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I noticed some small cracks in the paint on my aluminum frame bike. The cracks are clustered around the rear brake boss, and only on one side (NDS). The bike is from 2011 and has been ridden over 10,000 km. Just wondering if it's just cosmetic or something I should be worried about.

cracks in paint

  • It wouldn't surprise me if there turned out to be an element of chemical contamination. This can weaken paint and make it brittle – Chris H Apr 30 '19 at 17:53
  • Scratch one of the cracks with a fingernail and see whether the paint flakes away. Looks like a really badly done over painting. – Carel May 1 '19 at 12:00
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I believe the cracks are just in the paint, not the alloy.

Only the white paint is cracked, not the black, the cracks are evenly spaced and run in all directions rather than the single direction a stress crack would follow.

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    Concur - it looks like the white paint is not as forgiving as the black. White appears to be layered on top of the black, so perhaps a case of bad preparation between coats or maybe not enough time was left between coats. Reinforcing this is that its only on one side of the bike. This spot by the brake boss is going to have the most flex on the whole seat stay. Fixes - ignore it, or prep it/mask and paint some enamel white on with a small brush. Or some modern reflective paint could be cool. – Criggie Apr 30 '19 at 11:50
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    I'd not ignore it - recently I've found a crack on my fork which I thought was a paint issue. So maybe scratching those paint cracks and looking what is underneath can be a better idea – k102 Apr 30 '19 at 14:52
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    Whilst I agree its almost certainly just a paint issue, i'd probably be sanding that back to take a close look to be safe. – Andy P Apr 30 '19 at 14:58
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I would at the very least check the inside of the brake boss attachment for cracks. The way the paint is cracking, it appears as if there is some large-scale movement of the aluminum, which could be caused by a crack on the inside. And if that is the case, you need to stop riding this bike immediately!


Background

Brakes put a lot of stress on the structure of the frame, especially cantilever- or V-brakes. They push the seat stays apart, and this pushing force is not aligned with the center of the seat stays. Rather, they produce quite some torque along each seat stay, which is transmitted at the attachment of the boss itself. Thus, the welding of the bosses and the chain stays, as well as the aluminum parts directly next to those weldings, are prime candidates for cracks. And you definitely do not want your seat stays to fail you!

Also, the fact that only one side is afflicted is a huge red flag. If the issue was just with the paint, you'd expect the symptoms to be all over the place, or at least symmetric. Not only at one side where there is a lot of structural stress.

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  • While there's no way realistic way to tell without removing the paint and examining the actual frame, I don't think those cracks in the paint are caused by flexing of the brake boss. Note the same crack pattern over the very thick and quite inflexible weld bead. Were that cracking caused by flexing, I'd think the thinner, more flexible areas would be cracked while the paint over the weld bead wouldn't be. While it could be a crack on the inside of the boss as you say, and I'd definitely check for that, to me it just looks like a bad paint job or bad paint prep in that area. – Andrew Henle Apr 30 '19 at 20:18
  • @AndrewHenle I sincerely hope you are right. – cmaster - reinstate monica Apr 30 '19 at 20:28

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