TGR-ECOSSE
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| posted on 31/5/08 at 09:42 AM |
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Roll cage bending in a crash?
I read this statement on another forum from somebody giving advice on rollcages for autograss cars and dont think it is quite right!
quote:
The purpose of a cage is to absorb energy in a collision. If a cage gets bent in a collision, - that's a good thing, because the energy which it
took to bend that bit of metal was energy which did not have to be absorbed by any other part of the structure.
I realy wouldn't want the cage to bend at all especially near the driver or during a multiple roll where the cage can get get
bounced off the same spot. Now i am no expert but i have built a few cages and rolled a few times and the cage has not "bent"
What do you lot think?
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iiyama
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| posted on 31/5/08 at 09:54 AM |
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If it bends to absorb energy then that would make it a deformable structure, or a crumple zone. Dont think Id want a cage to act as either of these!
If its broke, fix it. If it aint broke, take it apart and find out how it works!
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Hammerhead
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| posted on 31/5/08 at 10:00 AM |
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sounds dodgy advice to me.
I am pretty sure that each roll is unique and infinately variable, as you say what if you hit the same spot in a multiple roll?
The thing meant to save you could cause you the most damage.
Also why would you bother bracing the cage with triangulations if you want the bugger to bend?
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smart51
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| posted on 31/5/08 at 10:27 AM |
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I want my roll cage to stay the shape it is. It is there to stop your head hitting or being dragged along the ground.
Luckily, the 2" OD (ish) thick wall tube used in roll cages for sevens are made from stronger material than the 1" 16 SWG ERW tube used
for the chassis. In a bad crash, I'd expect the roll cage to be the least likely to bend.
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iank
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| posted on 31/5/08 at 10:27 AM |
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A cage in action would have to absorb some of the energy of the crash - it's inevitable (assuming you aren't deforming the ground
significantly) and bending will inevitably form part of that. I think the question is really one of the elastic limit of the steel - for most crashes
a well designed cage shouldn't reach that limit so will snap back and not be bent in the deformable structure sense of the word.
You'd be amazed at how much deformation goes on in the simplest thing if you drop it - but by the time you've picked them up they're
back to normal. I was involved with drop testing a mobile phone at one point and it's incredible how they flap like a birds wings when viewed
with high speed cameras as the bounce off a hard floor.
If the crash is big enough to overcome the elastic limit of the steel, I'd rather have a cage bend a little than snap/shatter which is the only
alternative. But I agree that a bend in a cage after a crash either means it's badly designed, or it was a really big crash.
[Edited on 31/5/08 by iank]
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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Liam
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| posted on 31/5/08 at 11:05 AM |
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To give him the benefit of the doubt, I dont think he's neccessarily advocating designing a roll cage to deform in a crash - he's just
saying that if one bends in a crash (and that'd have to be a damn serious crash!), then that's good as it's absorbing energy,
which as iank says is better than snapping/shattering! It's just his first statement that's wrong - the primary function of a roll cage
is not to absorb energy (that's the job of the rest of the car), it's to maintain the integrity and shape of the cabin to protect
the occupants - i.e. to not deform.
Liam
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Hammerhead
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| posted on 31/5/08 at 11:07 AM |
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quote:
Originally posted by iank
A cage in action would have to absorb some of the energy of the crash - it's inevitable (assuming you aren't deforming the ground
significantly) and bending will inevitably form part of that. [Edited on 31/5/08 by iank]
I'm not sure it's a case of one material having to deform significantly or the other. I think that deflection of the vehicle would be more
likely.
I agree that the steel should bend, ripple or resonate, as it is a property of the material and as you say you would have to photograph it with motion
capture to see it.
A good example is the advert where the car is dropped vertically to the ground, you can see the ripple of the bonnet, looks like water.
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mark chandler
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| posted on 31/5/08 at 03:45 PM |
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Having bent a couple of cages he is completely right, you cannot avoid this. Even on a gentle roll you will get some give.
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Jon Ison
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| posted on 31/5/08 at 04:32 PM |
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I think you would be very lucky, or involved in a very light roll if nothing (on the cage) "bent"
There as to come a point where bending is better then no bending surly, bit like a bridge, if they didnt flex they would snap in half ?
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The Baron
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| posted on 31/5/08 at 05:12 PM |
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I saw something on one of the sky channels about a year ago, and yes they are designed to absorb energy, the clever part is where & how. Which is
why you can’t just knock one up from some scaffold tubes and go racing.
Cheers,
B
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