eli5 Why do trucks and busses do the “tsshhh” thing when they stop?
In: Engineering
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.
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