Carbon Fibre repairs

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Tinkley
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Carbon Fibre repairs

Post by Tinkley »

Hi fellow Bx'ers.
I am probably going to have to do some fairly serious repairs around the rear sub frame and sill on the ns. I don't have the welding gear thoogh I have done silver soldering (for bandsaw blades etc) and brazing before. However I do have the gear for carbon fibre/woven glass fibre with structural epoxies to do quite a range of boat repairs etc. Is it OK to replace the steel with this type of material. It is more than strong enough and does not rust!
I'm not sure the MOT tester would like it though all painted up you would not know. Do you feel it would be OK to use this as a repair method? Also is it legal?
With a foam/timber/ply core it could be made many times stiffer than teh original where space is not an issue.
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mat_fenwick
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Re: Carbon Fibre repairs

Post by mat_fenwick »

Only welded repairs are acceptable for the MOT in structural areas, regardless of whether the repair is technically strong enough. I assume that an inspector can be reasonably expected to judge the strength of a welded repair visually, but not necessarily for an alternative repair.

The other consideration would be bond strength to the metal, which although may be OK initially, could deteriorate repidly if the metal started rusting. Plus energy absorbtion in an accident...
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Vanny
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Re: Carbon Fibre repairs

Post by Vanny »

carbon fibre reinforced epoxy might be strong on one direction, but it is brittle to hell and gives no energy absorption properties. You only need to see an F1 car in a crash to see the inevitable results. And F1 cars have a composite 'tub' to absorb energy and protect the driver because the CF isn't up to the job.

The epoxy generally won't just 'bond' sufficiently to metal so you have to make inserts that then bolt into the metal.

I like the idea though, and certainly where weight is an issue it is heavily utilised in a lot of industries. I guess if the car had a tubular crash structure underneath it might be safer to do?
Tinkley
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Re: Carbon Fibre repairs

Post by Tinkley »

Thanks for the heads up. Pity 'welding only acceptable' for the MOT.

Actually the strength of carbon depends on its fibre orientation and there are many different designs of cloth and tapes. Some are not uni material, so a carbon/kevlar/diolen(glass) will have some of the properties of all three materials. I had in mind a 6k plain weave and maybe twill (bias stretch) 3k material. When you see a strut break in F1 it's most likely a unidrectional material hence a side blow will snap it relatively easily. However the main chassis seem much stronger and most likely a plain weave, with very complex compound areas being from a twill cloth. I've made quite few acoustic guitars from a laminate 1mm thick bowlback using a twill cloth. Just try breaking a 3 or 6mm strip of plain weave carbon or even better a 15mm balsa or polyurethane cored piece.
They used to leave them bare so you could see the weave giving away the construction. Now, despite a slight weight penalty, they paint them, probably for better UV protection as well as getting the sponsors names on them.

As for epoxy to steel bonding, I'm not at all worried. If the area is rust free and fully clean there is no reason not to achieve a 100% good bond. The trick is to use 80 grit emery to grind the epoxy into the steel surface and then lightly heat with a hair drier to exclude any air bubbles. Don't forget the 'primer' on most cars is an epoxy. I've repainted a barrel on a m/cycle with epoxy then silver and it was better than the factory finish! (the barrel was recoated with nicasil plating as it had worn). All 2k paints.

Trouble is so few MOT stations would even understand the strength of these materials even if you gave them all the figures and samples. Boeng 787 anyone...

Personally even a timber and plywood car would not bother me especially if glass or carbon sheathed and reinforced. Remember the Africar with a GS engine?
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Re: Carbon Fibre repairs

Post by mds141 »

I remember the Africar very well indeed. In fact the factory where it was built, is 10 mins away from my house!!
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Re: Carbon Fibre repairs

Post by citsncycles »

Personally even a timber and plywood car would not bother me especially if glass or carbon sheathed and reinforced. Remember the Africar with a GS engine?
Or the Marcos with a GRP body bonded to a plywood chassis.

While still sat on a steel chassis, my Falcon's bodyshell is about 80% plywood, with a thin ally cladding for cosmetic purposes only, and a GRP nosecone, bonnet, scuttle top and rear corners.

There was an Africar for sale locally a few years back - another I should have bought!
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Re: Carbon Fibre repairs

Post by mds141 »

And talking of 'Africar's', funnily enough there is a small article on them in this weeks Autocar, on page 19.
Mark Smith

Is it just me or is everything shit?

1989 BX GTi 16 valve. Blanc Alpine. Completed the Citroen Classic Challenge Ecosse and 1337 miles without a hitch.
2000 XM VSX 2.1 td Auto. Rouge Magenta.
TGD saloon many years ago.

1990 Swift 'Corniche' 12/2 aka BXClub HQ.
Honda Firestorm. Gone, but not forgotten.
2015 Triumph Tiger Explorer XC.
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