The Bicycle Transmission and the Compact Crank Set

December 13, 2016

Simon Butterworth


tagged in:

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.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 CranksetThose 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 discussionOne 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.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 (52/36 are also on the market) teeth on the outer and inner ring vs. 53/39 teeth on road bikes and 54/42 or bigger on Tri bikes (Very very few amateur athletes should give any thought to a 54/42 chainring combination). There have been several articles about compact cranks in the Tri and Bicycling press in the past few years.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 optimum cadence as wind gusts and or small changes in elevation cause small speed changes.• Some maximum 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 (see sample weights of cogs and cranks below).None of the articles this author has seen have quantified the difference between conventional Cranksets and the Compact Crankset. Below is a examination of the differences between a 53-39 Chainring and a Compact (50-34) Chainring. The calculations are based on a 700c wheel with a cadence of 90rpm. Bold print marks a change in cassette size.For those of you wanting to play with different combinations of chainrings and cog sets here is the spreadsheet I created to do the calculations below. The spreadsheet is protected so you do not mess up the formula. It is limited to products from Shamino and FSA.53-39 Chainring, 11-21 Cog 50-34 Chainring, 11-21 CogMPH Biggest Gear 33.2 31.3MPH Easiest Gear 12.8 11.2Comments Changes between each gear are the same (cassettes are the same). The trade off is easier climbing as the expense of a small loss of Top speed. This is the lightest configuration for either set up.53-39 Chainring,11-23Cassette 50-34 Chainring, 11-21 CassetteMPH Biggest Gear 33.2 31.3MPH Easiest Gear 11.7 11.2Comments Low speed now closer. 11-23 cassette is heaver with gear ratios wider apart.53-39 Chainring,12-25Cassette 50-34 Chainring, 11-21 CassetteMPH Biggest Gear 30.42 31.3MPH Easiest Gear 10.74 11.2Comments 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 Chainring, 12-25 Cassette 50-34 Chainring,11-23CassetteMPH Biggest Gear 30.42 31.3MPH Easiest Gear 10.74 10.18Comments With slightly wider gear ratios the 11-23 cogs further ease climbing with the Compact Crankset while maintaining top speed.53-39 Chainring,12-27Cassette 50-34 Chainring, 11-23 CassetteMPH Biggest Gear 30.42 31.3MPH Easiest Gear 9.95 10.18Comments 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.53-39 Chainring,12-27Cassette 50-34 Chainring, 11-26 CassetteMPH Biggest Gear 30.42 31.3MPH Easiest Gear 9.95 9.0Comments 12-26 One of the new cog sets from SRAM, ideal I think for many racing the Peak with a Compact Crank.It is easy to see why the Compact Crank would be a big plus for older age groups or anyone who is not an elite athlete. 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 their cadence in a narrow ideal range.Selecting optimum gearing for a bike needs to consider 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 or 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 age grouper a 12-27 cassette with Compact cranks would eliminate the need for a triple Crankset (a much heaver solution sometimes used to get low gearing).Anyone wondering what to put on their new lightweight aero flyer they are buying this spring should seriously consider Compact cranks. Even if you don't have funds in the budget for a new bike you might want to consider a helpful upgrade. Compact Cranks are available from FSA, Shimano , Zipp (v expensive but very light and stiff) and others. The downside is you cannot just replace the chainrings on your standard cranks as noted above.Weights of Components discussed above (2004 data, products are now generally lighter).Shimano Cassettes11-21 145g 11-23 155g12-23 160g 12-25 171g12-27 177gFSA Carbon Pro Elite Compact Crankset 515g*FSA Carbon Pro Superlight Crankset 500 SHIMANO Dura-Ace 7701 594g*While the FSA conventional Crankset is slightly lighter than the Compact the rotating mass is less on the Compact because of the smaller ring diameter. The difference between these two Cranksets performance is probably negligible.

Coach Simon Butterworth has an experienced philosophy about coaching.  The key ingredients in a good coach/athlete relationship are regular and open communication, mutual respect, and keeping it fun for the athlete and their family.  His training programs are developed with those ideas at the forefront. He works with athletes to develop both short-term and long-term objectives that work well within the context of the other things they have going on in their life.

Coach Simon is a 2X World Ironman Champion and has 16 Ironman World Championships races to his credit. He has finihsed on the podium 7x.  He is a USAT Certified Coach, USMS Swim Coach, FIST Certified Bike Fitter and Training Peaks Certified Coach.

schedule a call