowed down fast enough to prevent running into the back of a car that pulled out in front of me on a two lane road near Springfield, Illinois. The trailer only had 8000 pounds of household goods inside, which amounted to about half of the 32 foot trailer I was pulling at the time. I was traveling about 50 mph and this car pulled out in front of me as best I can remember about 500 to 700 feet in front of me and even that distance was not enough time to keep the tractor from running into the back of the car that was involved. A car that was driven by a 90 some year old man who obviously had clear vision of my presence before he pulled out. The only thing that explains his reckless behavior was, besides his age, was his depth perception and reaction time to existential threats, or possibly did not even look in my direction. Before running into the back of his car (I merely clipped the rear driver's side of his car before veering off to the oncoming lane on a two lane bridge, wiping out about 20 some feet of bridge guardrail before coming back onto the road surface and eventually laying the whole truck on its left side and well off the bridge which spanned the Sangamon River.

  • Air brakes were originally developed for trains in the late 1800s, and are also still used on the majority of them.

  • Another reason that air brakes are not used on automobiles is that air brakes require a sizable air tank that takes up space. On a semi truck there is plenty of room for air tanks. On a car an air tank would take up a sizable amount of trunk space.

    • yep, besides the fact that such a sufficient compressor would a) weight too much for a small car and b) would drain too much power from set car.

    • They'd never use air brakes on a car for the simple fact that they are noisy. No one wants to hear a compressor as they're driving down the road. But a truck is expected to be noisy, so they put up with it there.

    • @nixon2tube Wrong, trucks in the EU aren't noisy et all and so aren't the air brake systems as well. We've pretty harsh (actually the same) noise regulations on trucks like on cars.

    • @Bruce Carbon Lakeriver compared to a car a truck is loud even in europe and every brake that air needs to go somewhere 120psi Escaping makes noise I know in europe trucks have like dampers on the blow out But still noisy

  • For a multitude of reasons, but the main one is that the desired failure state for a semi is stopped. Air brakes in these vehicles use air pressure to decouple the brakes from the rotor or drum and, thus, if you experience a compressor failure, brake line failure, etc. then it will fail to stopped rather than fail to a lack of ability to stop.

  • As a trucker for over 50 years, this has been long in coming. Air disk brakes WILL stop the truck quicker and easier than drum brakes. So glad they are here!

    • I've done many brake jobs on my own cars. Changing worn out pads on a disc brake system is far easier than changing shoes in a drum brake. I assume that the brakes on a semi truck are just larger versions of automobile and light truck brakes.

  • They do overheat, but another added benefit is that their default position is locked, eliminating the need for a parking brake.

  • Disk brakes were not used on heavy commercial vehicles for a few reasons...one is the shorter service life...they usually have less friction material of the pads in contact with the rotor at once which causes the pads to wear quickly...two is the metals available at the time were much less able to deal with the massive forces and heat without warping and damaging the rotors, once warped and gouged they tend to shorten pad life even faster, cause excessive vibration, uneven braking, possible damage to other brake components....the third reason, which is still relevant today is the fact disc brakes are expensive to manufacture, they require expensive rotor materials which can be difficult to work with, they require tighter tolerances, they are used much less often, so on large commercial vehicles there is a need to design an entirely new system which will require a lot of expensive time consuming testing and then have to given the ok by whatever country you want to use them in... again they aren't used often so Mechanic's have to be taught the new systems diagnosis, repair and replacement procedures....one thing I don't know how they would get around...when drums overheat and expand they expand away from the friction material...a brake rotor would expand into the friction material...and if you make the pads larger with harder material and the rotor expands into it you will have trucks stuck in places or the rotor expands beyond the limit of the caliper breaking it and the truck losing brakes...how do you implement air actuation into a disc brake system?? Along with an emergency spring system to lock the brakes up on loss of air pressure? Like it's a lot more than just bolting some stuff on a truck involved in keeping you safe when you decide to cut off an 18 wheeler and brake check it thinking it can stop that quickly....it's a mistake folks don't do it...the truck driver will not be at fault because of a simple fact....he cannot actually stop fast enough to not run you over...I've seen the accidents closer than you want to be driving a flat bed wrecker doing state police calls...I've seen whole families taken because the head of household got road rage and thought "hey I'll cut this truck off and slam on brakes that will show him who's boss" and that was it 2 adults 3 kids gone....but not all of them instantly...3 were partially there and witnessed the rest of the family like that before leaving in a very painful sad way...so again don't do it folks that truck cannot stop that fast...

  • As others are pointing out, the critical reason to use air brakes is that it takes a lot of force to apply heavy truck brakes, and an 18-wheeler has ten of them. This requires _much_ more force than could be put by a human with their leg and foot into a hydraulic system. The system is applying about a hundred pounds per square inch of air pressure to a quite large rubber diaphragm inside the brake chamber, which in turn applies it to the brake shoes. Since the diaphragm has a surface area of about 30 square inches, something like 3000 pounds of pressure is being applied to the brake shoes. This makes sense if you think about how much kinetic energy the brakes have to transmogrify into heat energy in just a few seconds. Also, don't mess with the spring in spring brake chambers. They don't have to apply quite as much force as the service brakes, but more than enough to kill you if their energy gets released onto your body.

    • Aircraft use hydraulic brakes but the system is typically pressurized to 3000 psi via Engine driven pumps as well as AC motor pumps. Hydraulic brakes can have some serious stopping power under certain applications.

    • @Flickyman175 It's true. I guess the question for our purposes is why an air compressor to add power to the system instead of a hydraulic pump? My guess is compressibility. The air tank gives the system a huge buffer compared to what you could get with hydraulic buffers, because liquid is not compressible. In a large aircraft you need that hydraulic pressure at all times to run the control surfaces; it also runs them. In a highway truck you only need it intermittently to run the brakes. The buffer of the tank means you can use it for other systems that draw far less energy from it, like shift solenoids and suspension airbags and seat and cab suspension, but these are very small and uncritical draws. The compressor on a highway truck runs for a pretty marginal fraction of the time the truck is driving compared to what an large aircraft demands, I'ma wager. It could be that what we're looking at here is the amount of energy we're trying to control. All large aircraft are dealing with several orders of magnitude more kinetic energy than a heavy truck, just because of how much faster they're going. It seems obvious when you consider how much more powerful are their powerplants. There are road trains in Australia that are as heavy as some large airliners. And, of course, all actual rail trains are, which also use air brakes. Yet no road vehicle has anywhere near as much power as any basic airliner. Three mile long trains can have as many as six four thousand horsepower locomotives. But a single medium airliner produces twice that much power. A single 747 produces the equivalent of 80,000 horsepower.

