Q: I am a new cyclist who watched the Tour de France on TV. I’m confused about gearing. I have no idea what’s meant when commentators use numbers to talk about gears. And I’m not sure how this applies to my riding, either. Can you explain? — Sally P.
A: COACH FRED: Gear inches give you a way to compare gearing that comes from different combinations of chainrings and cogs. For example, if you’re in 39×15, how does that compare with the 53×21 gear that your friend is using? Even though he’s in the big chainring, you’re actually in a bigger gear — 70 gear inches to 68.
Also, if you calculate the gear inches for all the chainring/cog combinations on your bike, you can spot gaps or overlaps. You may want to remedy these things by changing to different cog or chainring sizes. This was more important back in the days when all we had were 10 or 12 gears. Modern drivetrains with 18 to 30 cog/ring combos make it a lot less necessary.