Alignment

Have I mentioned the importance of alignment already . . . If you missed it the first few times then here it is again.

There are several important aspects to alignment, as distinct from frame geometry:

1. The tubes of the front triangle must be in a single plane.
2. The axis of the bottom bracket must be perpendicular to this plane.
3. The two halves of the rear triangle (i.e. the left and right seat and chain stays) must be equidistant either side of this plane so that the rear wheel lies in the same plane as the front triangle. In addition, the dropouts must also be aligned so that the wheel is held in this plane.
4. The front fork stays must be equidistant either side of this plane, and their dropouts aligned to facilitate this.

The simplest method to align the front triangle is to use a straight edge and construct the frame joint by joint, starting with the HT-DT, then ST-BB, DT-BB, and finally installing the TT. Or you can use a flat piece of wood with shims under the smaller diameter tubes, then clamped so that they all lie in the same plane. In the old days pinning was used to hold the tubes in place while they were brazed in a brazing hearth. This generally required cold setting of the joints afterwards - basically, bending them back into aligment if they had moved. Steel is very forgiving of this type of treatment and it was commonplace to cold set before the advent of 'precision' framebuilding.

By far the simplest method is to use a jig in which the whole frame can be assembled in the correct alignment. The frame can then be either brazed in situ or tacked and removed for brazing. Some people say that brazing in situ causes unwanted stresses to build up because the metal is not allowed to move naturally under the heat of the torch, and indeed, if the frame is tacked and brazed out of the jig brazing must be performed in a definite sequence to minimise movement. What this sequence is I have yet to entirely figure out - suggestions on a postcard please.

In the end I decided to make a jig, tack, then take the frame out for brazing. As you will see in a later post there was some movement, about 1 mm out of plane. But let's face it, the only person who will notice this is me, and I think it's not too bad for a first timer.