How Strava Calculates Power

Rosie

February 07, 2012 21:40

Our Power Equation

The power produced while riding is made up of several components:

Power produced to overcome the rolling resistance of forward motion.
Power produced to overcome wind resistance.
Power produced to overcome the pull of gravity (in the case of climbing hills).
Power produced to accelerate from one speed to another.

The total power produced, P(total), is the sum of all four power components.

P(total) = P(rolling resistance) + P(wind) + P(gravity) + P(acceleration)

P(rolling resistance)

The power required to overcome rolling resistance can be described by the formula P = C_rr x N x v, where

P is the power required.
C_rr is the rolling resistance coefficient. We define this based on the type of bike (road, mtb, cross) you used.
N is the normal force of the bike and the athlete against gravity.
v is the rider velocity.

For more information about rolling resistance, see http://en.wikipedia.org/wiki/Rolling_resistance.

P(wind)

The power required to overcome wind resistance (drag) can be described by the formula P = 0.5 x ρ x v³ x C_d x A, where

P is the power required.
ρ is the density of air.
v is the rider velocity, relative to the wind.
C_dis the drag coefficient.
A is the the surface area of the rider facing the wind.

Because we do not know wind speed or air density during your ride, we assume no environmental wind conditions and an outside temperature of 15C. The drag coefficient is determined by the type of bike you are riding (TT bikes have less drag than mountain bikes). We use a constant for surface area. For more information about wind drag, see http://en.wikipedia.org/wiki/Drag_(physics)

P(gravity)

The power required to overcome the pull of gravity while riding up an incline can be described by the formula P = m x g x sin(arctan(grade)) x v, where

P is the power required.
m is the mass of the rider and the bicycle.
g is the gravitational constant, 9.8.
grade is the slope of the hill.

P(acceleration)

The power required to accelerate from one speed to another within a ride sampling window can be described by the formula P = m x a x v, where

P is the power required.
m is the mass of the rider and the bicycle.
a is the acceleration between your starting speed and your ending speed within the sampling window.

Strava Calculated Power vs. Power Meter

We have seen that in most cases our watts number are very close to the numbers provided by a Powertap or SRM. Note that Strava calculated watts are not the watts produced at the crank but the watts produced by the rider-bike system, this will create a slight difference between the powermeter data and the Strava watts. Lack of good chain lubrication and low tire pressure can rob you of the watts you see on your Powertap or SRM. Other reasons watts can be inconsistent include strong winds and bad elevation data reported by the Garmin. Our calculations are most accurate when climbing given accurate rider and bike weight.