how semi truck trailers don’t just roll away when disconnected from a semi truck
In: 8
Air pressure from the truck keeps the brakes off. When they disconnect the trialer the air is removed and the brakes are On.
The brakes on the trailer are fail-safe – meaning that if they lose air pressure from the truck, the brakes on the trailer default to locked. This prevents the trailer from rolling but also prevents the driver from continuing to pull the trailer without good air pressure.
You can see this sometimes on the show Ice Road Truckers – if they get a little moisture in the air lines and it freezes, the trailer becomes immobilized until its fixed.
So, the last 2 responses talk about the brakes and how they’re designed to fail-safe, that is, when they fail, they fail into breaking mode, and that’s all true.
What I’m not sure was mentioned was that, when properly disconnect, trailers also have landing legs towards the front that are extended when parked and retracted while driving, and, well, things in legs tend to stay put, even when there are wheels on the other end. Unless the load is extremely unbalanced
Semi truck trailers use air brakes. When the trailer is disconnected, a spring in the air brake chamber keeps the brakes on, so the default condition for the brakes is ‘on’. To release the brakes you need to pressurize the air brake chamber, which can allow the spring to move, releasing the brakes.
Ex trucker here. These answers are kinda right but wrong.
Although air pressure does keep the brakes off, trailers have their own air tank which is supplied and filled with air from the unit (cab section).
When you park up you have to go to the trailer and pull a lever which activates the trailer brakes so you can pull out without the trailer rolling away.
The landing legs will not always stop a trailer from rolling away and shouldn’t be relied on to keep the trailer in place. They can and will bend if parked loaded on a slope with the trailer brakes off.
A couple of reasons;
The first is that trailers have braking systems in their wheels, connected to the brakes on the tractor. So before a driver unhitches they ensure the brakes are on and the trailer safe.
It is also worth noting that these brakes are fail-safe – unlike normal car brakes which are off by default and you push the brake pedal to apply pressure and turn them on, the brakes in a trailer are on by default, and then purposely held off by a pressurised air system from the tractor. If that air line breaks then the trailer brakes will lock on and the trailer will not move, rather than locking off (and I have seen the results of this line breaking while a truck was on the move – locking the trailer wheels which then caught fire due to the friction and heat).
There is also the fact that large trailers need to be supported before the truck can release them – there are a set of legs at the front of the trailer that need to be lowered to support the front and prevent it just landing on the ground as soon as the truck moves out from underneath it. These legs normally just have flat plates on the bottom, so they will stop the trailer moving to some degree (just like the fact you need to lift up a wheelbarrow before you can easily push it).
If a driver is worried, you can’t also use things like blocks under the wheels to stop it moving too.
Ultimately though, this is something that is considered by the driver of the vehicle – even if you move down to smaller scale trailers that are towed behind a car, you still have to be careful when unhitching that it is safe to do so – using manual brakes on the trailer, making sure the ground is level, having enough people to manually maneuver the trailer and so on. Act irresponsibly and even a relatively small trailer can run away from you and cause significant damage.
So, no one mentioned parking on a level surface?
Between the landing legs and sliding tandems, it helps them support the load. I’ve seen many drivers pull a unit with a bad landing leg that couldn’t drop and they have to stay hooked until a mechanic can help fix it or put up support to keep it up.
In the US a trailer has two airlines that connect it to the tractor.
One airline supplies air to open the brakes. There are very powerful springs in the brake canisters that keep the brakes applied when there is no air pressure.
The other airline supplies air to the braking side of the brake, you know, when you press on the brake pedal. Or when you pull on the “Johnny bar” which will apply brakes to the trailer.
With both airlines disconnected from the trailer the strong springs will close the brakes and keep the wheels from turning.
Your Car breaks when force is applied via the break pedal, the break is open at rest.
So assuming your break pedal disappears you can’t break.
Trucks do it the other way.
The breaks are engaged while at rest (done by a big spring) and are only kept open by hydraulic or pneumatic pressure.
So when their “break pedal” disappears the breaks simply engage.
In Short:
Car uses force to engage breaks, they are naturally open.
Semis use force to keep them open, they are naturally closed.
With the trailer disengaged there is no force to keep the breaks open, they stay engaged.
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