Originally Posted by
jonwvara
But I'm kind of confused about the concept of third-step gearing. What's the shift pattern? If you shift them like a normal half-step you would get alternating big and small jumps. There's obviously something about this approach that I don't understand. But then, that's true of so many things. So very, very many things....
Shift pattern, from small / small, upshifting by 1/3 steps:
(F/R): 1/1, 2/1, 3/1 --> 1/2 , 2/2, 3/2 --> 1/3, 2/3, 3/3 --> ... --> 1/5, 2/5, 3/5
Basically 5 sequential ~10% 3-speeds. To shift it sequentially, which I doubt anyone really would, every 4th shift is a triple / double. Up or down one in the back, and all the way down or up in the front. The levers even move the same direction.
In practice, I'd probably start in a slightly-too-low inner-ring gear, like the 42x22, giving me 2 10% upshifts before I'd have to triple-clutch. Kind of like how I shift the half-step part of a 1/2-step triple, but without having to finesse the front downshift to avoid dropping onto the granny... that's what you're trying to do!
Given the appropriate bike, I'll probably end up building one. 50.4 BCD 5-pin cranks would seem to be ideal, since this is how the triples go together anyway. 48/44/40 gives a 101-29" range, which is pretty close to ideal for an old-guy road bike. And if I hate the shift pattern, which I might, you just lose the middle ring for a 48/42 half-step. Which I already know I like. You just trade 9.4% average steps for 14.1% ones. Whether that's a noticeable difference at all, let alone one worth getting weird for is left as an exercise for the reader.
My legs care, and I'm weird. And bored. And broke. So here we are.
--Shannon