Originally Posted by
Road Fan
I think Ultra-6 was a way to sell the 6-speed innovation to owners of 120 mm bikes as the lowest cost upgrade - no cold-setting or axle replacements, just change the cluster and chain, which were seen as wear items, anyway.
As a side benefit it let SunTour lead the way in getting field experience with narrower chains and narrower rear pitch.
Sorry, no experience with 6's specifically in the '60s. What I will say, is that the racing community was very conservative, because broken bicycles don't finish races, and one cannot win a race that one does not finish. So the racing world was not the agent for change across the board. I doubt that axles and spokes were weaker by much than what we have now - stainless was an available as a material, as was CrMo for axles. So upgrades to stronger spokes and axles were available to the industry. Perhaps dishing was an issue before the pitch of a 6 was reduced to fit the total width of a 5, but I really don't know.
Whether 6-speeds would lead to wheel rim problems with 120 spacing would vary greatly, so it was up to the bike's mechanic to avoid using too light of a rim, insufficient tensioning or too much variation of spoke tension.
With attentive tensioning, I believe that most of my converted wheels are stronger than they were when they came into my hands as five-speeders.
The durability of spokes has a lot to do with their processing, including quality control and testing.
Also, the purity of the metal and not just the alloy specification. That (and the exact processing), determines the grain structure within the spokes which has an enormous effect on the spoke's lifespan.
That said, quite-good spokes have been around for a long time by now, and I have yet to break even one of my older Schwinn's butted, galvanized (and hard-ridden) spokes.
Lower-priced hub axles also are potentially much better today, with automated manufacturing processes making it much easier and cheaper to achieve better control of things like the concentricity of the hole within the axle. Many of today's axles omit any slot along the threads (not that breakage is a common problem with today's freehubs).
Quality-control practices have improved greatly in mass-produced parts, quality control that formerly relied on a factory worker's energy level throughout the day.