Gear Inches

Over a century since the death of the penny-farthing we still quantify bicycle gearing in reference to a high wheel bicycle. Like the terms horsepower and album, gear inches quantifies today’s technology in terms of the past, left over from when safety bicycles with equal sized wheels took over the market towards the end of the 19th century.

High wheelers are the first and most efficient of fixed gears, with the crank arms directly attached to the axle and no drivetrain to speak of yielding a 1:1 ratio of wheel to crankarm revolutions. Given the design, it was the size of the wheel that determined and described the relative speed and pedaling effort of each bike. The larger the wheel, the harder to pedal and the faster a high wheeler can ultimately go. With chain driven bicycles the gear ratio (front chainring : rear cog, ex. 46:16 or 3:1) represents how many times the rear wheel turns for each crank revolution, with a 3:1 ratio meaning the wheel revolves three times for each turn of the pedals. As bikes with smaller wheels and chain drives appeared and gained popularity, with the rear wheel turning at a speed different than that of the cranks, it was necessary to describe them in the familiar terms of the high wheeler.

Gear inches is an expression of bicycle gearing equivalent to the diameter of a high wheel—one crank revolution of a modern bicycle geared at 70 gear inches moves the bicycle forward the same distance as a penny-farthing with a 70” wheel. Note that gear inches is not an expression of the distance forward a bicycle will travel in one crank revolution but merely a measure used to equate modern bicycles to high wheelers. Simply multiply gear inches by a factor of π to calculate the inches of rollout or “development.”

Gear Inches = Wheel Diameter in Inches X (Number of Front Chainring Teeth / Number of Rear Sprocket Teeth)