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