Large chainrings and small rear sprockets result in high gears for going fast. Small chainrings and large rear sprockets result in low gears for climbing hills.
Before you alter your gearing, it helps to be able to see the consequences of different combinations and to compare them with other cycles. To display the gear chart for your cycle and use it to choose your chainrings and sprockets...
The number of Gear inches shown for each chainring/sprocket combination indicates how easy or hard it will be to push that particular gear. Small values are easy to push and high values help you go faster. Use the chart to compare different combinations and to select those appropriate for your kind of cycling.
For derailleur gears:
For hub gears:
You may come across other ways of specifying cycle gearing:
Gear inches, used in the charts above, are normally used in english-speaking countries. It has its origins in the 1890's and specifies the diameter, in inches, of the front wheel of an equivalent penny-farthing bicycle!
gear inches = wheel diameter (ins) × no. of chainring teeth no. of rear sprocket teeth
Développment is used in french-speaking countries. It's the distance travelled on the ground, in metres, for one revolution of the cranks.
développment = wheel diameter (m) × π × no. of chainring teeth no. of rear sprocket teeth
Velocity Ratio or Gain is not in common use, but is more scientific than either of the above methods as it takes the important effect of crank length into account. It's a true ratio, so you can use any units as long as you don't mix them - a gain of 5 means that your cycle moves 5 metres along the ground for every metre you move the pedals, or 5 inches for every inch.
gain = wheel radius × no. of chainring teeth crank length no. of rear sprocket teeth
© Chris Bell