What is Power?

Power is the total energy or work an athlete is able to produce in a given time frame. On the Power Tap, work is measured as the force exerted on the rear hub times the distance traveled by the hub (Work = Force x Distance) while power is simply the total work divided by time and expressed as a watt or joule per second (Power = Work / Time = Force x Distance / Time = Watt) Since a given distance divided by time is essentially speed, (Power = Force x Speed). In practical terms, power is a combination of how hard and how fast a cyclist pushes on the pedals or how fast an athlete can overcome all the forces holding her back. Those forces include wind, gravity, and the junction where rubber literally hits the road. Accordingly, the power an athlete needs for a given speed is dependent upon factors like weight, aerodynamic drag, the road surface, and tires. Because these factors are different for everyone, when comparing athletes, power is best expressed as a power to weight ratio for climbing or a power to drag ratio for riding on the flats (Table 1).

Key:

Power = Work/Time = (Force x Distance)/Velocity = Force x Speed = Torque x Angular
Power = Joule/second = Watt
The Watts Calculator takes into account the main variables that effect power output on a bike.

Power Output to Weight (Watts per Kg) at Lactate Threshold

Gender	Female	Male
USCF Category 4-5	2.5 to 3.0	3.0 to 3.5
USCF Category 2-3	3.0 to 3.5	4.0 to 4.5
US Domestic Professional	3.5 to 4.0	4.5 to 5.0
Successful Pro Tour Pro	4.0 to 4.5	5.0 to 5.5

Approximate ranges for power output to weight ratios at the lactate threshold for men and women of varying ability.

Inputs Required Include:

1. Distance.

2. Elevation gain if any (a negative elevation gain can also be inputted and power or the decrease in power required will be calculated).

3. Body weight.

4. Bicycle weight.

5. Height.

6. Time in hours, minutes, and seconds.

The calculator assumes aerodynamic resistance based upon height and weight using a standard road bike. The rolling resistance for standard clinchers at 120 psi is used. Finally, it is assumed that the rider is not drafting, there is no wind, and that speed is constant (i.e., no accelerations or decelerations).

Based on these 6 Inputs, here are the Outputs:

1. Power required to overcome gravitational resistance.

2. Power required to overcome rolling resistance.

3. Power required to overcome aerodynamic resistance.

4. Total power required.

5. Watts per Kg.

6. Watts per Kg with bike.