Are compact cranks the secret to conquering tough courses?
By Ian Buchanan, fitwerx.com
May 14, 2008 -- While they do not offer gearing quite as low as that found on a triple-chainring crank, compact cranks do offer lower gearing options than traditional double cranks, and compacts are typically less expensive, lighter and less mechanically complex than triple cranks.
Before switching to a compact setup, however, you should understand what you will give up and what you will gain. While most traditional double cranks have a 53-tooth big chainring and a 39-tooth small chainring, most compact cranks come with a 50-tooth outer ring and 34-tooth inner ring. Unless there are rear cassettes available that offer a higher gear than what you are currently using (an 11-23 instead of a 12-23, for example), expect to sacrifice your highest (fastest) gear in exchange for the lower compact gearing. That said, for those who want to maintain as high a top gear as possible with a compact drive train, we have successfully set up systems with aftermarket 52/36 chainrings.
In addition to changes in gearing, compact cranks change your bike mechanically. From a parts perspective, going compact can involve as little as simply swapping the cranks. However, this is not always the case. Other potential mechanical changes to be aware of before switching to compact gearing are outlined below.
Gearing jumps: The tooth difference between the inner and outer chainrings on a standard double crank is usually 14 teeth or fewer. For example, a 53-tooth outer chainring paired with a 39-tooth inner has a 14-tooth difference. Most compact cranks come from the factory with a 34-tooth inner ring and a 50-tooth outer ring, a 16-tooth jump. The larger the jump between rings, the less consistent shifting can be and the more likely the chain will rub the derailleur in more gears. If you are experiencing shifting or rub issues with compact gearing, changing the inner ring to a 36-tooth, to reduce the tooth difference to 14, can help.
Chain length: Smaller chainrings need less chain, and a link or two of chain should usually be removed when switching to compact to keep the chain taut and quiet in most gear combinations.
Front derailleur: Front derailleurs are notoriously finicky devices regardless of gearing. While Shimano says its standard front derailleurs work fine with compact cranks (and they usually do), there are situations where the results are not consistent. To address this, companies like FSA have introduced compact-specific derailleurs that are Shimano compatible, and Campagnolo has been making compact-specific derailleurs for its systems for more than a years.
Crank/bottom bracket selection: Stiffer cranks and chainrings almost always produce the most precise shifting, and the importance of stiffness to shift quality is magnified on compact designs. Use cranks, chainrings and bottom brackets that are as stiff as possible for the best results. Campagnolo’s carbon compact cranks and chainrings are stiffer than average, while external-bearing crank designs are some of the stiffer compact cranks on the market.
Over-shifting and chain suck: Outside of derailleur rub, the most common shifting issues we have seen with compact-crank designs include a greater tendency for the chain to over-shift and an increased incidence of chain suck, which occurs when the chain gets sucked up and wedged between the chainstay and the chainrings.
Wear and tear: Just as a 10-speed drive train uses less metal than a nine-speed drive train, compact chainrings are smaller and can wear more quickly. Expect to replace chainrings more often on a compact system, and be sure to keep your drivetrain clean and well lubricated.
650c issues: In a given gear combination, 650c bikes are already geared lower than 700c; so, many riders may not need a compact chainring on 650c. However, for those looking for a lower gear on their 650c bikes, be aware that the shorter and tighter rear triangles on many 650c frames can sometimes be less adaptable in this regard than their 700c counterparts. We have successfully installed compact systems on 650c bikes, but some have proven finicky. For this reason, all the items discussed above become even more important on 650c bikes.
Shifting technique: A smooth and consistent pedal stroke and shifting technique can go a long way toward allowing a bike to shift smoothly while lowering the likelihood of a thrown chain. This is even more important with a compact crank. When shifting, plan ahead, shift early, soft pedal (reduce your pedaling force) and focus on pedaling smooth circles at a moderate cadence for the best results.
Compact cranks are an excellent option for those who want lower gearing without the added complexity of a triple crank. When selecting a compact system, focus on the compatibility of the system with your other components, and consider systems that maximize crank, bottom-bracket and chainring stiffness for the most consistent results.
Ian Buchanan is co-owner of Fit Werx, a triathlon and road-cycling company that offers specialty cycling products and services. Fit Werx offers cycling and triathlon products, specialty fitting and analysis services, consultation and technology research. Ian can be reached at (802) 496-7570 or through the Web at www.fitwerx.com.