Got some damage in the braking surface of my rear carbon clincher rim on my road bike. I first noticed it about a year ago, when it looked like this:

[url=https://flic.kr/p/2gjKBae]Untitled[/url] by njee20, on Flickr

I’ve done about 2,000 or so miles on it since (it’s probably done 5k in total), it now looks like this:

[url=https://flic.kr/p/2gjKfa1]Untitled[/url] by njee20, on Flickr

You can get a finger nail under the fibres and it appears there’s a small void (possibly from whatever impact originally caused the damage) and then some more material behind.

No noticeable change in braking, it doesn’t make any noise or anything. I assume the degradation is purely from the wear. As you can see it’s basically smack in the middle of the brake pad contact patch. My worry is that the hole may get bigger until the brake pad ‘grabs’ on the edge, at which point things could go south rapidly.

Thoughts:

A) holy shitballs, how are you still riding that, you maniac, replace it now!
B) that doesn’t look great, maybe a bit of Araldite on the area sanded down, and order a new rim sometime soon enough
C) that’s got another 10k miles in it, easily

There’s a fair bit of material on the braking surface, but I’d sooner not have some explosive failure of the rim really.

It’s only the rear. What’s the worst that can happen? Big skid? 😉

If it was a front I’d be binning it straight away.

The brake pad has worn a pretty deep groove in that rim. I’d fill the hole with Araldite and get looking for a new rim or wheel.

It’s only the rear. What’s the worst that can happen? Big skid? 😉

If it was a front I’d be binning it straight away.

That was a significant part of my thinking frankly! 😳

Also leaning toward the Araldite thing, there’s a definite groove as you say, circa 2″ around.

+1 araldite.

I’d continue riding until evidence of a similar hole starts appearing somewhere else – at which point presumably the entire brake track is wafer thin 😀

Perfect excuse for a new bike perhaps with discs?

I’ve run alu rims till they wear through and burst.. the bang is normally from the tube going, and both times I avoided being killed to death, but it was a bit buttock-clenching. Completed the ride both times too, tho had to disconnect the relevant brake.

I dunno if carbon ones would go the same way, or worse though. I think the risk that it might be worse would have me replacing it. And TBH I’d rather avoid it happening again on alu rims too. You’d think i’d have learnt my lesson the first time…

Perfect excuse for a new bike perhaps with discs?

Half the reason I CBA to replace it is that yes, I am continually flirting with replacing the bike with something with discs!

I think the risk that it might be worse would have me replacing it. And TBH I’d rather avoid it happening again on alu rims too. You’d think i’d have learnt my lesson the first time…

Mmmm, I imagine carbon shards to the heart and all that. I don’t think the rim is about to explode catastrophically. But that’s what they want you to think.

The rim won’t explode of it’s own accord, but remember there’s 100psi of pressure behind it, pushing out all the time.
I’d araldite that and use it in the mean time, but I’d also be looking for something to replace it . . .quite soon

Obviously I’m nu-school in my thinking, so there’s only 85psi pushing out on it. But yes… agreed.

Whilst i would probably look to repair it I have seen that repairs to carbon rims and in particular the braking area is not recommended. Even if you use a resin that can cope with high temperatures it is likely to fail again due to temperatures experienced in use.

http://www.carboncyclerepairs.co.uk/your-questions_4.html

so there’s only 85psi

And the additional pressure created by rider weight and later on, heat transfer.    Still, I wouldn’t worry 😀

I’m going to go against the grain and say that there is no way I’d be using that.

There’s a hole in my carbon braking surface, dear Liza, dear Liza…

Then why don’t you fix it dear njee, dear njee?

if you have water, a stone, a knife and some straw it should be an easy fix. Assuming your bucket is in better shape than your wheel…

I can’t imagine having much fun riding my bike knowing that was there. I’d be putting a spare wheel in there until I could get a replacement.

My worry with araldite, or any epoxy is they cure with temperature, so what would make a perfectly good repair at ambient temperatures might go a bit soft at braking temperatures (then cure and go even harder, but also shrink).

I’d swap it for a new rim, then keep the old one for building onto a disk wheel.

My worry with araldite, or any epoxy is they cure with temperature, so what would make a perfectly good repair at ambient temperatures might go a bit soft at braking temperatures (then cure and go even harder, but also shrink).

