Single Speed Gearing

Posted: January 30, 2015 by Mark in Uncategorized

One of the challenges in single speeding is picking your gear.

I made the following chart to help me pick my gearing.
Gear Chart

The “perfect” gear for a course can give you an advantage. The wrong gear can be frustrating. Gear choice adjusts time spend walking vs. pedaling, mashing, spinning, and coasting. On a single speed “perfect” is all about finding the balance. With the chart, I can find my gear ratio then look at the ratio table to see the speed that the gear produces at cadences from 120 to 40 RPM.

On a 29er with a 2.25″ tire the bike moves 0.00142 miles each time the rear wheel makes one full revolution. This means that the math for speed is simply 0.00142 * Ratio * Cadence * 60.

For example, a 32/16 gear is a 2.0 ratio. With a 2.0 ratio, one revolution of the cranks produces two revolutions of the rear wheel. This means that pedaling 80 RPM will cause the bike to travel at 13.7 MPH. The 2.0 ratio works well on gravel roads. A ratio of around 1.8 works well for a fit rider on hilly single track. A 2.4 ratio (or more) is good for pavement with 20 MPH at 100 RPM.

The ratio for a 32/16 is exactly the same as a 28/14 or a 36/18 but they ride differently. Rings and cogs impact drive train performance. Small cogs have problems holding the chain as it is bounced and torqued. In general the best cog size for off road is 16 tooth or larger. An 18 tooth cog will hold the chain far better vs. a 14 tooth cog so the 36/18 will perform better then the 28/14. I try keep the rear cog as 17 tooth or larger when riding off road.

The gear I use most often is a 1.78 ratio with a 32/18. For races, I switch gearing based on the race course. Last weekend I ran a 2.0 ratio because it was a smooth fast course with a lot of gravel roads.

My next race is the Beaver Dan and New Light challenge. The race has two trails with a short fast road segment to connect them. The key to the race will be the single track segments. With a lower gear, I can ride the single track segments faster but it will leave me under geared for the road segments. Picking a gear for this race is going is a fun challenge. I think something around a 1.8 or 1.89 ratio is the way to go for my current fitness level. This gear will be slow on the road section but will be fast on the single track.

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