Gear Inch Calculator Chart

What’s a “gear inch calculator chart"?

You can see one below all this text.

It's nothing you NEED to know about. On the surface it's geeky, but it's actually helpful or useful or at least interesting.

Gear INCHES is the classic American way to figure out whether a 36 front x 28 rear combo is harder or easier than a 40 front x 32 rear. A way to objectively, instantly, compare how hard or easy different gear combinations are. 

For instance, if your current bicycle has a low gear of 28t front and 32t rear, but you're getting a new bike, maybe even from us, and the crank we suggest for it has a 24t small ring...what size rear cog should you get to have the same pedaling grunt?

Or if your current high/hard gear is a 48t front x 13t rear and you're getting one of our bikes with a 44t front and you want a little bit higher gear on this bike, should you go with an 11t or 12t small cog?

A gear chart (short for "gear-inch chart") will tell you instantly. It's even better to know how to figure it out yourself mathematically, and it is easy as pie: You divide the rear cog's teeth into the front chainring's teeth. You can either look at the decimal number, or you can multiply by a nominal wheel diameter in inches: 29 for a fat 700c wheel, 27.5 for a fat 650B wheel, or 26 for a fat 26-inch wheel. 

Like this: a 42 front x 21 rear: 42/21 = 2.

a 36 front x 18 rear: 36/18 = 2

So a 42 x 21 and a 36 x 18 is the same goddamned gear. 

If you have a 40t front ring, what size rear cog would you need for the same gear as a 42x21 or a 36x18?

Yes, a 20t rear cog.

 In the '70s through the mid-'90s when most riders knew about this, the standard multiplier was 27, because a standard racing wheel was around 27-inches in diameter. With any of those combinations, the rear cog goes into the front ring exactly 2 times. Two x 27 = 54. The answer is expressed in inches, so...54-inch gear.

Do a 24t front x 36t rear combo

First divide 24 by 36. Then multiply by 27. Whatja get?

It should be eighteen. Any gear below twenty is considers  "pretty damn low."

How did "gear inches" come about? You don't have to know this, but:

Back in the High-Wheeler years from about 1870 to about 1887, the pedaling gear you had was determined by the diameter of the wheel, because the crank was attached to it, so one revolution of one meant one of the other, too. In that case, Wilt Chamberlain would win all the flat-road races, because his shorter competitors would be twirling too fast on a smaller wheel.

When chain-drive bikes came about with their cogs attached to the rear wheel, the English “gear-inch” concept came about, which allowed a rider to pick front and rear sprockets and be able to compare the distance-traveled per revolution to that of the high-wheelers.

On this chart, the numbers derived by dividing the rear cog into the chainring, then timesing that by the wheel diameter, is the equivalent wheel diameter of your great-great-grandfather’s high-wheeler. That is combo bike history and trivia, but is relevant if you wanna know which is lower, a 34front x 36rear, or a 24front x 32 rear. The lower the gear-inch number, the easier the gear. 

In Europe they have another way, more complicated to us Americans. You don't have to learn either, and for sure not both, so we're going with "gear-inches."

Sheldon Brown had problems with both ways, and developed a better way, called "gain ratio," that, typical of him, was simultaneously ahead of its time and too late and overestimated anybody who wasn't Sheldon Brown or a mathematicial/physicist to understand it, but you can look it up.


Download the .xls file of the gear inch calculator chart here