Join Date: Sep 2008
Location: Loveland, CO
Bikes: Two LOOK 585s, one KG461
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Think in terms of percentages, not gear inches. At the top end, a shift from an 11T cog to the 12T cog is 8% or 10 gear-inches. At the low end, a shift from the 23 to 25 is the same 8%, but less than 5 gear-inches. Percentages make the most sense, when you consider that an average cadence is in the 100 rpm area. Each shift would create the same change in cadence or the same increase in available torque.
My other favorite formula is what I call "equivalent gears". When comparing different crank setups, a simple formula allows an easy comparison. For example, let's say you have a 53/39 with a 12-27 and want to know what setup with a compact provides about the same low gear. 27/39 x 34 = 23.5 That tells you that a 34/23 is not quite as low, but a 34/24 would be lower. 27/34 x39 = 31 That tells you that a 39/31 would be needed to match the low gear of a 34/27. The same forumla works nicely to compare triple cranks to a compact.