    • As Ron Hawkins had said they are technically spring brakes. Depressing the brake pedal releases air pressure that hold back springs that are actualy applying the brakes. If all air pressure was lost all the brakes would lock up!

    • @Charles Nixon That's true of the parking brakes actuators. Not true for the service brakes. You can see that by looking at the steer axle brakes on any truck or bus. You'll see a brake chamber that's one third the size of the brake chamber on any other axle. That's because they don't have a parking brake chamber, with its integral spring.

  • It's not about the fade -- any braking system will fade if applied sufficiently, be it disk or drum, pneumatic or hydraulic. Air brakes are on trucks, as other commenters said, for two primary reasons -- they're tolerant of poor maintenance (small leaks) and it's very convenient to provide an air supply (and signal) to (potentially multiple) trailers for braking capability. However, brake reaction time is significantly poorer than hydraulic and brake feel is virtually non-existent. These two items make air brakes undesirable in passenger car roles. Can you use electric brake controllers to provide braking control to the trailer? Yes, but that system requires a priori knowledge of the braking topology (i.e., how many trailers will be connected) (which is not a problem for modern electronics) and would be less fault tolerant than air brakes.

  • THIS IS HOW AIR BRAKES ACTUALLY WORK: In the trucking air brake systems there’s two types of brake chambers, dual and singles. Single chambers only have a service brake or foot brake. While duals have a spring or parking brake and a service brake. The spring brake works by removing all air from the chamber to engage, if you introduce air to the system, around 40 PSI it will over power the spring and move a diaphragm holding the brakes on to release them. Keep in mind not every axle has these so most brake chambers are single. A service brake works as follows. You push the brake pedal, and air fills the chamber. This pushes on a separate diaphragm moving a push rod. This connects to something called a slack adjuster to ensure your brakes are always adjusted properly. The slack adjusters twists something called an S-Cam which pushes the linings into the drum to engage the service brake. This is the only type of brake a single chamber has. A dual chamber has both types of brakes and two separate diaphragms, 1 adding air to engage the foot brake, and the other removing air to engage the parking brake. This might be a little complicated to understand so look up some diagrams on the internet if I didn’t explain it. Well Enough.

  • Wrong, when the truck has no air in its reserve, the brakes are locked, hence it's impossible for the brakes to be activated when air is injected in the chamber, it's the complete opposite, air is released from the chamber which activates the brakes.

    • thanks for sharing your opinion

    • You are also only halfway right. The principle you describe is only true for the emergency brakes. Trucks normaly use 4 circuits to operate their airsystems. Circuit one is for the truck's front axle, circuit two is for the drive axles, circuit three is for the emergency brakes and circuit 4 is for al the auxiliary air systems like air assist servo for the clutch and so on. Trucks commonly use what is called a double acting booster except on the front axle. These double acting boosters has two features. First the emergency brake feature, a large spring located in the rear part of the booster, and the service brake that is located in the front part of the booster. The service brake uses a rubber diaphragm in a sealed chamber. When the service brakes are applied, when you put your foot on the brake peddle, the chamber is pressurized and the diaphragm press against a rod that moves the slack adjuster and rotates the s-cam. The emergency brakes aka park brake is activated by that large spring. The spring pushes in the same direction as the service brake diaphragm and has a diaphragm of it's own in a seperate chamber that when pressurized pushes the spring back. So when the air is low or when the parkbrake is applied it is the sring keeping the brakes engaged, but when the parkbrake is released and you depress the brake peddal it is the service brake diaphragm that engages the brakes.

    • Yep, this video is all wrong...

    • In the chamber is a spring which air needs to win and release brakes. When there are not enough air, the spring "activates" brakes and locks them until the preserve are filled. So the spring presses brakes. If I do remember correctly you can brake eight times without refill before the spring activates and brakes the vehicle

  • In 9-1/2 years of driving, I have lost my air brakes three times. Each time, one of the air lines from my cab to my trailer broke. Twice, the brakes did not safely lock up. Instead, I was forced to downshift and safely get to an area where I could get the air lines repaired. Once, this happened in Houston Texas right as I was crossing over a tall bridge, going down the steep side, in rush hour traffic. By the grace of God, I was able to cross multiple lanes and exit off the highway without hitting anyone. Then drive my truck 5 miles to a repair shop. Without brakes. Only once has the system worked and my truck locked up around a busy and dangerous curve. So I drove my truck about fifty feet into an empty lot, dragging the trailer tires. State Trooper arrived, inspected my truck and air lines and cleared me of any wrongdoing. The relatively new red air line simply exploded. Air brakes are great but they do not always work as suggested

    • Switch to the old school heavy rubber airlines instead of those cheap ass hard plastic lines and you will not have those issues. Those hard plastic lines fail in extremely cold weather as well. Check the slack adjusters on your brakes, they should have locked up if properly adjusted and maintained.

    • @Mark Davis The company I used to work for did not care to maintain their trucks that well. We got the cheapest fixes possible.

    • During a pre trip inspection you should be putting the vehicle in gear without releasing the parking brakes and try to move, if it moves then the brakes need to be adjusted. I’ve been a commercial driver for over 30 years and a safety trainer, if it isn’t right then don’t drive it, if your boss tries to force you to drive it then file an OSHA complaint, you’d be surprised how fast they fix things.

  • Air only replaces the fluid, brake material is the same, so they can overheat and fade. Old airbrake chambers (single chamber) would not stop or hold the truck without air, modern systems lock the brakes when air is lost due to a strong internal spring. Air is needed to over come the spring to release while air pressure is also used to offset balance between chambers to slow or stop the truck. If all pressure is lost, and truck must be moved, there is a caging bolt that must be inserted to cage or release the brakes.

  • Disc brakes are almost always the best choice.cost is more, but stopping power is all that counts here.Yes, being in the right gear is essential and so is Engine brake.

    • Disc brakes don't work as well in wet conditions, which is why some cars have discs on the front wheels and drums on the rear wheels.

