Gearing part III: Gear sequence

The third installment of my gear cogitations addresses gearing systems and how to get low-enough gears without too much compromise.   

First, let me opine that the great majority of cyclists do not need gears as high as what comes stock on bikes these days.  In fact, most don’t need more than 100” ( the old-school 52×14, or with modern hardware, 41×11 ).   A 100” gear allows 27 mph at 90 rpm (27” nominal diameter wheel); most of us only hit this speed going downhill, and if the descent is too steep to pedal with this gear, it is more efficient to tuck.  This even goes for low-level competition cyclists.  As a category 3 rider, I once wore myself out pushing a 108” gear on a long downhill in a race, only to be passed and dropped by some of my team mates who had tucked on the descent and were relatively fresh for the subsequent long climb.  As for top speed and sprinting, a little practice gets a rider’s maximum cadence well about the 90 rpm that is sustainable for long periods — Charles Murphy set his paced record of 60 mph in 1899 on a 104” gear (at 198 rpm).  The 130” gear (52×11; 35 mph at 90 rpm) that comes stock on high-end racing bikes is good only for downhill sprints and for developing bad pedaling habits, unless you can keep up with Mark Cavendish.  For touring and town bikes, a high gear of 85” is not unreasonable, though most of will probably want to stick with 95” to 100” on the touring bike.

So here are my criteria for an all-purpose gearing system:

1)  As explained at length in the last post, with my topography, fitness level, and purposes, I like to have a low gear in the low 20’s or even lower.  People who live in flat places or who are strong climbers (and plan to stay that way) can adjust accordingly.

2) A high gear of 100” or a little lower works for me on the road

3) Gear ratios should be spaced closely enough to allow the rider to maintain a cadence within his/her comfort range throughout the gear range (or at least the most-frequently used portion of the gear range).  As a reference, 5% steps are real tight, allowing a cadence that stays between 90 and 95 rpm.  Most of us are happy with steps of 10-15%. 

4) There should be a logical shift sequence that is easily executed.  It used to be common for people to have a gear chart taped to their stems so they knew how to get to the next gear.  While it is not a bad idea to give some thought to how your gears are laid out, they should not require a map.

Figure 1. Hayduke diagram of 3-speed internally-geared hub with 44-tooth chainring and 19-tooth cog. 25% steps between gears; 178% total gear range.

Internally-geared hubs and single-chainring derailleur systems have the simplest  shift sequence.  Three speed hubs have a relatively narrow range and large steps between gears, which is okay for flat areas, and IMO a lot better than single speeds.  I was happy with a 3-speed commuter bike when I lived in flatter places and had a higher level of fitness than I do now. 

Seven speeds have smaller steps and enough range for short trips in hillier areas.  I currently use a Shimano Nexus 7-speed on my commuter/utility bike geared down for the local hills.  High gear is 80″; low gear is 33″.

Figure 2. Hayduke diagram of my Nexus 7-speed internally-geared hub with a 42-tooth chainring and 22-tooth cog. Average 14% steps between gears. 244% total gear range.

The wide-range double “compact” crank seems to be dominating the market.  This setup has a good range for many situations, though it does not go low enough for my purposes.  For me, the fatal flaw with this approach is the shift sequence.  The shift between front rings is big (30% or so), so usually when the rider make this shift,  it is necessary to correct 2 or even 3 cogs on back to get a reasonable-size step.  This shift is right in the meat of the riding range (50” to 80”) so the clumsy big front shift and rear correction happens frequently.  When I tried a similar arrangement, I hated it because the gear I wanted to shift into always seemed to require this awkward sequence of shifts.

Figure 3. Hayduke diagram of typical compact double with 48- and 34-tooth chainrings and 12-27 tooth 9-speed cassette. Gear range 108" to 34" (318%). Three pairs of near-duplicate gears; 13 usable unique gears. 7.8% average step between gears (neglecting duplicates).

The best way I have found to meet the criteria I laid out above is the step-and-a-half triple with granny.  In this arrangement, the step between the big and middle chainrings is about 1.5 times bigger (in percent) than the steps on the freewheel.  For example, a 9-speed 12-27 cassette averages 10% steps between the cogs; 39 and 46 tooth chainrings are about 15% apart (note that since we are confined to integer number of teeth, this frequently needs some trial-and error to find a combination that works best).

Figure 4. Hayduke chart for step-and-a-half-plus-granny gearing system. 20-36-45 chainrings and 12-36 nine-speed cassette. 15" low gear to 101" high gear (675% range). 7.4% average step between gears on middle and large chainrings; 14% maximum step. 24 usable non-duplicate gears.

Of course most riders are not likely to shift sequentially through the gears.  Usually, one would still use the chainrings as high-range low-range (with a double) or high-medium-low ranges (triple).  However, the  shift between chainrings is not as big as in a compact system, so you don’t need to correct as often on the cassette when you change chainrings; when you want the next sequential gear, you only need to correct by one cog.  The shift from the middle to small rings is still a big step, but this shift does not happen as often as the similarly large step between rings on a compact, because it is at the extreme of the gear range.

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  1. #1 by Jack on December 3, 2011 - 6:20 pm

    Great article. Really appreciate the effort – I’m new to cycling and had wondered about so many questions you answered. I did have to look up several things to read the article – like “step-and-a-half triple with granny”. However I managed to create my own gear calculator from your article. Thanks again for explaing in such detail.

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