Originally Posted by
mikeread
A tig welded frame has nothing to distribute the stress so why does a lug need it? could a lug just be a short round collar - like a big tig weld?
I think it's just as you stated with there being higher stress on the top and bottom of the tubes, where the points are. This is mostly just theoretical though and doesn't matter in the real world. I also think lugs are the shape they are because of tradition. A "short round collar" would function just fine for the application.
Personally, I like lugs that have a fair bit of surface area on both tubes being joined because they are more fault tolerant of a poor fill job and/or a gap between the tubes that gets hidden/covered up by the lug. The shorter the points (or collar as it were) the more exacting the brazing and fitment needs to be.