An Introduction to Gear Ratios

Have you ever found yourself out on a training ride and encountered conditions in which you just couldn't find a comfortable gear? Shifting into an easier one makes it feel like you're just spinning the pedals around, but shifting into a "harder" one causes you to slow down and makes your legs burn?

Or have you ever been standing next to a couple of gear-heads talking about the relative benefits of "compact cranks" versus "standard" and been embarrassed to ask?

Don't worry, you're in good company. Gears are simultaneously one of the most essential and misunderstood components of a bike. Knowing a bit more about how they work and the terminology that describes them can significantly help your performance. Choosing the right gear based on the circumstances is the mental component of cycling. Working smarter can prevent you from having to work harder.

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Let's cover the basics first. Your standard racing bike (triathlon or road) has 10 gears attached to the rear wheel, collectively known as the cassette. Each one of those gears is a cog. Up front, attached to your cranks, are the gears we call the chain rings. Typically you have two of those, known simply as the "big" and "small" rings, but some bikes are equipped with "triple chain ring" systems, using three rings. We'll stick with the two-ring model for this discussion.

In total, that gives you 20 different gearing combinations. The size of all these gears, whether a chain ring or a cog, is measured in terms of the number of teeth around its perimeter. A "12 cog" has 12 teeth on it. A "14 cog" has 14 teeth, and so on.

Chain ring sets come in two varieties, "standard" and "compact." A standard set typically has a big ring size of 53 and a small ring size of 39. By contrast, a compact set will have rings of 50 and 34 teeth, respectively.

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There are several variations using different sizes and combinations, however, so you might want to check your owner's manual or ask your local bike shop mechanic what gears you're running. The difference between 53 and 50 or 39 and 34 may seem tiny, but take those few teeth for granted at your peril! We'll see the impact the difference has shortly.

Moving onto your cog, the gear size may vary, but here are a few standard size orders:

  • 11/12/13/14/15/17/19/21/23/25 (this would be called an "11-25" cassette)
  • 11/12/13/14/15/17/19/21/24/28 (this would be called an "11-28" cassette)
  • 12/13/14/15/16/17/18/19/21/23 (this would be a "12-23" cassette)

There are others, as well, but there are three key factors illustrated by these variations. First, the typical lower and upper limits for cog size are 11 and 28, respectively. Second, there are options to go with a narrower range of gears. Third, there are multiple arrangements within the range you establish. All of these factors depend on what type of racing you do. We'll discuss that bit of nuance later.

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