| **Gear Ratios --> what is the math behind it?** | jimsawino
*Feb 12, 2003 2:20 PM* | | Currently, I have a 39/53 crank and a 12-23 cassette (dura ace, if that matters). I think the gears are 12, 13, 14, 15, 16, 17, 19, 21, 23 (9-speed). What is the formula to find the ratio? Thanks in advance. |
| **re: Gear Ratios --> what is the math behind it?** | DougSloan
*Feb 12, 2003 2:23 PM* | | Here's a spreadsheet that is easier than calculating:
http://www.midcalracing.com/gears.xls
# front teeth/# rear teeth x tire diameter in inches = "gear inches"
"ratio" is simply front divided rear |
| **circumference instead of diameter? Or measure your rollout** | Spunout
*Feb 13, 2003 4:19 AM* | | I mostly use ratios, that is front/rear. If you switch wheel sizes alot, the development (rollout) will be different, so there is that to think about. |
| **You might find this formula useful as well...** | joekm
*Feb 13, 2003 5:44 AM* | | If you have a typical 700x23 tire then, according to the documentation that came with my bike computer, that's a 2105 cm circumference. So, with a little playing with numbers, you can get this formula.
1 mph = 12.82 rpms at the rear wheel
So,
Speed X 12.82 X (# teeth in chainwheel)/(# teeth in cog) = CADENCE
...adapt as req'd if you're not running a 700x23 tire. |
| **Correction to above....** | joekm
*Feb 13, 2003 5:47 AM* | | Sorry about that, it's
(# teeth in COG)/(# teeth in CHAINWHEEL)
....I really should refrain from math until after I've had my morning coffee... ;) |
| **Make it easy.** | MR_GRUMPY
*Feb 13, 2003 8:49 AM* | | Use this: Front chainwheel/ Rear cog X 27 = Gear inches. An example would be: 53 X 12 = 119.3 Gear inches. Another example would be: 39 X 23 = 45.8 Gear inches. If you have a Mtn bike, just use 26 instead of 27. Yes, I know that some tires are fatter than others, but the difference is ignored in calculations. |
| **re: Gear Ratios --> what is the math behind it?** | jimsawino
*Feb 14, 2003 7:47 PM* | | all this infor has been very helpful to me. i like having more to think about than the girl on my stem when i'm on the trainer (i live in ohio - for now). thank you all. |
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