pickup truck crash, would you want toolbox stored up against the cab, or at the back?

[supernova87a]

Hi all, this is not highly technical, but more of a basic physics question.

Take a pickup truck, about to be involved in a head-on collision. A few friends and I are debating why the manufacturer recommends that you store heavy loads up against the cab rather than at the back of the bed. I agree that you should store them up against the cab, but not about the reason.

Friends say that obviously, if you store, for example, a heavy toolbox at the back of the bed, when it goes flying in the crash, it will have all this "momentum" that causes severe damage when it hits the cab (the body panel where the small of your back is located). I suppose they mean that this is worse than storing the toolbox hard up against the cab, because it has all that distance along the truck bed to "build up force" that is "worse".

I think this is a load of crap. When the truck stops, both a toolbox at the back of the bed and up against cab were going at the same velocity v, and have to be stopped by the cab body panel. It's not like somehow the toolbox at the back of the bed gains some extra "force" by flying through the air. In fact, if you take it to an extreme and imagine air instead as a thick fluid, if anything it would decrease the toolbox's velocity/momentum.

My explanation for the reason is as follows:

When you store something hard up against the cab back wall, in a crash it will probably more evenly distribute its mass against the body panel (because you've set it up that way), thus making the impact force of the toolbox spread over a larger area, and better using the body of the truck to protect you.

If the toolbox was at the back of the bed, it has a higher chance of changing angles while flying through the air, and presenting a sharp corner or edge on impact, and making the force much more concentrated into a point. Or in flying during a crash, it might go through the cab window aimed at your head instead of the body panel.

So I think that it is because of these more "practical" reasons than their notion of increased "force".

Am I wrong?

Thanks!

 

[CycloWizard]

I think you are both right. If you store the box at the back of the truck, its velocity will tend to be whatever the truck's was before the collision. If the truck is completely stopped, the box is then moving velocity v relative to the cab. This is not really an accumulation of force per se, as the box is not accelerating. However, if you look at it from the opposite perspective, the truck is decelerating rapidly, which is where the force comes from.

Your argument is also correct in that the back of the cab will prevent the box from achieving any velocity which is different from the truck and, in the case of an imperfect (non-rigid mounting), would distribute the load across the metal of the truck's body more or less evenly rather than in a random manner if the case were stored at the back of the truck.

 

[canis]

..

 

[f4phantom2500]

aside from the aforementioned points, it's also worth noting that the vast majority of the weight of the truck, outside of the toolbox, is near the front of the truck, so mounting the toolbox at the back of the bed might give the truck a sort of sling effect during a collision, possibly causing (more) whiplash for the passengers; consolidating the weight distribution near the passengers, i think, would minimize this effect. further, it may provide for greater stability while driving, thus reducing the chances of spinning out and causing a wreck to begin with.

 

[Gibsons]

If it's in the back of the truck, the tools or the box might impact the rear window instead of the back of the cab.

 

[William Gaatjes]

Don't place the toolbox at the end. If the car stops, the toolbox will still have the same speed and the car will slow down. The toolbox will gain kinetic energy when compared to the car. And the car will tilt because the energy from the moving car is partially converted in a swinging motion where the backside moves up. This energy is also given to the toolbox.
It is the same as driving in a sedan with a hammer or a bowling ball in front of your rear window or on the backseat. When you make an emergency brake the hammer or the ball will come flying at you. In the end it has nothing to with causing extra damage to your truck. The important thing is that you or bystanders do not get hit by tools from a flying toolbox or a toolbox itself. There is any way only one proper way, make sure the toolbox is locked in position and cannot move even with an emergency stop. Then it does not matter where you place it. The only thing is that when placing it at the end, you have to keep in mind that the lock or restrains have to be stronger at the end of the car because of more momentum.


Apologize for the amount of text, but i have difficulty formulating.

 

[supernova87a]

Don't place the toolbox at the end. If the car stops, the toolbox will still have the same speed and the car will slow down. The toolbox will gain kinetic energy when compared to the car. ...The only thing is that when placing it at the end, you have to keep in mind that the lock or restrains have to be stronger at the end of the car because of more momentum. ...

Umm. yeah. This is exactly the kind of strange thinking I'm talking about.

 

[William Gaatjes]

Umm. yeah. This is exactly the kind of strange thinking I'm talking about.