  • The main reason air brakes are used in semis is that various trailers are constantly being connected and disconnected from the tractor (no leaks or purging), and air never needs to be flushed and replaced as brake fluid does so there is no mixing of old and new hydraulic fluids. It is primarily a maintenance issue where interconnection of poorly maintained equipment to well maintained equipment will not degrade either. The fail-safe nature of air brakes where they lock on when disconnected is a valuable design feature but is still independent of the underlying technology. Other factors such as overheating are even more unrelated to the actuation technology.

  • There is also the system of air actuaded hydraulic brakes , slowly becoming phased out but sometimes still around

  • I like my multiple hydraulic disc brakes on my 400 ton haul truck. Stops on a dime perfectly if needed and I’m not worried about stopping distance. It just works.

  • I've read a lot of comments about why semi trucks use air brakes for their trailers, and some people are missing the point. The reason why is that when you couple and decouple a trailer from the tractor, there is NO way to prevent the slightest amount of air from entering a hydraulic system. A small air bubble in a hydraulic braking system greatly reduces or even totally prevents hydraulic pressure from building. This is why standard hydraulic brakes on cars need to be bleed. Think about it: the time it would take to bleed a hydraulic brake system every time a driver coupled a trailer would be too significant of time wasted. Not to mention, decoupling the trailer would result in hydraulic fluid loss. This would even be worse for the yard hostler who couples, moves, and decouples countless trailers every day.

    • That's a reason but it's not *the* reason. Buses and full body trucks also use airbrakes for the reasons others have discussed; pretty much all heavy vehicles do.

  • I believe air is used because air doesn't overheating like hydraulic brake fluid, and also it is easier to couple and decouple trailers.

  • The biggest problem with disc brakes is that it's like going back to the days of inboard drums. You have to pull the hubs to replace the rotors. I hadn't seen inboard drums in 30 years except for a couple of old trailers.

    • Are inboard disc breaks like the militaryhumvee?

    • @Andrew Rosales never worked on a humvee, after my time

  • In London, the famous Routemaster bus uses mainly the Lockheed power hydraulic braking system. The mineral oil fluid is pressurised in the lines by a pump and continuously flows around and into two storage accumulators which have sliding pistons inside The fluid pushes the piston against a pre charge of air to give the storage of pressure for the system. A cut off valve which gives off a "clonk" tells you that full pressure has been reached. Also, a flag in the Driver's cab with STOP cut into it, drops into the line of sight, and as well as a warning light warns of low pressure. Handbrake is the traditional pull on mechanical type. When the Driver depresses the brake pedal, the pressure is applied to the wheel cylinders, and a smooth application of the shoes is made. London Transport chose this system as they found air brakes could freeze and have air tank capacity problems in heavy traffic. An alternative supplier was Clayton, who used spherical accumulators with rubber diaphragms inside rather than the cylindrical ones with sliding pistons. On the maintenance side, occasionally a compressor called an intensifier, was connected to the Schrader valves on the accumulators to restore the air pre charge. On the other hand, I know Engineers who detested this system, dismantling meant them getting saturated in Tellus, (the mineral oil) Is this system used in the States, apart from possible aircraft where it was derived from?

  • "No brakes fade from overheating" What is commonly known as brake fade occurs when the binder that holds the brake pad material together melts, and the resultant liquid lubricates the the interface between the brake material and the drum or disc. That's why disc brakes are less prone to brake fade, because the disc is surrounded by air, which cools the disc much more effectively than the enclosed drum.

    • Are you sure about that? I’d be asking isn’t it the outgassing holding the pads away from the disc. Which is why the high performance cars run slotted and drilled rotors. The outgassing also heats the pads more cause you have to push harder on the pedal to overcome it. When it reaches the critical point the brake fluid will boil.

    • Truck brake shoes don't use binders to secure the friction material to the shoe plate, they are riveted. There is a wear line on the side to let you know when you are on the rivets and need to change them. Usually 1/4 " of shoe left, toss them out

    • ​@Kyle Derry The binder is in the brake pad material itself, it's not what holds the material to the shoe, it's what keeps the material from crumbling in your hands

    • @Richard M. Nixon I got you now, I have seen shoes on little trailers with electric brakes overheat and the material separated, I didn't consider the actual binder holding the material together

    • ​@Copes Performance cars use slotted and drilled rotors for different reasons. The holes allow for more airflow through the rotor, keeping them cooler, which helps to prevent offgassing before it even starts, and the slots help to "trim" the brake pads to keep them flat. Have you ever gone to replace a set of brakes and seen the uneven surface on the brake disc as well as on the brake pad? That results from the natural inconsistencies in the brake pad wearing more in some areas and less in others, causing peaks and grooves in the pad material, which then wears peaks and grooves in the brake rotor. So when you have a slotted rotor, the slots effectively work like a razor, knocking the high spots off of the brake pad and resulting in more even wear. They also serve to prevent glazing of the brake pads, which again improves breaking consistency. A side effect of the slots is that they allow the offgassing to dissipate through the slots, but that's not the primary reason they exist.

  • Air brakes allow connecting and disconnecting from trailers without having the problem of getting Air in a hydraulic brake system... Down side Air brakes have a slight lag from brake pedal application to brake application about 1/3 of a second on tractor brakes a little more for the trailer...

    • Love it when the comments are more informative than the video

    • Which in turn is the reason you start slowing down early and braking just a tad bit early.

    • you actually starts braking on your trailer first ... split seconds but the trailer first .. to keep the whole rig stretched out in a line .. that's why the trailer has a pre-idling valve ...

    • You wouldn't have the lag if the truck and trailer would support EBS :)

    • @Bruce Carbon Lakeriver yes you would as the brake only has security pressure aka safety release pressure

  • I think the main reason is, with a semi where the weight limit is critical, any excess weight that can be removed will be. Sure, air lines are easier to couple to a trailer, but you can make valves that make coupling easy and leak-free. But if you scale it up like everything else, say comparing 5 quarts of oil to what a Cummins uses, and then apply that to the added weight of all that brake fluid (remember the trailer too), and you see you'd lose maybe a 100 lbs from your carry weight. Might not seem like a lot, but when you figure losing 100 lbs of chargeable weight to every trip, and you see why air brakes make a lot of sense in trucking.

