Sunday, April 15, 2007

Gears and Other Interesting Numbers

I used the Gear Inch & Shift Pattern Calculator just to crunch some numbers. Don't get the idea that I'm obessed with this whole gear ratio thing - I mean it's just a bike. Who really cares what the gear ratios are and how many shift combinations copy each other? What matters is whether the bike is fun and ridable or not. Anyway, just in case years from now someone else is deciding whether to build a bike and they're surfing the web for ideas and numbers and copying pictures to a special folder like I did... here are some more numbers.

My chain rings have 28, 39 and 50 teeth. The gear cluster on the back wheel has 14, 16, 18, 21, 24, 28 teeth. My rear wheel diameter is 26.5", so my gear-inch range is 26.5 to 94.64".
If you're not familiar with gear-inches, that means in low gear my wheel would feel like a 26.5" diameter wheel if the pedals were attached directly to the wheel (like a unicycle). In high gear it would feel like a 94" diameter wheel. The really useful numbers that come out of this is that at 50 pedal strokes per minute, in low gear, I would go 3.94MPH, and in high gear at 70 strokes per minute I would go 19.71. Those are pretty reasonable numbers -a good hill climbing torque, but a good downhill high gear with a few gears in between.

Whl Dia: 26.50 inches
Chainring: 28/39/50

Cluster: 14-16-18-21-24-28

1> 28x28 = 26.50"
2> 28x24 = 30.92"
3> 28x21 = 35.33"
4> 39x28 = 36.91"
5> 28x18 = 41.22"
6> 39x24 = 43.06"
7> 28x16 = 46.38"
8> 50x28 = 47.32"
9> 39x21 = 49.21"
10> 28x14 = 53.00"
11> 50x24 = 55.21"
12> 39x18 = 57.42"
13> 50x21 = 63.10"
14> 39x16 = 64.59"
15> 50x18 = 73.61"
16> 39x14 = 73.82"
17> 50x16 = 82.81"
18> 50x14 = 94.64"


I think I could probably do without the middle (39) chainring if I were starting from scratch. The lowest range and the highest range overlap. But - it's just a bike, and what difference does a 6 ounce gear make when I carry way to many extra pounds myself. I read somewhere that the cheapest, bestest, most cost efficient way to make your bike lighter is to lose a couple pounds.

And still more numbers if anyone cares:
Seat bottom to crank angle = 8 degrees above horizontal
Seat back to crank angle = 112 degrees
hip joint to crank = 34"
crank bearing center is 1" lower than hip joint (drops 1.75 degrees)
X-seam = 40.5"

Jack

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