Optimizing bicycle gearing, chapter 42


*Warning — technical stuff ahead*

In terms of gearing, what a cyclist would really like is a more or less infinite range of choices, with more or less infinitesimal granularity. Those who don’t spend much time on a bike just take what they get. Those of us who do spend a fair amount of time on a bike (albeit less than in previous years) also take what we can get, but we look harder to see what’s available, and try to optimize for the kind of riding we do.

I fitted out my classic Richard Sachs bike for touring and mountains. It was (still is) a 30-speed, configured in what’s called half-step plus granny. The idea of the granny gear is pretty obvious: a set of low-end ratios for the uphill grinds. Half-step was once called Alpine gearing, back in the days of 10-speed bikes. The idea is to have a wide-ratio rear end, with the two chainrings at the crank selected to interleave the ratios. The crank then has three chainrings, the granny and the two half-step rings that are almost the same size. My half-step rings were 45 and 48 teeth, which looks pretty much useless to someone who doesn’t understand how it works.

Well, how does it work? Think of interleaving your fingers: the fingers of the left hand represent successive ratios on the one chainring; the fingers on the right hand represent the ratios on the other. This arrangement offers both a wide range and close spacing, which is great. The downside is that it requires a lot of shifting of the front derailleur, which is usually more trouble than it’s worth. It also requires considerable know-how, 99.9% of all riders don’t know and don’t care, and manufacturers no longer offer separable bits and pieces that allow this kind of thing to be built up.

When I damaged my Achilles tendons some years ago, I was no longer able to ride the hills, so when I got the new Trek bike last fall, I bought only a 20-speed configuration (2 x 10) and put a close-ratio freewheel cassette on the back. This set of very closely spaced ratios lets me optimize for speed and effort, but only within a fairly narrow range. As long as I was riding the flatlands, that wasn’t too bad: I didn’t need the range.

2008 10-speed 50-34, 16-27

Here’s how it looks, graphically. The red lines show the ratios available with the 34-tooth chainring; blue is the 50. Not a whole lot of overlap, and nice, fine spacing.

But… but… I do more than just commute to work, where the afternoon headwind is the hardest challenge. And it turns out that the current generation Shimano rear shifters only shift down by 2 ratios when I sweep the lever all the way across its range. For a flat cassette, that’s pretty unsatisfactory (the one on the Richard Sachs does 3, and that’s what I was expecting: boo, Shimano!).

The cassette that came with the bike was sitting on the shelf in the garage. Today, I swapped it out: 11-28, where the previous one was 16-27 (those are the number of teeth in the smallest and largest cogs, respectively).

Took it out for a ride (35 miles, 1200 feet of climb), and I think I’ll keep it. Observations:

  • The low end is just slightly lower, 28 teeth instead of 27. Lower is always welcome, so that’s fine.
  • The range on the small chainring (34) is as great as the range across both rings with the flat cassette. To do the same as before, I hardly need the 50-tooth ring at all. The small-small combination makes a little noise, so I don’t quite get easy use of all 10 ratios, but it’s close.
  • The 50-tooth chainring is pretty much wasted. Assuming a modest cadence of 95 rpm, I could push the high end ratio up to 35 mph. I might dream of needing that capability, but the fact is that it’s completely useless. Around here, grades steep enough for that speed are not straight enough. Maybe in the Rocky mountains, but I doubt it. On my check ride today, I did power a couple of downhills into the high 20s, 6 or 8 MPH faster than I could push the previous gearing. That’s enough.

So my next challenge is to see whether I can find a large chainring that will fit the Ultegra crankset. Looking at the graph below, going all the way from 50 down to 44 teeth would be okay, but anything smaller than 50 would be an improvement.


As before, the red ratios are for the 34-tooth chainring. Green is for the 50, and blue shows what a 44 would offer. Clearly, the high end of the green range is not useful.

(Soapbox: Most bikes, and most bikies, are geared too high. 53 teeth in front is not uncommon. As well as going slower than they could, there is a non-trivial risk of injury.)

We, who like to tinker, are always trying to optimize. Will this be the last round of optimization? Probably not, not until some clever manufacturer offers a semi-infinite range with semi-infinitesimal granularity.

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