    • The issue with that logic is that air brakes still need a compressor, dryer, air tanks, lines, and heavy springs to function, at a minimum. The weight of all those parts definitely exceeds that of any comparable hydraulic brake system. The main advantage I see is the standardization of coupling to a trailer. Coupling separate hydraulic systems is nearly impossible to do without introducing air into the system. If the system uses air to begin with, it's not an issue.

    • No it doesn’t really have anything to do with the weight. The number one advantage that air brakes have over hydraulic is simplicity and reliability. A blown hydraulic line would put a truck out of service and require a lot of work to repair. Air leaks and even severed air lines can be patched on the road and allow the system to continue to operate until the truck can be driven to a repair shop for new air lines. Also, something the video gets wrong, is the safety factor of air brakes in the event of a catastrophic failure. The system works by RELEASING air pressure from the brake chambers when the brake pedal is depressed. A spring forces the the pads to move and create friction against the drums. When the pedal is released, air fills the chamber and compresses the spring, which releases the brakes. So in the even of a catastrophic failure (loss of all pressure) in the braking system, the brakes will automatically apply. Trailer disconnects from the tractor? Brakes go on automatically. Cost efficiency and safety are the two most important factors in designing trucks, that’s why they’re built the way that they are.

  • You left out that air brakes are very noisy, which would be a huge drawback for a regular size vehicle. You also left out the alternative of electric brakes which are done on most towed trailers like RVs/campers. They are not used on boat trailers because the system gets submerged in water, and will destroy the components. Boat trailers use a hydraulic braking system where the weight of the trailer pushes against the piston up on the tongue and applies breaking to the trailer.

  • Correction: Drum Brakes are not only used for their price, since they are in a sealed container, they are preferred for Off-road use, since disc brakes can and will deteriorate in those conditions

    • I didn't know they made off-road semi-trucks.

    • @KX36 Most third world countries uses standard highway trucks in dirt roads or "off-road". Take the "highways" in the Amazon region for exemple.

    • @Filipe Arthur cool, thanks for the info

  • Here is what chatGPT says: Air brakes on a semi truck utilize compressed air to activate the braking system. When the driver presses the brake pedal, it opens a valve, allowing compressed air to flow from storage tanks to brake chambers located at each wheel. The air pressure forces a diaphragm in the brake chamber to push against a pushrod, which applies the brakes by pressing brake shoes or pads against the brake drums or rotors. Releasing the brake pedal activates a release valve, allowing the compressed air to escape, retracting the pushrod and releasing the brakes. This system provides reliable and powerful braking performance for semi trucks, ensuring safe operation on the roads.


  • Yes air breaks are most used but when you’re pulling an load that can weight up to 80,000 Pounds your not gonna use the air breaks as much your gonna spam the Jake break to slow down because the Jake break is more affective at slowing down 80,000 lbs then the air breaks especially when going down an steep hill

  • I'm missing the two most important reasons for the use of air brakes. One being the size off the brakes, the brakes on a truck are to big to be operated manually with the use of a vacuum brake booster for a hydraulic system. The second is the connection to a trailer for a hydraulic system would a risk for introducing are in the brake lines when connecting which is a bad thing .

  • 4:05 - Brake fade happens in drum brake -air brake system. Most truck run aways while coming down a mountain happens due to brake fade. Brake fade is much reduced in a disc brake setting.

  • It seems like it has more to do with cost than effectiveness. Disc brakes resist heat better since they're ventilated, and can apply huge amounts of force with the hydraulic system. But the cost to install and maintain disc brakes in all of a semi's wheels would be too high. Instead, leave the discs only for the front, and use cheaper drum brakes that'll get the job done good enough in the rest, since they're working in multiple axles at the same time. Plus they already have the air compressor for the tires anyways, so it's two birds with one stone. But that's my guess.

  • As someone who just got their cdl permit this is very helpful to know. Thank you!

    • Do more research, this video has many inaccuracies about both systems. BTW Gratz on your cdl. I've been a flatbedder for 8 years and can drive a car,motorcycle pretty darn fast as well lol. Done a ton of research on rotors, pads, fluid and brake lines over the years. Research everything and take everything a trucking company tells you at face value. About half the drivers or more are full of crap as well. Be careful out there and listen to old timers and not the kids with 2 to 3 years exp. They are still learning themselves

  • Air brakes only activate with no air on rear axles due to a second piggyback chamber with a huge spring , these are called Maxi’s which are also the parking brakes , (yellow knob) , brake fade is a function of brake pad/shoe friction on the drum or disc which happens with air brakes as well and not due to boiling of brake fluid .

  • Only one issue, Hydraulics give more force, this has been proven using comparable size if you will. Air can provide the same force but with a larger size unit. Air breaks were chosen because of the safety issue.

  • I’m not sure, but intuition tells me that air brakes are easier to maintain on the go. If there is a minor issue that you can actually fix yourself curbside with basic tools, then you can just do so, have the system re-pressurize and off you go. With hydraulic brakes, you could fix things all you want, but in the usual cases you’d still have no hydraulic fluid on hand to refill.

  • You forgot to mention dynamic breaks that are used on heavy equipment. Also, it is impractical to use hydraulic braking system on a tractor-trailer configuration or on a train.

  • Hydraulic and air brakes both can overheat. Difference being the hydraulic fluid also overheats, air will not. The air system is essentially hydraulic also just air is used instead of fluid. Both use surface area to push and dow work. Air systems are far more complex in my opinion. Many fail safes built in.

  • I was taught that air brakes are always braking when without pressure for safety reasons. And adding air pressure releases the brake. Then I found tons of comments mentioning this. Yes. The method I was taught was correct.

  • I have worked on class 8 trucks for 40 years I always thought airbrakes are easier to bleed out

    • Just what are you bleeding out of the airbrakes?

    • @ARK blood obviously

    • @ARK exactly

    • @ARK I refuse to believe you are that dense to let an obvious joke go over your head. But then again....

  • You missed the main fact why trucks have air brake systems instead of a hydraulic brake like cars have: the energy reservoir A truck has more axles and higher weight, so you need a lot of force to activate all drum / disc brakes. There is more energy needed than the driver can create by pushing the brake paddle. Therefore you use the energy stored in a reservoir and release this to the brakes with an valve. The main advantage for using air is that you can store it easy and cheap by compressing it into a tank. For nearly incompressible hydraulic fluid, reservoirs are much more expensive, so an air system is the cheaper solution for trucks. By the way, you can use air also for other components in a truck like air suspension, gearbox actuation,...

