Flexibility in lugged bottom brackets for first build?

[sirskialot22]

For all of the frame builders out there.

About to start my first build, and was curious how much leeway do I have with adjusting angles from the set lugged BB angles? I'm sure there is a bit of play, and I know I can grind down metal with a file to change angles slightly, but is 1 or 2 degrees off of the given 60 degree (DT to ST) and 62 degree (ST to CS) acceptable?

[8aaron8]

Two degrees is pretty easy to adjust without doing much, often just mitering the bb ends of the seat tube and down tube mixed with prepping the lug for the tubes give you enough leeway.

[eBAUMANN]

yea you'll be fine, I've squeezed 5* out of a few BB shells...it was a pain in the ass but it worked.
2* isn't a big deal but for more than that you end up doing some serious "blacksmithing" to get the tube/socket fit right.
hardest fit up I've done was s-bend true temper chain stays (which are NOT perfectly oval...surprise!) into a lugged shell...on 3 frames...never again haha

[Doug Fattic]

Yes it is possible to adjust the angle a couple of degrees between the down tube and seat tube in a bottom bracket shell, There are a variety of ways to make a frame and I have my frame building class students braze the seat tube into the bb shell as a 1st step in making a front triangle. After filing or grinding and maybe bending the sockets to a more agreeable angle, I spot braze the 2 pieces together at the needed angle before before proceeding to fully braze them together. Of course there needs to be alignments checks during that process.

[sirskialot22]

Ok great, thanks for the insight here everyone.

As a next step, I know lugs can be cutout and shaped, but is there anything I should be doing to them, inside and out, in terms of surface finish, etc., that would be helpful?

And other than aesthetics and weight, is there any benefit of sculpting out a lug? How does one know how much material can be removed?

[Doug Fattic]

I know lugs can be cutout and shaped, but is there anything I should be doing to them, inside and out, in terms of surface finish, etc., that would be helpful?

And other than aesthetics and weight, is there any benefit of sculpting out a lug? How does one know how much material can be removed?
The inside of the lugs should be reamed or ground or filed so that there is just a nice slip fit between the tube and the lug. The 56% silver we use in building requires a tolerance of something like 0.003 to 0.004". in other words enough to move but not have any slop. The outside doesn't matter before brazing.

The shape of lugs is mostly decorative if your miters are properly tight. You need to leave something like a quarter of an inch but nobody does a lug that tiny. Right after WWII British builders did a lot of fancy lug work and advertised that the extra shorelines distributed stress better and help prevent a break. I think that was mostly marketing to allow manly men to have pretty frames.

Blank lugs were difficult to find and Haden the last lug company to make them went out of business 10 or 15 years ago. I convinced the Nikko lug company in Japan to make some more. I am giving a lug cutting demo at the Philly Bike show this week end at 3 on Sunday. I'm bringing a 9 page handout to give away to those really interested. I know it is impossible to remember every detail. Many of my frame building class students like to cut some kind of design out of these blank lugs.

[nicrump]

my 2cents. work your angles in both directions before any brazing as you're likely to loose that adjustability in the ST/BB shell. in other words you'll have more flexibility if all is adjusted and fit prior to any brazing.