Bicycle Gearing part 1

I want to post about gearing, because that is what I am obsessing about.  I think that people enjoy riding more if they understood how to operate their gear systems better and if they have the gears they really need.  So I plan about four posts, starting with some general concepts about gearing, moving on to choosing the highest and lowest gears for your bike, and finally, some discussion about ways to optimize the gears that are in between.  There will be nothing particularly original here, but I might just put it together in a way that is useful for someone.

The function of gearing is to match power output from a motor (in this case, the bike rider) to the work being done at a speed (in revolutions per minute, RPM) that is most efficient for the motor.  That optimum speed varies for different riders and different skill levels somewhat, but it is usually pretty close to 90 RPM.  Riders can generate a lot of power at lower RPM standing on the pedals climbing a hill, or at higher RPM sprinting at top speed, but a cadence of 90 RPM or so is usually the most efficient for the long haul.

The biggest variation between riders is not their cadence, but their power output.  And anyone can generate more power for a short period of time than for a longer period.  Figure 1 provides a partial idea of how much variation there is between different people and the amount of power they can generate, along with the variability of this power output over time.

HumanPowerOutput2

Figure 1. Human power output. From D.G. Wilson, Bicycling Science, Third Edition. 2004.

When I was in my mid 20’s, I raced bikes pretty seriously for few years.  I was a category 3 rider, and with my results and experience, I probably would have advanced to category 2 the following season had I continued racing rather than going back to school. (In those days, category 4 was beginner, category 3 was local, category 2 was regionally competitive, and category 1 was nationally competitive.)  I was riding about 200 miles per week, with a regular rhythm of speed, endurance, recovery, and competition.  I remember estimating that I could hold about 23 mph for an hour, maybe a little better.  That calculates out to just about 200 to 280 watts for an hour, depending on what source you use for frontal area and drag coefficient numbers. The lower number falls right on the lower curve in Figure 1, the NASA “healthy men” curve.  Clearly, I was never a “first class athlete” (the middle curve in figure 1), but that “healthy men” curve is a pretty high bar for most of us. 

I am twenty six years older than I was when I last raced. I ride half as much as I did then, and half as hard.  Now, I estimate that I cruise for long distances at closer to 100 w.  When I do centuries, I am not at the front, but I am usually not the last finisher, either, and I usually have a lot of company.

For those of us without power taps, here is the calculated speed for different power levels on a flat road and no wind.  For the calculations, I used the frontal area and drag coefficients measured in a wind tunnel and reported in Bicycle Quarterly, autumn 2007.

  Drops Hoods
100 w 16.1 15.7
200 w 20.8 20.3
400 w 26.6 26.0

 

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  1. #1 by fubarbicycleproducts on December 17, 2010 - 5:38 pm

    Great blog ! awesome info! great work!

  1. Gearing Part II: In praise of low gears « Rolling and tumbling

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