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Asked by jmyerz
This is a great example of an old fashioned measurement system that is still clinging to life in the modern era, despite the fact that it’s hard to use, difficult to understand, and inaccurate.
So this should be fun!
Here’s the deal. Back in the 1800s, the first real bicycles were horrifying contraptions, aptly nicknamed “boneshakers.” Imaging cruising around the cobblestone streets of London on a bike that weighs fifty pounds, sporting iron (!) wheels. Not such a great experience, eh?
Regardless, it was the state of the art at the time, and one of the things people were concerned about was how far the bicycles would travel with one turn of the pedals. Since the pedals were fixed to the front wheel, one turn of the pedals produced one turn of the wheel. Smaller wheels pedaled easier, but didn’t travel far. Larger wheels were harder to turn, but took you further.
The measurement that people bantered about was the diameter of the wheel, measured in inches. On some of the penny farthing bicycles the front wheels where over fifty inches, and were raced by would-be Lance Armstrongs on tracks at a whopping fifteen miles per hour.
Over the last hundred years, bicycles have evolved. We’ve added gears and standardized wheel sizes, which really throws off the notion of using the size of the wheel for determining the ratio between pedaling and distance.
Never the less, human ingenuity has bent, broken, and resuscitated this notion of wheel inches into gear inches, which converts a gear setting on your modern bicycle to the diameter of a fixed wheel from an iron beast from the 1800s.
So, here we go!
Today, you can calculate your gear inches as follows:
Dividing your front gear teeth by the rear gear teeth gives you the ratio of how your pedaling is modified by your gearing, so you simply apply that to the diameter of your wheel and you’re good to go.
Working in metric? Hold on to your top hats. Europeans use a completely different (and similarly inaccurate) system called “meters of development” which measures how far your bike travels with one turn of the pedal. To convert, multiply the gear inches by pi, and convert to metric units.
So why do I keep saying inaccurate?
The problem is that both of these measurements completely miss an important variable: the length of the arms on your pedals (“crank arms,” as cyclists call them). The strength required to heave you bike up a hill is directly related to the ratio between the distance your feet travel and the distance the bike travels, and if you lengthen your crank arms, you change that ratio. In other words, you can have completely different gear inch measurements that require the same amount of strength and go the same distance because of different length crank arms.
If you want to compare your carbon fiber and titanium superbike to a iron wheeled boneshaker from the 1800s, go ahead and use gear inches. If you’re interested in the real relationship between your feet and your bicycle, use a measurement called “gain ratio:”
This accounts for all of the components on your bicycle that effect how hard you’re pushing when you’re rolling down the road: your gearing, the size of your wheels, and the length of your crank arms. As a bonus, it’s expressed as a ratio, which needs no conversion whether you’re measuring in metric or imperial units!
Next time someone asks you about gear inches or meters of development, I encourage you to politely bring them into the 21st century. Gain ratio is where it’s at — even if you’re racing boneshakers and penny farthings!
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