Cycling UK Forum

I put Strava on to record part of my ride today (Penarth to Barry Island) http://www.strava.com/activities/104850102
It suggests that at one point (which would be for a very maximum of 17 seconds) I was developing 947 watts and my average for the ride was 150 watts. Does anyone know how Strava measures such things just from GPS? I assume it is rubbish as I feel very unfit at the moment.

I don't use such things that much but do like a record of rides every now and then.

if you feed Strava with GPS data only then it will calculate power etc based on the GPS data. GPS data is notoriously glitchy and depending on the algorithms used to process the data, the result might be meaningful (which the average might be) or might be complete nonsense (which some segments containing glitchy GPS data might be).

GPS is least accurate in the vertical axis, and the way your GPS unit will display bad data will vary. Some units default to the last accurate data reading until there is new good data available. This can look like you have climbed a long way in a short distance . Similar issues can make it look like you have done some climbing when riding on a flat road, or more climbing that you have actually done in a ride.

IIRC Strava knows nothing about the drag of your particular bike on any given day, or what your Cda is, or indeed if the wind is blowing. In this respect it is next to useless, for measuring power because in any given fast ride, ~3/4 of all your power output might be against aero drag, and this can go up and down 30% net with changes in wind speed and direction, even on the same loop ride. So my take on it is that, really (without a load of additional data from a powertap or whatever), it is 'just a bit of fun' I suppose, not a serious training tool in its own right.

cheers

That sounds sensible, thanks

Unless you have a power meter all power reported on strava is a guess, you can tell if someone is using a power meter and the power is 'real' as strava puts a lightening bolt symbol next to the figures.