# Strava Power estimates



## Mr645 (Jun 14, 2013)

I can't really figure out how this information can be of any use or even remotely accurate. I don't have a power meter and use Strava to keep track of my rides and milage. As far as power, how does it know if I am riding alone, or in draft for the whole ride. Wind conditions?

There are times when 22 mph behind 9 other riders is relaxed and I can recover from a 25mph sprint and other times I could be riding alone at 17mph into a headwind on the same road and be worn out from that pace.


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## Fireform (Dec 15, 2005)

It doesn't know. That stuff is worthless.


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## duriel (Oct 10, 2013)

I have a GPS speedmeter and Strava coordinates my location with NSA satellites to determine if I am riding alone or with a group. A MIT master program student from India wrote the code to determine how many of those other riders I could drop if I wished.


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## tom_h (May 6, 2008)

Strava is reasonably accurate on estimating power, when climbing steady grades of 5-6% or steeper, on a windless day, and if you're not drafting (small effect even on a 5-6% hill).

Other than that, it's not possble to estimate reliably, there are too many variables for aero drag and tire rolling resistance.


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## sandiegosteve (Mar 29, 2004)

Can't speak to Strava accuracy, but power is actually a pretty simple thing to calculate and can be estimated knowing a few variables. There are a lot of variables, but not that many with dramatic effects on a bicycle. When the extreme folks talk savings, it is in the nature of a few watts over an hour. The biggest influencers in power for a ride are elevation and the riders weight, both of which Strava can have a good estimate of. That said, the estimate is good for a whole segment/ride, not parts of it (instant).


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## den bakker (Nov 13, 2004)

sandiegosteve said:


> Can't speak to Strava accuracy, but power is actually a pretty simple thing to calculate and can be estimated knowing a few variables. There are a lot of variables, but not that many with dramatic effects on a bicycle. When the extreme folks talk savings, it is in the nature of a few watts over an hour. The biggest influencers in power for a ride are elevation and the riders weight, both of which Strava can have a good estimate of. That said, the estimate is good for a whole segment/ride, not parts of it (instant).


keep telling yourself that into a 20mph headwind


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## jmorgan (Apr 13, 2012)

sandiegosteve said:


> Can't speak to Strava accuracy, but power is actually a pretty simple thing to calculate and can be estimated knowing a few variables. There are a lot of variables, but not that many with dramatic effects on a bicycle. When the extreme folks talk savings, it is in the nature of a few watts over an hour. The biggest influencers in power for a ride are elevation and the riders weight, both of which Strava can have a good estimate of. That said, the estimate is good for a whole segment/ride, not parts of it (instant).



Who stays in one position on the bike for an entire ride? (changing position can easily influence speed by 1mph or more) Like stated above changing wind conditions also heavily impact your speed (easily resulting in 5+mph differences). Even if you were able to sort out all of the above and calculate a corrected speed, I hope you were never in anyones draft at any point during a ride because your calculation would still be off. 

As a power meter owner, Strava does get into the ballpark for overall longer climbs, its useless on most segments as wind and drag play such a large roll and none of that is accounted for in Stavas calculation and there is no easy way to derive that information. It is also dependent on weight and not everyone reports it accurately.

To the OP you are right it provides little to no significant value. You are not able to use the data on a ride to help train a certain zone and can be influenced heavily by outside factors. 

Here is an image from Alex Simmons showing the influence of variables on speed.
View attachment 292031


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## cxwrench (Nov 9, 2004)

Mr645 said:


> I can't really figure out how this information can be of any use or even remotely accurate. I don't have a power meter and use Strava to keep track of my rides and milage. As far as power, how does it know if I am riding alone, or in draft for the whole ride. Wind conditions?
> 
> There are times when 22 mph behind 9 other riders is relaxed and *I can recover from a 25mph sprint* and other times I could be riding alone at 17mph into a headwind on the same road and be worn out from that pace.


Wait...what...?:skep:


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## jmorgan (Apr 13, 2012)

cxwrench said:


> Wait...what...?:skep:


Some people arn't as fast as others


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## Alex_Simmons/RST (Jan 12, 2008)

sandiegosteve said:


> , but power is actually a pretty simple thing to calculate and can be estimated knowing a few variables.


Well that's true, but the problem is many of the important variables are not being measured or accounted for, and so poor assumptions are made.



sandiegosteve said:


> , There are a lot of variables, but not that many with dramatic effects on a bicycle.


Hmmm. Wind and aerodynamics has a dramatic impact on determining power from speed.

e.g. take a nearly imperceptible wind of 0.5m/s (that is with a wind speed rated as very calm)

Say rider maintains 32km/h / 20mph on flat road.

