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.

Now we come to the part of actually measuring gear size. Because we always use two gears in cycling (a chain ring and a cog), we assess gearing by the ratio between the chain ring and the cog. For instance, if we use an 11-25 cassette with a standard chain ring, our available ratios would be:

53:11 / 53:12 / 53:13 / 53:14 / 53:15 / 53:17 / 53:19 / 53:21 / 53:23 / 53:25

39:11 / 39:12 / 39:13 / 39:14 / 39:15 / 39:17 / 39:19 / 39:21 / 39:23 / 39:25

If you're thinking that this doesn't look very helpful at all, you're right. It's terribly uninformative. We know we have 20 different possible combinations, and now we've given them some ratios to identify them. So what? Ah, so what indeed. If we do some simple division on those ratios, we can express them as more easily read numbers. Take a look at what they look like when we do the conversion.

4.82 / 4.42 / 4.07 / 3.79 / 3.53 / 3.12 / 2.78 / 2.52 / 2.30 / 2.12

3.54 / 3.25 / 3.00 / 2.79 / 2.60 / 2.29 / 2.05 / 1.86 / 1.69 / 1.56

See anything interesting? It looks like some of those numbers are very close to each other. Let's highlight the similarities.

4.82 / 4.42 / 4.07 / 3.79 / 3.53 / 3.12 / 2.78 / 2.52 / 2.30 / 2.12

3.54 / 3.25 / 3.00 / 2.79 / 2.60 / 2.29 / 2.05 / 1.86 / 1.69 / 1.56

Aha! What we discover is that eight out of our 20 possible gear combinations are redundant! In other words, there's virtually no difference between 39:17 and 53:23. Why the redundancy? Well, because of the way your derailleurs align your chain, going with a 53:23 actually causes a situation known as cross-gearing, in which the chain has to bend laterally so much that it increases friction and causes unnecessary wear and tear on your drive system.

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The rule to follow is to always avoid "big-to-big" or "small-to-small." In this case, the 39:17 is the combination you want to achieve this ratio.

In fact, the design impetus to avoid cross-gearing explains the redundancy in gear ratios. Notice that the redundant combinations occur at the extreme upper end for the 53-ring and the extreme lower end for the 39-ring, which are the exact spots you would develop those undesirable "big-to-big" and "small-to-small" situations. Only one of those two ratios is the one you want.

So the next time you find yourself pedaling up a hill or into the wind and shifting between your chain rings as much as your cogs in search of a good gear, take a look and see if you're in a cross-gear situation. If so, decide on a "final answer" for which chain ring you want to use and then finesse your rear derailleur to an appropriate position. Odds are that if you get your gears straight, the rest will sort itself out with a few clicks of the shifter.

Safe (and smart) pedaling!

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