The search function on here isn't helping me much so forgive me for asking a question that may have been asked before.
What is the thinnest GRP layup that I can get away with for car body panels? Someone I work with suggested 300 GSM plus surface tissue and gelcoad is
enough. I want the body to support its own weight and not be punctured by stones (or people) but be as light as possible. I won't be using a
mould, so no gelcoat. Instead I'll make a buck out of foam and glass over it, with filler and paint on top. The foam will be removed from the
inside, apart from thin strips that will be glassed over for stiffening.
Tbh dude I struggle to see the reason why you want to remove the foam as leaving it will add greatly to the strength of the body and allow you to use a very thin glass cloth and the weight of foam is so little. I have many model planes built this way and their strength is quite incredible, without the foam core they would be flimsy useless structures.
PU foam weighs, I believe 42kg per cubic metre. A square metre of foam 50mm thick (residual thicknes from a 75mm block sanded to shape) is going to
be 2.1kg. A 300 gsm fibreglass skin will weigh 0.9kg on one side, giving a total weight of 3kg. A layup of 900g CSM and resin would probably be
2.4kg without foam and would be tougher. I want the lightest skin possible that will be strong enough for the job.
I would use ceconite cloth and dope as used on small aircraft but I want nice compound curves and taughtened cloth gives straight lines on at least
one axis. It is very light though, 100 gsm plus dope and paint.
You do gain though by making the GRP thick. Core matt is quite a good idea in this situation. If you make the buck out of foam won't you have to use epoxy???? I think polyester resin eats polystyrene??
On paper I'm sure removing the foam sounds the way to go but in practice doing to will leave your shell like a flimsy balloon plus it will be like a noisy drum. If you really want to minimise the weight make the core thinner where loading is low and then increase it around mounting points etc. I was going to use a flapper wheel on a grinder to blast away at the foam and carve it. A very blunt drill will allow you to drill holes through the foam (without damaging the outer skin) to insert depth gauge rods, I was going to use balsa for those. Once done just add the inner skin and there you have a perfect structure and one that’s safe in crashes.
quote:
Originally posted by BenB
If you make the buck out of foam won't you have to use epoxy???? I think polyester resin eats polystyrene??
quote:
Originally posted by BenB
If you make the buck out of foam won't you have to use epoxy???? I think polyester resin eats polystyrene??
Hmm interesting, just been looking through your photo's how are you going to remove the foam as you seem to be working from the inside out? I was
going to do exactly the opposite, basically make a big foam mould, sheath it, cut the doors and windows out then carve the inside foam before skinning
that too. In the end I'd have one huge crash helmet kind off thing that I could drop on to a backbone chassis.
[Edited on 4/8/08 by Mr Whippy]
The plan was to fasten foam blocks to a plywood box that sat around the car, with strategic joins so that the parts could be separated. Glass over
the foam to form a shell, remove part of the foam and glass in from behind to form right angles where the panel joins will be, remove almost all of
the rest of the foam and glass in more right angles, so that there is a 2mm fillet of foam between the right angles and cut out the individual panels.
Remove any remaining foam save for beads where I want box sections and glass over that.
There will be a separate nose cone, lower side on both sides, a rear wing - roof over the boot, a rear bumper skirt and a 1 piece windscreen / roof /
doors arrangement that pivots below the windscreen.
The whole idea of the project is to make the minimum amount of car that would serve as all weather transport. It is to be as light as possible then
as aerodynamic as possible, with the minimum practical room for 2 adults and a small quantity of luggage. With such a car, a small engine will be
sufficient and so the least quantity of fuel can be used whilst maintaining personal freedom from public transport.
I have make numerical computer models in excel and have found that, surprise, saving even more weight is beneficial. Sure, a heavier foam filled body
may be better in a number of ways. More is more after all. What I want is less. No, make that the least possible.
So, is a single skin of 300 gsm CSM (with strategic reinforcement) enough or do I need more? How much more?
Looking at the amount of work you are going to have to do, why do you not want to make moulds?
You are basically making a pattern anyway, and if you make moulds you can:
- Experiment with layups to get the optimum strength to mass ratio
- Make replacment panels if any are damaged
- Sell panel sets if you desire/any demand
Cheers
Fred W B
I would say that a single skin of 300 gram csm would be about the stiffness of thin card.......especially with no gel coat . It would allmost certainly sag and distort in between the reinforcing ribs , and need supporting from behind if you need to rub down for paint .
I expected so. 600 gsm then? 900?
Well.............my bonnet bulge is gel coat and two layers of 300 gram csm - and I am having trouble rubbing it down to spray it as it is just too
flexible........
I once made a drag-bike fairing from two layers of 300 gram woven cloth , and put the paint job in the gel coat - it wouldnt stand up on it's own
, but it was very light 
My usual construction for such things was normally one layer of 300 csm , one layer of 450 csm , and finish with 300 woven - light , but stiff enough
and still a bit flexible .
Thanks very interesting how your going about it, I suppose there is no one way. make sure you keep taking pictures as you go. I'm going to phone
Ultima tomorrow and get an exact price for a windscreen, think its about £400 as my car would be based round it. I need to have a look round the local
building sites for lonely blocks of blue foam
damn it sisters dogs are barking have too go