Cycling UK Forum

Afternoon all, need a bit of help. I bought a carbon tt bike last year (guru cr 901) which was in excellent condition bar a confirmed paint crack below the seat.

I've recently noticed a small 1 inch crack on the frame, now I've only gone over 2 very small potholes in the last year, so other than that, I've not hit the bike or gone into anything likely to cause a crack.

Now the bike is white, and I'm lead to believe that white paint is applied thicker than other paints and is prone to cracks, being 10+ years old, I would have thought this could be the main reason.

Now, I've done a couple of coin taps on both the confirmed paint crack, and the new crack, and both sound the same. However, I've contacted a carbon repair company and they've said that on bikes, with monolithic laminates, it's sometimes difficult to establish if it is a crack in the carbon.

Anyway, just wanted to know, would it be ok to carry on using it, or should I get the necessary repairs done, I just don't want to be fleeced by the company telling me there are cracks when there aren't. Any advice much appreciated.

gary.mcevoy.75 wrote: ↑25 Sep 2024, 12:53pm Anyway, just wanted to know, would it be ok to carry on using it, or should I get the necessary repairs done, I just don't want to be fleeced by the company telling me there are cracks when there aren't.

No-one on a forum can look at a pic and give any sort of definitive answer that a bike is or is not safe to ride unless it's a blindingly obvious failure.

At it's most basic, if I (or anyone else) said on here:
"that looks fine, carry on..."

and the next day the frame fell into pieces with you on it doing 30mph, you'd probably be a tad miffed (or dead...)

A repair place shouldn't just go and do a whole load of fixing stuff without discussing it with you and agreeing a plan first. Your only safe and reliable option is to take it to them and get them to examine it via ultrasound or X-ray or whatever procedure they use. There'll be a charge for it but then they'll be able to give you a report and a series of options from there. Best case, it's fine and you've paid for peace of mind. Worst case, it's a crack but repairable at extra cost.

Thanks for the reply. Makes sense. Was just wondering if anyone on the forum had used the coin tap scenario, and was later proven wrong.

What rareposter said^^^

Scrape the paint away and have a look ?Am I being too simplistic ?

Was just wondering if anyone on the forum had used the coin tap scenario, and was later proven wrong

There's a reason that Airbus engineers don't go around tapping their aircraft with a coin

This Australian is an expert and has a Youtube channel of CF testing and repair. This example is why you shouldn't rely on a visual inspection alone https://carbonbikerepair.com.au/article ... scan-video
He's not a video editing expert, so stick with it

Probably not directly relevant (unless you plan to go underwater cycling) but the Titan submersible that imploded near the Titanic was partly made of carbon fibre and I assume they had used ultrasound scanners etc (although perhaps not based on their apparently lax approach to safety). I guess the point is that the structural stability of carbon fibre is tricky to assess.

I guess the point is that the structural stability of carbon fibre is tricky to assess.

Titan was the first deep-sea submersible pressure hull made with CF, normally they'd be made with steel, aluminium or titanium.
There isn't the practical experience and understanding of CF under repeated deep sea pressure conditions and the associated testing regimes

By contrast, goodness knows how many bicycle frames and other components have been made with CF (and steel, aluminium or titanium).

jimster99 wrote: ↑26 Sep 2024, 1:15pm Probably not directly relevant (unless you plan to go underwater cycling) but the Titan submersible that imploded near the Titanic was partly made of carbon fibre and I assume they had used ultrasound scanners etc (although perhaps not based on their apparently lax approach to safety). I guess the point is that the structural stability of carbon fibre is tricky to assess.

Nothing about Titan is remotely relevant to the use of carbon fibre in bicycle frames.

Structural assessment of carbon fibre is pretty straightforward - most modern passenger aircraft have at least some carbon fibre parts (often the wings and tail) and they are routinely tested and examined. Although as tim-b mentioned, not by tapping them with a coin...

The only safe and reliable option here is a proper carbon fibre repair place with the tools / tech etc to examine the frame correctly. It's not difficult but it is specialist.

a chap I know sanded all the paint off his CF frame. It did save a lot of weight, but presumably the prospect of UV damage doesn't worry him.

rareposter wrote: ↑26 Sep 2024, 2:25pmNothing about Titan is remotely relevant to the use of carbon fibre in bicycle frames.

I realise this and probably it wasn't a helpful comment especially as I know nothing about carbon fibre bikes but it has been in the news recently so my simple mind made a connection. I guess if carbon bikes were constantly failing they would eventually be banned and they haven't been so presumably they're reasonably safe in practice.

arguably the UCI weight limit is to stop people riding excessively lightweight CF equipment.

tim-b wrote:

Was just wondering if anyone on the forum had used the coin tap scenario, and was later proven wrong

There's a reason that Airbus engineers don't go around tapping their aircraft with a coin

This Australian is an expert and has a Youtube channel of CF testing and repair. This example is why you shouldn't rely on a visual inspection alone https://carbonbikerepair.com.au/article ... scan-video
He's not a video editing expert, so stick with it

Fair enough. I haven't crashed my bike though, just gone over a couple of small pot holes. The bike is 10 years old though, so that video puts things into perspective. Cheers.

I got nervous about these cracks on carbon forks on an alloy / carbon fork road bike and replaced them.

A failure in the forks would be very unwelcome, was my thinking. Elsewhere on frame I might have been less cautious. No idea whether just the paint got stressed over a bump, or the carbon was damaged.

I replaced the forks with black ones, so now not so obvious if there is a problem.

Bice wrote: ↑27 Sep 2024, 5:28pmI replaced the forks with black ones, so now not so obvious if there is a problem.

The Ostrich solution?