What do you think is strange ?

Because when a car stops during an emergency brake, you can see simplified the front wheels as an axis where the car rotates around on.

I wish i could explain it better but at the moment i can not even understand the programs i have written myself.

 

[jaha2000]

It will of course hit with more force if its at the back.
Think about it this way.
Against the cab the delta V of both the box and the truck will be the same.
Assuming the restraints break and its at the back, the truck most likely will be at a much lower velocity than the box by the time it hits the cab. Its not that the box is accelerating, its that the truck has decelerated so much that the difference in velocity at time of impact is going to be quite large.

 

[CycloWizard]

Umm. yeah. This is exactly the kind of strange thinking I'm talking about.

It's a bit of a mis-statement as the toolbox gains its kinetic energy while the truck is accelerating up to speed. It has that kinetic energy when the wreck happens, as does the rest of the truck. The truck's kinetic energy is transformed into strain energy (i.e. energy used to deform the truck during the collision). The toolbox's kinetic energy will be transformed into strain energy when it hits you in the back of the head as well, but its elastic modulus is much higher than your head's, so you will come out on the losing end of things.

 

[ModestGamer]

toolbox mounted to bed at back of cab = energy dsitributed through cab/frame assembly during collision.

toolbox free at the end of the bed is a missle that now has the kinetic energy stored and seeking point of disappation. Not to mention if the toolbox turns or a corner rpesents to the cab or you skull.

duct tape a hammer to a piece of plywood.Throw it on the ground.

Now throw the hammer at the plywood with the plywood laying on the ground.

get the picture ?

distrobution of force is the difference. The amount of energy is the same.

in scenario #1 the force is spread more evenly along the body structre.

in scenario #2 the force is focused and depending on how it is aimed could do more damage.

lots of crash test data on this subject.

 

[Murloc]

I think that the theory they told you is right, you cited some things that can make it even worse though.

When a car crashes, it will absorb the hit and slow down, you will go flying through the glass if you're not using the belt. If you use the belt, you will be a part of the car and you will be slowed down with it.
Usually cars slow down thanks to their chassis, it deformates.
Now we know that trucks are very bad because they're hard but still.

If the toolbox is touching the cabin and is stopped there with strings, it will slow down with the whole chassis, its momentum will result in more force being applied to the wall or whatever you crashed in.

If it's on the rear, it will break any string and go flying at 50 km/h or whatever towards the now still cabin.
It will penetrate it because it applies a strong force on it and kill you.

this is important when just loading your car too, because a bottle of water flying can kill you, same for people not using the belt.
If you load baggage leaving space between them, they can go through the rear seats and kill the passengers.

 

[Pulsar]

If the toolbox is up against the rear of the cab of the truck, or even close to it, the force of the box striking the back of the bed is an impulse that is stretched over the time of the collision. Collisions are not instantaneous.

If the box is in the rear of the bed, it gives the truck time to decelerate while the toolbox continues moving forward. This causes the box to strike the back of the bed with an instantaneous impulse - a much shorter duration for the total transfer of energy takes place.

You can also think about it terms of work. It's the TIME PERIOD over which the toolbox is decelerated that is important.

W = F * T.

Since the work remains the same in either case (the tool box ends up being stopped), in the situation where it strikes the back of the cab has a much smaller time to decelerate the box, and hence the force imparted is much much higher.

 

[Murloc]

I agree with pulsar

every moving item has momentum, which is given by mass * speed.
the transfer of momentum between items is called impulse.

To transfer the whole momentum of the box in very little time, you apply a huge force, which will break the cabin.

if it's already there, after very little time the cabin will be slower because some of its kinetic energy has been absorbed by the chassis with the deformation, this means that the force will be little, because the cabin is slower than the box but not still.
Over time the cabin slows down and the box does too, but in any moment the force applied will be lower than if you put the box in the rear.

 

[mutz]

If the toolbox is up against the rear of the cab of the truck, or even close to it, the force of the box striking the back of the bed is an impulse that is stretched over the time of the collision. Collisions are not instantaneous.

If the box is in the rear of the bed, it gives the truck time to decelerate while the toolbox continues moving forward. This causes the box to strike the back of the bed with an instantaneous impulse - a much shorter duration for the total transfer of energy takes place.

