Compact Chainrings - Why and How
The popularity of "compact" cranksets has caused us to install some sets, and to compare features and trade-offs. The most popular ratio is a 50/34 double chainring on a 110mm bolt circle, as opposed to the 53/39 on larger bolt circles (130mm for Shimano, FSA, others, and 135mm for Campagnolo). This compact chainset is designed for climbing and to provide less redundancy in gearing between the two chainrings. Here are a few observations that we have found:
- The cassette cogs need to match the new chainrings. If one uses a 13-26 (Campy) or 12-27 (Shimano), a 12-25 will be the proper choice. If one is using a 12-25 with the larger rings, an 11-23 will be a good match.
- The choice of front derailleur with the crankset is very important. We are finding the Campy compact front derailleur will only work with Campy cranksets. The FSA compact front derailleurs will work well with FSA and Stronglight. IRD makes a compact front derailleur and we will provide data once we try it.
- The shifting technique is a little more critical, with the need to "soft pedal" while shifting from the smaller to larger ring. This is good technique with any chainring, but is just more important with the compact rings.
With less redundancy in gears between the large and small rings, one will need to re-learn the gear choices for a particular terrian. This is not a bad thing, but does mean that one may be climbing a hill in the big chainring, or may need to use more chain cross by using smaller cogs in the small chainring, or larger ones in the big chainring. Modern chains are more flexible, so this crossing is not so bad, with exceptions. The use of the smallest cogs in the small ring may cause the chain to rub on the big ring, and the pins may cause the chain to jump. Be aware of what gear is being used, and learn consistent "shift points" so the decision to switch to the other chainring is automatic.
One last point is that a shift from one chainring to the other will require a larger span of cogs to get the "next" gear. With Campagnolo systems this is very easy in both directions, but with Shimano systems, a lot of clicks will be needed when going from the large ring to the small, with the transition from a large cog to smaller ones.
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