Hi everyone,

Three weeks ago I bought a rare first-generation Stumpjumper on Craigslist. The previous owner had built it up with a hodgepodge of poor quality parts which I quickly stripped off. I intend to build it back up with the original components once I repaint it. Upon closer inspection, I noticed that he had used a smaller diameter seatpost than it was designed to handle. Then I saw that a small crack had formed on the top front of the seat tube at the TIG weld. Is there a way to fix this without replacing the entire seat tube? Thanks for any advice you can give me.

that's really hard to tell without seeing the crack. Not sure I believe that a crack on the front is due to a seatpost size issue. Mountain bikes crack at the seat tube top tube joint because people ride them off of drops.

I thought the 1st gen stumpy had lugs?

I wish I could post the pic I have, but apparently new users must have 10 posts under their belts before they are allowed to post pictures.

They started building Stumpys in October of 1981 with TIG-welded frames and by the end of 1982 they were doing lugs.

I haven't stripped the paint around that area, but the crack appears to run vertically through the weld that connects the top tube to the seat tube and down the seat tube for maybe an inch. I'm sure it's related to using the wrong seat post diameter. Head smack.

Is this something that can be brazed?

Thanks.

The same thing happened to a friend. For whatever reason, he had a smaller diameter seatpost in there and the seat tube cracked right by the collar when he tightened down.
His wasn’t a first generation stumpy, tho. Believe it was a mid 90s metal matrix iteration.
Anyway, a bike mechanic buddy was able to round it out enough to get the right diameter seatpost back in there but it wasn’t pretty.

I had a '94 S-Works M2 that broke along the weld between the seat tube and the top tube, but that wasn't from a seatpost issue.

Do you think brazing would be enough to fill the crack on the seat tube if I ground down the weld where the crack also is, and re-weld the joint area?

If the crack is like the one on my friend’s bike, I think you could do it successfully.
But I am no expert. Can you post a pic? Hopefully someone with a lot more knowledge of such will see and reply.

I have shots of the area in question. Unfortunately, I need to have at least 10 posts on the forum before I can post linked shots or URLs per Bike Forum rules. Quite frustrating.

You can post a bunch in “introductions” to get your count up.

Okay, here's the area in question:

Strip the paint take it and have the seat tube junction and the crack tig welded, file down the weld on the tube and ream out the inside of the seat tube to remove the excess weld material. It will be close to good as new. Plus, from the sound of you probably won't be subjecting it to anything too severe once it is restored.

Thanks, Steve. That sounds like a fairly easy thing to do. Care to hazard a guess as to how much something like this cost to repair? If anything, I thought I might do some light riding on it. Nothing too extreme, and definitely not dirt. More of a museum piece than an actual usable bike. Thanks again for your response.

I agree, TIG is the way to go for a cracked tube. Brazing is only a little better than bondo in this situation.

That crack could be TIG welded and filed back to round again. I did something similar, when I noticed one of my chainstays was cracked from some aggressive dimpling, on my current gravel bike frame. I didn't see the crack until after the whole frame was welded, so I welded up the crack and blended the weld bead into the tube with a file. I've been riding it for 2 years without an issue.

On the other hand, the easiest thing to do is nothing. Put the correct size seat post in and just ride it. Check to see if the crack grows over time, but it will probably be fine.

While I was deciding what to do, repair wise, I'd probably drill crack end holes. I'd also think about removing the paint around the area and applying nail polish as a temp. coating that's real easy to remove later for any repair. This allows current and future monitoring with out much effort. I see the real issue is will the crack continue to travel. The ST/TT joint is one of the least stressed ones on a bike and the post adds some support (if sized and inserted properly). Andy

Sounds like a pretty standard procedure. Thank you, Unterhausen. I had no idea that brazing was that weak.

Andy, you have to exercise one of my pet peeves this early in the morning? Don't drill "stop holes." Doesn't usually stop anything. Reduces the strength of the frame. Encourages crack growth. Nobody in aviation would ever drill stop holes and not fix the underlying condition. The end of the crack is actually under compression, which keeps the crack from growing as fast. Drilling that out removes that. It's one of the factors that make ductile metals so useful. There is nothing driving this crack to grow, so leaving it alone is a good idea. Maybe paint over it.

Next someone will bring up shimmy and I'll have to take a timeout.

Thanks for the applying clear nail polish tip. I will definitely try it. I'm looking forward to seeing what lies below the paint.

Eric- I do understand pet peeves. I have always wondered about the mechanics of cracks. I suppose different causes have different mechanics. Perhaps some day you can educate me about this more.

As far as shimmy... that's about to be visited in another thread over in Bike mechanics. Once again. Andy