Bike Forums - Threading History

Quote:

Originally Posted by Amesja (Post 12631873)

Back to threading -it is fascinating stuff.

The question I ask is why did the Taiwanese continue to produce bikes with 26tpi threading when they scaled up production for the early Raleigh-USA bikes? Did they have a big stock of English-made cups they wanted to use up? 24tpi cups were readily available as that is what everone else was using in the Asian factories. I would have thought it would have been a good time to make the transition. Tooling up for 26tpi bb's and forks must have been actually harder than using the standard of the area, but I've worked on these bikes and they still have 26tpi threads for some reason. I can't believe they shipped the tooling all the way from Nottingham to set up their factories. Did they? Or did they spend extra money to make new machines to conform to a backwards standard? Why?

I can't answer "why?" But it is likely that your first guess is correct. They might have shipped their equipment and inventory to Taiwan and used it up. Seems a reasonable assumption. They probably had literally tons of BB shells and steerers. You could sell them for scrap, but that's for ha'pennies on the pound (Sterling.) More economical probably to ship it and use it.
Even if they didn't, it's not like you'd have to make new machines. Just change the part that does the forming. If you're using a lathe to cut them, it's just a change of gearing for the pitch, and a cutter for the form. Cutters, rollers, taps and dies are consumables, they get worn in the forming process anyway, so you'd always be needing new ones. No big thing for a toolmaker.
Raleigh dealers and warehouses in the USA and worldwide would also have had old inventory, and would have had no problem continuing to sell and work on the proprietary Raleigh stuff no matter its country of origin.

By the way, the Raleigh headset and BB threads were not just different threads per inch than BSC, but also different form. They used the Whitworth thread form, like the Italians did. "Because they could" is right! Their production and distribution was huge, and they could get away with using something nobody else did.

There are mechanical reasons for using finer pitch. First, it is less likely to unscrew from vibration. I don't know if there's a world of difference between 24 and 26tpi. But the original CEI standard for 1" thread was 26tpi. Second, it cuts less deeply, leaving more material for strength.

The latter is also a reason to use a 60° vs. a 55° angle. The steeper angle gives a deeper thread, and thus a smaller core. On a tube in particular, like a steerer, this leaves less metal in the threaded area. The depth of a 55° 24tpi thread is .0267". The depth of a 55° 26tpi thread is .0246". The depth of a 60° 24tpi thread is only .0222". So Raleigh, using the Whitworth form, used 26tpi to keep enough material for strength. (Interestingly, 1" x 26tpi, Whitworth form, is also a British Standard Brass (BSB) thread.)
Admitted, it clearly wasn't a huge issue, because the Italians were using 55° 24tpi all along for steerers. I suspect it had to do with the quality of steel and the precision of machining Raleigh was willing or able to use on its cheaper bikes, and the extra .0021" brought it into the margin of safety.