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I am trying to calculate the power needed to hold a steady speed on an indoor trainer, so as I can train using power in watts. I set the bike up and put the crank at 90 degrees, then put a weight of 2.695kgs on the pedal to overcome the resistance, just on the verge of moving. The faster you pedal the more power is needed.

The crank length is 175mm The weight to overcome resistance is 2.695 kgs (converted to Newton’s 26.429) Cadence is 80 rpm

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Unfortunately, the resistance is unlikely to be constant with speed. But power is easily calculated as force times speed. –  Daniel R Hicks Jan 20 '13 at 14:07
    
What @DanielRHicks said is especially true with something like a fluid trainer, where the faster you turn then mechanism, the more resistance it gives. You'll also need to include your gear ratio in you calculations. If you switch to an easier gear, you shouldn't need as much weight (force) on the pedal. –  Kibbee Jan 20 '13 at 15:42
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As others have pointed out, this is unlikely to bear any resemblance to the actual resistance at 80 rpm, but if you wanted to turn your numbers into watts, the formula would be:

P[W] = F[N] * l[mm] * w[rpm] * 2 * pi / 60,000

P[W] --> Power [in Watts]
F[N] --> Force [in Newtons]
l[mm] --> Crank length [in mm]
w[rpm] --> Cadence [in rpm]

For your example, it comes out to just under 40 W.

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Take a look at TrainerRoad.com to see if your trainer is on their list and you may be able to use a Speed/Cadence sensor with an ANT+ USB stick in your computer to use what they call "Virtual Power". They don't actually measure your power output, but based on known power curves of the trainer at given speed/cadence they run through some formulas to give you a number as you spin away on the trainer.

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