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Crack on seat tube lug

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Crack on seat tube lug

Old 08-09-22, 10:38 AM
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Gods lonely man
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Crack on seat tube lug

Would it be safe to ride this frame with this crack on the seat tube? I am going to use a pretty long seat post, on the long term, brazing the crack would be the best idea.

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Old 08-09-22, 11:12 AM
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I can't see that being much of a problem. Just keep an eye on it.
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Old 08-09-22, 11:20 AM
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Originally Posted by cxwrench View Post
I can't see that being much of a problem. Just keep an eye on it.
Thanks for the comforting words, I am fairly light weight so I hope it's doesn't go further.
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Old 08-09-22, 12:15 PM
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Makes me wonder if your seat post is the right size, or did some previous owner jam something down the tube that wasn't the correct size and later corrected it.

But otherwise, I don't see any immediate fear. Just keep an eye on it as @cxwrench said.

If the crack turns horizontal and starts running around the circumference of the tube, then that will get to be a problem faster.
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Old 08-09-22, 12:16 PM
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I would extend the slot down to close to the lug's bottom edge and round off the bottom of the slot. Seat posts can be harder to make tight is the slot is short. I like a slot at least an inch long. A warding file is what I use. Andy
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Old 08-09-22, 12:21 PM
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Originally Posted by Andrew R Stewart View Post
I would extend the slot down to close to the lug's bottom edge and round off the bottom of the slot. Seat posts can be harder to make tight is the slot is short. I like a slot at least an inch long. A warding file is what I use. Andy
I thought it a little odd that the slot stopped in the lug. While I can't say for certain, I thought my lugged steel bikes had the slot going through the lug entirely and a little further down the tube.

But since I got rid of my older bikes, I can't double check that.
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Old 08-09-22, 12:29 PM
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Originally Posted by Iride01 View Post
Makes me wonder if your seat post is the right size, or did some previous owner jam something down the tube that wasn't the correct size and later corrected
To be honest I don't know how long It's been there, maybe previous owner ignored the minimum insertion line, seatpost size 27.2.
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Old 08-09-22, 12:36 PM
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Originally Posted by Iride01 View Post
I thought it a little odd that the slot stopped in the lug. While I can't say for certain, I thought my lugged steel bikes had the slot going through the lug entirely and a little further down the tube.

But since I got rid of my older bikes, I can't double check that.

This is from my Razesa pista, like Andrew said , the sloth is longer, the one of the cracked frame is really short , I didn't though much about it.
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Old 08-09-22, 01:06 PM
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That slot should have looked like this from the start. It looks like the builder filled the hole in the lug with brazing filler material and then made the slot too short.


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Old 08-09-22, 01:24 PM
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Originally Posted by dsaul View Post
That slot should have looked like this from the start. It looks like the builder filled the hole in the lug with brazing filler material and then made the slot too short.


Thanks for the editing, that actually makes sense , is kind of funny that the crack ends like is turning into a circle, just like it was supposed to be.
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Old 08-10-22, 07:06 PM
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Originally Posted by Gods lonely man View Post
Would it be safe to ride this frame with this crack on the seat tube?.
I have a couple thoughts. First, a crack like that would most likely form or, at least, propagate as a result of tensile stresses. But this particular location should be mostly under compressive stress unless there are some unusual circumstances. So I wouldn't expect this crack to go anywhere very fast if at all. Could it have formed as a result of brazing the lug?
Second, even if it did propagate, that crack isn't likely to cause a catastrophic failure anyway.

Here's an extreme example of a seat tube slot crack gone wild. A folding bike I bought in 2006 developed cracks extending circumferentially from the stress-breaker hole at the bottom of the slot. This seat tube was exposed to at least four cycles of latch opening/closing every work day for several years. I failed to notice it in time to increase the radius of the stress-breaker, and it eventually circled the top part of the seat tube at the weld joining the seat tube to the frame tube. This left the latch and its collar attached to the seatpost. It didn't interfere with riding the bike, as my weight held the seatpost in place and the crack didn't propagate anywhere else. Of course, if I lifted the bike by the saddle, the seatpost would come out. A couple of home-made aluminum stays connecting the seatpost clamp to the frame took care of this. I've ridden this bike for ten years since this repair without any problems. See the attached image.

My "winter" bike, with the jury-rigged stays to hold the seatpost in place when lifting the bike by the saddle.
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Old 08-11-22, 10:11 AM
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That's a pretty cool Mad Max solution , I hope I don't have to do anything like that though, I have seen a lot of aluminum frames cracked at the seatube, not so many steel frames.
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Old 08-11-22, 11:31 AM
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Not a frame builder or pro mechanic, so my opinion is likely not worth much. Maybe @gugie could offer some advice - he's built and modified frames, and is by reputation very good at it. He's a regular in the C&V forum but this might pique his interest.
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Old 08-11-22, 11:43 AM
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Originally Posted by Hondo6 View Post
Not a frame builder or pro mechanic, so my opinion is likely not worth much. Maybe @gugie could offer some advice - he's built and modified frames, and is by reputation very good at it. He's a regular in the C&V forum but this might pique his interest.
All I can say is ditto to this post and this one. When I do framework I typically check for a squared off seat tube lug slot, then take a small round file to the bottom. The intersection of two straight lines makes a stress raiser. Campagnolo Record and Nuovo Record crank design has a stress riser on the drive side arms in much the same fashion, the "upgrade" is to round off the sharp corner with a round file as well.
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Old 08-11-22, 11:46 AM
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I'd drill a hole (two, actually) at the bottom of the crack to prevent spread. Start with a tiny hole and then go one bigger.
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Old 08-11-22, 12:28 PM
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↑↑↑ This as the forward edge of the crack is a massive stress riser which is relieved but drilling the leading edge out.
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