Although I am not yet convinced that the pt is something I can really justify given I ride recreationally only, I am still considering it. I get the general concept and applicability, but have a question though. Does it, in any way, account for wind resistance as in windy rides? How would it know I am only doing 18 mph but putting a ton of effort because of a 20 mph headwind?

By the sharp increase of wattage.

+1. I think you're confusing the powertap, which measures your output, with one of it's jack-monkey competitors that estimates your output (poorly).

iBike's ears are burning.

no. I know which measure vs which estimates. My question still is how does the unit measure power at 18 mph with high wattage and wind vs same speed due to lack of cyclist's power. I don't know if I am making the question clear enough.

Power is force times distance divided by time. Force is how hard a rider pushes on the pedals. Distance is the distance the pedals travel rotating around the bottom bracket and time is how long it takes to make the revolution. None of this has anything to do with what the environment is doing to the bike and rider. The balance between the power produced by the rider and that needed to overcome the external forces; gravity, wind and friction determines how fast the rider goes.

Your mistake in conceptualizing the operation of the PowerTap is assuming that it cares about any external factors such as wind. It simply measures the torque applied on the hub by the chain and the rotation of the hub. When you work harder into a headwind you apply more torque to the pedals which transmits that to the chain and the hub. Note that the SRM, Ergomo, and Quarq power meter measures the torque at the crank instead of the wheel, while the iBike measures all external factors (such as speed, wind, grade) and calculates the power, and the Polar power kit also calculates the power indirectly by measuring chain harmonics.

Edit: read this

Keep in mind that all power meters measure some other stuff to estimate power. The PowerTap is just at the price point where accuracy begins to become reliable.

I would not say the SRM, Power Tap, Ergomo, or Polar estimate power. They directly measure components of power and calculate power from them using simple, exact arithmetic. There is no estimation in that.

Except for the Ergomo, which takes simple math and doubles it and the Polar which only works if you have everything weighed and at the proper angles. Which makes them estimators to me. Agreed on the PT and SRM though.

I would say multiplying by 2 is about as simple as arithmetic gets, and the issue of weighing the chain and sensor position goes to functionality and accuracy. In principle, the Polar and Ergomo do just what PT and SRM do, but with different terms.

Its a glorified bathroom scale......

The Polar unit most certainly does not directly measure components of power.

What about a leg imbalance? It's assuming that both legs are equal. If they're not, your power is an estimation(or actually, just plain wrong). I know(thought) you train with power, have you used the polar? If you hit a bump and the sensor moves angles, or you put the wrong chain weight in there, it is also wrong*. So, while the polar is not an estimation, it certainly is not as accurate day to day as the SRM or PT.

*I rode with the Polar and PT on my bike for 1 month. At 80-275w they were dead on most days. When I really got on it and went at/above threshold, the polar got farther out of whack as the watts went higher. While it may be a good meter for reproducability, it's not a great choice overall(when looking at powertap prices compared).

Powertaps are omniscient, WR. They don't need to measure.



OP: PT's use a strain guage to measure deflection of the hub, this is (what I would consider to be) a direct measurement of torque. Couple that with the measure of speed, and you have power. Wind doesn't matter, grade doesn't matter. The amount of deflection and rotational velocity are the only things that matter.

Only in binary, asgelle.

FYI, my personal experience measuring harmonics to calculate force make me doubt the accuracy of the Polar. YMMV.

Forget about buying a PowerTap or similar power units unless you are serious with your training, and is willing to spend the time learn about the technical aspects of power analysis. Or you are seriously wasting your money.

One can always work with a coach experienced in training with power.

It can be fairly simple to train by power

This answered my question completely. Thanks.

Yep, just like when a headwind occurs, you can feel that you are working harder even if you are going the same speed. The PT just shows you that you are producing more watts to maintain that same speed. If you were going 18mph into a headwind at 300w, then all of a sudden the headwind went away, you would still be producing 300w, but without the headwind working against you. You would then be going faster because you are applying the same power. Without the headwind, you may only need to produce 250 watts to go 18mph.

In regards to the ibike i've had both at the same time. the ibike and PT. and when both are calibrated corrected surprisingly the ibike numbers and PT are very similar w/ very slight variances. Most who knock it have not given it an honest try and or compared it in the real world against a PT or SRM. I am not a fan of the ibike but I try to be honest instead of making things up like most do on these forums. On a different power forum I did a complete write up about it. Certainly the ibike gets the information by different methods but at the end of the day it work. All power system are not 100 % accurate and watts is just a measurement output. at the end of the day all that matter is how its translated or interpreted. I found that the PT was actually distracting me from having as much fun and that regardless of how much I read Coggans " training / Racing w Power" and no matter how much I got home uploaded it to cyclingpeaks it was all fruitless. I truley believe that a coach is needed to take advantage of training with power and keep your goals and training regimen aligned. At first it was exciting then it was simply a drain. You can acheive most recreational goals alone. A garmin a great tool if you like looking at numbers. I went thru 3 powermeters this year and it was a very expensive way to find out I didn't /want/need/ any of them. Alot of people on this forum (including myself) want to believe that this stuff helps and are sold by marketing but getting stronger is not a end result of the powermeter. a proper coach, diet, training regimen, and finally a powermeter might give you those results.

I would agree with cslone in regards to the headwind question.

I'm just saying there's no direct power measurement, so you measure what you can and apply it to an equation. There is error propagation, and the PowerTap is about where the error is acceptable. In the F*d/t equation, the d is still a bit error prone.

Read my first sentence. I am almost in complete agreement with you.