The Bicycle Transmission and the Compact Crankset

The Bicycle Transmission and the Compact Crankset

I first wrote this article for Slowtwitch in 2004. At that time very few companies made Compact Cranks with FSA the only one selling well. The response from some of the Guru’s of the sport on the Slowtwitch Forum was to say the least scathing. I knew I was right and after getting support from the editor Dan Empfield some voices started to come around grudgingly.

compact cranks
In 2008 all the major component makers made Compact Cranks in their line up and there were many more of the less well known manufacturers making these cranks. This made me feel fully vindicated and I have now updated the article to discuss present day.

The Bicycle Transmission and the Compact Crankset
Those old enough to remember the muscle cars of the 60’s will remember that at most they had 4 speed manual gears and with automatics there was only three. The engines in these cars could put out high torque and horsepower over a wide range of rpm. This made the need for more gears unnecessary. Today’s high performance car engines produce their maximum power in a narrow speed (rpm) range. To get the most power to the road the transmission of these cars have closely spaced gears. They also have enough gears (5 and sometimes 6) to keep the engine turning over in the ideal rpm range up to their max speed. Human power output is a bit like a modern high performance engine but with tiny amounts of power. We suffered in the good old days of 10 speeds and less but now with 20 speeds we are getting what we need.

So what is the ideal human engine speed? While some research suggests the most efficient cadence (rpm of the pedals) for cycling is in the 75rpm range top cyclists typically maintain a cadence of 90-100rpm. Slower cadence needs more strength for one revolution and recruits more Fast Twitch muscle fibers. Conversely Slow Twitch fibers are mainly used at higher cadences. Since the fuel in our bodies used to fire the Slow Twitch fibers is much more abundant (fat) the endurance racer should ride around the 90rpm range. Here is a link to a detailed cadence discussion http://www.slowtwitch.com/mainheadings/coachcorn/cadence.html

One of the changes Lance Armstrong made to his technique after recovering from cancer was to significantly increase his cadence. Watching him power up hills at 95 rpm is awesome and clearly effective. Tyler Hamilton also made the case for maintaining a high cadence in his breakaway stunning performance in stage 16 of the 2003 Tour de France. He was using a “Compact” Crankset for this race, and he was racing with a broken collar bone. The rational for use of Compact cranks follows.

The conventional solution to maintain high cadence on a climb is to increase the size (number of teeth) of the large cog on the rear wheel. The production 10 speed cassette (the group of gears on the rear wheel) has 11 teeth on the smallest cog and a minimum of 21 on the largest (an 11-21 set). The next size up is an 11-23 which spreads the gears apart; not a desirable change but it does give you a slightly easier low gear. To get a 25-tooth cog gear most manufacturers configure the set 12-25 and the next size up is 12-27 (Sram is an exception to this making a 11-26 and 11-28, ideal for many riding the Rockies).

The problem with a 12-27 cassette is the gears are spread apart even more than the 11-23 and you have lost some top speed potential. However, if you need the big cog to get up a hill you have no other choice (unless you went with a triple chainring). That is until the Compact Crankset came on the market.

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