crash box of vehicle

[m.piron]

I'm studying the bumper crashbox of vehicle, especially for formula SAE (where I find lots of information), and there are various design: square box, triangle, cone, foam, honeycomb...
But I see that the australian bull bar, for this purpose, are bolted to the chassis in slotted holes, so during crash the bull bar could slide along these holes.
I know that the crash box must be done to ensure a constant force during deformation. This constant force is easy to obtain using the "sliding bolts", because it depends on the tightening force and friction coefficient.
So, why these "sliding bolts" are not used, but will used deformable box of various type?

Thanks

 

[MintJulep]

On paper, the sliding bolts look good because the simplistic assumptions make the formula so easy.

In the real world:

Tightening force is variable because:
It is difficult to control accurately during manufacture.
It will change over time from relaxation, creep and thermal expansion.
It can be changed by vehicle maintainers.

Friction is variable because:
It depends on many factors of all of the surfaces involved.
It can change over time due to corrosion and contamination.

 

[GregLocock]

As MintJulep says. In practice sliding joints are used as part of the crash, but more as a breakaway feature than to generate force or absorb energy. Typically the rear subframe mounting bolts are a sliding joint, so when the front of the engine hits the barrier it can slide back relative to the body. Bull bars fit to slotted holes because of tolerances, not crash.

Cheers

Greg Locock


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[m.piron]

Thanks for your answer.
So, using deformable parts, you can ensure that the behaviour will remain the same.

Bull bars fit to slotted holes because of tolerances, not crash.

Maybe you're right, because I'm not mother tongue english so I don't understand completely, but in this video looks like the bolts are used to absorb energy during crash:

https://www.youtube.com/watch?v=i2foHYCS92w

 

[TugboatEng]

Sales video so take everything with a grain of salt. There is lots of deformation to absorb energy going on in the video. Imagine the bolts in the slotted holes have lock nuts and aren't actually right. This will help you picture the system better.

 

[gruntguru]

Yes those slotted holes are obviously a deliberate design to absorb energy. You can easily estimate how much energy from bolt clamping force, friction coefficient in the sliding joint and length of the slot.

Yeilding steel tube or box section absorbs a lot more energy than can be easily obtained with friction.

je suis charlie

 

[GregLocock]

Direct quote from the video, "the sliding joint is there to reduce force". This is so that in a minor collision the stiffness of the bar does not cause a spike in force sufficient to set the airbag off.

Cheers

Greg Locock


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