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An $8 Power Meter?!

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An $8 Power Meter?!

Old 02-13-14, 11:28 AM
  #1  
j814wong
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An $8 Power Meter?!

I just found an app of Google Play that claims to "without calibration (only entering rider's weight and choosing bicycle type), average power for a given ride typically correlates to within 15% (with no drafting) of a conventional power meter." Of course, my first reaction was "Wait... WHAT?!" After all, normal powermeters are so damn expensive because of all the fancy secret technologies tucked inside them or maybe they are also just overpriced because companies know cyclists would pay good money for such things. But in any case, a $8 power meter app is intriguing and while serious cyclists won't care for the app because of the +_15% difference in accuracy to normal power meters, I think that the concept could be a good replacement for those of us who don't have the money for a $1000 power meter.

The reviews for the app are quite good (Average 4.9 stars). I don't have a power meter myself so I can't compare the accuracy of the measurements. But to be honest, I a bit suspicious.

Oh, it also can estimate cadence and % braking energy. Yeah, color me skeptical but when the weather lets up in my area, I'll give it a go and compare the cadence measurements to that of my Garmin.

Quoting their FAQ on cadence...
"How is the app able to detect cadence without a cadence sensor?
The phone's low power accelerometers along with advanced signal processing techniques are used to detect the small acceleration differences seen due to normal rider's output power variation through the crankshaft cycle. Figure 5.7 in the book High Tech Cycling by Ed Burke illustrates this effect. The effect is more challenging to detect for mountain bike riders (more constant power vs. angle and more vibration noise). Wearing in a jersey pocket has produced the best (lowest noise) cadence results, but a handlebar mount can also work reasonably well. Noise and errors are common when the power to weight ratio drops (i.e. coasting or soft-pedaling) since accelerations can be dominated by other effects in this case."


Thoughts?

Note: Google Play has a crappy 15 minute return policy on paid apps (It ought to be at least 30 minutes in my opinion).

Last edited by j814wong; 02-13-14 at 11:34 AM.
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Old 02-13-14, 11:35 AM
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Probably as reliable as Strava's best guess power estimate. A heart rate monitor would be more beneficial for training if you can't afford a powermeter.
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Old 02-13-14, 11:38 AM
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Sounds like it's worth exactly what it costs.
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Old 02-13-14, 11:42 AM
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+/- 15%...thats like getting eye glasses that are +/- 15% the prescription. just save the money and download yourself a flappy birds clone
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Old 02-13-14, 11:44 AM
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If you care about tracking power output, then I would think 15% would be a huge variance.

The cadence thing has me curious as to how well it works ... not $8 curious but maybe $0.08 curious just for kicks & giggles.
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Old 02-13-14, 11:46 AM
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15% is huge dude.

300W could be anywhere between 255 to 345...

you best be trollin with this thing!
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Old 02-13-14, 11:51 AM
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Well, we all know that everyone in the 41 pedals perfect, smooth circles, so it shouldn't be able to pick up our cadence from micro-surges.
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Old 02-13-14, 11:56 AM
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Originally Posted by j814wong View Post
I just found an app of Google Play that claims to "without calibration (only entering rider's weight and choosing bicycle type), average power for a given ride typically correlates to within 15% (with no drafting) of a conventional power meter." Of course, my first reaction was "Wait... WHAT?!" After all, normal powermeters are so damn expensive because of all the fancy secret technologies tucked inside them or maybe they are also just overpriced because companies know cyclists would pay good money for such things. But in any case, a $8 power meter app is intriguing and while serious cyclists won't care for the app because of the +_15% difference in accuracy to normal power meters, I think that the concept could be a good replacement for those of us who don't have the money for a $1000 power meter.

The reviews for the app are quite good (Average 4.9 stars). I don't have a power meter myself so I can't compare the accuracy of the measurements. But to be honest, I a bit suspicious.

Oh, it also can estimate cadence and % braking energy. Yeah, color me skeptical but when the weather lets up in my area, I'll give it a go and compare the cadence measurements to that of my Garmin.

Quoting their FAQ on cadence...
"How is the app able to detect cadence without a cadence sensor?
The phone's low power accelerometers along with advanced signal processing techniques are used to detect the small acceleration differences seen due to normal rider's output power variation through the crankshaft cycle. Figure 5.7 in the book High Tech Cycling by Ed Burke illustrates this effect. The effect is more challenging to detect for mountain bike riders (more constant power vs. angle and more vibration noise). Wearing in a jersey pocket has produced the best (lowest noise) cadence results, but a handlebar mount can also work reasonably well. Noise and errors are common when the power to weight ratio drops (i.e. coasting or soft-pedaling) since accelerations can be dominated by other effects in this case."


