The horizontal scale is nominal gear-inches (see end-note*) and each box is a ring/cog combination. Ring size is the number in the box and cog size is on the blue vertical scale. The graph illustrates gear ratios of different ring and cog combinations relative to one another and to the nominal gear-inch scale.
Click a box to choose a gearing or enter values in the form below.
Chain length – wheel position
|Chain length||Bottom bracket to wheel axle (c-c)§|
|Nominal gear inches*||= 27 × ∕|
|Actual gear inches||= /π × ∕|
|Rollout‡||m ( inches)|
* Gear inches has no current physical significance; it corresponds to the diameter in inches of the main wheel of an old-fashioned penny-farthing bicycle with equivalent gearing. [wikipedia]
Track racing convention is to speak of gear-inches assuming a nominal 27 inch diameter wheel. Thus gear-inch is simply 27 × ring∕cog. I follow this convention here in only two places: the horizontal scale of the Ratio selector/display graph and in the Ratios summary (marked with *). All other values are calculated using the actual wheel circumference specified by the user.
† The value of wheel circumference here is the basis for calculating the cadence, speed, time relations. Measuring circumference with a tape measure (or diameter and multiplying by π) is not very accurate. Nor is measuring forward movement of the bike without load. Measuring movement of the bike with rider in racing position is better. Counting wheel revolutions over one lap of race track’s measurement line is pretty accurate. This is the same problem as calibrating a bicycle computer.
If you choose a wheel size from the selector, the form enters a nominal circumference for you. This is a convenience – not a reference! The wheel diameter values are from Sheldon Brown’s Gear Calculator and he did not provide references on how he calculated these numbers.
‡ Rollout is how far forwards the bicycle moves under one revolution of the cranks.
§ The calculation of distance between rear axle and bottom bracket axle disregards chain elongation (which lengthens the distance) and slack in the chain (shortens it).