Curious about the calculations used in a true tailwind condition - where wind velocity is actually greater than measured vehicle velocity. In a true steady-state condition (zero acceleration and zero gradient) with zero wind, the PowerPod should be calculating power required to overcome aero drag and rolling resistance only, correct? So, if you introduce a tailwind to the steady state condition, once the system equalizes (pedaling power reduced and assuming speed and all other variables remain unchanged), the PowerPod should be calculating the same power required to overcome drag and friction again. But, now the calculated power value is total power required including the tailwind, and is actually higher than the power output of the rider - because the PowerPod cannot measure the 'negative' wind value.
So my question is, how fancy do you get with the math in a tailwind condition? I realize that you can calculate a difference between wheel speed and airspeed. But, as far as I know, in a tailwind condition the PowerPod will only see a wind value of zero. Do you have a method of calculating (or measuring?) the tailwind component velocity and subtracting the resultant power input from the power value? Or, do I just have to remember that in a tailwind condition, the power value displayed could be higher than my actual power output because the wind is helping me?
Power Calculation with Tailwind
Re: Power Calculation with Tailwind
While standing at rest, "tailwind" is the ambient wind that is blowing from behind you.
Let's suppose you're riding down the road at 20 mph and there is a 5 mph tailwind. PowerPod measures a net opposing wind of 15 mph.
When you turn around and ride into the wind, PowerPod measures a net opposing wind of 25 mph.
In either case, the power calculations are correct.
When tail winds are stronger, cyclists naturally ride faster--enough, in our experience, such that the bike speed is in excess of the tailwind speed. As long bike speed exceeds tailwind speed, power calculations will be accurate.
Let's suppose you're riding down the road at 20 mph and there is a 5 mph tailwind. PowerPod measures a net opposing wind of 15 mph.
When you turn around and ride into the wind, PowerPod measures a net opposing wind of 25 mph.
In either case, the power calculations are correct.
When tail winds are stronger, cyclists naturally ride faster--enough, in our experience, such that the bike speed is in excess of the tailwind speed. As long bike speed exceeds tailwind speed, power calculations will be accurate.
John Hamann
Re: Power Calculation with Tailwind
Yes, but I'm specifically talking about a situation where the tailwind is greater than vehicle speed.
Re: Power Calculation with Tailwind
Any case where the net opposing wind is less than zero.
Bicycle speed of 15mph and direct tailwind component of 25mph (today's conditions around here).
Bicycle speed of 15mph and direct tailwind component of 25mph (today's conditions around here).
Re: Power Calculation with Tailwind
If a cyclist rides in a 25 mph tailwind, on the flats, at a bike speed of 15 mph, the cyclist is pushed forward by a net tail wind of 10 mph.
A bike moving at 15 mph on the flats requires 39W of propulsive power to overcome the forces of road and tire friction. But a 10 mph tail wind produces more than 46W of "pushing" power. So, at a bike speed of 15 mph, the pushing power of the wind is enough to overcome the opposing frictional force and thus the cyclist would not have to pedal. PowerPod would read zero, correctly.
As I mentioned in my original post, when the wind is at their back most cyclists ride faster, enough so such that there is a net opposing wind. In fact, in 13 years I've never seen a ride file where these "in the field" conditions have been met, so I'd be very interested to look at one.
A bike moving at 15 mph on the flats requires 39W of propulsive power to overcome the forces of road and tire friction. But a 10 mph tail wind produces more than 46W of "pushing" power. So, at a bike speed of 15 mph, the pushing power of the wind is enough to overcome the opposing frictional force and thus the cyclist would not have to pedal. PowerPod would read zero, correctly.
As I mentioned in my original post, when the wind is at their back most cyclists ride faster, enough so such that there is a net opposing wind. In fact, in 13 years I've never seen a ride file where these "in the field" conditions have been met, so I'd be very interested to look at one.
John Hamann