Is it possible to take my GPS tracks and estimate my power output so I can avoid buying a power meter? I was looking at my recent rides and noticed that I'm spending more time at higher speeds and also climbing faster, but sometimes it doesn't affect my average speed. I realize I would have to put in the weights of my bike, body, and cargo to get a rough estimate.

Strava does this. But their rough estimates are not very accurate, therefore, not very useful.

I realize that comparing my power output with yours is not useful, but is it good enough to compare my output from today with that of yesterday?

I think GPS does not have the accuracy to do it correctly. At best consumer GPS receiver has maybe 20ft error and consumer GPS device typically takes only one reading per second. I have seen some that sample at 10Hz, but I don't think they are common. GPS then calculates your speed based on the distance you moved between current position and position one second prior so there is a bit of lag. If both of those are off by 20 ft, then you could have a bunch of error in that calculation. If you average it out over several seconds then you get a better idea. But it may not give you the resolution you want.

A long (over 20') steady climb can approximate it.

Strava is free so try it out to see if it fits your needs. But Strava can't tell a 5 mph tailwind from a 5 mph headwind on any given ride or segment.

how would they know you didn't have a massive tailwind, or more likely, a massive headwind? It's not that easy to do this calculation. Even taking wind into account, they don't know if you are wearing baggy clothes, if you pumped up your tires, or anything like that.

Too many variables. Riding on rollers I've noticed just by consciously breathing deeply I can knock my HR down a dozen bpm over the course of 10 seconds while maintaining consistent cadence. So even trying to incorporate HR for input as a measure of effort is all over the place.

It depends how good you need it to be. I'm not trying to be a jerk with this answer. Yes, you can estimate your power from GPS data. Yes, you can do that and not buy a power meter. Can you get the same numbers a PM would have given you? No. Can you chart fitness gains or losses over time? Sure. Can you use it to inform your diet? Probably, with a little effort.

On just any random ride you do, probably not. But then I'm not sure you can do that on just any ride with a power meter, either. I mean, I just did a 4 mile round trip to the grocery store because it's easier than parking a car. It was anything but an all out ride.

Now if you have a big hill, and you don't mind going all out when you climb it, you can go back through your data and see how long it took yesterday and today and tomorrow. Wind can make or break a sprint on the flats, but aerodynamics don't usually have such a big effect going up hill. It'll be the same amount of steep every day and paved just as well today as yesterday. Strava will do all the math for you, make it a segment.

Good responses. Thanks, folks. You're right especially about the wind. I deal with a lot of it here. I wish I lived in a hillier and less windy area.

yeah, wind is evil. I prefer hills

I have a power meter and I find Strava's average power (and thus work/calorie burn) estimate pretty accurate. But this only works if you do an out/back route to account for wind. And obviously there's no way for Strava to know if you're drafting someone so keep that in mind for group rides.

I will go along with this and say, Strava does a pretty good job of estimating power over a large amount of time. For example, if I average my power curve over the previous month, Strava's estimate is pretty accurate to what I see when I take a month's worth of power data from my power meter.

I also agree - but it's kind of like a +/-20-30% kind of accuracy. I also notice that when I ride and the wind is up, then it is not accurate at all.

Power meter pricing continues to come down. I will say that I learned a lot from having a power meter that I didn't expect - things like being able to fine tune or even understand technique differences was really enlightening.

J.

OK now the idea of having a power meter starts to make sense to me. I see the various types out there, and I suspect the kind in the rear hub makes the most sense for me. They seem to be about $600. [MENTION=190826]Dunbar[/MENTION] and [MENTION=219896]JohnJ80[/MENTION], what kinds do you have?

It may be a little off topic, but since you mention it: the higher speeds on a ride don't affect the average as much as the lower ones. Just model it out, start with the record of a real ride if you want and "what if" you increased the higher speeds by half? Then "what if" you increased the lower speeds by half. You'll see that bringing up the slack or resting speeds influences the average, but slamming it downhill does almost nothing for it.

Of course, hammering downhill is the fun part so that's mostly academic.

Strava has no clue about how un-aero I am, so Strava's power over a ride is a guess. But Strava knows pretty well how long it takes to go up a long, steep hill and going slowly means that aerodynamics is a very small part of it. So with real weights, and a long hill where Strava has good elevation data on, the power estimate will be very close IMO.

I have a Stages single crank. I looked pretty hard at the 4iii. I wanted something in the crank since I have multiple wheel sets. dcrainmaker has a great evaluation of pretty much any power meter out there.

J.

Thanks! That's interesting.

Good thoughts. I commute about 13.5 miles each way. I do this about two or three times a week. On other days, I take the subway to work (and back). My route is entirely flat for 9 miles. Then I have a short and steep hill and then a few small hills. The challenges are wind. Only the last 2.5 miles are in motor traffic. Before that, it's on a bike path or multi-use path, so there aren't many occasions to slow or stop. Still, given what you point out, my opportunities to improve my time are in the stops and slow-downs.

But time isn't really important as long as I get where I'm going on time. I want to be stronger. My time on the steep climb has been as low as 40 seconds but it typically takes me 1:10. I'm paying attention to how fast I recover after the climb: I try to build up speed as fast as I can, and I can feel myself improving there.

I've always been a half-decent (or maybe even fully decent) climber. One weakness for me is riding on flat ground. I pass people going uphill, and people pass me on the flats.

Today's commute was very unusual in that there seemed to be no wind. I ride along the Hudson River, which is very windy most of the time.

Here are my GPS tracks from this morning's ride. You can see things such as my elevation over time and my speed over time.

When I got a power meter I found that is was <5% difference on familiar routes riding solo. You have to do an out-and-back route to control for wind though. Otherwise that can throw the numbers way off IME. On a commute with ~8+ hours in between rides the wind could change enough to throw off the numbers.

I think we're saying the same thing. Wind is the big variable against the Strava calculations. The out and back strategy presumes consistent wind as well.

J.

It certainly does. My route has shifty winds, since the sides of the river have steep cliffs and the river is over a mile wide. It usually does change overall direction between morning and evening.

So yes, I see it's a huge problem, not possible without a local pressure sensitive meter. Wind is the greatest impediment on a bike, so the question is how well you are pedaling into it, and that depends on the wind speed and direction.

RideWithGPS does this, and the numbers at least are consistent with what I get with Zwift, and have the added merit of being realistically feeble. Changes in power are probably more reliable than the absolute number.

What does realistically feeble mean?

And as we have discussed, my effort varies a lot but my time doesn't, which means my power readings are not helpful to compare one day with another.

The low number probably accurately reflects my inherent weakling suckiness.