Join Date: Mar 2003
Location: Grand Prairie, TX
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Looking at Bill's article, a 26" tire will be of 2.5" less diameter than an equal width 700C tire. Multiplying the difference by pi, and given an 83.75" loaded rollout diameter for a 700 X 28, that works out to slightly more than the difference between a 12T cog and an 11T cog. Given a partner who is not too keen on higher cadences, it seems to me that the difference is sufficiently great to be a solely decisive factor.
Nominal front derailleur capacity is quoted in teeth of difference between the large ring and the small ring, so as you increase the ring size you are decreasing the difference in ratio. The Shimano STI-type front derailleurs are adapted to work best with particular sizes of chainring, though close produces performance sufficiently good for some people. I prefer to use 53-39-28 rings with my STI Ultegra (FD) systems to copy road bike systems and improve upon them at the bottom end. This helps keep the cog sizes from getting too large, and keeps the ratio difference between consecutive cog choices close. A 39T ring in the middle can prevent a shift to the small ring that you would have had to make with a 42T.
Given that the chainring gearing is relatively fixed, if you are going to ride a road tandem like a road bike, for example, in a pack of road tandems or road singles, it is likely that you are best served by 700C wheels and tires.