eli5 Why do trucks and busses do the “tsshhh” thing when they stop?


eli5 Why do trucks and busses do the “tsshhh” thing when they stop?

In: Engineering

They have what are called air brakes. Traditional cars use hydraulic brakes, but with larger vehicles, the are more dependent on a pressurized air system. The hiss you hear is the release of pressure.


They use a different type of brakes than normal cars do. Busses and big trucks will either use hydraulic or air brakes, which both will make a “tsh” noise, since they need to let air out of their systems when they brake.

Most air systems work in reverse. The pressurized air keeps the brake off, the brake pedal releases an incremental amount of air to apply the brake. The whoosh is the park brake releasing the air to set the bake on firm. The pshh to release the park brake is the valve letting air back in the system to release the brake.

Edit: the idea is, if there was a total loss of the air system, the brakes would engage instead of no brakes.

As others have said, large vehicles such as busses use compressed air to apply pressure to the braking pads that stop the vehicle. However this doesn’t quite explain the hiss on stopping because if the brakes are on while stopped then surely they want to *keep* air pressure, not release it right?

The reason this happens is because there are actually two braking systems at work, the usual driving system that uses air to apply braking pressure and the parking brakes which apply pressure with springs. While driving normally the parking brakes are held open with air pressure, but if for some reason pressure was to drop below a critical point the parking brakes could close under spring pressure.

It is an automatic safety feature just in case the vehicle brakes fail, but this also is how the parking brakes are normally applied. Normal procedure is to apply the parking brakes at every pickup, and to do this the pressure in the lines between the storage tank and brakes must be released. The hiss you hear is that happening as the parking brakes are applied.

1. When the driver presses the brake pedal, compressed air is supplied to the brakes which is what makes the truck stop. When the driver releases the pedal after stopping (even if they just release it partially while still holding it down a bit) the air gets released under the truck making a “tsshhh” sound.

2. If the driver going to be stopped for a while they might want to apply the park brake. The park brake works the opposite way, it uses air pressure to release and springs to apply. So normally when the truck is driving, there is air pressure in the park brake system holding it off.
When the driver applies the park brake, it releases the air from the park brake system. This makes an even bigger “tsshhh” noise than just releasing the brake pedal.

3. Then there is air dryer purge. Trucks have an air compressor driven by the engine. When the air pressure in the tanks is too low the compressor turns on, when the pressure reaches the maximum amount it turns off.
But to prevent all the moisture in the air from filling the air tanks with water, most trucks have an air dryer fitted which absorbs moisture from the air as it passes through after the compressor but before the tanks. Each time the compressor finishes topping the air tanks up, a valve opens underneath the air dryer and high pressure air is blown though it, spraying moist air under the truck and drying the air dryer ready for the next cycle.
This makes a big “tsshhh” and can happen any time the engine is running. You might hear it when the truck is sitting still or when it is driving.

So as other people have mentioned, it comes down to the air system. You basically have 3 main reasons the pressurized air is making noise:

If a truck is coming to a complete stop and setting the park brake, a lot of air is dumped quickly (since the parking brakes are held open under pressure as a fail safe)

If the truck is slowing or stopping for any reason you may hear smaller releases of air as the driver adjusts their pressure on the brakes, each time they let off a bit that air the was applying the brakes is released.

Finally at any given point you may hear a quick blast of air leave one or both of the truck’s air tanks. This is the dryer/blower releasing air when the tank reaches desires pressure (~120psi) to blow out any moisture that may have accumulated.

They could be talking about the air dryer hiss, the brake air system takes in atmospheric air which has moisture in it, to keep the moisture from building up into water the system will purge a small amount, giving us the hiss noise while parked or stopped for a long time.

Larger commercial vehicles are operated by an air compressor, the name speaks for itself. Once the air pressure builds up to a sufficient level, like 120 PSI for 18-wheelers, the governor will regulate when to “pop-off” signaling that the air tanks have enough supply to operate the brakes. If an air hose is leaking then a buzzer (sensor) will sound and typically you can see on the A or B gauge that you have dropped below 60 PSI which is not safe to continue driving.

