More Obsessing on Gearing

posted Sep 10, 2009, 9:54 AM by Donald Vescio   [ updated Sep 12, 2011, 8:11 AM ]
After my recent experience at Timberman (see an earlier blog entry), I reconsidered the wisdom of using a single 60t chainring in front.  I realized that there may be times when it might be advantageous to be able to shift to a smaller chainring; the diffculty was that the 60t was so large that I couldn't fit a front derailleur on my bike so that it would shift correctly.  (The problem is both a clearance and derailleur geometry problem.)

The thing is, I really liked my 60t.  My optimal cadence is between 75 and 80 rpm, and the 60t accomodates this cadence band very nicely.  Also, on a subjective level, large chainrings just seem to role more comfortably for me than smaller chainrings, and I like having the option of having a really big gear for long downhills so that I can keep my cadence in control, while still being able to reach high top-end speeds.  (This is important, as I can catch a lot of riders on the descents who are better climbers than me.)

So what to do?  After Timberman, I swapped out the single 60t for a double 56,48 combination.  Even though I would seldom want to shift out of the 56, I thought that it would be nice to have a small gear option if the terrain becomes really, really steep.  I've been using this combination for two weeks now and find that it works well on relatively flat courses, but that I miss having the extra gear inches associated with the 60t when bombing down hills.    What I decided to do was run some calculations to see how fast I could go in different chainring combinations,  based on a specific cadence.  I chose a cadence of 80 rpm for my modeling, as this falls in my optimal range; I know that I can go at a much higher cadence if I have to, but this seemed like a good starting point.

Now there are two ways that you can calculate speed based on gearing and cadence.  One is to plug a formula into Excel and enter your variables; the other is to go to Sheldon Brown's Gear Calculator page (http://sheldonbrown.com/gears/) and work with his simple to use online application.  I chose the latter.



Sheldon's Online Gear Calculator

Sheldon's Online Gear Calculator is pretty simple to use.    In the gear units drop-down menu, I selected the MPH@80 RPM option; for chainrings, I entered 60, 58 (the largest chainring that I can use with a front derailleur that will shift), and 56.   For the cassette, I chose the stock 11-21 nine speed, which I normally use. 


Data Entered into Calculator


The resultant calculations are interesting:




What the chart above tells me is that at 80 rpm, I will be traveling at 29.7 mph if I'm in a 60x13; at the same cadence, my speed would be 27.7 mph in a 56x13.  Assuming that I could carry the same cadence for both of these gear ratios, the two mph difference could be significant.  Note that for a given cadence, the delta in speed grows greater as the gearing becomes larger: 35.1 mph for a 60x11 versuse 32.7 for a 56x11.

Is this delta significant in real world terms?  For me, I think so only if there are a lot of descents in a course.  On a flat course, I'll average between 29 and 31 mph; in this scenario, a 56x11 would fall nicely in my optimal cadence range.  If there is a large downhill--think the back side of Marsh Hill at Timberman, for instance--then I would want a much bigger front chainring.  Given that in most instances one needs to climb at some point in a loop course if it has great descents, a small chainring might be necessary, too, which excludes the use of a 60t (see above).

The goal, of course, is to match gearing to one's natural cadence and power bands.  By observing what gears I'm using during competition, I'll have  a better sense of whether I need to go with a bigger or smaller chainring in front.  Ideally, I like to have two bailout options: a small gear for going up hills, and a large gear for going down.  Based on the data above, it would appear that (for me) a 58t (the biggest chainring that I can use and still have a functional front derailleur), along with a 48 small, might be the best all-around combination to use for most of the courses in New England.   


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