Carbon rim repair

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fatbikephil
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Carbon rim repair

Post by fatbikephil »

Fixing own gear, rather than making it....

So after my jaunt the other week, my mate noted a large gouge in the back rim. On closer inspection there was a rough hole about 3mm across punched through the rim wall, in the middle of the gouge. I've no idea what caused this - I certainly wasn't aware of smacking it off a rock. I think it may have been a rock pinging up off the tyre and hitting the rim. This happens often but I guess in this case the rock had a particularly jaggy bit which did the damage. Slightly frustrating as I've used these rims on a lot of very hard terrain and apart from lots of scratches, nothing like this has happened before. I guess it's the trade off for using carbon fiber. It's light but more damage prone than aluminium.

The rim wall is very thin here - you could flex it with your thumb either side of the hole. We plugged it with epoxy for the ride home but my mate (who fixes canoes and kayaks for a living and uses carbon fibre a lot) suggested doing a patch repair to be on the safe side. He kindly gave me some carbon sheet and some epoxy resin, as well as advice.

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The damaged area filled with epoxy. I decided to leave this as is.

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Remove a couple of spokes, clean the area up and give it a good going over with emery paper to roughen it up and allow the epoxy to key to it.

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This is where it gets tricky. The carbon woven sheet is tricky to deal with as it comes un-ravelled very easily. To cut it you need finely serrated (and very sharp) scissors. I used my Wengen Swiss Army knife scissors and carefully cut three sheets of increasing size. You then mix the epoxy, which is very runny, unlike Araldite, and paint on the area to be patched. Lay the first sheet on carefully and then dab it down with a plastic spoon until the resin has soaked right through. Then dribble more resin on, add the next patch, repeat.

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The sheets and resin need to be pressed firmly down with no air bubbles. I used sheets of cling film at first stretched over the patch and then wrapped round the rim. It all moves around a fair bit, but this is fine. The keen eye amongst you will note the patch extends over two spoke holes. This is probably well OTT but it weighs very little so I consider this to be a belt and braces approach.

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This was the clever bit - wrap with insulating tape as tightly as you can. Excess resin was squeezed along and out the end as you did this. This is wiped up, along with some that had run into the rim bead. You then leave it for 48 hours.

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Quite pleased how this came out.

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Smoothed down with course to fine emery paper. I'd cut the excess carbon sheet away from the edge of the rim but after sanding this was a bit ragged. So I painted over with glass fibre resin as a gel coat, then smoothed some more, followed by a light over spray of clear lacquer.

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The finished article. Re-drilling the spoke holes was slightly stressful but it went OK. Its a bit rough but I'm not bothered about that. time will tell if it works!
redefined_cycles
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Re: Carbon rim repair

Post by redefined_cycles »

Looking forward to seeing how this goes Phil. Hope you stay safe mate. Mad idea, love it.
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gecko76
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Re: Carbon rim repair

Post by gecko76 »

Nicely done. Top tip: wrap the electrical tape sticky side out so it comes off more easily. Maybe you didn't have that problem though. Looks great.
Lazarus
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Re: Carbon rim repair

Post by Lazarus »

Disappointingly no use of a spoon
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Bearlegged
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Re: Carbon rim repair

Post by Bearlegged »

Nice job Phil, always and good to see repairs over replacement. 👍
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fatbikephil
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Re: Carbon rim repair

Post by fatbikephil »

Lazarus wrote: Tue Sep 17, 2024 2:15 pm Disappointingly no use of a spoon
Oh contraire - I was ruminating on what to use to dab the resin into the sheet and found an old disposable plastic spoon in the draw - this worked a treat as well as for dribbling resin over the next layer :-bd

It'll be fine Shaff, hence the huge patch over a small hole.
gecko76 wrote: Tue Sep 17, 2024 1:23 pm Nicely done. Top tip: wrap the electrical tape sticky side out so it comes off more easily. Maybe you didn't have that problem though. Looks great.
My mate recommended the cling film as it doesn't stick to the resin. The insulating tape was cheap rubbish so it pealed off no probs!
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gecko76
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Re: Carbon rim repair

Post by gecko76 »

Aha, tape over cling film, got you now. Will remember that.
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thenorthwind
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Re: Carbon rim repair

Post by thenorthwind »

Nice work Phil :-bd Though slightly nervous since I built a new carbon rim into my rear wheel after your suggestion that it would be more durable as well as lighter!
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fatbikephil
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Re: Carbon rim repair

Post by fatbikephil »

It was Alpinum who convinced me so we can blame him :grin:

To be fair I've ridden a lot of rocky trails since I fitted these rims and apart from lots of scratches they have been fine despite many, many rock / rim strikes...
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Bearbonesnorm
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Re: Carbon rim repair

Post by Bearbonesnorm »

Nice work Phil, The great thing about carbon is that it requires very little resin - unlike glass. :-bd
May the bridges you burn light your way
substandard
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Re: Carbon rim repair

Post by substandard »

good work!
Having repaired a few carbon frames, the resin somehow doesnt stick to the tape so no need for the clingfilm,
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