Is the compressed air also released when the clutch pedal is pressed during gear changes? Does the clutch system use air pressure as well?

Automatic air dryer. Let’s a spit of air out of the primary air tank. It connected via piece of quarter inch tube to the air governor on the air compressor so when the air governor shuts off the compressor it sends a signal to the dryer.. before that we used to have a valve on the bottom of the tanks with the cable you pulled.. before that we just had a petcock you had to open by hand. Before that we just had a tank full of water.

May someone please answer this; I thought the hissing was related to the suspension change in the schoolbus, when it starts hissing the suspension drops to allow easy access into the first step of the bus. I haven’t seen a reply yet covering this system of the bus. Is anyone sure the hissing isn’t the release of air suspension? Thx

A lot of Buses also use air suspension too, but of course this hissing sound you’ll only hear when the bus stops, usually to let ya grandma on or someone with a wheelchair for example.

Heavy duty vehicles have heavy duty brakes that function differently than normal cars and trucks. They use an air system to stop instead of regular pads and rotors or drums.

They also work backwards of a normal car. When you hit the brakes in a normal car they squeeze on the rotor slowing the car down. In a semi or a bus they pump air into the brakes to get them to release and when they hit the brakes it bleeds off this air pressure so the brakes will apply. When the vehicle comes to a complete stop the pshhh sound you hear is it just releasing the air so the brakes will reapply.

This is important for heavy duty vehicles because if their braking system fails they don’t have a vehicle that weighs 80,000 pounds screaming down the high way unable to stop. If it fails it just applies the brakes bringing it to a stop.

Car and light truck brakes are based on positive pressure. When you apply your foot to the brake pedal, you’re also directing pressure to apply to the brake pads against the rotors to slow and stop the vehicle.

Large, heavy vehicles like large trucks and busses need brake systems that are fail safe, meaning if the brake system completely fails, the brakes are still applied to slow and stop the vehicle. So, their brakes are actually activated by a large, powerful spring. In order to disengage the brakes to move the vehicle, a supply of pressurized air is applied to overcome the spring pressure pressing the brakes together, thus pulling apart the mobile and stationary portions of their braking systems to allow them to move independently.

In those vehicles, pressing your foot on the brake pedal is applying negative pressure to the air brake system, meaning the air pressure keeping the brakes off is lessened, applying the braking force to the vehicle in proportion to your control input on the brake pedal.

The reason you hear the blast of air when they park is that that is, essentially, their parking brake. You put your car in park and a device called a parking pawl engages with the output shaft of the transmission, locking the drive shaft to keep the car from moving. Again, large vehicles need more safety assurance. So, they just dump all of the pressure in their air brake lines to atmosphere, thus insuring all of their brakes immediately go under spring pressure lock down, and the vehicle can’t move.

There is the large tsssss and the quick tssss
Large is when the brakes are set so the truck and trailer won’t move and the smaller sound is the air governor which regulates the air pressure

We just got a 43 foot diesel pusher today. I’m 53 and every one we looked at I asked my husband if it went phsssttttt at the end. All the sales people got a kick out of it. But that was what I wanted! To pull up somewhere and go phhsttt!

Big trucks have air compressors. When air tank full – go psh. When brakes are applied and released it go psh. When you set the parking brake it go psh. Trailer brake psh is most satisfying 🙂

The bus uses a lot of air to hold the brakes up. <**Breath in while holding the kid up**> So when they need to park, they release all that air! <**Blow out of your mouth while slowly putting the kid down**>

Our car doesn’t do this because we don’t use compressed air in our cars (we use a really cool oil network instead but that isn’t suitable for a bus of this size).

The real question is why they don’t muffle the sound. It would be simple to do and would save a lot of ear damage in bystanders. Sound guy here, high frequencies damage hearing and lead to tinnitus much more than those booming lows that are coming out of subwoofers