Braking force of a truck

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  • #26
256bits
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He says a pallet on the truck will move backwards after a crash???
After the crash, possibly, due to the relaxation of elastic compression forces on the pallet from the bulkhead, or from the elastic compression properties of the contents within the pallet.
There is not a statement where one can say with certainty that all pallets of material will re-coil backwards AFTER a frontal crash.
Anyways, most crashes are messy things, and your pallets will be jostled around during the crash, and thus not in supreme condition when inspecting their integrity afterwards.
 
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Whiplash is a neck injury. It results from a victim's head and torso being subjected to significantly different forces - most commonly in the 'fore and aft' direction. Anything that keeps head and torso in constant relative positions will prevent that particular injury. In the simplest case (a rear-end collision), the torso is accelerated forward (by the seat) and the (unsupported) head isn't. I don't see any useful similarities to a pallet of bricks.
 
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  • #28
sophiecentaur
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After the crash, possibly, due to the relaxation of elastic compression forces on the pallet from the bulkhead, or from the elastic compression properties of the contents within the pallet.
All these scenarios seem to depend on a significant amount of resilience. A major part of vehicle design for dealing with collisions is permanent distortion of the structure to dissipate the energy. I can't think of any element of the tractor and trailer that might have longitudinal resilience, built in, except the driver's seat.

The only mechanism I could think of that could produce a backlash effect could be if the pallets were not secure and could carry on travelling forward after the trailer had stopped. They could then hit the inside front of the trailer and nudge it forward. That would cause the driver to feel a forward impulse.
 
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  • #29
hmmm27
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He says a pallet on the truck will move backwards after a crash???
Sounds reasonable. I'd be curious as to what the greatest movement he's seen, is.

Between the bumper and the load, there are plenty of bends in the frame and structure to make a de-facto spring system (including of course the actual leaf-springs between bumper and frame). All of which will compress, then spring backwards. They stop when they've returned to their unstressed "rest" position, but the pallet will keep moving.

And, that's before we get to what most of it probably is : the bumper is much lower than the load : in a collision the frame tries to pivot upwards around the impact point, during which time the sprung undercarriage - which hasn't tried to rotate - pulls it back downwards... but not the pallet which relies only on gravity. Might be a bit different for an 18-wheeler due to the tractor/trailer hinge.

Per "0.8" : formulae's uncomplicated : you need to know engine output and gross vehicle weight.
 
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  • #30
256bits
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The only mechanism I could think of that could produce a backlash effect could be if the pallets were not secure and could carry on travelling forward after the trailer had stopped. They could then hit the inside front of the trailer and nudge it forward. That would cause the driver to feel a forward impulse.
Put the pallets on wheels, which they are not ( usually ) and the floor- pallet reduced friction would have interesting effects.
 
  • #31
sophiecentaur
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Put the pallets on wheels, which they are not ( usually ) and the floor- pallet reduced friction would have interesting effects.
Shifting loads are a real embarrassment. Even a pot pf paint in the back of your car can give serious grief if it's not secured. Some of what's described in this thread is very frame dependent and a lot of statements should be tidied up before they can be taken seriously. People 'feel themselves thrown forward' and that colours a lot of descriptions - from teachers as well as the general public.

I'd be curious as to what the greatest movement he's seen,
So would I. Elastic distortions would surely be a lot less than plastic, if there is any significant movement. The exception could be vertical 'bouncing' as suggested.
 
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  • #32
hmmm27
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Shifting loads are a real embarrassment. Even a pot pf paint in the back of your car can give serious grief if it's not secured. Some of what's described in this thread is very frame dependent and a lot of statements should be tidied up before they can be taken seriously.
Ah, well... my assumptions are of an impact with a bridge abutment at suburban speeds ; also a straight-bed (2-5 ton) truck (amended to include a tractor-trailer, after I reread the thread) ; carrying, a pallet of bricks about 4'cubed (which turns out to be wrong : a pallet of bricks is only 5 bricks high, massing about 1.25t).

Elastic distortions would surely be a lot less than plastic, if there is any significant movement. The exception could be vertical 'bouncing' as suggested.
True, but - even though the effect is much less with a squat pallet (ie: low CoM) - I think it would still be significant enough to count.
 
  • #33
sophiecentaur
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- I think it would still be significant enough to count.
What component of the chassis / body would distort elastically? Vehicles are designed to suppress flexing because that affects behaviour with instantaneous loads (as with the collision and bumps in the road) `In collisions, the requirement is to absorb the energy and not to store it and deliver it back into the situation. Freak conditions can induce vertical motion (one vehicle ending up on top of another) but even that is dealt with by the barriers hung down from the tail of trucks.

I think we could be chasing the real meaning of a chance remark of a teacher who may have loads of experience but not necessarily a depth of physics. We'd really need to go back to the beginning and examine the source of the information in greater depth. Every explanation that's be suggested has relied on a significant amount of 'interpretation' of the presented description. That's not usually a reliable way to getting a meaningful answer.

That's not to say that the chat in this thread has not been fun. (Double negative - sorry.)
 
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