edit: Solved. Thanks guys
In: Engineering
Many have said brake canisters or suspension, but there’s also the air dryer that purges excess moisture from the air system. That’s the sound you hear the most when a truck is idling or driving down the interstate without touching their brakes.
Also, the TSSSS sound you hear from a truck or bus shifting gears and speeding up is a blow off valve, releasing pressure from the engine built up by a supercharger, or turbocharger.
Took a sound design class a while back where there was a homework assignment that involved going out in the city (New York) to measure the decibel level and compare it to the city’s guidance on sound levels. That hiss sound from the buses was wayyyy higher than the limit it was even higher than a subway passing through a station.
Air suspension usually. Fun anecdote: My dad was a long haul semi driver. Something in the suspension blew out, flipped the truck on it’s side and his head went through the window and hit the pavement. He nearly died and is mentally disabled now. He still thinks he got fired because we hid his truck keys from him.
Heavy duty vehicles like buses and trucks use air brakes, unlike the typical care or small truck which uses hydraulic brakes. There is an onboard compressor that is driven off the motor. There are also storage tanks to keep about 100 psi of pressurized air. The occasional “TSSSS” sound could either be the air governor “popping off” or releasing air when too much has built up in the storage tanks/system. This air is used to hold the brake pads off the brake drum or disc, hence the brakes are released and the vehicle is ready to and able to roll. When a heavy duty vehicle’s driver pushes on the brake pedal, air is released from the system releasing the pressure thru valves located near the brakes on each axle which allows the brake pads to then engage the brake drum or disc at each wheel, slowing the vehicle. Applying the brakes will also cause the “TSSSS” you when your near a heavy duty vehicle.
Atleast for semi trucks. They have air brakes (atleast on the trailer. Tbh i only worked on the trailers). Air goes to the break to open it up. Releasing the air is how they brake. They release it because the air would release and the brakes would lock if the trailer became disconnected from the truck. Specifically the glad hand became disconnected .
Air brakes are common on larger vehicles for various reasons, but mainly because the air is constantly recycled, bleeding off system heat as they go, unlike hydraulic systems which just use the same fluid which can get hot and even boil to failure.
Also, the way that airbrakes work is to “fail on” rather than “fail off” like hydraulic brakes do. Here’s what that means.
In a hydraulic system, system failure means you got no brakes. That’s bad.
Airbrakes, however, have a failsafe built in called a spring brake, which, upon system failure, applies the brakes, hard. There’s a very stiff spring that pushes the brakes on, until air pressure pushes against the spring and holds it open. If you lose air pressure, the brakes turn on, not off. The vehicle stops, hard, upon system failure. I think the benefits of that are obvious.
The compressor that provides the compressed air for the system is engine driven, so whenever the engine is running, it’s making compressed air.
What this means is that the PSSHT sound comes from three sources:
1.) The operator releases the air from the system to set the spring brake, which also functions as the parking brake of the vehicle. This will happen whenever the operator pulls a yellow, diamond shaped button on the dash out. It sets the spring/parking brakes and makes the vehicle immobile. Buses do this for safety whenever they stop at a bus stop to ensure the vehicle doesn’t move as people get on and off. PSSHT
2.) If the operator applies the brake pedal at all, when they let off the pedal, the air that was fed into the system to apply the brakes is vented so that the brakes unapply. So, anytime you hit the brake pedal and then lift off, PSSHT.
3.) Because the compressor is engine driven, it is constantly making compressed air. Eventually, it makes more than the system needs, and a pressure actuated relief valve will blow some un-needed air out of the system to maintain the proper pressures and prevent overpressure. PSHHT.
There are a number of different tssss that a truck or bus can make.
Usually if it is sitting there with the engine running or driving, you are hearing the air dryer purge.
There is an air compressor driven by the engine that pumps air into the air tanks, but first it usually goes through an air dryer that removes moisture from the air.
When the air tanks have enough air, the compressor keeps pumping a bit longer and this extra air pressure is then blown through the air dryer which opens a valve on the bottom to blow some of the water out that is has collected.
If you look under the truck when it is doing this, you might be able to see a wet spray of air going to the ground.
Another tssss is when the driver releases the brake pedal. The pedal applies air pressure to the brakes, so when the brakes are released the air pressure gets let out.
The biggest tssss is when the driver applies the park brake. The park brake works the opposite to the foot brake, it is applied by big springs and uses air to release it.
When the driver pulls the park brake, all the air from the park brake chambers is dumped and the big springs are allowed to tightly apply the brakes.
Because heavy vehicles use air for so many things, there are a lot of things that can go tssss.
There is the air being released from air suspension, like when a bus lowers itself at the bus stop.
There are air powered and air assisted clutches.
There are even air powered or assisted gear shifts.
Rule 2.
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