Wednesday, March 9, 2011

Crankset Selection: Part 1

When it comes to selecting a crankset my main consideration is gain ratio.  There are a couple of  web sites out there that allow you to calculate this, but I have found that this one by Mike Sherman the most useful.  It not only  gives you the calculation for gain ratio (the Sheldon Brown calculation), but also gear inch, speeds at a particular cadence and a roll out table.  Basically what all this can help you determine is the appropriate cassette/freewheel and chainrings sizes to use.

Sugino Mighty, 144BCD, 53/42

For my sportif bicycle I am going to be using a six speed cassette with a double compact double crankset.  The reason for selecting a double crankset instead of a triple is because where I live and ride, there just simply aren't that many inclines, and the ones that are around can easily be tackled with an appropriate sized inner ring.  Initially I was pretty set on using a 1970's, 144 BCD (bolt circle diameter) Sugino Mighty crankset I had acquired a while back, which was fitted with 53/42 chainrings.  52/42 chainrings are pretty common for road bikes, but I feel that gearing is not too useful.  After reading up on 144 BCD cranksets I found that the smallest chainring I would be able to get for this crankset is a 41 tooth, which when matched with say a 48 tooth chianring would make for a lot of very close gear combinations.  What I am hoping to achieve with this crankset/gears is a setup where I can ride in the large chainring for at least 90% of  the time and only have to shift into the smaller chainring for climbs.

6-speed cassette, 13-15-17-19-21-24

To achieve a suitable chainring size I first figured about what gear I ride with most often.  On one of my bikes I ride at a gear ratio of 4.6 and on the other 4.75.  That being said, I would be trying to achieve this gear ratio in the middle of my cassette, meaning that when in the larger chainring and in the middle of the cassette I should be able to ride with a gear ratio close to my most common gear ratio.  The cassette I will be using has the following cog sizes: 13-15- 17-19-21-24, therefore, the large chainring should be 46 to achieve this.  Here's what the gain ration chart would look like for this setup:

         13     15      17      19      21      24
46   7.12   6.17   5.44   4.87   4.41   3.86

Enough about technical data and chainring selection.  Next posting I'll outline my options for possible cranksets based on chainring compatability and quality.

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