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I bought a used BMC Granfondo 01 which has a carbon frame.

The frame is solid (no chips or cracks), but it's full of weird discolouration. It looks like there were be stickers in the places where discolouration appeared. The paint is solid in the places of discolouration

The owner told me he bought it with the stains being there already and he assured me that he had no issues with the frame.

What could have caused this?

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  • They look like sticker marks, but would be strange thing to pull off.
    – Criggie
    Sep 13, 2016 at 3:13
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    There is no discolouration, its a feature, not a defect. This is what UD carbon fibre actually looks like without paint or cosmetic sheets applied over the top.
    – Lachlan
    Sep 13, 2016 at 6:30

4 Answers 4

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Edited see comments below

This is from the BMC website. Cut-and-paste for reference.

BMC’s frames are made mostly using UD carbon fibre. UD stands for unidirectional. Unlike more traditional carbon fibre products that use a woven carbon finish (criss-crossed finish like a woven piece of material), UD carbon is literally a piece of carbon material with the filaments all running in one direction. BMC’s frames are made from many different pieces of UD carbon that are woven together, similar to what is done in textile production. Some of our frames contain around 350 individual pieces of carbon fibre that go into the mould before heating and curing takes place. These material sheets - some very small, some very large - are laid at different angles across each other to achieve different results: torsional strength, compliance areas, rigidity, and so on. The layup of these sheets is extremely important to the end result. This is actually the art of building a carbon fibre frame.

On our unpainted frames, you can see the result of these carbon fibre sheets overlapping each other through the clear coat covering the frame. We are actually very proud to be able to show this real effect on our frames. It shows not only the true workmanship of the frame, but also the resulting quality as the frame exits our moulds. Many bike manufacturers prefer to paint their frames because there are often many imperfections in the finish that need to be re-worked using a filler before waiting for it to dry, sanding it, and then giving it a final paint job (a bit like a panel shop). This is a cheaper way to finish the frames, ensures you have no reject quantity, and allows you to use less stringent production techniques.

On unpainted BMC frames, when you look 'into' the frame, you see the actual quality of what’s on the inside – we are showing you our finish and quality and we are very proud of that.

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    This is terrible advice. The frame is supposed to look like this. Here are several examples you can find with a simple google search google.com.au/search?q=bmc+naked+carbon&tbm=isch
    – Lachlan
    Sep 13, 2016 at 10:45
  • I'm not convinced. The photos (particularly 1 & 3) don't look like a manufactured finish. And your link doesn't show any BMC frames with the above finish. If the OP could post more photos from different angles and confirm the marks are stains on the finish rather than tricks of light,
    – OraNob
    Sep 13, 2016 at 11:58
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    Just found this link - and sounds like Lachlan is correct in his answer. This is how it is with BMC frames. They do apply a clear-coat over the carbon but seems unlikely this is what is showing in your photos. forums.roadbikereview.com/bikes-frames-forks/…
    – OraNob
    Sep 14, 2016 at 9:01
  • I suggest editing your answer to include all your (new) knowledge. Either keep the first version or not as you choose, since it will always be available to anyone who browses back through the edits.
    – andy256
    Sep 14, 2016 at 9:20
  • Different generations like different things, partly because the other generations don't. If BMC and others can sell unfinished products, I say good on 'em. But they won't sell to me.
    – andy256
    Sep 14, 2016 at 9:22
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I have the identical bike and in sunlight I see the identical shimmering carbon sheets under the clear lacquer. It's perfectly normal. I love the copy and past above form the BMC website. It's just so Swiss. Perfection. Which is why I love my BMC GF01. SOOO COMFORTABLE and reliable and superb handling.

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I know this is an old post but I want to respond. This is absolutely a normal appearance of UD carbon. Layers of paint can add hundreds of grams of weight. But if you cannot sleep at night because of UD appearance, buy a glossy bike. A showroom floor of matte carbon frames will all have some degree of these imperfections. Compare it to a marble kitchen top. If you don't feel any cracks, stop worrying. Take it to a real bike shop of carbon if you doubt it.

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  • Hey Tim - this is true. Thank you for your response! Unfortunately the bike was stolen couple of years ago.. :-(
    – Taks
    Apr 20 at 14:03
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I just bought a gf01 frame and on first inspection, it doesn't look like that, so I don't think it's supposed to be like that. Having said that, it looks mostly cosmetic that speaks more to the lack of care from the previous owners than to anything specifically wrong with the frame.

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  • That is a worry - lack of care is bad for carbon. Are you able to take two photos of your frame, from the same angles as the original question's photos? I think you can only post two until you have more rep. Please take photo outside in sunlight like the original ones for comparison.
    – Criggie
    Sep 16, 2016 at 22:17
  • sorry, i didn't see this note. i just started building up the bike, and upon closer inspection, i'm seeing similar characteristics. i'll pull some pics in the morning.
    – teemoleen
    Oct 12, 2016 at 5:56

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