    • Your last point (and only that one) is valid, but air brakes were in use before air suspension was common and long before air was used for shifting.

    • Not To mention it would be a pain hooking and unhooking a trailer. And if the glad hands leak, or you hook up to a trailer with leaks, it's a heck of a lot cheaper to replace air than hydraulic fluid.

    • @Wes Howell Yes, the only serious reason air brakes are used is so trailers can be quickly dropped and hooked. Everything else is arguable.

  • Air brakes definitely suffer from fade, it’s the braking material that fades, not the system type

  • Back in the early 80s when I first started driving trucks with air brakes every night except for winter we had to drain the air tanks. Their first attempt at making a dryer did not work. Back then I never once saw a tank that did not have water inside of it. In the winter you have to have alcohol in your air system. When we would use alcohol it was to stop the lines from freezing from all the water in the system. The system that we have today started sometime in the mid-90s. When I had a new tractor I was treating it just like the old one. Every night I was draining my tanks but nothing was coming out. After about a month went by and nothing was coming out I decided to ride down to the shop and talk to the mechanics. That's when they popped the hood and started showing me all the different parts of the air dryer. They told me if I felt like it I could still do it every night but they said once a month would be okay also. So that's what I had done. Even after a month there was no water whatsoever coming out. All of us who have our CDL license we all know there's a big difference between air brakes and hydraulic. You have to take a written test on air brakes to even get a learner's permit to start driving. In my 40 years of driving I absolutely do not remember having to have brakes done. Now when it would be time to go in for its p.m. they could have changed them then. I was the type of driver who took care of things myself. I carried tools in my truck with me. Whenever I wasn't pulling my own trailer the first thing I would do is check the brakes and see if they needed adjustment. I even carry extra rubber for the glad hands. Whenever I would run out the mechanic had no problem giving me more of them. They said they'd appreciate that I would do that without anybody having to tell me that. I always had a great reputation on how I cared for my equipment.

  • Just like my grandfather once told me: every Bus and Lorry has a Airbrake, and stops the vehicle immediately

  • You didn't mention that with disconnecting and reconnecting the trailer there is always some spillage and the systems would need to constantly be needed to be topped up. This is true for air brake systems too but the compressor handles that with the free fluid , air.

  • The air pressure keeps the breaks deactivated and I thought it was the other way around, now it makes sense why trucks make the air release sound when it breaks or park, thanks for the video

    • Nope. Wrong. First, it's brakes, not breaks. The blast of air one hears when a semi truck shuts parks is the air which holds pushes against the brake springs. Those springs serve as parking brakes and emergency brakes, in the event of a system failure, such as an air line rupture or an accidental disconnect of the glad hands (a connection point between tractor and trailer). So, the red air line supplies air to the trailer to release the parking brakes on the trailer. That's all the air in that line does. Think of it as a dead man switch to try to stop the trailer in case it disconnects from the tractor somehow, or to keep the trailer from moving while parked. The blue air line, however, functions in a manner similar to a car's hydraulic brake lines, using pressure sent with application of a brake pedal, to each wheel, to force the brake shoes or pads, to the drums or rotors. From around 1973 or so, semi trailers have been required to have such safety brakes. The brake chamber down by the wheel is two chambers in one. One chamber takes red air line air and resists the parking/emergency springs, keeping them from trying to apply the brakes. The other chamber is the service brake chamber. That one takes air from the blue air line, which is regulated by the driver's foot applying pressure to the brake pedal, and that air pressure pushed a diaphragm which moves mechanical linkages to apply braking force. Truck Tropia should, as a public service, delete this video and not replace it unless and until the author learns about truck brakes.

    • @Planet Hedgehog slight correction: pressure to the service brakes doesn’t come directly from the blue line. The blue line sends a pressure signal to a valve which then opens to allow pressure to flow from the air tank to the service brakes.

  • As an european truck mechanic, I also need to add that maintenance on air brakes is also way easier!

  • Good explanation about both brakes. Hydraulic brakes has a help of the booster. What about the air brakes?

    • The author of this video has barely a clue; don't waste your time trying to commit anything he said to memory. The air brakes rely on an air compressor and air tanks, which the compressor fills to about 125 psi. So no booster is needed, unlike a car's hydraulic brake system. For about the past 70 years, semi trailers have used a two-in-one chamber system to convert air pressure into mechanical movement of metal parts which then force brake shoes and pads against brake drums and rotors. One side of the brake chamber gets filled with air to push against heavy springs which otherwise apply the brakes to keep the trailer from moving in the first place (to remain parked) or to slow and stop the trailer in case all sorts of bad shit starts to happen. The other half of the brake chamber uses service brake air, supplied by the compressor (and stored in tanks) and controlled by application of the brake pedal, to apply pressure to the brakes to stop the vehicle.

  • Another advantage of air you didnt mention is that air is free while brake fluid is not.

  • The real reason and only reason is purely to have a simple and reliable way to connect and disconnect the trailer braking system. This is easily done with air actuation and is much more difficult and expensive to do reliably with a hydraulic system

  • The size of vehicle doesn't determine using air or hydraulic brakes. Large aircraft such as Boeing 747s use hydraulic brakes. Air brakes fade due to heat the same as hydraulic brakes. It's the braking material, sweep area, cooling ability and many other aspects that determines brake fade due to high heat not the braking activation mechanics. Semi's are very much known for overheating and loosing their friction brakes due to brake fade. It is not considered viable for friction brakes on large heavy vehicles with comparatively small surface area air resistance to be used for long duration retard braking such as on long descending grades. It is virtually a requirement that large semi trucks have engine braking for doing the bulk of braking when descending long grades due to their air brakes fading completely if dependent on them. The strong engine braking capability of Diesel engines is a major reason to keep using Diesel engines for large trucks even if a heavy duty gasoline engine maybe cheaper to fuel when Diesel is at $5 gallon and gasoline at $3 gallon. If it wasn't for the strong dynamic Diesel engine "Jake" brake the Diesel engine wouldn't be the undisputed choice for large trucks. The only good substitute is a BEV large truck with regenerative braking. That works much better and allows large trucks to descend grades at much higher speeds with high force dynamic braking. The "Jake" brake of a Diesel engine requires running the engine in about the same RPM and gear the truck used to clime the grade as it does to descend the grade so if the truck climbed at full throttle doing 30 mph it would have to descend at about 30 mph in the same gear it used to clime. A very good "Jake" brake Diesel engine typically can produce almost as much braking horsepower as is can produce horsepower.

