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