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Old 12-12-2012, 05:04 PM   #3238
LexLeroy
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Joined: Jan 2011
Location: Lexington, KY
Oddometer: 1,544
Quote:
Originally Posted by Benesesso View Post
Beautiful looking, but there could be a problem with them. Brazed joints work very well when stressed/loaded in shear (lap joints), but not so well in tension. Tension is where welding shines. Your lugs appear to me to relying on the tensile properties of brazing, and they may fail.

Well made brazed joints have lots of overlap/shear area, as do well made adhesively bonded joints.
Good points. Here's the head tube lug jigged and heat-sinked prior to brazing - inserted rather than butted in order to get a better mechanical joint with some overlap. The brass flowed nicely into the joint - so much so that I needed to chuck the thing into the lathe afterward and clean up the ID with a boring bar.



And here's the head tube with the finished lugs silver brazed into place -



The homemade lugs join the sloping top tube to the (steering) head tube and the seat tube using silver brazing (56% cad free). The sloping top tube is mitered on both ends and butts against the head tube and seat tube inside the lugs. Since the top tube is under compression and the ends butt solidly against the head and seat tubes I'm thinkin' that I'm OK - the lugs aren't carrying much of any sort of load.

The bottom (lower sloping) tube is under tension, but that's silver brazed into an American-made investment cast lug at the head tube and into an American-made investment cast bottom bracket (where the pedals go).

But to your point, at least one builder that I know of who uses brass fillet brazing instead of lugs has experienced a failure of the junction between the head tube and the down tube on a bike that got used hard - a mountain bike. Fortunately I wasn't involved.
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LexLeroy screwed with this post 12-13-2012 at 08:22 AM
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