    • The preference for diesel engines is primarily about moving the truck. The fact that they can work well with a jake is far secondary to that. Even if fuel costs significantly more, an equivalent gasoline engine would use so much more fuel that there would be no cost savings. Semis don't purposely use an engine design that is thirstier just for braking action - the reality is that these diesel engines consume less fuel, apples to apples.

    • @Christopher Burtraw Even if Diesel was slightly more expensive per mile than gasoline there just isn't a proven well recognized heavy duty dynamic braking system for heavy haul trucks other than the Diesel engine Jake brake. Real world experience has been better than the theoretical 30% higher efficiency for Diesel over gasoline, but 30% higher efficiency is the common figure. At 30% higher efficiency Diesel could cost $5.20 a gallon and be on even cost per mile with gasoline at $4.00 gallon. Diesel has been much more expensive than that compared to gasoline. Diesel at near $6.00 a gallon with gasoline around $4.00 a gallon has taken place around Indianapolis. There really is no choice between heavy duty Diesel engines and heavy duty gasoline engines for heavy haul trucks, there are just Diesels. With lighter large motorhomes there is a bit of choice between gasoline or Diesel, but they are typically totally difference vehicles you can't just select a gasoline or Diesel engine. One of the reasons for getting the Diesel engine is better fuel millage, but the deciding reason is frequently to have the dynamic braking of the Diesel engine Jake brake when driving the RV on such highways as Vale pass on I-70.

    • @Douglas Engle I agree that there's no choice in practice, I disagree with the reason for it. My understanding is that it's basically entirely down to motive performance and durability, not braking. Jakes are not the only feasible auxiliary braking devices and they aren't equipped on all diesel semis. For starters, hydraulic retarders exist and work well, without the Jake noise, which btw is prohibited in a lot of places. Yes, Jakes are a great option for auxiliary braking in any case, but it is probably not a primary reason that diesel engines are used in semis, it's more of a side perk.

    • @Christopher Burtraw I believe the choose of a Diesel over gasoline heavy duty engines for heavy vehicles such as fully loaded semis and dump trucks is heavily restricted to the dynamic braking available with a Jake Diesel engine brake. There are no proven alternatives for providing the needed mile after mile dynamic braking required by heavy vehicles on long highway downgrades other than the Diesel engine Jake brake despite a century of research. This is why heavy duty gasoline engines have not come back into consideration when the cost of running Diesel or gasoline fuels has the possibly of being comparable and Diesel possibly more expensive. Lack of a Jake brake like system in the opposed piston Achates Power Diesel maybe the reason for the lack of excitement for an engine with at least 15% higher efficiency than others and possibly a lot higher on average. The Achates Power opposed piston Diesel also has a gasoline prototype that gets a reported 37 mpg compared to its Diesel fueled version at 42 mpg when used in a Ford F150 prototype. That is 1.135 X more efficient for Diesel over gasoline, which is really at the normal 15% higher energy content per gallon in Diesel than gasoline. In most markets it would be cheaper to be running the Achates Power opposed piston gasoline engine because Diesel costs much more per gallon than regular gasoline and far more than the increase in fuel millage. One advantages of the Tesla BEV semi to alert observers is the vehicle's ability to do regenerative braking at extremely high levels. Not only can the Tesla Semi clime very steep mountain grade interstates at 55 mph - 60 mph it can down those grades at 65 mph while other fully loaded semi's are having to clime at 40 mph and go down at 35 mph - 40 mph. That is considered to possibly be a big time saver in some applications. If proven and reliable the Tesla Semi may revolutionize mountain grade interstate trucking more due to its extremely advanced regenerative dynamic braking than any other aspect.

    • ​@Douglas Engle there is nothing about the operation of a Jake brake that would prohibit it from being installed on a large gas engine so it's not really a choice in the first place they could have a gasoline engine with just as much engine braking power as an equivalent diesel

  • Air brakes use air to compress a spring in the brake chambers, also known as "cans." It is these springs that provide the brake force that is applying the brake shoes/pads. When the driver steps on the brake pedal, the air is released and the springs apply the force. This is also why the vehicle will slow down/stop if you lose your air pressure. Because the springs are no longer being held back.

  • I’m curious in air brakes , if it is difficult to graduate the braking pressure and to tune each wheel so no one wheel decide to lock up under medium braking pressure etc ? An advantage I’m guessing: under no braking each shoe is lifted high and dry from its disc/drum surface, unlike a car

  • A: we need to be able to hook and drop trailers without introducing contaminates to the brake system. Disc brakes are better because when metal get hot metal go expandy and when drum surrond thing gap get bigger whereas when metal is surronded gap get smaller.

  • The only advantage of air-brakes over hydraulic-brakes is they are massively easier connect and disconnect part of the system such as when changing trailers.

    • It's not the only advantage, they are just better for heavy vehicles in general which is why - for example - most buses use them.

    • @Michael Woods Or articulated trucks are the default heavy vehicle which got the funding in brake development resulting in air brakes being the default heavy vehicle brake despite being inherently inferior in unarticulated vehicles.

  • And remember, it takes special training to use air brakes. Why? Because air brakes are not instantaneous. Hydraulic brakes transfer your foot pressure at the pedal to the brakes immediately, because you are moving the fluid. Air brakes fill the system each time, and depending on the size of the vehicle, can take a little bit of time to fully pressurize. The first time going from hydraulic to air is terrifying, because it feels like for a split second the brakes are dead. However, once you learn, it's not hard at all. And boy, talk about powerful!

  • As someone who used to drive trucks, I’d use gears to slow down. Cannot rely completely on the brakes. Especially going downhill! But ya better get it into that gear as you transition to a downgrade or ya won’t be able to once the truck gains momentum. Truck’s transmissions were designed to last a million miles. The truck will only shift gears if the rpm’s and speed are exact for that gear. Not like a car manual transmission where ya can jam it in if necessary. Nope. Once I became more comfortable with truck trannys, I could float the gears. And essentially used my cruise control and jake brakes to travel. Thank goodness I never had to use a runaway truck ramp. Driving semi requires loads of common sense and a strong work ethic. Because that is singularly the toughest job I’ve ever endeavored. So, it’s not about what type brakes are in a tractor. It’s the operator.

