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designing a GRP covered foam crash absorbing structure
smart51 - 11/12/08 at 12:56 PM

I want to design a foam cored crash absorbing structure for the front of my new car. How do I go about it? There's probably a better way to do it than cover a square block in GRP and bolt it on. Does anyone have any hints / tips / links that might be useful?

twybrow - 11/12/08 at 01:05 PM

Think in cones of increasing wall thickness... A certain well known McMerc uses that to great effect.

Try this link....

Mr Whippy - 11/12/08 at 01:06 PM

The drawings you've done look fine so I'd not change it and tbh I see nothing wrong with just carving a single block of foam. Possibly using a hot wire to do the big initial trimming then a flapper wheel to get the ruff shape then finish off by hand. The light cut outs at the back might be a bit tricky though. I'd glass the back of the panel too so as to stop it splitting in an impact and to help spread the load across the chassis. It’s coming on well

Volvorsport - 11/12/08 at 01:19 PM

a big thick aluminium radiator .

or some honeycomb .

Mr Whippy - 11/12/08 at 01:30 PM

quote:
Originally posted by Volvorsport
a big thick aluminium radiator .

or some honeycomb .


good idea, he could bond that inside the foam to help prevent sharp structures from another vehicle penetrating the foam core. They can take a huge beating to get through them as I’ve tried before with sledge hammers and picks, just seem to get stronger the most they are squashed

balidey - 11/12/08 at 01:31 PM

multi cells is one answer.

If you want it 300mm long, then instead of one block 300mm long with grp around it, break it down into 5 or so blocks, each 60mm thick, each inside grp skins, stuck together.

smart51 - 11/12/08 at 01:36 PM

quote:
Originally posted by Mr Whippy
The drawings you've done look fine so I'd not change it and tbh I see nothing wrong with just carving a single block of foam.


The design has a big empty space under the headlights, inside the front bumper and infront of the chassis to be filled with a "crash absorbing structure". I'll be starting the front bumper soon so need to know what to do with the crash.

quote:
It’s coming on well


why thankyou. Glass over foam bodywork is proving to be a real PITA but it is coming along slowly.

Mr Whippy - 11/12/08 at 02:01 PM

oh I've been following it quite closely as I'm going to make a new bodyshell for the falcon using the same method. What's the problem you’re coming across? is it getting a smooth finish? I was going to use a soft filler mixed with microballs then once sanded smooth roll on some finishing resin to make the filler hard. I’ve use that to make foam core wings for some r/c planes and got a good finish

have a look on some of the web site for large model boats and planes as this is the most common method of construction these days.

[Edited on 11/12/08 by Mr Whippy]

02GF74 - 11/12/08 at 05:15 PM

carvingg foam??? waht about the stuff from a can (no more gaps?) or you mix in a bucket?

when I was playing with this stuff in the bathroom, once set, it looks and feels like hte stuff you see under the plastic covering of tintop bumpers.

smart51 - 11/12/08 at 06:37 PM

quote:
Originally posted by 02GF74
carvingg foam??? waht about the stuff from a can (no more gaps?) or you mix in a bucket?

when I was playing with this stuff in the bathroom, once set, it looks and feels like hte stuff you see under the plastic covering of tintop bumpers.


More or less the same stuff, only pre made in 8x4 sheets.

Mr Whippy - 11/12/08 at 07:15 PM

quote:
Originally posted by 02GF74
carvingg foam??? waht about the stuff from a can (no more gaps?) or you mix in a bucket?

when I was playing with this stuff in the bathroom, once set, it looks and feels like hte stuff you see under the plastic covering of tintop bumpers.


yip though its not remotely UV stable

prob is you need a mould to pour it into and it does generate a lot of internal pressure as it expands which can distort even strong moulds. It tends to be used more as emergency buoyancy in boats, especially plywood hulls and it does eventually (like expander white polystyrene) absorb water