Compact cranks and gearing
4.28.04 by Simon Butterworth
(www.slowtwitch.com)

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 were 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 race car engines produce their maximum power in a narrow speed (RPM) range. To get the most power to the road the transmissions 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.

While some research suggests the most efficient cadence for cycling is in the 75 RPM range top cyclists typically maintain a cadence of 90-to 100 RPM. Slower cadences need more strength for one revolution and recruits more fast twitch muscle fibers. Conversely fewer fast twitch fibers are recruited 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 90 RPM range.

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 last year’s Tour de France. He was using a “Compact” Crankset for this race. Here is a link to Tyler’s own account of the day. 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 smallest production cassette (on a 9 speed) has 11 teeth on the smallest cog and 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. To get a 25-tooth cog gear manufacturers configure the set 12-25 and the next size up is 12-27 (other cassette manufacturers offer addition options, such as SRAM's 12-26).

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. That is, until the Compact Crankset came on the market.

Compact cranks have chainrings that are significantly smaller than conventional cranks and are the polar opposite of the big chainrings seen so often on Tri Bikes. Compact cranks typically have 50/34 (sometimes 36) teeth on the outer and inner ring vs. 53/39 teeth on road bikes and 54/42 or bigger on Tri bikes. The ability to use these smaller chainrings occurs because of the crank's smaller bolt pattern, 110mm instead of the usual 130mm or 135mm spacing. There have been several articles about compact cranks in the Tri and Bicycling press in the past year. The argument put forward in these articles has been:

• Higher RPM’s can be maintained on steep climbs because of a lower low gear.
• Closer spacing of the gears makes it easer to maintain the optimal cadence as wind gusts and or small changes in elevation cause small speed changes.
• Some maximal speed potential on down hills is lost, but unless you are sustaining speeds of over 33mph on the flats this should not be an issue.

• The combination of Compact cranks and appropriate cassettes has less rotating mass (lighter) than a conventional set up.

None of the articles this author has seen quantified the difference between conventional Cranksets and the Compact Crankset. The following examines the speed differences between a 53-39 Chainring and a Compact (50-34) Chainring. The calculations are based on a 700c wheel with a cadence of 90 RPM. Bold print marks a change in cassette size.

...::: 700C Wheel At 90rpm :::...

53-39 Rings, 11-21 Cogs 50-34 Rings, 11-21 Cogset
High speed 33.2 MPH 31.3 MPH
Low speed 12.8 MPH 11.5 MPH
Comments Changes between each gear are the same (cssettes re the same). The tradeoff is easier climbing as the expense of a small loss of top speed. This s the lightest configuration for either set up.

53-39 Rings, 11-23 Cogs 50-34 Rings, 11-21 Cogset
High speed 33.2 MPH 31.3 MPH
Low speed 11.7 MPH 11.5 MPH
Comments Low speed now almost the same. 11-23 cassette is heaver with gear ratios wider apart.

53-39 Rings, 12-25 Cogs 50-34 Rings, 11-21 Cogset
High speed 30.42 MPH 31.3 MPH
Low speed 10.74 MPH 11.5 MPH
Comments The 12-25 cassette lowers the top speed on the conventional Crankset and makes climbing easier but at the cost of even wider gear ratios and weight.

53-39 Rings, 12-25 Cogs 50-34 Rings, 11-23 Cogset
High speed 30.42 MPH 31.3 MPH
Low speed 10.74 MPH 10.18 MPH
Comments With slightly wider gear ratios the 11-23 cogs further ease climbing with the Compact Crankset while maintaining top speed.

53-39 Rings, 12-27 Cogs 50-34 Rings, 11-23 Cogset
High speed 30.42 MPH 31.3 MPH
Low speed 9.98 MPH 10.18 MPH
Comments 12-27 is about the biggest (heaviest) cassette seen on Tri Bikes. Almost the same climbing can be achieved with the 11-23 cogs with a compact Crankset with a higher top speed, closer gearing and reduced weight.

It is easy to see why the Compact Crank would be a big plus for older age groups. So why would Tyler Hamilton choose Compact Cranks? Gear spacing and weight are probably the answer. Even on flat roads there is usually some variation in elevation and the wind is usually not constant. Closer gear spacing allows small changes in gearing, letting the elite rider to keep his cadence in a narrow ideal range.

Selecting optimal gearing for a bike needs consideration of the abilities of the athlete and the bike course (wind and hills). It is not unreasonable to have different gearing for different races. With conventional chain rings on a flat course an 11-21 cassette might be ideal. On a hilly course (Lake Placid IM) a 12-27 would be much better. With compact cranks you could use an 11-23 for both races with similar results. If you are an older or less powerful age grouper a 12-27 cassette with Compact cranks would eliminate the need for a triple Crankset (a heaver, more complicated solution sometimes used to get low gearing).

Compact-style cranks are available from FSA, Pinarello, Stronglight, Campagnolo and others. If you want to switch to smaller rings, because of the required 110mm bolt pattern, you can't just buy new chainrings. You must buy the whole crank.

*(Simon Butterworth is a USAT Level 1 coach and F.I.S.T. Certified Bike Fitter. He has completed 6 IronMan races, three in Hawaii and has achieved All American status in USAT ranking for the past 6 years. He can be reached at tricycle@optonline.net).