Originally Posted by
Timmi
I've seen mentions that reaming was necessary from distortion after welding... but for the most part, in these forums, we are usually talking about better quality bikes made from better alloy tubing, which required brazing, so as not to overheat the tubing. If any reaming was needed to correct significant deformation from welding, then it would either have been over-heated in the first place or made from very low-grade tubing that is thick enough to allow for such temperatures and more leisurely manufacturing methods.
Brazing can also distort the tubing. Brass in particular requires the steel to be hot enough to become quite soft and thus susceptible to distortion. This is particularly an issue at the top of the seat tube, for a number of reasons. First, there are several tubes that all join in the region (seat tube, top tube, and seat stays), so the seat cluster gets quite a bit of heat while the frame is being brazed together. Second, seat tubes, as you note, are frequently single-butted, with the thin end at the seat cluster (because a seat post will be inserted and reinforce the area). The thinness of the tube make it more susceptible to distortion. Finally, the top tube is fixed at both ends, and when heated to brazing temperature expands, gets longer and exerts pressure on the softened, thin seat tube, distorting it. That's why seat tubes frequently need reaming. This is less of an issue with silver brazing, but silver is more expensive and requires closer tolerances for the joints.