Originally Posted by
smontanaro
Something I've wondered: Why in the world make the spacing different between different cogs? Is it just vendor lock-in? You've got a derailleur which I believe moves in a linear fashion, and a shifter with a circular groove for the cable to ride in. Given a certain angular movement of the shift lever, you should see the same linear movement of the derailleur pulley cage, right? If I'm screwed up in my thinking about one or the other, wouldn't a Shimano, SunTour, Campy, or other derailleur move in almost exactly the same fashion?
Please clue me in.
I am pretty sure that the wider spacing between the smaller cogs was to improve shifting.
Even with a slant-parallelogram rear derailer, with most sizes of freewheels there will be an increase in chain gap as the chain is shifted to smaller cogs. I've looked at this very closely using Suntour derailers.
So Suntour figured out that they could use slightly narrower cog spacing between the largest cogs and still get reliable, precise shifting performance, which allowed them to use the wider cog spacing where it was needed (between the smaller cogs) for good shifting, while keeping the overall freewheel width narrow.
As far as implementing this, they had to create an indexed shifter with uneven detent spacing, a freewheel with cog spacing to match, and find or make a narrow-enough chain.
They pushed the limits with chain width, cog spacing and the shifter's overshift freeplay however, which all conspired to make shifting to the larger cogs rather touchy as the chain often contacted (or shifted momentarily to) the next-larger cog than the one the rider was trying to shift to, and the adjustment range tolerance was positively miniscule by today's standards, even with fresh cabling.
This is why Suntour Accu-7 freewheels enjoy so much better shifting with narrower (outside width) 9s chain.
By comparison, the original Accushift (which was standard 5s/6s spaced) gave much more robust shifting action than the subsequent Accu-7, if a decent chain was used. The problem back then was that only Shimano made a decent-shifting chain, and really only after their UG-Narrow chain was introduced for use with 5, 6 and their (Shimano's) new SIS-7 cogsets. And yet the Shimano UG Narrow chain was
still on the wide side for use on Suntour's Accu-7 freewheels (which had both thick teeth and asymmetric cog spacing).
A stock Accu-7 drivetrain, with only the addition of a modern 9s chain, shifts much better and stays within a decent adjustment range far, far longer than it ever did back in the day, so much so that I think that a better chain alone might likely have saved the company.