How does Strava calculate power?

In this guide, we will talk about how Strava calculates power. We will also see a breakdown of how Strava calculates power such as the formula for power to overcome rolling resistance, the power to overcome wind resistance, and so on. Even though you may see some formulas, there is no need to panic since it is not as complicated as it seems.

How does Strava calculate power?

If you wonder how Strava calculates power, here we have the answer. When we think about measuring power, you may picture it from a physics class or how movement through cycling produces Power. To measure Power, Strava considers that the total power produced or P(total), is the sum of the following components:

  • The power produced to overcome rolling resistance or the resistance of forwarding motion.
  • The power produced to overcome wind resistance.
  • The power produced to overcome gravity (when climbing hills).
  • The power produced to accelerate from one speed to another. 

Subsequently, we have the following equation:

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

Now let’s take a look at each of the components separately and as we have mentioned, it is all we won’t talk about physics but we will mention some of the formulas behind the Power measure.

The Power to overcome rolling resistance

The power to overcome rolling resistance is said to be described by the formula P = Crr x N x v, where the P in the equation is the power required, the Crr is the rolling resistance coefficient defined based on the type of bike you normally use, the N is the normal force of the bike and the athlete against gravity and the v is the velocity you are going at. 

The power produced to overcome wind resistance

The power to overcome wind resistance can be described by the formula P = 0.5 x ρ x v3 x Cd x A, where P is the power required, ρ is the density of air, v is the velocity you go at, Cd is the drag coefficient and A is the surface area when facing the wind. 

You may wonder how we could possibly know the wind speed or air density during your ride, and normally we can assume there will be no special environmental wind conditions and a temperature of 15C. Moreover, the resistance coefficient is determined by the type of bike you are riding and we assume a constant surface area.

The power produced to overcome gravity 

When I think about gravity, I imagine Newton’s apple falling down the tree. Yes, the same gravity that keeps our feet stuck to the ground, preventing us from flying away or the same gravity that we know from function movies. In this case, when we talk about the power produced to overcome gravity, we are referring to the gravity while riding up a hill. It can best be described by the following formula:

  • P = m x g x sin(arctan(grade)) x v, where P refers to the power required, them is the mass of the rider and bicycle, the g is the gravitational constant which is 9.8 and grade is the slope of the hill.

The power produced to accelerate from one speed to another

When we talk about the power produced to accelerate from one speed to another within a ride sampling window, it can be described by the following formula:

  • P = m x a x v, where the P is again our power required, m is the mass of the rider and the bicycle, and a is the acceleration between the speed at the start and the speed in the end within the same sampling window. 

How can I get the Power for my rides?

You may have noticed when using Strava that there is a lightning bolt next to the listed Power for a given segment. This will mean that the power value has been recorded by a device called a power meter. A power meter is a device that is usually integrated into a bicycle’s crankset, bottom bracket or a real wheel, and will calculate the power (or watts) being generated by you.

As indicated by Strava Support, When data from a power meter is not available, Strava can estimate power using information about athlete’s weight, speed, and elevation change.

Moreover, before Strava can estimate your power, you will need the following:

  • Go into Strava.com> Account settings and enter your bike information under the option ‘My Gear’. Then you need to edit the activity and select your bike. However, if you don’t set the information about your bike or activity, a default road bike weight of 9kg (or 20 lbs) will be selected.
  • Go into your profile settings and enter your weight. It can become more accurate if you enter your weight when you are using your cycling clothes, shoes and even your water bottle.
  • Finally, Satrava will need the most accurate elevation data for your activity. You could get this info either from a device with a built-in barometric altimeter or automatically calculated from Strava’s elevation base map.

Average Power Calculations

Measuring power can become very useful if you want to know the average power. This gives you the average power amount during a ride, expressed in watts. This will be inclusive of the entire ride, and Strava includes zeros. 

As indicated by Strava support, “Average power is most accurately measured by a power meter, though if you don’t have a power we give a rough approximation through our power estimator”.  Strava is the preferred social media site for endurance athletes to share their rides and runs. There are many Strava users around the world, especially cyclists and triathletes. 

However, some prefer a powder meter instead of using Strava to measure power. Because Strava alone estimates power based on GPS data which doesn’t account for wind, making it less accurate. 

Why is this blog about How does Strava calculate power important?

As we have mentioned on how Strava calculates power, Strava considers that the total power produced or P(total) as the sum of the Power produced to overcome rolling resistance or the resistance of forwarding motion, Power produced to overcome wind resistance, Power produced to overcome gravity (when climbing hills) and Power produced to accelerate from one speed to another. 

Moreover, instead of manually getting measurements, you may have noticed when using Strava that there is a lighting bolt next to the listed Power for a given segment. This will mean that the power value has been recorded by a device called a power meter. Many Strava users may find this information important and useful, especially for athletes and professionals.

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Frequently Asked Questions (FAQs) about How does Strava calculate power

How does Strava work out average power?

According to Strava Support, “Average power gives your average power amount during the ride, expressed in Watts (a measure of how much energy you are placing into the pedals.) This is inclusive of the entire ride, and Strava includes zeros (coasting)”. 

How does Strava calculate speed?

“When a GPS file is uploaded, Strava takes the distance data recorded in the file and parses it into a data stream to calculate total distance, average speed, and max speed. Additionally, distance readings contribute to your average speed, as average speed is calculated from distance over your total moving time”, according to Strava Support. 

What is a good average Watts cycling?

A good average watts cycling will depend but for most pro cyclists,  200 to 300 watts on average during a four-hour tour stage is considered a good average Watts. 

What is a good FTP?

The average recreational cyclist’s FTP is about 2.5-3.0 Watts/Kg. However, people who race regularly may have a higher FTP of 3.0 to 3.75. You can get above 3.75 to 4.25 and those are local elite racers. An FTP above 4.25 is considered a domestic pro and it can go up from there. 

Is Strava accurate?

Strava can become quite accurate when your GPS works properly since it feeds the data to the app. If your phone has problems with the GPS signal then it can make the recording of the data less accurate. Many forums may have an interesting debate about whether Strava is accurate in their measurements or not but we could say that it is a good option to track your activities. 

References 

Support.strava.com 

How to Get Power for Your Rides 

Velopress.com 

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