With the ‘Ordinary Bicycle’, now more commonly known as the ‘penny-farthing’ or ‘high-wheeler’, one turn of the pedals equals one revolution of the driving wheel. The bigger the wheel, the further you travel. A bigger wheel was therefore equivalent to a bigger gear on a modern bike.
The size of the wheel gave a simple basis for comparison. The distance travelled is equal to the circumference of the wheel but, understandably enough, bicycles came to be described in terms of wheel diameter instead. The largest wheel size in regular use was about 60 inches.
As bikes driven by crank and chain developed, wheel size alone no longer allowed direct comparison. However, riders naturally still wanted to know how far they would travel for each turn of the pedals. In Britain and North America it became customary to describe bicycles in terms of their penny-farthing equivalent.
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Let’s take a simple illustration. A 60-inch ordinary will travel approximately 188 inches for each turn of the pedals. Now consider a chain-driven bicycle with wheels of 30-inch diameter (this is not a modern standard size but makes the maths easier). The circumference of this wheel is 94 inches. If one turn of the pedals makes the wheels rotate once, it will travel 94 inches.
However, having a chain drive allows different drive ratios to be used. A 2:1 ratio means this bicycle will travel 188” per pedal-revolution – exactly the same as a 60-inch Ordinary. A 3:1 ratio moves the bike 282” per pedal-turn. This is the equivalent of a 90-inch Ordinary wheel.
Of course a 90” Ordinary would only be rideable by someone wearing stilts, but the system of describing gear ratios as the equivalent of Ordinary wheel sizes persists to this today. Cyclists, particularly in the English-speaking world, still refer to a 90-inch or 100-inch gear.
An alternative system was and is more favoured in Europe, using the actual distance travelled for each pedal-revolution. The French tern for this is développement. On an Ordinary bicycle this would be equal to the circumference of the wheel. Naturally this is usually expressed in metres and centimetres rather than inches.
The European system is arguably more logical, but either system, used consistently, allows comparison between different bikes even if they have different wheel sizes. More significantly, for many of us, it allows us to untangle the relationship between different gears on the same bike.
These systems allow us, for instance, to compare an Ordinary bicycle with a modern one. A 60-inch wheel was big for a ‘penny-farthing’ but 60 inches is in fact a low-to-middling gear ratio on a utility or touring bike.
Road bikes will have top gears well over 100 inches. Most of us might use this only on a fast downhill stretch but pro riders will use these ratios regularly when launching an attack, in a finishing sprint or in time-trials.
At the other end of the scale, really low gears, 30 inches and below, are useful for very steep hills and are commonly found on mountain bikes and touring bikes expected to carry heavy loads.