Originally Posted by
mrfish
True, but jeff^d is suggesting just adding one cog behind the biggest cog, not drilling out the rivets holding the largest three cogs to their carrier and somehow replacing them.
Downside to jeff^d's suggestion is that when you use bottom gear, the full force of two tandemists is transmitted through the single cog spline, assisted by the leverage from a 24x34 gear. The cog will likely rip through the splines of the cassette and deposit you on the ground, face first. This is why high end cassettes use cog carriers for large gogs. Another idea is to use a, pinned shimano cassette if you want to modify the cluster as you can unscrew the pins then reinstall them on the modified cogset. This ensures torque is transmitted by all the cogs in the cluster onto the cassette body.
Take a look at the last cog of a Shimano 10sp cassette. It's not flat like normal cogs. You can't just take the last cog from say a 9sp cassette and stick it on the back with a spacer. The cassette won't fit properly on the wheel.
I thought higher end cassettes use an aluminum carrier for two or three cogs to save weight. The Campy Centaur cassettes have now switched to all single cogs. Wouldn't there be more torque on the smaller single cogs?