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QOTW: How Can I Estimate Watts Used to Achieve a Certain Speed?

This week’s QOTW comes to us from Reddit. User juderm asks the question below:

 

If I stayed the same weight can I get a rough estimate of watts needed to achieve a certain speed this way? Stats from weekend race: Average power: 244 19.25 MPH 244 watts/19.25 mph = Equal to 12.6 watts for each MPH If I had a goal MPH of 23.0 would that give a rough estimate of 291 watts needed to get close to that speed? I understand that wind will also play a factor, but not sure on this total effect.

 

The most important thing to remember is the relationship between power and speed is not linear.  If that’s enough of an explanation for you, please skip to the last paragraph. Still there? There are many variables in the relationship between power and speed including CdA (which is, in short, the drag coefficient multiplied by your frontal area), rolling resistance, and gravity.  The sum of these forces resisting you is the work which is expressed in joules.

 

If you want to achieve a certain speed (expressed as V for velocity), you have to expend the energy to do the work to move V feet each second. The rate here is power which is expressed in watts.

 

Pwheel (watts) = Fresist (Newtons) · V (m/s)

 

Now to add to all that, all of the energy produced by your legs does not actually make it to the drivetrain. Some is lost to friction in the drivetrain. Confused yet? The implications of all these factors mean that to go from say, 20 mph to 25mph, your power has to nearly double! This is because the power you have to produce is proportional to the cube of your velocity.

 

Okay so what should you do if you don’t want to go to the wind tunnel and you hate math? There are a number of tools online available to easily estimate your power at a given speed. The best on in my opinion is Best Bike Split. Using this tool you can easily upload a GPX of the course and estimate your speed on a given course at a given power. This is something we do for all our athlete’s A races as we can also back into their performances.

 

A recent visit to the A2 wind tunnel

If you don’t know your CdA you can go to a wind tunnel, take a guess, or do a field test. The how-tos of that are for another post, but I’ll add that the A2 wind tunnel experience is awesome! Best of luck to you at your race!




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