  • I'd argue that that the air systems biggest advantage is ease of repair, and the sheer amount of things that can be powered by the system. Windshield wipers, air windows, height adjust on seats, air suspension, and even the fan hub. As for repair, you gotta remember these trucks travel up to 200k or more miles a year. Anything that can fail will fail. When I pull into a roadside for leaking airlines, its a 15 min job max, that is just straight up not the case for hydro systems. When I change a brake maxi, it takes 20 mins tops. On a car, you gotta take off the wheel, re & re the old caliper and then do a bleed. The best part, I need to order no additional parts. Brake maxi's are standardized and drum based trucks only vary between 30-30 long stroke, or 30-30 short stroke. The whole point of a semi's design is to be as easy to repair and as reliable as possible. There is currently no car on earth that could live the life that an average semi lives, just in terms of sheer longevity. Name the last car you've seen that had over 6 million miles on the odometer, cause I can name dozens of semi's that I've seen personally, and they're still hauling day in day out.

  • In addition to added expense, if air brakes are used on a car or small vehicle, the gas mileage will be less because the engine now has the added task of powering the air compressor on top of all its other accessories.

    • What if it's an electric compressor rather than belt/chain driven

    • ​@adventureoflinkmk2 An electric compressor would have nowhere to get its power other than the alternator, which is powered by the engine, I think. Maybe you could have regenerative braking or something, but that seems like overkill if it would be there just so that you could use an electric compressor instead of directly driving it from the engine.

    • @adventureoflinkmk2 Then the juice comes from the battery that now has to be replenished by the car's alternator that is driven by the engine. Energy cannot be created or destroyed.

    • ​@adventureoflinkmk2 That electricity would still have to come from somewhere, and a direct mechanical drive is probably more efficient than turning that mechanical energy into electricity then back into mechanical energy

    • Your combustion engine car already has a compressor built-in: cut off the fuel injectors, leave the throttle body full-open and divert the exhaust to a compressed air tank when you lift your foot from the accelerator, engine braking as a compressor instead of vacuum pump will give you 1000X more compressed air than you need to operate air brakes. Even if you ran a separate electric air compressor, you wouldn't need much air to operate car-sized air brakes and that compressor only needs to run long enough to restore the minimum operating pressure after brake application, which isn't much. Also, when you decelerate by moving your foot off the accelerator, your deceleration is driving the belts and alternator. A system could easily be designed to run the compressor for practically free during that time to build enough pressure for two or more extra braking events, then the only time you would need to spend energy pressurizing the brake tank is when you brake like a maniac multiple successive times to deplete it faster than it can regen during deceleration.

  • You also have Hydropnumatic brake system where hydrolic fluid is pushed by air to activate the caliper or the shoes as the case may be !

  • Surprised they didn't mention a huge plus that if juice brakes were on trucks and trailers, you'd have to bleed them everytime you hooked up to a trailer.

  • The only reason trucks use air brakes because it requires big reservoir to store the hydraulic fluid and all the hydraulic line required. Same applies to sedans,you don't have a big space to fit an air compressor to your engine hood and to the trunk.

  • I always wondered why tricks don't use disc brakes , thanks for video bro

    • Well, keep wondering! This video creator doesn't know much. Semis and trailers often do have disc brakes. They're just most expensive. Fleet Equipment Magazine reports brake manufacturers estimate that 12 to 15% of new trucks are ordered with disc brakes.

  • In our country(India) both buses trucks and trains use air brakes

    • That's true worldwide.

    • At least in North America, the air brake system used in trains not only does not have the failsafe parking feature of the truck system, it is fundamentally flawed and unable to even keep the brakes actively applied for a long period with even slight leakage - this has caused major crashes.

  • They're actually spring brakes. They have to be supplied air to release the spring and then your foot pedal counter acts that force easier than the release force. As far as brake fade goes. It's a super hot gas that forms under the brake pad. It's like a plasma, hot as the Sun. Makes a cushion of gas so the pad and brake surface can't connect no matter the force. I may add that you can use the release valves in an emergency situation. Maybe some of the brakes will catch. Just depends on the situation. I think you're suppose to release the trailer first. Then the tractor. Of course it's dependent on the situation. You hit the trailer on snow and ice it may come around on you and jackknife.

    • It is air pressure that provides the braking force, not the spring. The spring just provides a “fail closed” feature that doubles as a parking brake. When you release the park brake, air compresses the spring and releases the brake. When you press the foot brake, a separate cylinder pressurizes and presses the brakes closed again.

  • Hydraulic brakes DO NOT press against the wheels. The pads press against the rotor or press out against the drum.

  • Well, 1:35 the pads or shoes don't press against the wheels. They press against a brake rotor/disc or drum.

  • You saw the two differences on Hydraulic Brakes Air Brakes An Yes the tractor's front (Steering Axle) has Disc Brakes which uses hydraulic brakes. On the drive axles use compressed air used to operate the brake drums. When coupled to the trailer also uses compressed air for trailer wheels. Ok ! There's a difference between the two systems , how effective possible cost to replace when it have to be replaced.

  • I am going to have to point out that air brakes CAN and DO overheat and fade. Ask any Driver that operated in the mountains....especially the ones that have used runaway ramps.

  • Besides brakes, trucks and trailers use air for other applications such as suspensions, lift axles, seats and even windshield wipers

  • Air brakes don’t need bleeding. Imagine every time you change trailer to bleed system 😂

  • You heard it here folks: disc hydraulic brakes are better in every possible way and should always be used forever except sometimes they’re bad, Air brakes are also better and good in every way and use drums that are worse but also good so use air drum gas hydraulic brakes on the thing that is better sometimes also for reasons. There you go

    • There ya go dude,thanks for straightening that out!

  • Thank you so much now am a BRAKE master after this video

  • disc brakes dont scale up very well. even in a big rim like a truck. drums can be made wider without hitting anything on the dual rear axles.

  • You need very powerful system to push that much liquid. Although there are trucks with liquid brake fluids and they are pleasure to drive because brakes react faster and at better precision

    • Not true at all. The fluid is the actuation method. The linings and disc's or drums do the actual work. The same pressure can actuate smaller or larger brakes to give less or more stopping effort. Hence why race cars fit bigger brakes to get more stopping power- the pressure is the same- or often less- if they remove brake boosters to give better feel.