Thoughts?

Note: Google Play has a crappy 15 minute return policy on paid apps (It ought to be at least 30 minutes in my opinion).
15% is useless, especially since it's +/- 15%. That's so inconsistent, it means nothing. Imagine the following scenario:

Your FTP is 300 watts. You're trying to do an interval within that zone (so about 288-315 watts). The app tells you, your watts each minute were (3-minutes for simplicity):

* 288
* 291
* 301

But given the error-level of this app, those wattage readings could actually be:

* 245
* 247
* 256

or they could be:

* 331
* 335
* 346

Either way you could be way below, in or way above the zone and have no idea. Useless.
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Old 02-13-14, 12:00 PM
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Originally Posted by j814wong View Post
I just found an app of Google Play that claims to "without calibration (only entering rider's weight and choosing bicycle type), average power for a given ride typically correlates to within 15% (with no drafting) of a conventional power meter." Of course, my first reaction was "Wait... WHAT?!" After all, normal powermeters are so damn expensive because of all the fancy secret technologies tucked inside them or maybe they are also just overpriced because companies know cyclists would pay good money for such things. But in any case, a $8 power meter app is intriguing and while serious cyclists won't care for the app because of the +_15% difference in accuracy to normal power meters, I think that the concept could be a good replacement for those of us who don't have the money for a $1000 power meter.

The reviews for the app are quite good (Average 4.9 stars). I don't have a power meter myself so I can't compare the accuracy of the measurements. But to be honest, I a bit suspicious.

Oh, it also can estimate cadence and % braking energy. Yeah, color me skeptical but when the weather lets up in my area, I'll give it a go and compare the cadence measurements to that of my Garmin.

Quoting their FAQ on cadence...
"How is the app able to detect cadence without a cadence sensor?
The phone's low power accelerometers along with advanced signal processing techniques are used to detect the small acceleration differences seen due to normal rider's output power variation through the crankshaft cycle. Figure 5.7 in the book High Tech Cycling by Ed Burke illustrates this effect. The effect is more challenging to detect for mountain bike riders (more constant power vs. angle and more vibration noise). Wearing in a jersey pocket has produced the best (lowest noise) cadence results, but a handlebar mount can also work reasonably well. Noise and errors are common when the power to weight ratio drops (i.e. coasting or soft-pedaling) since accelerations can be dominated by other effects in this case."


Thoughts?

Note: Google Play has a crappy 15 minute return policy on paid apps (It ought to be at least 30 minutes in my opinion).
Doesn't surprise me. You can do a pretty good job of estimating average power over a hill climb with a stopwatch, a GPS signal, and knowing your total weight. As long as the wind isn't blowing, you can do a reasonable job of estimating average power on the flat if you have a rough idea of bike type or position and a stopwatch and a GPS signal. And, in fact, if you know your speed and something about the terrain you can also get ballpark close on cadence.

However, the real reason for a power meter isn't for estimating averages. What's important isn't so much the average (though that's useful), what's important is knowing about surges and recovery. Power production is pretty asymmetric, so going 5% over your threshold is much more costly than going 5% under threshold. In addition, because we ride up and down hills and with and against the wind, our demand for power is pretty variable.
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Old 02-13-14, 12:07 PM
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Maybe the instantaneous reading is +/-15% but the error integrated over some distance would be much less - worth looking into.
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Old 02-13-14, 12:09 PM
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Originally Posted by halfspeed View Post
Sounds like it's worth exactly what it costs.
You're being generous.
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Old 02-13-14, 12:11 PM
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The instantaneous error can easily be much larger than 15%.

Let's plug some numbers into a power calculator. Let's say that you're going 18 mph down a fairly flat road, assume some typical numbers for body weight, aero drag and temperature. All that the app has to work with is your weight, bike type, speed and approximate altitude (from which it can calculate approximate grade). Since it's running on a smartphone, it does not have access to barometric altimeter but let's assume that it can somehow measure your altitude to +/-10 feet.

If you're on the hoods wearing a flappy jacket (CdA 0.45 m2) and there's a 2 mph headwind, you're spending 210 watt.
If you're in the drops wearing a race jersey (CdA 0.37 m2) and there's a 2 mph tailwind, you're spending 115 watt.
If you're on the hoods and you're going up 2% grade with 2 mph headwind, you're spending 340 watt.
If you're in the drops and you're going down 2% grade with 2 mph tailwind, you don't need to spend any power at all, you can simply coast at 18 mph without pedaling.

