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Don't try this at home. I am taking personal responsibility for what happens in my car.

Hi all, I recently purchased a fire extinguisher for my car (it weighs 1.5 lbs or 0.68 kg). I browsed the internet for a decent bracket to mount it inside my car, under the front passenger seat. Everything was fairly expensive which made me want to make a DIY version that would be cheaper.

I have been trying to find what sort of materials I can/should use. The last thing I want is the bracket to break and have a 1.5 lbs metallic projectile flying inside the car in the event of a crash.

The bracket should be 16" or ~40 cm wide so that I can use the bolts on which the passenger seat is mounted to attach the bracket. (See picture below)

enter image description here

Obviously I could use some thick steel and be safe but I don't really have easy access to tools to cut metals so I would like to try something else.

My requirement for the bracket is that it should be able to hold on to the fire extinguisher in the event of a frontal collision on the freeway. Which would be an impact of, if my memory serves me right, of

65 mph + 65 mph = 130 mph or ~210 kmh

If my math is correct this would be equal to

210000 * 0.68 = 3966 N

After that I'm not sure where to go... If I was considering using a PVC pipe, would I be able to relate those Newtons to the tensile strength of the pipe to figure out if it would hold up?

I had also thought about 3D printing the bracket but a friend pointed out that it's hard to make measurements with PLA because the printer layers the material so the "resistance" or "strength" is different in different directions.

I am also open to other material suggestions! Thank you.

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    $\begingroup$ you forgot 1km = 1000m. so the correct answer is 39669N momentum. $\endgroup$ – kamran Jan 28 at 19:55
  • $\begingroup$ can I just convert those Newtons to pounds and use the result to check whether the tensile strength of my PVC pipie is enough? $\endgroup$ – Goofynose Jan 28 at 20:03
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    $\begingroup$ Just attach it behind the driver seat. It is already designed for impact. $\endgroup$ – kamran Jan 28 at 20:05
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    $\begingroup$ You should check for any relevant car safety codes, which might require resisting a given load regardless of how much your extinguisher actually weighs. Also, you don't need to add the vehicle speeds in a frontal collision, just use the highest of the speeds. Due to Newton's Third Law (equal and opposite reaction), hitting a car at the same speed as your own in the opposite direction is identical to hitting a solid wall (losses in energy due to vehicular deformation non-withstanding). $\endgroup$ – Wasabi Jan 29 at 3:03
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I don't know what you thought you were doing multiplying $210000 \times 0.68$ but it certainly doesn't give a force in Newtons.

Measured crash data shows the maximum deceleration is of the order of $50g$. In other words your 1.5lb object has a force of $1.5\times50 = 75$ lb acting on it.

The simplest way to restrain it is to realise that in a frontal crash it will be trying to accelerate forwards, relative to the vehicle. Therefore if you mount it on something where it can't move forwards, such as the front of the footwell, the rear of a seat, or even inside the glove box, you don't have much to worry about.

Note: the reference was based on crashes at lower speeds, but that is not a critical issue. If the crash is enough to crumple the front of the car by a significant amount, the maximum force and acceleration will be more or less independent of the speed, but the duration of the impact and the total amount of deformation will increase. There is not much point worrying about the state of the fire extinguisher, if a high speed frontal crash deforms the car so much that you are trapped in the seat with the steering wheel pushed against your chest, and you can't reach the extinguisher to operate it even if you are still conscious!

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  • $\begingroup$ Out of curiousity, where did I go wrong with my calculations? Isn't F = ma correct? $\endgroup$ – Goofynose Jan 29 at 4:24
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    $\begingroup$ @Goofynose: If that's meant to be 210000m/h, that's a speed, not an acceleration (the $a$ in $F=ma$). The relevant acceleration is that speed (which has to be in m/s, not m/h, for the result to come out in Newtons) divided by the time it takes for the vehicle to come to a stop, which is very complicated to determine accurately. $\endgroup$ – Wasabi Jan 29 at 18:16
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A couple thoughts here:

Almost nothing will hold the bracket (and extinguisher) in case of a hard crash unless the fire extinguisher & bracket are mounted flat to a surface parallel to the firewall, i.e. perpendicular to front-back axis. The last thing you want is a projectile, so start with this requirement.

Then, a reasonably thick (say 8 mm) 3D-printed bracket made from ABS (or nylon if you have a printer that can handle that temperature) is the quickest and easiest way to go. Yes, the strength depends on layer orientation, but if you print it 90 degrees from the installation orientation, so that the layers are in the X-Z plane when installed, that should suffice. IANAME (mech Eng) so do not treat this as a guarantee.

Now that I think a bit, I would recommend nylon strapping with quick-release buckles. That's cheap, incredibly strong, and takes no time at all to install.

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