how semi truck trailers don’t just roll away when disconnected from a semi truck
In: 8
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.
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