Why does the truck hiss when braking?

Trucks stuck in traffic are a rich source of interesting questions from the back seat. For likes from behind you know the answers better. Today: What is that hissing sound when trucks brake?

From the popular Rubik: "Traffic jam questions from the back seat" today: "Mom, Dad, why is the truck always hissing? And why does it have so many colorful hoses?". Automatic air conditioning and ventilated seats don't help against the sweat on your forehead and back. The heirs need input. The front row of seats must deliver. Two questions, many explanations from auto, motor and sport.

What's hissing there?

Vehicles over 7.5 tons gross vehicle weight usually have air brakes instead of a hydraulic brake system like in a car. If the driver presses the brake pedal, previously generated compressed air is routed through a legion of hoses and valves to the wheel brakes. There, the compressed air presses the brake pads onto the discs via membranes or pistons. This excess pressure in the brakes escapes with a hiss when released. So we don't hear the hiss when braking itself, but when the brake opens again.

Now it's hissing again

Now the offspring could follow suit confidently: "Look, now he's standing and hissing again." Counter: The driver decides to use the hand or parking brake. A unique component of the air brake is used here. The so-called spring accumulator. A strong spring constantly presses on the brake and keeps it closed. It only detaches under high air pressure of five to seven bar. If the air pressure falls below a certain safety value, for example due to a leak, engine failure or if the trailer becomes detached, then this brake closes without compromise as an emergency braking system. In the case of the parking brake, the spring accumulator is vented and the compressed air escapes with a hiss. By the way: This is the reason why large rescue vehicles in the depot are permanently supplied with compressed air from an external source. You can drive off immediately and do not have to wait for air in the system.

What are those colorful hoses?

The rulers of the fund respond to the hoses dangling between the tow vehicle and the trailer. Alternatively, they point to the two spiral hoses between the semi-trailer truck and the trailer. They differ in color or the connections in red and yellow. The red hose or the red connection mark the reserve air, i.e. the air that is used for braking and also opens the spring accumulator. Yellow is the control line. It basically relays the driver's braking request. If you connect or disconnect the trailer and leave the red connection "alone", then you disconnect the trailer from the controlled braking system. Under certain circumstances, the trailer then rolls away on its own and only stops when the red line breaks off. Hence the golden truck rule "Red never stands alone".By the way: if there is a blue hose between the red and yellow hose, then that is compressed air to increase the tire pressure, for example.

Why do trucks brake with air?

Air brakes are not required by law. However, if a trailer with a permissible weight of more than 3.5 tons is to be towed, then this must be connected to the braking system of the towing vehicle, and compressed air is the ideal solution. Technically, however, the air brake is necessary because a hydraulic brake with the maximum permissible weight of 44 tons would be hopelessly overwhelmed, the brake booster would be the size of a garden shed and several liters of brake fluid would be in the system. The biggest difference is how it works: A car brake brakes purely through the force of the foot on the pedal: the harder the pedal is pressed, the more braking force is generated. With the air brake, it is not the force on the pedal that modulates the brake, but the distance that the pedal travels.

Electricity and water in the brake

Bonus round: Air brakes are not required by law for trucks or vehicles. From a certain permissible total weight, the number of different, independent braking systems is regulated. For trucks over 9 tons and for buses over 5.5 tons, a continuous braking system is required in addition to the actual service brake. It must be wear-free and be able to maintain the speed of the fully loaded truck at 30 km/h on a gradient of seven percent. Retarders that brake using eddy currents, cooling water or oil pressure are common today. There used to be dynamic pressure brakes: flaps in the exhaust that throttled the engine by accumulating exhaust gas. Since retarders lose braking power with decreasing speed, they are actually not suitable or intended for decelerating the vehicle to a standstill.

Weight to the rear

If you want to bring the finishing moove, you can explain to the inquisitive at the back why some trucks with trailers only have a box body at the back (so-called swap bodies) and nothing at the front. Parked swap bodies look like containers with stilts. If only one has to go further, it gets on the trailer. This is also due to the braking system: As described above, the compressed air brake works completely differently than that in a car. The brake pedal transmits the brake request to a sophisticated system of valves and control circuits. What happens after that depends on many factors. In the case of a towed trailer, the so-called trailer brake valve recognizes the attached load and supplies the trailer brakes with more compressed air. The trailer brakes harder and stabilizes the train. Without this lead or loaded vice versa, the trailer would start to lurch, since the towing vehicle brakes earlier or more strongly due to the dynamic axle load shift. Because: Trucks, semi-trailers or trailers brake differently depending on the load.When the load is light, the brake is applied with less force than loaded, increasing the effect and advantage of the weight on the trailer. Caution: This only applies to vehicles with air brakes and trailers.

Something else to say

1. A fully loaded truck with 40 tons only needs 36 meters more braking distance from 80 to 0 km/h than a car with 3.5 tons.

2. If a trailer is braked by a continuous brake system, i.e. compressed air, it may have 1.5 times the permissible weight of the towing vehicle: 15 tons of trailer weight should be pulled by a towing vehicle that weighs only 10 tons.

3. The pulling part of the truck is called the towing vehicle. Zugmaschine is colloquial and technically incorrect. A tractor only pulls. A towing vehicle carries part of the total load itself. The front part of a semi-trailer also carries the drawbar load.

4. The number of axles on a truck gives an indication of its weight. We add a good eight tons of weight to each axle. Single axles are always allowed to carry 10 tons, double axles between 11 and 19 tons depending on the distance and suspension.

5. When you drive past a trailer, you sometimes see two or three large yellow fields or "holes" on the side. These are lifting points for the crane when the trailer is lifted onto the train to cover long distances.

6. Trucks often use the same compressed air for the air suspension of the load-bearing axles. This enables level control under load as well as raising and lowering the superstructure for loading and unloading.


Game, set and match: If this concentrated information doesn't make the questioners open their mouths and appreciative looks from the next seat, you should work on your previous day's technique - because air brakes or pneumatic brakes are a fascinating technical masterpiece. Almost all chassis components of modern trucks or buses depend on an air system. In addition to the brakes, the suspension and the level control are controlled by air - without compressed air, modern truck traffic as we know it would be almost impossible.


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