Any wisdom with regards to how close a butt can be to a weld/braze joint?
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25mm is always what I used as a bare minimum. But I like to have quite a bit more where the action (ie head tube) is.
-Walt
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Is the 30mm valid for lugs as well? How long should the butt section go beyond the tip/point of a lug?
Thanks!
Others may have more experience based feedback but I always make sure the lugs tips are within the butted area. I’ll shorten the tips up a little if I need to.
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Thank you!
I guess that if it comes down to filing the lug, only a few mm within the butt section will do the trick? I currently have about about 3-4mm.
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You also have probably 40mm of transition so a mm or two should be fine. Also, if you’re tubes are so short that you’re running out of butted material, the frame may be for a light rider which will also be in your favor. The bottom lug extensions should also be broadly rounded, not pointy.
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Might be slightly off topic, but why do you want the bottom extension of the lug to be rounded off? I guess it’s to distribute stress over a larger surface?
I’m currently bulding a frame wiith a Long Shen 102R seat lug and the bottom is quite pointy.
Yes, I round those off. If you thin them, it seems the risk of them acting like a can opener is decreased.
To be clear, I don’t have any scientific data, but that point can’t be good for the tube (especially on the thin walled section!) and a rounded off lug just makes more sense to me. I have heard other builders say the same so, I’m going with Lore & Magic vs Engineering & Science. 
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That is the general understanding and I do that as well. Round and thin a little. Stress is always better handled with gradual changes. Avoiding steps in material and points/corners is always better. When you join a tube and lug they effectively become one. that said we have millions upon millions of lugged frames made over decades that are fine with pointed lugs.
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