Admittedly, most cyclists know little as to what a gear actually means beyond “is it harder or easier to pedal?” And although you’ve probably heard the terms gear ratios and gear inches, the question begs to be asked: What are they? Why are they important? And why should you care?
Let’s take a look back on the penny-farthing, or high-wheeler, in order to set the standard. The penny-farthing is driven by a crank that directly turns the front wheel. Therefore, each full turn of the cranks means that the front wheel travels in one full revolution. Working with this logic, a 60-inch wheel propels a bicycle faster than a 50-inch wheel when both are cranked at the same cadence.
Today, though, we don’t have the option of changing out the wheel size on our bikes in order to change speed. Instead, we have chainrings and cassettes to get the job done. So, to start with a simple example, if your bike has a 29-inch diameter wheel, and the front chainring and rear cog have the same number of teeth, one turn of the pedals turns the wheel one revolution. And because the gear ratio of the front to rear is 1:1, it’s equivalent to the direct-drive system of the penny-farthing. However, what’s more likely is that your front chainring has 40 teeth and that the rear cog has 20 teeth. This ratio is 2:1, which means that each turn of the cranks revolves the wheel twice, or 58 gear inches. It would be the same as riding a penny-farthing with a 58-inch wheel.
Therefore, the formula used to calculate gear inches is:
“(Drive wheel diameter [in inches]) x (number of teeth in front chainring / number of teeth in rear cog)”
Now, to add a further bit of math to the equation, it should be noted that the gear inches are based off of the diameter of the drive wheel, not the circumference. Therefore, the “inch” calculation is not the distance that your bike travels; rather it’s a standardized number that can be compared to other drivetrain systems.
If you want to know how far your bike travels in one pedal revolution, calculate the “development” of the gear. This is done by multiplying the ratio of the front chainring to the rear cog by the circumference of the drive wheel, or:
“(Drive wheel circumference) x (number of teeth in front chain ring / number of teeth in rear cog)”
*Note: Circumference is “diameter x 3.14″ (C=πd)
Going back to our first example, a 29-inch wheel has a circumference of 91.06 inches. Therefore, using the 2:1 (40t front to 20t rear) example, the distance that your bike will travel with one revolution of the pedals is 182.12 inches. You now begin to see the importance of cadence in relationship to gearing. A rider who’s able to spin the 58-inch gear at 90 RPM will travel 1365.9feet/minute, or 15.52mph, while a rider who pedals at a cadence of 85 RPM will travel 1290feet/minute, or 14.66mph. The formula for miles per hour is “(gear inches x cadence) / 336.” It should be noted that, while gear development is usually expressed in meters, I have converted it to Imperial for consistency.
Given the formula for calculating gear inches and development, it makes sense that tire size affects the results. For example, if you use a 19mm tire, your gear inches will be a bit smaller, whereas a 25mm tire results in larger gear inches. And while this may not make or break your road race, on a track bike, where cadence is highly important, racing in a gear that is a few inches too small or too large has the potential to kill your legs. Additionally, if you’ve been at any race with juniors, the official’s call to “gear roll-out” is familiar. Junior riders are restricted to specific gear development measurements. Having a larger tire can throw off the development measurement, even if the gears themselves are within allowable parameters. If you have a junior, skip the calculations and just “roll out” and measure.
Also worth noting is that many gear charts refer to a 700C wheel as a 27-inch — corresponding to an older British wheel size (ETRTO 630). In actuality, a 700C wheel can vary from a diameter of 27.5-inches for a 38mm tire, or as low as 26-inches for a 20mm tire. For consistency, calculate a chart using your exact wheel/tire size, or use a consistent wheel/tire size in an existing chart. And while the numbers may not be exact, you’ll always be comparing apples to apples. There’re also several smartphone apps that calculate gear inches, cadence, and speed, and these are all based on exact wheel sizes.
The general rule-of-thumb is that, for each tooth added to the front chainring, the gear inches increase by two. Meanwhile, for each single tooth decrease in the rear cog, you increase gears inches by six. It does vary a bit by given tire size, but it’s a quick and dirty way to calculate how big or how small you’re going.
So, why is all this important? Well, to begin with, knowing your gearing helps when you’re deciding on a new drivetrain, whether it’s a compact crankset or changing out your rear cassette. If you’re contemplating changing your mountain bike to a one-by setup, a simple chart will help you determine which front chainring gives you the optimal gear range for both climbing and descending. Secondly, if you choose to geek out, you’re able to figure out exactly what gear you need to push, and at what cadence, in order to hit the speed that you’re aiming for in your next time trial or triathlon. An interesting note is that smaller gears are easier to accelerate but harder to maintain at high speed. Meanwhile, larger gears are harder to spin up but easier to keep at speed. So, for all those off-season cadence drills that you vehemently dislike, you’re able to prove that they’re worth it.
If you love metrics, add these to your arsenal.