The gearing we have on our trikes is a most signifcant thing. Some trikes only have one gear, some have 3, 5, 8, 12, 14, 21, 24, 27, 30, 81, 90 or some other number which is obtained by means of a multiple speed internal hub in addition to the derailleur system. Of course, for a trike with only one gear there is not much involved in figuring out the “gear inches”. What are gear inches, you ask? Actually it is not all that simple to answer definitively. There are variables which complicate things. One such variable which seldom is discussed or taken into consideration is the length of the crankarms. Usually it is just the various sprocket sizes (number of teeth) and drive tire diameter and circumference measurements that are used in the calculation. According to Wikipedia the definition of gear inches is: “the diameter in inches of the drive wheel of a penny-farthing bicycle with equivalent gearing.”
The lower the low number is the lower the gearing is which means one can more easily climb hills. The higher the high number is the higher the gearing which means a faster top speed is obtainable. So the ideal gear inches would be very low to very high. On a derailleur system the rear derailleur is only capable of handling so much of a range. I have an article on this subject HERE. The only way I know of to get around this limiting factor is by employing internal hubs in addition to the derailleur/multiple sprocket system. The internal hubs can be in the rear wheel hub or in the crankset or both. One can greatly increase the gear inch range by using these internal hubs. You can lower the low number and raise the high number. Here are examples of internal drives:
3 speed Sturmey Archer rear hub 2 speed Patterson Drive in crankset
For those who want to figure it all out for themselves here are some formulas to use:
Gear-Inches = Chain Ring Teeth X Tire Diameter Divided by Rear Cassette Sprocket Teeth
Distance traveled per Crank Revolution = Chain Ring Teeth X Tire Circumference Divided by Rear Cassette Sprocket Teeth
And lastly … MPH at 60 RPM = 0.0568182 X Distance per Crank Revolution
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