On Sat, 10 Mar 2018 10:06:11 -0600, Tim McNamara
wrote:

On Sat, 10 Mar 2018 07:17:58 +0700, John B wrote:

Actually some of the old guys that used to build classic bikes used a
lot simpler fixtures then those sold by Henry James (cheaper too). I
remember a youtube about some old Japanese builder of classic steel
bikes who seemed to have a fixture for holding the seat tube and
bottom bracket at 90 degrees for brazing and then aligning everything
else with hand tools to the BB/Seat Tube assembly.

When I built my frame in the shop of a friend, his technique was to
start with that connection and to have it plumb and square to the faces
of the BB, which he considered the reference point for the frame. Every
other tube was added without rigid fixtures, using a flat table and
supports. Then each new tube was aligned relative to the faces of the
BB. His goal was to build a frame with no residual internal stresses,
so that if you were to saw through the top tube the ends would remain
aligned.

"Why" is something I didn't quite understand, except that when the last
joint was done the frame was almost perfectly aligned and required only
a few tweaks at the dropouts. The frame I built has held up for over a
decade now, but let's face it most frames hold up for many decades
unless they are damaged.

I think that using the Bottom Bracket as the basic reference point is
the common way of building a frame.

Although admittedly the Henry James "hold everything" fixture is
probably faster if you are churning out quantities of frames.

Yep. His complaint about this was that clamping the frame into a rigid
fixure and then brazing away resulted in a frame with internal stresses
that were not relieved. After finishing the brazing or welding the
frame usually has to be forcibly aligned. My friend thought that was a
bad thing. I am not sure it makes any difference in terms of longevity
and seems very unlikely to make any difference in the performance of the
frame

Given that the fork blades often are sold as a straight tube and the
frame maker then bends them to the proper radius to achieve the
desired trail obviously the act of simply bending a frame tube can't
be as evil as it sounds :-)



OTOH, the best frame builder around here had a Henry James fixture and
the bike he built for me is the best riding one I have ever been on (or
was until I mangled it in my cash last summer. Have a replacement on
order).
--
Cheers,

John B.