2 mph headwind is so mild that you may not even notice that it's there,
2% grade at 18 mph is about 0.5 feet per second of elevation change. If it's just a bump in the road rather than a sustained climb, your phone will most likely fail to notice that it's there.

Now, if we're talking about _average_ power over the entire ride, then some of these factors are going to cancel out, and you might end up getting a work estimate within 15% of the actual value. But you can't, say, do intervals with this app because its measurements are going to be too far off most of the time.
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Old 02-13-14, 12:11 PM
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Originally Posted by sced View Post
Maybe the instantaneous reading is +/-15% but the error integrated over some distance would be much less - worth looking into.
I can't think of what good that would do, other than (maybe) satisfying curiosity.
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Old 02-13-14, 12:45 PM
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Somebody with time on their hands made a toy app with colors and stuff, and some people bought it because they like to have more computer type stuff on their bikes. Kinda silly to over analyze something like this.
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Old 02-13-14, 01:00 PM
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This isn't an $8 sport.
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Old 02-13-14, 01:15 PM
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Originally Posted by j814wong View Post
Wearing in a jersey pocket has produced the best (lowest noise) cadence results, but a handlebar mount can also work reasonably well.
What is the point of a power meter in your jersey pocket? Even if it were accurate, that is.

If you want to guess at what your power was after the fact, just look at Strava.
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Old 02-13-14, 01:22 PM
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Oh man, horrible idea for an app.
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Old 02-13-14, 01:24 PM
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I can guess your ftp +/- 15% by looking at a picture of your bike and knowing your favorite flavor of ice cream. For free.
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Old 02-13-14, 01:25 PM
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Since nobody is commenting on my thread , I'm going to (partially) hijack this thread. Check out this: https://www.kickstarter.com/projects...ear-headphones.

Granted, not as cheap as $8, but for $200, it detects cadence, heart rate, oxygen, distance, etc. With the right app, this could replace the Garmins. Used in conjunction with a smartphone, the possibilities are awesome.
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Old 02-13-14, 01:29 PM
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Originally Posted by Gramin View Post
Since nobody is commenting on my thread , I'm going to (partially) hijack this thread. Check out this: https://www.kickstarter.com/projects...ear-headphones.

Granted, not as cheap as $8, but for $200, it detects cadence, heart rate, oxygen, distance, etc. With the right app, this could replace the Garmins. Used in conjunction with a smartphone, the possibilities are awesome.
All the appeal of the 8 dollar app for $200!!
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Old 02-13-14, 01:53 PM
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Originally Posted by TrojanHorse View Post
What is the point of a power meter in your jersey pocket? Even if it were accurate, that is.

If you want to guess at what your power was after the fact, just look at Strava.
This!

/thread
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Old 02-13-14, 01:59 PM
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Originally Posted by RPK79 View Post
All the appeal of the 8 dollar app for $200!!
And when they fall out of your ears while riding, you can spend another $200
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Old 02-13-14, 02:08 PM
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Originally Posted by j814wong View Post
Thoughts?
Useless except for bragging purposes (I did 400W!) if the 15% claim is accurate.

15% is the difference between a pace that's not unpleasant for a couple hours and being done in 10 minutes. +/- 15% totaling 30% is the difference between a one hour time trial and four hour endurance ride.

Of course it's not accurate - it doesn't know what the local wind conditions are, your personal aerodynamics, coefficient of rolling resistance which can vary by a factor of 2 between best and worst slick tires, etc.

If you really can't afford power (used second generation Powertaps with the wired little yellow computer dropped into the $200s four years ago) heart rate is a reasonable approximation for many purposes including pacing (except on shorter intervals, maybe 5 minutes or less?) and tracking training stress.

Last edited by Drew Eckhardt; 02-13-14 at 03:39 PM.
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Old 02-13-14, 02:21 PM
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Originally Posted by halfspeed View Post
Sounds like it's worth exactly what it costs.
Less, even, considering Strava is free to use with the smart phone you already need to own for this $8 app.
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Old 02-13-14, 02:24 PM
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Ya'll taking this thing way too seriously. It's a curiosity for those who don't have the interest or money for a power meter. Like me

I mean you don't really do anything with watts if you're not going to train with them. However it's always nice to now your ballpark average fitness. Average speed is worth practically nothing if not done in a velodrome. The calculators you find on the internet are possibly as useful as this app but the good thing in the app is that you can access it mid ride and be surprised about how well you are doing.

It's for fun people! Don't you know that fun means?
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