Bicycle Mechanics - Cassette Size REdux!

One of the lost threads from last week was about cassette sizes for a road bike.

A poster said that he/she swapped a 16 for a 12 and made a 13x27 cassette.

I'd love to try this as I rarely use my 12, but have a couple of questions......

1. I can't seem to find a solo 16 for my Shimano Ultegra 9 sp.

2. Is there anything special about the 12 being the last one in line? I don't think there's a replaceable spacer that could be placed on the end of the 13, but am interested in specifics.

This seems like a terrific idea, but I need some help.

I'd also like to do the same on my other cassette swapping a 16 into an 11x23 to make it a 12x23.

Thanks

yes the 13 is special......email harris cyclery (aka Sheldon brown) they will send you what you need for about $20 bucks.

Yes, the first position cog has a built-in spacer, so you'll need to remove your 12 and 13 cogs, and get a first position 13 and a 16. You can order individual cogs from Harris, or you can get them from other cassettes. When I first made this conversion, I used a 13-23 105 cassette that I had on hand - 105 cassettes have all individual cogs, where ultegra and dura ace have multiple cog assemblies that share a spider. This chart: http://www.sheldonbrown.com/k7.html#9cassettes
shows which gears are linked together in modules for the various cassettes. loosescrews.com also sells some individual cogs, but they're $10-12 each, so buying a 105 cassette ($30 at excelsports) to get the two gears you need is close to the same price - you could get the 13-25, and end up with both a 13-27 and a 12-25. Also, you can use 8-speed cogs - they're very slightly thicker, but not enough to matter.