By Fred Matheny
Q: I’m a new cyclist and confused about gearing. I have no idea what’s meant when other riders use numbers to talk about gears. And I’m not sure how this applies to my riding, either. Can you explain? – Sally P.
A: Good question! Lots of new riders are confused about gear designations. It’s pretty simple, though.
For example, a gear of 34×17 means the 34-tooth chainring in front (this would be the standard small ring on a compact cranket — another term for you) is combined with the 17-tooth cassette cog in back. When you see or hear a designation like this, the chainring is always the first number.
Now, there’s the matter of gear inches. Gear inches are derived by dividing the chainring teeth by the cog teeth, then multiplying the result by the wheel diameter. A 39×17 gear equals 62 gear inches (rounded off). So in this case, 39 divided by 17 equals 2.29, multiplied by 27 equals 61.94 gear inches.
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.
Why use 27 for the wheel diameter? It’s a throwback to when a 27-inch wheel was the common size for road bikes. Nowadays the nearly identical 700C wheel is most common, but 27 is still used to construct gear charts. If your bike has 26-inch wheels, multiply by 26, and so on.
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 day when all we had were 10 or 12 gears. Modern drivetrains with 18 to 30 ring/cog combos make customizing a lot less necessary.
During more than 30 years in cycling journalism, Fred Matheny has written hundreds of fitness & training articles for top bike magazines and websites. Many of his best eBooks and eArticles are on sale in the RBR eBookstore. As a rider, he has raced to medals in state and national championships, plus a senior world record in the Team Race Across America. As a coach, he has worked with hundreds of riders at PAC Tour Training Camps, Carpenter/Phinney Bike Camps, and Dirt Camp.