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How Does Strava Calculate Power Data?

How Does Strava Calculate Power Data? post image

In this post I’m going to look at how Strava calculates the power generated over the course of a ride, specifically where the rider (like me) doesn’t have a device attached to the bike for measuring power.

Dedicated power meters are still an expensive addition to your bike. Is it worth spending that extra money when Strava can provide the data for free?

Why Do We Care About Power?

The Power Of Love, A Force From Above – Frankie (Goes To Hollywood)

The use of power data as a training and racing tool seems to have grown in profile in recent years.

In the pro ranks (and in some amateur ones), time trialists use power meters to help them spread their effort over the full course of a race without blowing up. Team Sky use power meters in order to ride tempo up the steep passes of the Tour De France, preventing attacks from other teams (and, according to critics, sapping interest from the race).

On the consumer side, power meters are starting to come down to a price that’s within reach of amateur riders (admittedly, amateur riders with plenty of disposable income). Now that the likes of Garmin and Polar (but mainly Garmin) have easy-to-install pedal-based meters available (as opposed to cranks and rear hubs), you can only see them growing in popularity.

But do you need to spend £’000s on a power meter when Strava provides the ‘data’ for free?

To answer that question, we need to look at how Strava calculates its power data, and how accurate it is for you as a rider.

Wherein The Grimpeur Attempts To Explain How Strava Calculates Pow’r

Strava calculates the overall power output of the rider and bike ‘system’ rather than the wattage generated at the cranks, pedals or hub (as would be the case when you use a power meter).

It takes your measured performance (speed, distance, time), adds in some environmental factors (some actual, some assumed) and comes up with wattage numbers over the course of your ride.

Since the wattage is that of the whole rider/bike system, Strava’s data is most accurate when it knows more about the rider and the bike (e.g. rider weight, bike weight, type of bike).

Which ‘Components’ Does Strava Use To Calculate Power?

I am not a physicist (I’m a historian), so I don’t know whether ‘component’ is the correct word to use. It just felt a bit better than ‘things’.

Anyhoo, Strava uses the following, er, things in its power calculation formula:

  • Wind resistance
  • Rolling resistance
  • Gravity
  • Acceleration

What Strava Doesn’t Know (And What You Should Tell It)

Each of the components mentioned above has it’s own formula (i.e. to calculate the power required to overcome that particular thing).

Within the wind resistance calculation, for example, Strava doesn’t know the environmental conditions under which you were riding (or at least they don’t know at the level of detail required). So, according to the website, Strava assumes no environmental wind conditions and a constant air temperature of 15 degrees (as far as this relates to air density).

In addition, there may be things you haven’t told it.

The drag coefficient within the wind resistance calculation is based, in part, on the type of bike that you’re riding (which you set when you ‘Add Bike’ under Settings > My Gear). Presumably it makes an assumption on the drag caused by an average rider in an average position on the bike.

Similarly, if you’ve added your weight and that of your bike, these are used to calculate the power required to accelerate the bike and the power required to overcome gravity as you soar gracefully up that mountain road.

How Does Strava Power Data Differ From Power Meter Data?

As discussed above, Strava takes the measured output of you and your bike together and then backs out an estimate of the power you employed. Power meters measure the actual power produced at a specific point on the bike (i.e. where the measuring device is located).

Power meters do not need to take into account terrain data, weather conditions and how aerodynamic you are, all of which Strava has to contend with.

If it’s really windy, the elevation data being used is dodgy and you’ve decided to ride with drag-racing style parachute attached to your seat post, then Strava’s power data is unlikely to be very accurate.

That said, Strava claim that ‘in most cases our watts number are very close to the numbers provided by a Powertap or SRM’.

So there you go then, question answered….

So Do You Need a Power Meter?

Speaking for myself, a power meter is not top of my wishlist (well, it might be, but what I mean is that I ain’t going to be buying one any time soon).

Whilst I’m sure I’d find capturing the data interesting, to be spending that sort of money, I’d have to be using it to train properly. Many of the benefits of training with power can be achieved by training with heart rate zones. If I can prove to myself that I can use heart rate data consistently, maybe that will be the time to purchase a power meter (and perhaps by then the prices will have fallen).

If you do want to train with power then you need to be able to view your wattage output in realtime whilst riding. As far as I’m aware, Strava doesn’t do this, even if you have your iPhone mounted on your handlebars (correct me in the comments if I’m wrong on this). For this you’d need to buy a power meter.

Photo Credit: kevin dooley via Compfight cc

{ 8 comments… add one }
    • Rob September 3, 2013, 7:33 pm

      Good article! I do similar calculations, and make similar assumptions for the trains we build, to check we have the righht amount of installed power to meet the time table. We would call it “power at wheel”. And the “things” are know as “parameters”, but “things” works as well!

      • Andrew Montgomery September 6, 2013, 8:30 am

        Rob – I thought a train modelling enthusiast had stumbled upon my site… then I realised it was you!

        Glad that ‘things’ works for you. Maybe a career in engineering beckons…

    • Mike September 4, 2013, 7:49 am

      This is exactly why myself and two colleagues (software engineers all three) spent a bit of time recently working on this:

      It takes the TCX file from your garmin ride, and performs physics based calculations on the ride data given the input of your weight, bike weight, drag area and coefficient of rolling resistance.

      There are some sensible defaults (ok, my metrics, based on a Road Bike with 23mm slicks riding on average UK tarmac) in the app, but we found some nice research somewhere with different drag coefficients that we use as a look up table when we switch bikes, or take a more “touring” riding position.

      There are some issues with the current build around the output from a 705 (each garmin varies a little in the content) as the guy who last checked in made sure it worked with his 800 and broke it for my 705 🙁 works for a 500 currently I believe (conveniently we each have different garmins).

      We did it as with a Strava Premium subscription and a real wire tap, you get loads of interesting graphs and figures (http://app.strava.com/activities/53834726).

      But, as your weight varies over the ride (water bottles empty, gels swallowed, toilets visited, cakes eaten) and the wind changes direction etc it’s all an estimate.

      We also get some extreme spikes in the power as glitches in the altitude make it look like you rode up a 90% gradient for a few seconds etc.

      But interesting to look at, if you view it in context.

      (oh and you can set the days temperature too, which will adjust the air density and get you “more accurate” inaccurate estimated figures :))

      • Andrew Montgomery September 6, 2013, 8:29 am

        Thanks Mike. I wonder if Strava are doing the same work in house? If not, maybe you should see if they’ll buy it off you (or maybe one of Strava’s competitors would).

        How do I use it from the github site (or are we not meant to)?

    • Mike September 4, 2013, 9:38 am

      And you can play physics here:

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