strava accuracy

[stephen.rooke]

how accurate is the power estimates on strava, just been looking over part of my ride and it says i hit 700watts as a maximum but for the 20 seconds before that its showing 0watts, although that part, is that because i was going downhill so assumes i wasnt pedaling even though i was.

700 watts seems high for me as im only a beginner

 

[stephen.rooke]

another part of the ride before strava cut off for some reason, shows me hitting 853 watts, now i definately think somets up

 

[Hacienda71]

Probably not very accurate. Have you inputed your weight rather than relying on the default 390 pounds. That could skew your results.

 

[stephen.rooke]

yeah ive inputted my weight, dont usually look at power, i just use strava to track my distances and times / av speed etc

 

[jdtate101]

Are you using the phone app or inputing from a GPS file? Strava seems to peak and zero for me quite a bit. For example on a straight level bit of road I use for time trial practice, where I'm on power all the time, the website often shows zero, followed by a huge peak, then back to zero (repeat until the end). The only way to get correct power readings is to use a power meter. According to their support forums, they think that the AVG power reading over the length of segments and rides is within 10% true, but I have my doubts.

As an example, me and my brother rode the same segment together, we are very close in weight and height (within 3lbs), no major difference in bike weight either. He recorded by phone, I recorded by Garmin. His power came out at 385w, mine at 335w, even though we rode side by side at the same pace and finished with the same time.

I think their power calculation is way off personally.

 

[stephen.rooke]

i use the phone app on my samsung galaxy nexus

 

[stephen.rooke]

seems to me that any slight downhill gradient and power goes to 0, i pedal downhill though as i like the speed

 

[jdtate101]

This is typical of what I get :

You can see from the data (collected by Garmin 800, with HR strap and GSC-10 speed/cadence module), that over the 9 min TT stretch I'm pedelling all the time and at not a inconsiderable speed. Therefore I'm generating power all the time, but the power line repeatedly drops to 0 and peaks up to 1000w+ sometimes. It seems as if it is lacking a 'smoothing' algorithm to avg over the time/distance.

Strangely other people do not seem to have this issue at all. When I presented this output to Strava for an answer, all I got was a email 'shoulder shrug'.

 

[fossyant]

Not accurate. Pinch of salt.

 

[fossyant]

Power is only accurate if you have the kit, ie very expensive hubs or cranks.

 

[MrJamie]

From what ive seen of it, its just that it doesnt smooth out the gps error very well.

Say you do 100m in 10 seconds at a constant pace, if gps perfectly detected each second's movement it would measure a 10m move at 10m/s, but in reality it could read 14m at 10m, 18m at 20m, 33 at 30m giving you 14m/s followed by 4m/s followed by 15m/s. The 4m/s interval would have to assume 0 power, then a big power spike to have you at 15m/s right after. The speed graph is usually smoothed in gps tracking apps to give a more realistic feel of your speed ie. average of the last 3 speeds so spikes arent so dramatic. I suspect the power spiking is down to it thinking your speed is constantly changing because of gps error.

The segment average power should be more realistic and easier to calculate. Im not sure if they use the elevation data saved for the segment or what the gps records, which always tends to be quite poor at pinpointing height.

 

[Hacienda71]

Power is only accurate if you have the kit, ie very expensive hubs or cranks.

Something like this http://www.koolstof.co.uk/SRM-Sram-Power-Meter-GXP

 

[fossyant]

Something like this http://www.koolstof.co.uk/SRM-Sram-Power-Meter-GXP

Aye unless you are elite level then this is just a waste. Yeh if you can afford a 10k bike, but putting this on is like wasted.

Great for number nuts though.

 

[cloggsy]

Nowhere near accurate; I use the Cyclemeter app on my phone, but have the added extras of the ANT+ Speed, Cadence sensors and the heart rate monitor (which improves accuracy greatly, compared to using it without!)

 

[2Loose]

As you say, the power graph is completely out of place when compared to the other measurements in the graphic.
Ignore....