Kinetic Edge has developed a power meter app for android and iPhone that estimates power output from GPS and weather station data. Here are a few quotes from their website.
“Mechanical power meters on the market They use sensors to measure applied or opposing forces and then convert this force to a power output for the cyclist. This is an indirect power measurement.”
“A rider’s kinetic energy, the loss of energy to aerodynamic drag, weather conditions, and potential energy differences in ascending or descending terrain are accurately factored in, resulting in the most reliable power reading for any rider.”
"PowerEdge is the most sophisticated, newest power meter available on the market. It uses the GPS, data from wireless sensors, as well as a microprocessor to provide the most direct measurement of the real-time power expenditure of a cyclist. Its ingenious algorithm relies on basic equations of physics. Extensive testing against other hardware based power meters over 5000 miles of riding showed the same power output between then within the noise level. Read more about the amazing accuracy in our technology pages. "
Here is a graph comparing a traditional power meter to the PowerEdge app. Notice how although it is relatively close on average power, it does not follow all the major peaks and troughs to the proper amplitude.
Website at http://www.kinetic-edge.com/index.html
Really neat idea; essentially the same as an iBike, but cool that it’s integrated as just part of a smartphone
Reliably accurate? Not really, the sensors for that stuff in your iphone aren’t anywhere near what they need to be to get that algorithm on par with the “real” powermeters
Now, is this going to be better than just a bike computer with speed? Yes, but $120 better? Mehhhhh
Bingo to the previous two posts. As RChung (or is it Coggan) said, it’s when PM DON’T agree that you should be worried.
Also, 115-117 W? That implies speed has to be really low so that any it masks a lot of deficiencies in the model. You’d have to presume that this is intentional data cherry-picking.
also, This, afterall, is just a real time Strava estimate, and those things are often way off.
I really do like the basic idea of it though. The struggle is just getting accurate enough inputs that don’t cost a bazillion dollars to install.
Some of the more out of the box thinking on this topic in the last few years has been pretty cool, just changing up like only measuring one leg, moving location to the pedals, significantly reducing cost of PT, etc.
Bingo to the previous two posts. As RChung (or is it Coggan) said, it’s when PM DON’T agree that you should be worried.
Also, 115-117 W? That implies speed has to be really low so that any it masks a lot of deficiencies in the model. You’d have to presume that this is intentional data cherry-picking.
also, This, afterall, is just a real time Strava estimate, and those things are often way off.
Yeah I was wondering how this would be different (better) from Strava, How would they calculate downhills? at 60 kph you could be at 0 watts or 300.
Biggest issue here would be accurate drag estimates (to determine how much power is needed to go x speed) it looks like it has drag inputs (and rolling resistance). Drag changes based on yaw angle and if you sit up on hills etc it will change. The math they are doing is not very hard so it really comes down to how accurate the inputs into the calculation are for gradient, temp, air pressure, wind speed (at ground level) and direction (with some variance), road condition, altitude, weight, etc… In any case $120 is just silly. Would be interested to see what they are trying to patent with this as all of the equations are well known and already in use in many products.
constant velocity = 0 acceleration
It has a algorithm that guesses drag (which scales with velocity) and compares it to GPS-measured slope. Depending on those two variables, it decides whether power is needed (and if so, how much) to maintain 0 acceleration.
It acceleration (not velocity) to calculate power.
Is that what strava uses? We are talking about virtual or fake power, this might be less fake than strava, though at 120$ vs free I am not sure how they would add value.
FWIW I have found strava power to be sort of close on a few steady grade climbs around here that are sheltered from the wind. In general though it’s not close at all.
If people are interested in power then I tell them to get a real PM.
I am aware, not much new in physics You are oversimplifying as it should take drag due to air resistance, rolling resistance, gravity (slope) etc into account. Air resistance and Rolling resistance obviously vary based on multiple variables such as bike/position/wheels, tire/tube type, air pressure, wind direction, wind velocity, and yaw angle (which is a bit more complex). Like I said the calculations are nothing new, getting good inputs is typically much harder. The auto weather input is a cool feature that will take care of some of the estimates, however it wouldn’t account for the terrain of the course which can vary the wind seen by the rider from that of the collection (typically ~10 meters off the ground in clear open area).
Did you create the app? Just curious as I had a few questions not answered in the FAQ.
Does it only use GPS or altimeter as well? How is noise filtered on GPS elevation accuracy? Also when you say guesses drag, is that based on data from the ride (which would be really cool) or from presets?
Agreed. If you want “accurate” power it’s best to get a powermeter. I think it is possible to make a close approximate app like the other poster mentioned, but you really need some good drag estimates at a lot of points (various yaw angles in various positions) and rolling resistance of the road. You would also want access to a good altimeter inside your phone (iphone doesn’t have one that I am aware of but Galaxy S4 does I think). If it was $20 or had a free trial it would be cool the test out, but at $120 (without an apparent free trial) think I’ll pass.
Nope, I don’t have anything to do with it.
I’d like to see whether this app actually takes any of those rider-specific variables into account, and if so, whether the accuracy of rider inputted data could be trusted to be any better than whatever best-fit data Strava uses on theirs.
Like you, at first, the weather input seemed great! A stiff headwind makes a much much bigger impact than anything else, but as you said…that wind would be stymied by terrain.
As for GPS vs. baro altimeter…who knows? How many phones actually have a hardware altimeter? It could even be calling out and downloading topo data.
At the end of the day…at over $100, this is crazy talk. You’d be well on your way to an actual power meter.
Agreed. Got a used powertap for like $550. My phones battery with GPS on last about 3 hours… If they do a 30 day free trial it would be fun to test out though.
You are oversimplifying as it should take drag due to air resistance, rolling resistance, gravity (slope) etc into account. Air resistance and Rolling resistance obviously vary based on multiple variables such as bike/position/wheels, tire/tube type, air pressure, wind direction, wind velocity, and yaw angle (which is a bit more complex). Like I said the calculations are nothing new, getting good inputs is typically much harder. The auto weather input is a cool feature that will take care of some of the estimates, however it wouldn’t account for the terrain of the course which can vary the wind seen by the rider from that of the collection (typically ~10 meters off the ground in clear open area).