I’m normally the first one to recommend a bit of Araldite but I wouldn’t go for it on the braking surface of a wheel.

I don’t think it would have the adhesion or abrasion resistance. I’ve never actually looked at it but I guess the Tg of the standard ones is around 60C. Under the normal braking you’d hit that and post cure it. I don’t think shrinkage would be noticeable but it would become more rigid and brittle. Its a common failure mode. Wheels aren’t really my area but I assume they are vacuum formed and heated to cure.

There will be technology out there that will work but it will be poorly researched and niche because the value is low and the consequences of failure high.

Maybe contact https://www.fibre-lyte.co.uk/ but I’d be thinking about a new rim.

Technical FAQ: Resurfacing carbon brake tracks, disc/caliper combos, and more

What about some black Milliput? That stuff sets super hard!

Black epoxy repair filler is the stuff you want – used it loads of times to repair and bond carbon – it dries black and can be polished smooth. I’ve repaired numerous dinks in carbon rims – you can’t even see the repairs – leave it proud and rub down with wet’n’dry

Black epoxy repair filler is the stuff you want – used it loads of times to repair and bond carbon – it dries black and can be polished smooth. I’ve repaired numerous dinks in carbon rims – you can’t even see the repairs – leave it proud and rub down with wet’n’dry

That stuff is largely just cosmetic though. The issue is either “will it explode because its weak” or “will the temperature when braking have an effect on the epoxy”.

Then why don’t you fix it dear njee, dear njee?

if you have water, a stone, a knife and some straw it should be an easy fix. Assuming your bucket is in better shape than your wheel…

Amazeballs, well done!

Interesting thoughts on the temperature of Araldite curing being an issue TINAS, hadn’t really considered that. Would any sort of shrinkage actually do more damage though, or just render the ‘repair’ ineffective? I can see that if it became soft that could add some ‘fun’ to proceedings.

Stevious – it’s been there in some form for over a year, but yes it did play on my mind a bit as a hauled on at 40mph on unfamiliar roads at the weekend :s

Jonba – great insight, thank you. It’s a FarSports (Chinese) rim, so I imagine even consulting Fiber-Lyte would be more expensive than a replacement! Great bunch of guys though.

Dunno, my only experience of watching epoxy shrink is when people leave black dinghies upturned in the sunshine, you end up seeing the mast steps poking through the bottom of the hull as the hull shrinks around the internal structure.

I’d just do a cometic repair and demote it to disk only or a track bike or something.

Having said that, if it does go, I’d imagine it’s going to go around the circumference, just going to be like the tyre letting go suddenly. Probably just a code brown moment, unless you were braking into a sharp corner then it might hurt a smidgin as you fly off box hill in a ball of flames, with splintering carbon fibre shards killing any baby robins in the area.

I’d not be riding with that – I’d be too worried about the consequences if it fails at high speed. Inagine you’re doing 40mph ish down a big hill and the brake pads goes through the rim – doesn’t bear thinking about.

You definitely need a new bike, preferably one with electronic shifting and hydraulic disc brakes….

Probably just a code brown moment, unless you were braking into a sharp corner then it might hurt a smidgin as you fly off box hill in a ball of flames, with splintering carbon fibre shards killing any baby robins in the area.

Pfft, there’s a huge baby robin overpopulation issue around Boxhill anyway, I’d be helping.

Inagine you’re doing 40mph ish down a big hill and the brake pads goes through the rim – doesn’t bear thinking about.

Yes, I did imagine exactly that at the weekend. It wasn’t much fun, although it was only a passing thought.

You definitely need a new bike, preferably one with electronic shifting and hydraulic disc brakes….

This one’s got electronic shifting, and I’ve already got one with electronic shifting and disc brakes. Dead up to date me. But yes, a nicer one with discs and Di2 seems the obvious longer term solution.

I popped in to see Ben at Sharp Precision Wheels with it earlier, because I pass them on the way to take mini-njee to nursery. 1) top bloke, never met him, but he was totally happy to chat and proffer an opinion. 2) he thought it was probably a void in the rim where there was an excess of resin, and given it hasn’t got that much worse in a year he said he’d not have any immediate concerns about. 3) he sent me links to a load of rims he’d recommend, or said if I procured another FarSports rim he’d happily build it. Absolutely no connection to them, but was really impressed with that; actively told me not to spend money!

