More on gearing

[dmaze]

Google found me this page on cadence and gearing. I know a very very long time ago I made a gearing chart for my old bike, internalized a couple of details ("one front shift is worth about 1.5 rear shifts") and lost the chart. Since everything is on the Internet now, you can find, say, the gear sizes for SRAM cassettes without getting down and counting, and find out which one your bike has by looking at its Web page. Comparing my bike to the locally popular Bianche Volpe, in gear inches:

Specialized Sequoia Elite					Bianche Volpe
	30	39	50			28	38	48
26	30.4	39.5	50.6		32	23.0	31.3	39.5
23	34.3	44.6	57.2		28	26.3	35.7	45.1
21	37.6	48.9	62.7		24	30.7	41.7	60.2
19	41.6	54.0	69.3		21	35.1	47.6	60.2
17	46.4	60.4	77.4		18	40.9	55.6	70.2
15	52.6	68.4	87.7		16	46.1	62.5	79.0
14	56.4	73.3	94.0		14	52.6	71.4	90.2
13	60.7	79.0	101.2		12	61.4	83.3	105.3
12	65.8	85.5	109.7		11	67.0	90.9	114.8

(The numbers are gear ratio times wheel diameter in inches; I'm assuming both bikes have the same wheel circumfrence of exactly 210 cm and working from there.)

The differences appear to be all about the rear cassette. "11-32T" doesn't sound that different from "12-26T", but that difference means the Volpe has a 23% lower lowest gear (even a little lower for having an 8% smaller chainring) and an 8% higher highest gear (again slightly less for the smaller chainring). The flip side of this is larger steps: the Sequoia has a geometric mean of 10% between gear steps, but the Volpe 14%. So I have better gear resolution, about the same top speed, but less climbing power. My understanding is that replacing the cassette is a reasonably routine bike-shop repair; it might be something to keep in mind if I find hills routinely defeating me.

Comments

[knell.livejournal.com]

Granny on my Roubaix (which has a 50-34T compact chainset) is 33.2" (retro unit, the "gear inch") and I use every bit of the lower range when climbing proper hills (such as the one my house is rather inconveniently at the top of). I'm not the world's best climber, so maybe I should give up and replace the 12-27T on the back with something a little more generous.

Changing cassettes is a pretty easy DIY job, but as you need to have the right tools (a cassette remover and a chain whip) it might well be easier and quicker to ask the shop to do the swapout when you buy a new cassette

[dmaze]

I will note that the aforementioned Web page mentions that civilized folk use meters-per-pedal-revolution as their base unit, and the spreadsheet does have that (as something imminently comprehensible). I was eyeing the Roubaix, but didn't actually try it.

[fredrickegerman.livejournal.com]

Thanks for looking that up again for me. :-)

I did a casette replacement on my old bike as I'd worn the first one out. Replacing with a stock cassette was cheaper than I'd expected (around $20, about the same cost as a chain) and labor with a tune-up included was trivial. No idea how much custom cassettes run.

I'll note that the difference between 26t and 32t at the bottom means that the Volpe requires a mountain shifter (Deore, I think with some letters) in the back. These derailleurs have the longer cage for greater chain take-up. The Tiagra shifter on your bike has a much shorter cage if you compare them.

As I noted elsewhere, the loose spacing at the top is a bit tough, esp as I'm not skilled at chainring shifts with the STI levers yet (and they seem to require double-pumping to move across the middle ring, or very deep shifting compared to the gentle touch on the back).