You can also think about it terms of work. It's the TIME PERIOD over which the toolbox is decelerated that is important.

it should be the same,
even if the collision is not instantaneous or not, the box would let's say, slide on the back of the pickup, slowly, while the driver squeezes the breaks,
at the moment of collision , the box would fly (depending on the weight and speed), straight at the cabin.

at another scenario, when the box is held tight to the cabin, it would squeeze to it with the same force while breaking, and that momentum would remain, till the actual 'hit'.

so it's like placing an iron bar at the window level tight to the window, when the car breaks, the bar would get squeezed to the glass with the mass*force and at the actual collision, it might break it,
the same actually as it would fly if it was held a certain distance from the window,
the air would actually even slow it down a bit.

it is strange that we happen to think, that if an object is set far from the other, the actual movement through the air would multiply it's hitting force, yet as the OP was referring, this is nothing but a myth,
the force implied at the object if far or close, would be ~exactly the same force..

 

[jaha2000]

it should be the same,
even if the collision is not instantaneous or not, the box would let's say, slide on the back of the pickup, slowly, while the driver squeezes the breaks,
at the moment of collision , the box would fly (depending on the weight and speed), straight at the cabin.

at another scenario, when the box is held tight to the cabin, it would squeeze to it with the same force while breaking, and that momentum would remain, till the actual 'hit'.

so it's like placing an iron bar at the window level tight to the window, when the car breaks, the bar would get squeezed to the glass with the mass*force and at the actual collision, it might break it,
the same actually as it would fly if it was held a certain distance from the window,
the air would actually even slow it down a bit.

it is strange that we happen to think, that if an object is set far from the other, the actual movement through the air would multiply it's hitting force, yet as the OP was referring, this is nothing but a myth,
the force implied at the object if far or close, would be ~exactly the same force..

The force would not be the same because the objects momentum would not be the same.
Think about it this way, against the cab the two objects velocities at impact are the same. If at the back of the bed, the two objects velocities are quite different. You have MUCH more momentum in the second case.

 

[Modelworks]

Glass in vehicles is tempered glass. All that is needed to break tempered glass is a decent amount of pressure on a tiny area. ice pick , put against a car window and lean on it. no more window.

All the boxes I have seen installed on work trucks will not mount directly against the cab because they need clearance for the tops to open. They are also bolted down so tight you could probably lose the entire bed of the truck and have the box still attached. Add to that the really big boxes often are used on trucks that also have wire mesh over the back glass.

 

[mutz]

The force would not be the same because the objects momentum would not be the same.
Think about it this way, against the cab the two objects velocities at impact are the same. If at the back of the bed, the two objects velocities are quite different. You have MUCH more momentum in the second case.

what gives the iron bar "force" is the actual change in speed versus it's own mass,
if the environment is the same, let's say, the driver is slowing down from 60 to 40 and then collides, at the tight window scenario, the bar would squeeze against the glass at power = (60-40)/time(sec/ms/etc.) it takes to slow down, *mass etc. etc.,

at the other scenario the same thing happens, the only difference is that the bar is flying through the air or sliding toward the window, at the same mass and at the same speed decrease.
take these variables,
and the formula should be the same (counting out small differences like air resistance & whatever).

what hits the window, is the actual force held within the bar, the same force of breaking, it doesn't matter whether the bar is near or far.

 

[jjmIII]

So I think that it is because of these more "practical" reasons than their notion of increased "force".

Am I wrong?

Yup....your wrong . Jump off a 1 foot step ladder. Jump off a 10 story building next. Then decide.

 

[waffletten]

It would not seem logical to mount at the back gate as it would restrict the function of opening the gate and placing large items (like plywood) into the truck bed. Isn't the point of a truck to place large items into the bed?

If I were to postulate on collision and tool storage safety I would probably focus on the mounting point of the tool box. A bolt in the frame (like a utility body) would be much stronger than a bolt in the truck body (like your standard bed mounted toolbox). Take a hammer to a truck frame vs a truck body at a junk yard and you will see the difference in the strength between the two.

If you want to reduce the dangers of flying tools or the box shearing off in a collision, probably would be best to get a utility truck bed where the tools are in cabinets on the side and the whole utility bed is attached directly to the frame (not the tool box being mounted to the body of the truck through the wimpy holes they provide on the top of the truck bed body).

