Deep paint scratch or frame crack?

[Zenifir]

Hi, everyone! I'd like to ask you about your opinion on whether or not this is just a deep scratch (scuff?) on the paint or a crack on the frame of my aluminium road bike (please see photos below). For context, the area shown on the photos would be the seat tube, right about where it is joined by the seat stays. I already did the "coin soundcheck" test to help me diagnose the problem but I'm still not convinced by it. Anyway, the sound coming from the alleged crack as well as nearby parts of the frame i.e. the seat tube, seat stays, and top tube were the same. Also, the "crack" did not flex or show signs of movement when I smacked my butt on the saddle a few times. While both tests lean away from a case of structural damage on my frame, I fear that I am not experienced or knowledgeable enough to conclude anything yet.

Your expert opinions on how to go about this would definitely give me peace of mind.

-Z

   

   

   

[Criminal]

(12-05-2020, 11:43 AM)Zenifir Wrote:  Hi, everyone! I'd like to ask you about your opinion on whether or not this is just a deep scratch (scuff?) on the paint or a crack on the frame of my aluminium road bike (please see photos below). For context, the area shown on the photos would be the seat tube, right about where it is joined by the seat stays. I already did the "coin soundcheck" test to help me diagnose the problem but I'm still not convinced by it. Anyway, the sound coming from the alleged crack as well as nearby parts of the frame i.e. the seat tube, seat stays, and top tube were the same. Also, the "crack" did not flex or show signs of movement when I smacked my butt on the saddle a few times. While both tests lean away from a case of structural damage on my frame, I fear that I am not experienced or knowledgeable enough to conclude anything yet.

Your expert opinions on how to go about this would definitely give me peace of mind.

-Z

Hello Z,

Tough one! It looks more like a scratch, but it seems deep enough that you would see some change in paint color from top color to primer or even bare metal (I really don't know how they paint newer frames). If it was steel you could have it "magnafluxed", but it doesn't work on non-ferrous, non-magnetic frames (as far as I know). If you take the seat post out until the bottom is just above that area and do what you were doing with your "butt test" by trying to flex the material at that point then I think it would you may see (or not see) it widen if indeed it is cracked. I assume that the bottom of your seat post is well below that area during use. I have also used very thin oil before (penetrating oil, et al.), and by capillary action the oil will penetrate into the crack and essentially disappear; if only a scratch it should remain on the surface and just sit in the groove of the scratch. Use very little oil, you do not want excessive oil to fill the crack and surface groove around it since it will appear that no oil has penetrated the crack; I use a small needle which will only have a small amount of oil adhere to the point; less is best! The oil on the end of the needle should hardly be seen; you are not using an average sized drop of oil, but maybe one tenth or less of that amount.
Did you have an accident that could have caused it? Do you, or did you have something mounted in that area which could have caused it? Scratches tend not to go around the bend of tube so my instinct would be towards the crack diagnosis, but a guess from seeing the photo only.