What special considerations are necessary when selecting the steel for an S&S equipped frame? Since the couplers are positioned where the tubing is typically very thin I was curious if a specific tubeset is necessary. Does fillet brazing vs tig welding affect the tubing selection? I plan on having a custom builder make me an S&S frame this year and wanted a little info before contacting the builder.
On tubing sellection vs. method of assemble not really. There are some tubings that require particular processes, and other tubing like air hardening that respond differently to high heat. But most tubing you would want in a touring bike is not too high tech and would be happy with either method of assembly. ( I just noticed that, I;m always thinkin touring, but perhaps you want something like an ultralight race bike, in any case most materials can be joined any of the three ways.
On the S&S site there is a section on retrofits, and they state that not all bikes are suitable. They mention that some tubes may not have a long enough thick end before the thin mid-section, to be fit with couplers. In other words, they seem to need to be fit over the thick end sections, and not over the ultra thin tubing in the middle. This would limit some tubing relative to the length of the end sections and the length of the top tubes. But it should allow even light tubing to be joined if the coupler falls on the end section. S&S only sell to the trade, so anyone you approach should be clear on these kinds of issue.