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Old 05-17-06, 04:39 PM   #1
huhenio
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Cranky about cranks ....

Using PowerCalc to predict performance due changes in crank arm length.

So I got a hold of a set of 175s in really good shape for 20$. The difference in the actual cranks and my older cranks is that the new ones have the five arm configuration, so changing rings for fine tuning my gear inch development would take 15 minutes versus impossible.

This are my findings using PoweCalc

Problem: 7% hill on 74 g.i. is a bit tough

170 = 51.14 pounds @ pedals @ 52 rpm = 08.00 mph. 291 watts @ pedals

175 = 51.14 pounds @ pedals @ 36 rpm = 11.22 mph. - 425 watts @ pedals!

I do not get it I will use less pressure on the pedals, yet wattage increases.


Problem: 10% hill @ 5 mph on both cases.

170 = 71.28 pounds @ pedals. 247 watts.

175 = 68.87 pounds @ pedals. 247 watts.

Is Wattage a function of pedal pressure?

In my opinion, it should be . But PowerCalc seems to disagree.

Who knows this stuff well?
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Old 05-18-06, 09:31 AM   #2
Eggplant Jeff
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This part:

170 = 51.14 pounds @ pedals @ 52 rpm = 08.00 mph. – 291 watts @ pedals

175 = 51.14 pounds @ pedals @ 36 rpm = 11.22 mph. - 425 watts @ pedals!

Is broken. You changed gears in there somewhere. With the same gears, the MPH will be the same at the same crank RPM, no matter how long the cranks are.

Somehow, while changing the crank length, you also doubled (or halved depending how you look at it) your gear ratio. I.E. you went from 48/18 to 48/9.

The only thing crank length affects is input torque (which doesn't affect speed because a bicycle has a fixed transmission, no torque-based slippage) and distance travelled by your foot.
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