Interpreting Strava power(watts) - MTB
I know that Strava's power calculation isn't completely accurate but I have found that as an average over a segment it is fairly repeatable. Is there any rhyme or reason when comparing power output between two riders on the same segment? For instance, let's say I have KOM on a segment and my power output is 285W. Number 2 rider's time is 5 second less but his power output is 200W. Does that mean he wasn't working as hard and has the potential to improve quite a bit? Or is rider to rider power comparison meaningless due to differences in rider and bike weight? I also realize that strava's power calculation is even less accurate for mountain biking due to rocks and obstacles not taken into account, but if two riders are theoretically facing the same trail I think it shouldn't matter.
I think its based off of rider weight. I am 200 lbs and I put that in my strava and my power output is always higher than my friends even when there times are way faster than mine on the same night. They weigh between 160-180 pounds.
Some measurements don't measure much of anything.
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The Strava power estimates are often a long way away from the actual power output. You can't rely on them to be even vaguely accurate for comparison purposes between different riders.
Unless there's a small lightning icon next to the figure the wattage figure in Strava is only an estimate. The lightning icon in Strava shows that the power data came from a power meter, rather than being an estimate based on what the user has entered.
I believe Strava calcuates power from elevation profile and weight of the bike and rider. I believe you enter both of these values. Strava gets the time and elevation. Power can be calcuate by knowing how long it takes to lift a weight of certain height. That said it is subject to errors and this method does to not work well for instant power. Comparing to other riders is interesting, but not always valid as they may not have entered their weights properly.
'12 Santa Cruz Highball 29", '13 Santa Cruz Solo 27.5", '06 Rocky Mtn Switch 26" XC, AM, blah blah blah.. I just ride.
On the road, the power calculation is "fairly" accurate in most cases, assuming you have all your weights and bike weight entered accurately. For example, I've done a few tests with my PM on my road bike and running the Strava app on my phone. Usually, they're within about 20 watts average power of each other. However, there are times when it won't be as accurate - it depends on wind. I was riding into a tough headwind, and it was very inaccurate there on the average.
On a mountain bike, I'd assume it's going to be more inaccurate due to the rolling resistance of what you're on. If you're grinding up a loose rocky trail, it's going to be harder than going up the same grade on nice, smooth pavement. I've never had a PM to compare to though, so I'm not sure how accurate/inaccurate it is.
I'd be interested in how Strava does its calculations. In comparison to my power meter, I'm about 50-70 watts higher on Strava for the same segments. Another thing I've noticed is that singletrack has a consistently very low average wattage on Strava. Conversely road rides up a long gentle hill is the best way to have a crazy high average.
I've often wondered if my calibration is off for my power meter, but it is extremely consistent from ride to ride or even on a trainer with my RPE about the same.
Bike weight, rider weight, and topography. The estimates aren't bad for the most part. I find that they're most useful when comparing a ride of mine with a previous ride on the same route/segment and they do seem to reflect the amount of effort I've put out well enough for my purposes.
Headwinds and mtb, I agree, the accuracy is suspect. But not bad for an estimate.