Originally Posted by
davidad
If you go to a road long cage der. you can use a 30t cog on a 12-27 cassette. To make the change you will need a 13t first position 8pd cog. You remoive the 12 and 13 and use the spacer on the 30. Because the 13 and the 30 are at the ends of the travel the 8sp cogs will work fine.
Why use an 8-speed cog? I use 9-speed first position 13's on my 9-speed cassettes. In one case I used an 8-speed 13 and ground down the spacer to the 9-speed thickness for better shifting.
My favorite 9-speed cassette is a 13-26 that I put together from various parts including the middle cogs from an XTR. Very light with close ratios.