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Posted By : Adrian Lester, 07-Mar-2005, 04:14pm
I have been meaning to post this for some time, here it is at last. A cautionary tale as to the use of GRP, or other FRP bonnets on spitfires. This one was
bought from a large manufacturer of such items. The standard of the fibreglass lay-up and gel coat seemed pretty good, however the ability of the bonnet to
represent the one it was replacing has been somewhat dissapointing. I will now describe the attached photographs and what they show.
Top left: Reinforcement added by myself because there was so little rigidity in the bonnet, the GRP web formed over pipe insulation which runs across the
width seemed like a good idea at the time rather than using the standard stiffening bar from the old bonnet. Unfortunately the edges show on the outside of
the panel, my mistake and a better designed web might well solve it. Note also the prop (again glass on pipe insulation) between the wheel arch and bonnet;
this I deemed necessary to transmit the load from the bonnet stay because of the extremely weak join between the wheel arch and bonnet.
Top Right: See the wooden block between the wheel arch and hinge tube. Need I say more? This was a result of the wheel arches being nothing like the same
shape as the originals. You can also see a crack running forward from the rear edge of the wheel arch, this is a consequence of an interaction with the
wheel when turning, despite my already having cut the arches back in order to let the bonnet close! They have now been cut back more.
Bottom Left: Ooh lovely. You can see from the quality of workmanship that I was running out of patience by this point. Beefing up around the weak and
pathetic support points. The plywood provides a nice stiff and level place to bolt through.
Bottom right: Yes it's meant to be a narrow little picture. You are looking at the inside of a wheel arch and the elegant bit of angle-grinder work needed
to let the hinge tube through it. Necessary again because of the fact that the GRP arches are nothing like the steel originals.
This is by no means an exhaustive list of the modifications and bodges I have had to make, from the outside it now looks OK from about 10 yards away. Some
FRP bonnets may be fine well made items but others (mine included) are certainly not. If you have found this message because you are thinking of buying one
be very careful what you are getting and try to send it back where it came from if it doesn't measure up right on delivery. The 4 days it took me to
achieve these rather disappointing results were really not worth it.
Adrian.
*****
Glass Fibre Bonnets -- A warning
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Posted By : John Davies, 07-Mar-2005,
08:15pm
Adrian,
I'm sorry that you were so dissatisfied with the GRP bonnet you bought - have you taken your criticisms to the manufacturers?
As a user of, and builder with GRP, I know that the properties of steel and GRP are so different that they can't satisfy the needs of all. Steel is
stiff and heavy, while GRP is springy, light and very cheap. Making a GRP copy as strong and stiff in all respects as the steel original will incur an
unacceptable penalty in weight and cost to the market aimed at by the GRP manufacturers.
I know of one fully GRP bodied Spitfire that is as heavy as a steel one, because the builder, like you perhaps, was determined not to compromise on
strength. He was quite happy with the result, because he wanted a rust free Spitfire, another GRP virtue!
I built my SilverBack Vitesse with as many GRP panels as I could - only the bulkhead and tub are still steel. They weigh about one third of the
original metal and by careful design they are almost as stiff - in the case of the roof, many times stiffer! Composite construction using honeycomb
achieves this. But I did buy a commercial bonnet, and it is not a perfect shape nor as light as it could be - one day I shall make a bonnet mould and
try to do better!
John
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Posted By : Jonathan Binnington,
07-Mar-2005, 10:17pm
Adrian, I also used a GRP bonnet on the supersix and had to make a number of post-purchase modifications to finish the bonnet, they may be visible on
the website below
I too observed that the fit of the glassfiber bonnet as bought wasn't as good as the steel ones i have used before (although the price was less than
1/4 of steel ones) but with some careful thought and careful work I achieved a fit which was far better than it was straight out of the mould, perhaps
as good as a steel one now
I think it is a matter of paying yer money and... although, of course, i have not had experience of evry product from every mabufacturer
just my threehapennyworth
binman
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Posted By : Nigel Gibbins,
08-Mar-2005, 11:33am
To be perfectly honest GRP panels are always a poor fit (ish), as John has previously stated.
But you should also consider that even metal panels are a poor fit too (even bonnets), but they can be (and are) 'fitted' with a set of body
forming tools. (effectively bent into the correct shape). Of course you can't do that with GRP.
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Posted By : Jonathan Binnington,
10-Mar-2005, 10:16pm
adrian, you also need to include th eother bonnet tube from the steel bonnet and add a few webs and gussets
bin