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Standard 53X39 or Compact Double 50X34 Chainrings?
When buying a bike, one consideration is which chainrings to choose. I absolutely prefer a double front chainring, having ridden a triple front chainring for years, which I deem too fiddly. Plus, it won’t work with DI2 electronic shifters as far as I know.
While there are some other atypical standard double choices, the choice boils down to either:
- 53 X 39 standard double chainrings or
- 50 X 34 compact double.
Along with an appropriate cassette to match to it— I recommend the 11-28 for anyone doing much climbing.
Update May 28, 2011: my 2nd Trek Madone 6.9 SSL uses a 50 X 34 compact double, and I can say that the friendlier 50 X 34 gearing is of great value on climbs, so much so that I suspect that eventually I’ll convert my first bike to a 50 X 34 also. I’m running the 11-28 cassette on both bikes.
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Compact double advantages
The compact double has some key advantages:
- The compact double weighs a bit less, and allows a smaller cassette for similar gearing, thus cutting weight from the cassette also (e.g., a 25-tooth gear instead of 28).
- The loss of 3 teeth on the big front ring can be compensated for with an 11-tooth sprocket, versus a 12-tooth for a 53 ring. At any rate a 53/12 is a ratio of 4.42 versus 50/12 for a ratio of 4.17 or 50/11 for a ratio of 4.55.
- Considerably lower and more tightly-spaced gearing is possible with the 34-tooth front ring, making an 11-25 a very attractive option, and allowing a 12-27 for when really low gearing is needed. Even a 12-23 is a bit easier than a 12-28 on a standard double.
For the graph below, the gear ratio is plotted: higher values are more difficult pedaling. For steep climbs, concentrate on the right side with gears 5/4/3/2/1.
The 11-25 has a tight spacing for cruising in just the right gear on moderate terrain.
Dropping to a 50x34 + 12-27 offers a very comfortable 1.26 ratio, a big help on really steep climbs. The 53x39 + 11-28 offers a more difficult 1.39, which is 10% harder, a crucial difference for some riders on very steep climbs.
In short, use of a compact double offers a big plus for both climbing and for cruising in just the right gear. Even an 11x23 cassette might make sense for some riders where there are modest hills.
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About 1.3 gears more for a compact double — 14%
As the graph below shows, a compact double essentially moves the gearing down one gear, giving you one more gear for climbing with a 34 X 28 instead of a 39 X 28 — a 14% difference in gearing that might be enough to save your quadriceps and knees.
The downside is that the 50 ring yields about 6% less top end: 50 X 11 instead of 53 X 11 — a fair trade since ferw riders spend much time in the 11-tooth ring.
All 10 gears
The graph below shows the entire gear range in the small front chainring (39 or 34 teeth).