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Discussion Starter · #1 ·
It's pretty quiet around here, so thought I'd try to stir up some discussion. When I was out on my shakedown ride on the new frame I posted a couple of days ago, I got to thinking about what I would do if I got home from the ride and found a crack (or some sort of "failure") in the HT/DT junction. (I'm still skeptical that I can actually braze a joint strong enough to withstand that amount of force, just a confidence thang. this is frame #3 and no failures yet).

So if there was a failure at the HT/DT:
1. What would it look like? A crack in one of the fillets is possible?

2. What would you do? Options I thought of were (the frame is unpainted at this point):
a. remelt the brass and try to reflow in the area of the crack
b. file off as much of the brass as possible and try to "rebraze"
c. cut off the headtube, clean up the miter, and braze on a new head tube
(these were assuming there was no damage to HT or DT)

I would lean more toward b or c, but would like to hear your thoughts? Again, it's hypothetical. I promise.

Thanks.
 

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Old school BMXer
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I've thought about this as well as I'm also finishing up frame #3. My first frame was a learner and was never actually ridden. My wife has been riding my second frame for a few weeks.

If this frame cracks at the head tube, I'd have to evaluate the type of crack. At the joint? in the tube? and if in the tube, how far from the joint? So it really depends on the type of failure. Atlhough this frame has a 38 mm, 9/6/9 DT, I doubt it would be the tube (unless I really cooked it, which I don't think I did).

Ultimately, I probably wouldn't repair it and just make another using the information I learned. I've already learned a few things on this frame that I want to do differently on the next, anyway.
 

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Gabe.....
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Every crack is bad.........but for a small, non-symptomatic, fresh hairline crack in the brass on my frame I would try "a"........."a. remelt the brass and try to reflow in the area of the crack", after thoroughly cleaning (which is next to impossible if you've spayed some sort of frame saver in the tube :madman: ). But......if in doubt about your work, chop it up and keep practicing!!!!!!!!!!!!!!!!!
 

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Randomhead
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you aren't going to have a ht/dt failure, your frames are going to break at the bottom bracket.

If you're seriously doubting your ability, for $100 you can make enough practice miters to equal 50 frames. I really don't know why people don't do that. I like to practice as much as I can.
 

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While option 'a' (and 'b' now I think of it), seems like a simple straightforward fix, you're not actually solving the problem. Whatever forces caused the crack to appear will still be present, so unless it was just a bad brazing job in the first instance you could well be looking at a repeat failure down the road. You might need a bigger/thicker/oval downtube for added stiffness, or some other method of reducing the stress at that joint. My $0.02.
 

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Randomhead
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even with bad brazing, a crack is going to propagate into the tubing before you notice anything is wrong with the frame. So you're talking about a tube replacement
 

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Discussion Starter · #8 ·
unterhausen said:
you aren't going to have a ht/dt failure, your frames are going to break at the bottom bracket.

If you're seriously doubting your ability, for $100 you can make enough practice miters to equal 50 frames. I really don't know why people don't do that. I like to practice as much as I can.
I was probably a little harsh on myself - I'm not doubting my abilities so much as have a new appreciation for how much stress is put on these joints and how strong a weld/braze can be and has to be. I spent last fall with a box of 4130 cutting miters, practice brazing, and cutting them apart to understand penetration/flow. Probably did 50+ practice fillets working with a buddy who's experienced with a torch. So I didn't just jump right in to frames.

So the most common place for a frame by a new builder to fail is at the BB? Typically chainstays, or where?

Thanks for your input. I agree practice is key.
 

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Randomhead
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from my observation, the seat tube/bb joint is the number one failure spot in any frame, beginner builder or not. My experience is light in the mtb arena though.
 

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Google Suzy Jackson and her 'Little Fish' projects. Very nice write-up of her build process, complete with ST-BB failure and cutaway pics after the break.
 

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Randomhead
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a novice lugged builder is most likely to fail to get penetration at the bb, no doubt about that. I think a better measure is doing a google search for "is this frame cracked"

as far as how to fix it once it does crack, you can watch this video
 
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