Gear Inches

Most people would have the presumption that riding on a bicycle with smaller wheels equates to a "slower" riding experience as compared to one with larger wheels - with the same number of strides when pedaling, the distance travelled on a bicycle with smaller wheels would be less than that on a bicycle that has larger wheels.

This is not entirely true. A bicycle with a smaller 20-inch wheel size may not necessary be slower (or will travel less distance) than a bicycle that has a 26-inch wheel. Wheel size is certainly one of the important factors for cadence, but not the only factor.

What could be the other factor? The answer is gear inches.

From http://en.wikipedia.org/wiki/Gear_inches, gear inches can be expressed in terms of the following equation:

where

gi = gear inches

dwd = drive wheel diameter

fct = number of front chainring teeth

rct = number of rear cassette teeth

As you can see from the equation, the gear inches (gi) can be increased by using any one or all of the below:

1. using a rear cassette with lower teeth count (or decrease rct)
2. using a front chain ring with higher teeth count (or increase fct)
3. using a larger drive wheel (or increase dwd)

The modifications done to my Bike Friday tikit are item (2) and (3) and these make its "vital statistics" to become as follows:

1. the rear cassette has between 11 to 28 teeth (stock, unmodified)
2. the front chain ring has 60 teeth (upgraded from the previous 52 teeth)
3. the drive wheel is 18 inches (upgraded from the previous 16 inches)

My highest gear inch is therefore 18*60/11 = 98.2 and my lowest gear inch is therefore 18*60/28 = 38.6, which is pretty decent and comparable to a fast Tern Verge X20 or Tern Link P9, both having 20-inch wheels. I will have a tougher time climbing up steep slopes when riding at the lowest gear inches though, which I do not anticipate as a reality anyway.

Depending on the material cost that you need to incur, you may opt to use different combination of the above method to modify your gear inches.