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possible dangerous crack at lugged seat tube/BB join? (pictures)

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possible dangerous crack at lugged seat tube/BB join? (pictures)

Old 12-10-06, 03:20 PM
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TallRider
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possible dangerous crack at lugged seat tube/BB join? (pictures)

Today I was doing some touch-up (with clear nail polish) on my 1987 Centurion Ironman Expert frame (Tange #1 butted cromoly tubing), and noticed a crack in the paint along the lugged joint between the seat tube and bottom bracket. The crack is on the back of the seat tube, toward the left (non-drive-side). It's a slight crack in the paint, and I don't know if it heralds any danger of a crack in the brass brazing material that's directly underneath the paint at that point.
I bought the bike off of eBay, but it's been in good shape and so far as I know has never been crashed. I did slightly spread the frame's rear triangle (using the cold setting method recommended by Sheldon Brown) but only went from 126mm to 128mm spacing. I think of this only because Sheldon's method requires the lever arm to be braced against the seat tube in order to apply force. But it's not like I stomped on the frame or anything; I was quite careful.

I'm guessing this isn't something to be very worried about (though I'll be sure to watch it) but would like to hear feedback. I know it's possible to sand away the paint just at that point and look at the brazed joint, which I may do but would rather not if it's unnecessary.

Anyway, here are three successive pictures zooming in on the same shot.




Last edited by TallRider; 11-21-08 at 10:56 AM.
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Old 12-10-06, 03:26 PM
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Originally Posted by timcupery
Today I was doing some touch-up (with clear nail polish) on my Centurion Ironman Expert frame, and noticed a crack in the paint along the lugged joint between the seat tube and bottom bracket. The crack is on the back of the seat tube, toward the left (non-drive-side). It's a slight crack in the paint, and I don't know if it heralds any danger of a crack in the brass brazing material that's directly underneath the paint at that point.
I did slightly spread the frame's rear triangle (using the cold setting method recommended by Sheldon Brown) but only went from 126mm to 128mm spacing. I think of this only because Sheldon's method requires the lever arm to be braced against the seat tube in order to apply force. But it's not like I stomped on the frame or anything; I was quite careful.

I'm guessing this isn't something to be very worried about (though I'll be sure to watch it) but would like to hear feedback. I know it's possible to sand away the paint just at that point and look at the brazed joint, which I may do but would rather not if it's unnecessary.
This is most likely only a paint issue. If it is an actual crack, it isn't "dangerous" in the sense that it could cause a crash. If a crack like this were to grow to the point of total separation, the symptom you would notice would be bad shifting in front, and/or noise from the chain rubbing on the front derailer when pedaling hard.

Such a trivial spreading as you did would not damage any decent steel frame, and if you were to spread it far enough to do damage, the damage would be to the chainstays, not the seat tube.

Sheldon "Don't Worry" Brown
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Old 12-10-06, 03:29 PM
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Looks to me like just a crack in the paint. I would scrape the paint off at the crack and see what could be seen underneath.

If the crack does extend under the paint, it might be in the brass of the joint or in the steel tube. It would be relatively easy to fix a failing joint by flowing in some more brass, but if the crack is in the tube, the only fix is to replace the tube...
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Old 12-10-06, 03:38 PM
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Not that I have anything to add, but was the bike shipped in a Thule Bike Box?
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Old 12-10-06, 05:11 PM
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Since when does paint crack? The bad news is that the metal, be it steel or braze material underneath is cracked. It may be fixable by a pro, but not by your average home wrench. bk
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Old 12-10-06, 05:33 PM
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Originally Posted by bkaapcke
Since when does paint crack? The bad news is that the metal, be it steel or braze material underneath is cracked. It may be fixable by a pro, but not by your average home wrench. bk
Paint, especially old paint can be more brittle than brazing metal and can most certainly crack. It's probably only a paint crack. If you want to be obsessive about it, sand or Dremel it down and look at the base material. I wouldn't worry about it though.
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Old 12-10-06, 07:18 PM
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Originally Posted by bkaapcke
Since when does paint crack? The bad news is that the metal, be it steel or braze material underneath is cracked. It may be fixable by a pro, but not by your average home wrench. bk
Any coating can crack given a difference in expansion rates or a poor initial application.
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Old 12-10-06, 07:22 PM
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Originally Posted by bkaapcke
Since when does paint crack? The bad news is that the metal, be it steel or braze material underneath is cracked. It may be fixable by a pro, but not by your average home wrench. bk
You must not live in the South like timcupery and I do.
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Old 12-10-06, 07:29 PM
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That's a spot of thick paint, and which is prone to cracking and peeling. I would call it paint, but if you want to, do as suggested above and dremel it down.
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Old 12-10-06, 08:12 PM
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I scrubbed a bit of the paint away with a file and a razor blade, and the joint seems fine. Apparently it's just the paint. I've never seen cracks in paint before, but it appears to be nothing more than a crack in the paint. And that yellow glob of paint there was pretty thick, apparently in attempt to get the yellow vs. pink done correctly.

I think the paint job on the frame is ridiculous, but it's in good enough shape and I don't mind riding it like this. I'd thought of powdercoating it, and may do it in the future, but for now I've got my newly-powdercoated Comp TA frame (just treated it with frame saver today, buildup next Saturday probably) and it's beautiful but weighs the same and isn't any faster or slower than before. Just not going to rust, which it was doing a lot of before. And the Ironman doesn't have a rust problem which is why I'm not going to get it powdercoated for now.

Incidentally, the Ironman's paint job and framebuilding detailing isn't nearly as clean and pretty as the Comp TA's (original) paint job, and the Comp TA didn't have the same brazing imperfections. Now, the Ironman is not a worse bike from a functionality or durability standpoint, but the Comp TA clearly had more care taken of it in finishing than did the Ironman.

Last edited by TallRider; 12-10-06 at 08:29 PM.
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Old 12-10-06, 10:37 PM
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Puke purple and yellow. My favorite Centurion color scheme. Would love to meet the person that came up with that. Hope your frame is OK.
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Old 12-10-06, 11:02 PM
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That's not a crack, that's a fine hair in the paint.
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Old 12-10-06, 11:21 PM
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Originally Posted by DMF
That's not a crack, that's a fine hair in the paint.
No, it actually is a crack - I see the fine hair (vertical orientation) though. It's more clear (or was more clear) that it's a crack after looking from a few angles.
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Old 12-11-06, 02:13 AM
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I wouldn't worry about it. Cracks on these types of frames typically start on the tubing just beyond the lug, about 2-5mm. It'll start cracking around and follows the contour of the end of the lug. This is due to the stress riser at that point, flex to no-flex. That's why lugs are points and are often filed thin afterwards to spread out the area between flex to no-flex.

From the photos, it looks like paint just got worn off the sharp corner. Then the metal rusted underneath and spread out, peeling up some of the adjacent paint. AGian, filed lugs with rounded edges would've not given any sharp corners for the paint to wear or chip off.
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Old 12-11-06, 02:43 AM
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It is possible that rapid changes in temperature caused the seat tube and bottom bracket crown to expand or contract quicker than the paint was able to compensate, causing a crack where the paint was thickest. Do You know the chemical composition of the paint?
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