I used to work for a large construction company and these utility beds are very strong, more so than your standard factory truck bed. We had trucks with tool boxes mounted in the beds and they were a pain as you had to hop in the bed to dig tools from the bottom and you couldn't stack as much material in the bed as there was only about 4-5 inches under the toolbox to slide large panels/drywall sheets/etc.

 

[Pulsar]

what gives the iron bar "force" is the actual change in speed versus it's own mass,
if the environment is the same, let's say, the driver is slowing down from 60 to 40 and then collides, at the tight window scenario, the bar would squeeze against the glass at power = (60-40)/time(sec/ms/etc.) it takes to slow down, *mass etc. etc.,

at the other scenario the same thing happens, the only difference is that the bar is flying through the air or sliding toward the window, at the same mass and at the same speed decrease.
take these variables,
and the formula should be the same (counting out small differences like air resistance & whatever).

what hits the window, is the actual force held within the bar, the same force of breaking, it doesn't matter whether the bar is near or far.

I will try to explain this once more.

In the case where the tool box is tight against the back, the force applied to stop it is the time period of the crash.

In the case where the tool box is at the gate end of the bed, here is what happens. The driver of the truck hits the brakes. The tool box continues moving forward at roughly the speed of the truck. Now the truck plows into another vehicle, drastically decelerating it.

The tool box continues moving forward at it's original speed until it contacts the bed of the truck. The velocity which it impacts the bed will be very, very different than in the initial case, where it decelerates WITH the bed of the truck.

In this case, the truck has stopped decelerating and stationary (let's imagine a very big concrete wall that it). Now, the time that the tool box itself is decelerated (or negatively accelerated, if you like) to a stop will be much, much shorter, and the impulse imparted upon it will be much much bigger. While the work done will be the SAME, the time over which the box is decelerated to rest is much much smaller. By necessity, that means the Force will be much, much BIGGER.

Work done to stop tool box = Force of bed on tool box (much much bigger) * Time of tool box crash into truck bed (much much smaller).

Smack a baseball with your hand, as hard as you can. Now throw it. Assuming they go the same distance: which hurts more?

 

[canis]

If the toolbox is up against the rear of the cab of the truck, or even close to it, the force of the box striking the back of the bed is an impulse that is stretched over the time of the collision. Collisions are not instantaneous.

If the box is in the rear of the bed, it gives the truck time to decelerate while the toolbox continues moving forward. This causes the box to strike the back of the bed with an instantaneous impulse - a much shorter duration for the total transfer of energy takes place.

You can also think about it terms of work. It's the TIME PERIOD over which the toolbox is decelerated that is important.

W = F * T.

Since the work remains the same in either case (the tool box ends up being stopped), in the situation where it strikes the back of the cab has a much smaller time to decelerate the box, and hence the force imparted is much much higher.

Work=F*d if F is constant. I thought someone else would point this out. Same for your last post.

 

[Pulsar]

Work=F*d if F is constant. I thought someone else would point this out. Same for your last post.

Thanks for that. Seems things do get a bit rusty with non-use.

Just to refresh I looked it back up again. It's actually the impulse that I should have been naming, and not the work. Being that impulse is the integration of force over a period of time, my explanation is correct if you replace the word work for impulse (and correct the formulas).

20 years tends to make one forget things =/.

 

[WoodButcher]

I've been a carpenter since before I could drive and have been in and seen many wrecks cars, vans, and trucks with tools, equipment and materials of all sorts. Tied, loose, strapped, bolted and welded boxes and bits involved.
The one thing I can tell from over thirty years of observation you is that I'll pay the $37 dollar fine every time for the lack of seat belt and hope I get thrown clear of that mess. I try to tuck things down and away but if someone thinks he's "safe" by welding his toolbox to the tailgate and putting on the padlock will soon meet Murphy and his legalpad.
That said, IMO the best place to mount a box or carry tools on a truck is on someone elses truck. Failing that you put them where you use them most and are readily available and adjust as needed. Most of all drive defensively, use the tools you were given and pay attention. Shit happens, expect it.

 

[C1]

Seems like this has been made more complicated than needed. Isnt the issue simply that you dont want objects moving relative to the vehicle including yourself and that's one reason for wearing the seat belt?