finally finished getting everything set up on my first conversion attempt. i put on a 42t chainring and 16t cog, bust out a new sram pc58 chain and find out that it doesn't fit. the bike is an 80's pinarello with horizontal dropouts, but they're the later medium length kinda ones, not the older really long dropouts.turns out the links come up just a bit short with the wheel all the way forward, but if i add a link i don't have enough room to get proper tension. i assume this would be a situation remedied by a half link. i've never used one, is there any downside to having to employ one?
i pulled the 42t chainring and put on a 48t i had, added a couple links, and behold it happened to be just right. 48x16 is ok but i like spinning it a bit more. i guess it's a matter of finding ring/cog combo's that'll fit? and/or just getting a halflink to help accomodate ring/cog combo's that fall in between?
You can keep the same chain length of your 48x18 by keeping the same total # of teeth. Keep 66 as the same total# of teeth. For example, 46x20 will give you the same chain length as 48x18.
There's also the rule of 4, meaning that you can add/subtract 4 teeth total from cog and/or chainring, and with chain length adjusted by adding or removing a link, your dropout position should stay the same.
Yes, I figure you'd be well advised to get a half link, so you can put on any gear combo you want. But the pc58 is an 8-speed (narrow) chain and the half-links are made for bmx or track (wider chains). So I don't think you can make it work that way. Optimally, you should have a full wide drive train (cog, chainring, chain) and a half link.
So the easy solution remains using something like 44/16: close to 42/16 ratio and it has the same chain length as 48/16 (1 less link)
The so-called "rule of 4" isn't really. I tried to figure the math once, but the short version is that the effect of removing a link will depend on your gear ratio, which determines the angle of your chainline from front to back.
Those with a higher gear ratio (and thus a steeper chain angle) will see less horizontal movement from removing a link than those with a lower ratio (and thus a chain angle closer to fully horizontal).
Of course, like I said, the math got to gross (though it seems like a relatively simple problem, even when fully parameterized, so I suspect I was doing something the hard way), so I can't tell you what it really means.
Anyhow, I use a 3/32" on my mountain bike for exactly the reasons you're thinking about it, and it works fine.