Ok, thanks. I don't want to waste time either, or convince (or not convince) someone to buy the darn thing. So I'll just say my piece and we can call it good.
This device measures power by doing math. 1 W = 1 ((kg * m2)/s3). In other words, how heavy you and your bike are, how far you go, and how quickly you do it. That info is easy: you tell the system how much you and your bike weigh, and it reads your speed from your speed sensor and distance from your cycling computer. Do the math, and you've power. Now, what about wind?
If there is zero wind, then the velocity of the air entering the wind port is equal to your speed. Headwind? The velocity of the air entering the wind port is greater than your speed. Tailwind? The velocity of the wind entering the wind port is less than your speed. Crosswind? If it is a truly perfect crosswind, then the device reads the same as if there were no wind, because in effect there is no actual headwind or tailwind component. But that pretty much never happens, so any crosswind would be registered by its corresponding headwind or tailwind component.
Thus, on a perfectly flat road, maintaining a constant 15 mph, it requires more power if you have a headwind; less power if you have a tailwind.
If you keep the power constant (200W), then you go faster with a tailwind; slower with a headwind. Hence, your position is irrelevant to measured power output. If you keep the power constant 200W and the wind constant (5mph headwind), but make yourself more aerodynamic (in the drops), then you go faster. If you make yourself less aerodynamic (on the hoods), you go slower.
The device also has an accelerometer, to measure rate of acceleration, and inclinometer (to detect grade), and a barometer (to detect changes in pressure, and hence, elevation). In other words, it's got all the necessary data inputs to do the math to determine your power output.
And this is demonstrated by the data provided by DC Rainmaker. If there
might be inaccuracies and errors, as you assert, then why does the Power Pod's numbers so closely match the other 4 strain-gauge based power meters that were also on the bike? Shouldn't there be huge discrepancy? Interestingly, strain-gauge power meters don't directly detect power either; they detect gauge factor and then do math to convert that info into power. Anyways, the fact remains that, based on the actual data DC Rainmaker provided, the Power Pod is just as accurate as any other major player on the power meter market.
Originally Posted by
Redbullet
1. A number of factors influence the power that is generated whilst pedaling: The front component of air resistance / friction, the lateral component of the same, the aero characteristics of the system (e.g. on the drops, on the hoods, clothes characteristics, etc), the road type (surface characteristics), tires pressure, tires quality, others. All of them change from one ride to another and most of them change during the same ride.
My understanding is that is not how things work. Your power is your power, period: 1 W = 1 ((kg * m2)/s3). No where in that equation is air resistance, bike position, tire pressure, etc. All of those things impact
how fast you go for a given power output, but they have zero influence on what power you actually put out.
Originally Posted by
Redbullet
Power Pod only measures the first component of the above and tries to aproximate the power used to overcome it. But the rest of components that are not measured have a large influence (e.g.: change in aero position easily account for 5-10%.). I think that failing to measure and include those parameters in a complex algorithm is a strong limitation that prevents obtaining consistent results.
Again, not really. None of those things have anything to do with power. They do, however, affect speed for a given power output. The device simply does math to determine power.
Originally Posted by
Redbullet
2. In practice, the difference between “on the drop” and “On the hoods” is around 1.5-3 km/h – another 5-10% change in air speed without modifying power. I doubt that the front hole of powerpod can identify your position and keep the power reading constant, as long as it perceives change in air speed.
You just proved my point. If you keep the power the same, but improve aerodynamic efficiency, you will go faster. And the Power Pod will detect that the bike speed and the air velocity increase at an identical rate and an identical amount, thereby recognizing the improving aerodynamics and will display the same power.
Originally Posted by
Redbullet
3. When weather prognosis shows lateral wind around 15 km/h (50% of my speed) on my favorite road, that usually slows me down with 2-3 km/h, which means 6-10% of my average speed in calm conditions. I only guess that a plane flying with 500-900 km/h does not encounter lateral wind of 50% of its speed, thus, the influence of lateral wind versus its forward speed (or versus frontal air speed) might be much, much lower than in our case. And in lateral wind, the planes rotate their position and have some kind of “plan parallel” movement; that angle and other parameters might be used to calculate the lateral component of the wind – I guess. But there is no such rotation and plan parallel movement of the bike.
Besides, I guess that airplanes do not use the measured air speed to calculate the power that is generated by their engines - do they? (although airplanes are free from road rolling resistance, tires quality and pressure, passengers’ clothes and their unexpected changes in aero position, etc…).
Depending on the aircraft, it is entirely possible to experience a crosswind that is 50% of forward velocity, especially in the takeoff/landing phases of flight. Specifically in the landing phase, it is necessary to align the longitudinal plane of the aircraft with the landing runway, so as to not land angled. Otherwise in flight, yes, planes do "crab" into the wind to fly drift-corrected. I don't personally know of a plane that determines engine power output via airspeed computations, but I digress.
Anyways, thanks for keeping a cool head and settling me down in a mature fashion. Kudos to you!
