Framebuilders - Modifying angle of oval chainstay ports on bottom bracket

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jharley
04-19-11, 02:04 PM
I am going to build my first frame using a Llewellyn slant 6 double-oversize lug set, its going to be pretty big (61cm actual seat tube, around 65cm virtual). I've been drawing and redrawing to get all my angles, top tube, fork length, etc. just right. The problem I am having is that to get the bottom bracket drop where I want it (70-75mm), I would have to increase the angle of the chainstay ports on the bottom bracket by at least 2 degrees. With a 73 degree seat tube, the chainstays are 8 degrees from horizontal, I need it to be closer to 10. There isn't really another bottom bracket choice with a better angle given the tube sizes I am using (31.7 seat tube, 35 down tube.) I am not confident enough in my brazing skills yet to do fillet joints to a blank BB shell.

I've seen a couple references to people using steel bar in the ports to bend them, then hammering them back into shape. However, these chainstay ports are oval, so what would I use as a lever?

Anyone ever modified a BB shell this way? Any ideas?

Thanks!

unterhausen
04-19-11, 06:52 PM
I doubt you actually have to adjust them. If you do have to adjust them, you can use a file. Oval chainstays don't fit into the sockets all that well.
jharley
04-19-11, 07:46 PM
I doubt you actually have to adjust them. If you do have to adjust them, you can use a file. Oval chainstays don't fit into the sockets all that well.

Do you mean file the inside surfaces of the ports? And what do you mean about oval stays not fitting well...usually too tight? Too sloppy? Wrong shape?

Thanks again!

unterhausen
04-19-11, 08:05 PM
it's best to just fit them up and see how much play you get. Generally you can get 2 degrees without major blacksmithing. Think about what material would have to move to get a good fit.
Six jours
04-20-11, 07:47 PM
I've never had to make significant modifications, even though my chainstay angle is usually a few degrees different than the shell was designed for. At most, I tighten up the lugs a bit by inserting the chainstays and shell into the jig at the proper angle and then whanging away at them with a short length of aluminum bar and a hammer.