  • In addition to air brakes some of the older semis I drove (back in the 60s) used vacuum brakes. I don't know if those are in use anymore.

    • I didn't think diesels made vacuum, that's petrol

    • @adventureoflinkmk2 all engines create vacuum in the intake

  • Heavy trucks can get too hot and boil brake fluid. So air is used to release the brakes.

  • Imagine using hydraulic brakes with all of those connections. You’d need a 55 gallon drum of brake fluid strapped to the truck to compensate for the leaks.

  • There's 3rd brake option. Air over hydraulic/ hybrid brake system

  • I'm not sure about this vid b/c one of the main reasons for air brakes is you can attach any other vehicle such a trailers or machines not only to the towing vehicle but also having no issues with bleeding the hydraulic system while doing so. 1) Hence you can have a full engineered braking system including the attached vehicle whithin a second. (just connect the yellow and the red line and you're good to go). Imagine that application with an hydraulic system, it would make the whole process a costly (mostly b/c of the needed time) mess just for attaching and/or detaching a trailer or another special vehicle. That's imho the main reason for an air brake system on trucks and most other special vehicles, including tractors at a certain size. 2) The fading issues is actually the same b/c it is an issue of the brake pad and the brake disc or drum. It doesn't care what is applying the force, if the brake gets overloaded it will fade with both, hydraulics and air. 3) The Tristop brake cylinders are able to stop a vehicle due to the loaded spring. The same system can be implemented with a hydraulic system with ease, even with the same principled function and mechanics. That's not a criteria in that issue, I guess. In the end it is about applications and dealing with additional braking systems in terms of attached vehicles. There exist (at least in Europe) very sofisticated systems like EBS which can manage the whole braking situations even on long road trains, b-trains or just semi trucks and tandems. It wouldn't be that easy to manage if there is an hydraulic system in use, mostly b/c of the attaching process. Furthermore there exists hybrid systems with hydraulics and air and guess what, the air system is mostly needed for attached vehicles. Hence the actual reason why using an air system is: Daily operated tasks which include attaching and detaching vehicles such as trailers and special machines witch brake systems to maintain a safe driving situation. With hydraulics it would be a mess in terms of time, effort and at least safety. Everyone who had to bleed a hydraulic system knows exactly this is a maintenance task and not a daily use case, by far. From someone who did the engineering school and additionally just passed all the practical and theoretical exams for big rigs, small rigs and all kinds of trailers for trucks of all kind. :) Cheers o/

  • A good way of explaining it to people don't know anything about brakes is that the default position of air brakes is fully engaged. You have to build up air pressure to release the brakes. Then when you hit the brake pedal, you are relieving the pressure that is pushing against the brakes to keep them open. With hydraulic brakes the default position is off. When you hit the brake pedal your building up pressure to engage the brakes.

    • When you hit the pedal, you pressurize a second cylinder which acts independently of the cylinder containing the spring. Nothing exhausts (or bleeds) until you release the brake pedal.

  • Love the comments section. Some are arguing that he is *completely* wrong, others are saying he’s *partially* wrong/correct, some are talking about trains, some random said there is no [performance] difference between drums and discs, some guy said air doesn’t heat up like hydraulic/brake fluid does (IDK if he meant air doesn’t heat up at all or if he meant air won’t heat up as fast as hydraulic/brake fluid. His comment had all sorts of typos and he didn’t elaborate enough, so I can’t judge which way he was going with it). I’m just here because YT recommended it. I got no skin in the game, though I did operate some military vehicles that utilized air brakes, when I was serving. Don’t ask me how the [air brake] system works because, I kinda brain dumped some of that and I had a very basic understanding of how they worked. If you could ride a bike and write your name, you’re pretty much qualified to operate any type of heavy machinery in the military. Don’t give up on you’re dreams 😂

  • Brake fluid needs to be changed every two to three years adding extra cost and downtime.

  • what about clutch brakes + down shifting. I hear all the trucks using this to slow down with out pressing the brake pedal.

  • You have hydraulic brakes, you pneumatic (air) brakes, and then you have a third system, air over hydraulic which the japanese use in alot of thier smaller trucks, not sure about the European manufacturers though

    • I've only seen that third one in the Unimog.

  • Another reason semis use air brakes is because the trailer needs braking power. In a tractor trailer with hydraulic brakes the driver would need to manually bleed the brakes on the trailer while hooking up to it. It's a time consuming process that drivers and companies became very frustrated with. When hooking to a trailer the driver can simply connect the air lines and push a button in the cab to release the brakes. This saves lots of time as drivers will haul several trailers in a week

    • Not true, on vehicles with hydraulic brakes. the trailer uses either electric brakes or electric over hydralic brakes. Trailers that use hydraulic brakes have their own self contained system that uses a electrically activated master cylinder to activate the brakes. Personally I prefer the straight electric brakes to electric over hydraulic because it is more reliable in my opinion. Virtually all of the pickups you see towing goosenecks or tandem axle trailers have trailer brakes, usually electric.

    • @SilverStar Heggisist that's true with new technology but air brakes are older than electric trailer brakes. They were not an option back when air brakes became widley used in the 60's. They had to manually bleed the brakes back then

    • @Tony Dierks which to be fair is kinda dumb considering the technology for electric trailer brakes has existed since the automobile.

  • Heavy vehicles get more miles on a single vehicle and when hydraulic lines rupture, they make a big mess on the road that's also unsafe to drive over. Changing brake fluid isn't cheap either.

  • I’d say its ironic how trucks use the exact opposite how train brakes work, people know how safe those brakes are, yet they are not implemented in an automotive situation

  • Besides the brakes, trucks and trailers use compressed air for other applications

  • Stopping a large vehicle creates an enormous amount of heat within the brake system. It gets so hot that the hydraulic fluid will boil, greatly reducing the braking ability

  • In my opinion, hydraulic brake is more reliable than air brake. The reason why semi truck uses air brake is mainly about cost. Also it is difficult to make a flexible liquid connection between the tractor and the trailer. Heavy airplanes can be over 100 tons and they still use hydraulic brakes.

  • I expected this video to be about actual airbrakes, not pneumatic brakes.

  • Having this two braking system, front disc brake and rear air brake the best option.