I don’t think it would have the adhesion or abrasion resistance

e issue is either “will it explode because its weak” or “will the temperature when braking have an effect on the epoxy”.

It’ll be stronger than fresh air. I would repair and keep an eye on it.

That stuff is largely just cosmetic though. The issue is either “will it explode because its weak” or “will the temperature when braking have an effect on the epoxy”.

It’s an epoxy with carbon filler – structurally no different to how the wheels are made and far better than just leaving it or using an epoxy only. I’ve been using this to successfully repair carbon wheels for 8 years.

Care to share you experience?

Mmm, that stuff does look interesting, as Al said it’s stronger than fresh air if nothing else, the only conceivable risk I can think of is that it gets hot, melts and then things get all sticky, causing more problems, which was what TINAS mentioned to be fair.

In the old days before disc brakes I had two mountain bike rims burst open on me but with low pressures all that happened was that the inner tube gets out and wraps itself around the stays like an anaconda. I did once see a picture of somebody with the broken-out sliver of aluminium stabbed through his calf muscle though.

That carbon rim, being worn equally, will probably let go one day with a ripping sound as you are pumping the tyre up.

I have to say there’s no way I’d ride that. Maybe it me being overly cautious but that’d be binned if it was mine.

Can I have your bin? Bet it’s full of potentially salvageable parts for those of us with more lackadaisical approaches to personal safety!

If it was the front I’d definitely be more keen to change it, or if I rode in the Alps. But for the flat lands of Sussex I don’t think death (or explodey sidewall calf stabbing) is imminent.

I’d be tempted to agree with your friendly wheelbuilder.

It appears only to be the outer layer of carbon that’d affected. One would hope that there would be some wear allowance on any rim braked wheel, so at least until that layer disappears it wouldn’t seem unreasonable to assume you’ll be fine. Most composite components (not wheels) have a sacrificial layer of carbon or glassfibre on the outside to handle normal wear and tear anyway.

To be safe, you could pump it up to 150psi and heat it with a blowtorch to simulate an alpine descent. Film it for us, would you? 😉

It’s an epoxy with carbon filler – structurally no different to how the wheels are made

Quite a bit different actually. A significant part of the benefit of carbon comes from the fact the fibres are directional – even with a woven rather than UD you’re not bothering to deal with through thickness stresses that should be low, thus making it stronger and/or lighter for dealing with the in plane and bending stresses it’s designed for. (you can get much more fancy than that, but paste with some carbon in doesn’t do the same as even a basic knocked up in the garage carbon weave component.)

@njee20 After reading one of your posts last year about wheel building I bought a set of dt Swiss 411 DB rims on tune hubs and my friend built an exact copy of your wheels. They have both been bomb proof, light and stiff and set up tubeless.

Ditch the dodgy Chinese carbon rims and build the hub onto a 411.

The 411s were fine, but they’re nothing special. These are lighter, stiffer, more aero and IMO look better. I was totally underwhelmed with road tubeless too. I commute into Central London on these, and they’ve picked up some damage somewhere in the 3 and a bit years I’ve had them, which aside from aesthetics has had absolutely no discernible effect to date. Not sure I’d call them dodgy really, and rebuilding on 411s would be a huge retrograde step IMO. Glad you like yours though!

@njee20 I’ve seen your commuter bike so I understand 😁

Haha, the 411s were for the commuter bike (which I took to the Alps), they were superb for that, but these are definitely nicer, I approve of the Tune hubs though – that’s what these are built onto!

So there are epoxy repair resins that would be better than araldite. In theory there are other materials that might also be better. Polyesters spring to mind. If you are happy that the rim will not explode than a repair might prevent it worsening. My concern would be a belief that you are repairing it back to orginal strength.

Gurit do one but it is commercial – you’d probably have to buy a lot and it would be expensive. A bit of time on google and you might come up with something?

Sika similarly do a 2k crack filler that I think is epoxy based.

Araldite/Huntsman might do some – They have a huge range of products but I’m not sure what is available retail.

Loctite would be the other company.

Boat repair companies would be a good place to look.

Nah, under no illusion that it’s ultimately under palliative care, and I’ll replace it (or the bike) at some indeterminate point in the future! Also keeping an eye on it.

I did look at replacement rims, buying one is so bloody expensive though, $40 shipping and the fees are virtually the same (or at least not proportional) versus $45 shipping on a pair! Tempted to get another pair of rims for “next time” as well!