There is ~20% more power required to ride at that speed into such a tiny headwind than with a tailwind. 

So yes, getting power from speed is trivial, provided you can accurately measure all the input variables. Last I knew, Strava doesn't know the second by second wind data a rider encounters.

Wind affects power estimates from climbing as well:
Alex's Cycle Blog: Windbags


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## Fireform (Dec 15, 2005)

Strava can't tell whether you're powering solo into the wind at 25 mph or goofing along in a pack with a tailwind at 25. One case you're cranking out 500 watts, the other 100. Strava doesn't know the difference and assigns the same watts to each.


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## JackDaniels (Oct 4, 2011)




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## bikerector (Oct 31, 2012)

I've been asked this before and after laughing at the calculated numbers I get when riding in the winter I'm pretty sure it's extremely inaccurate under many circumstances. I think for one ride I was at 45 watts because I was riding through a lot of snow on my cross bike for a long time. 200 watts is an easy pace for me as measured by a PM for reference (fat racer).

I think the calculation format is quite similar to what cycleops.com had on their website for a long time where it assumed rolling resistance, or all resistances, were constant and uniform and just used weight, elevation change, distance, and time as variable to get a very rough approximation of power. Can't find it anymore since they split into cycleops and powertap.


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## cxwrench (Nov 9, 2004)

JackDaniels said:


>


That right there makes it crystal clear that Strava's power 'estimating' formula(s) suck.


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## Fireform (Dec 15, 2005)

If it's not clear to the op, the power numbers with little lightning bolts are from actual power meters, while the others are strava estimates. Big difference.


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## SpeedNeeder (Aug 19, 2013)

sandiegosteve said:


> ...There are a lot of variables, but not that many with dramatic effects on a bicycle... The biggest influencers in power for a ride are elevation and the riders weight, both of which Strava can have a good estimate of...


As others have said, Strava doesn't have a clue what power you are really putting out. Wind and drafting have dramatic effects on how fast you go. If you are riding flat, wind is the only inhibitor - For me, I either ride 14 mph or 23 mph, depending on which way the wind is blowing! That's a pretty big difference!
I'm not sure that a calculated average power over an hour ride would tell you anything. I have days where I complete a particular road coarse in 65 minutes, and I have days where I complete it in 75 minutes. Cross winds mess you up too.


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## duriel (Oct 10, 2013)

SpeedNeeder said:


> I'm not sure that a calculated average power over an hour ride would tell you anything. I have days where I complete a particular road coarse in 65 minutes, and I have days where I complete it in 75 minutes. Cross winds mess you up too.


Anytime your riding and it is windy, you speed suffers overall on any ride including a out/back. On a closed out/back course, your no wind time will be faster than any other time with wind. As the wind goes up, your speed will go down.
Therefore the quantity of people you can drop on a no wind day is exponentially increased.


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## Alex_Simmons/RST (Jan 12, 2008)

duriel said:


> As the wind goes up, your time will go down.
> Therefore the quantity of people you can drop on a no wind day is exponentially increased.


Hammer alongside the gutter and I'll bet my house far more get dumped on a windy day than on a calm one.


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## cxwrench (Nov 9, 2004)

duriel said:


> Anytime your riding and it is windy, you speed suffers overall on any ride including a out/back. On a closed out/back course, your no wind time will be faster than any other time with wind. As the wind goes up, your time will go down.
> *Therefore the quantity of people you can drop on a no wind day is exponentially increased.*


No clue...not a single one.


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## robdamanii (Feb 13, 2006)

cxwrench said:


> No clue...not a single one.



He is hilariously clueless though...


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## cxwrench (Nov 9, 2004)

robdamanii said:


> He is hilariously clueless though...


Startlingly, surprisingly, ridiculously, I could go on all night.


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## duriel (Oct 10, 2013)

You guys better re-read that, I have no idea what your talking about.

We're talking Strava here.... On a windy day your power estimates go down. On a windless day your power estimates go up. Ergo.. my power on a windless day says I will rule the wed night crit training ride.

As per the real world... power on strava is virtually useless info. On a windy day, far more will be splintered off the back, on those days stay in the first 6 or be relegated to pack foder.


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## aclinjury (Sep 12, 2011)

oh not this Strava power crap again.

Let me just say that Strava consistently say that I put out about the same power as my buddy who's heavier than me (he's using a SRM power meter to get his power numbers). Yet he usually drop me in any sort of fast crit like group ride, with or without climbs. It's a joke. It makes me feel a little embarrassed whenever I post a ride and Strava shows my estimated power number to be 10W within him, yet I know he's a stronger rider in all condition. That's all I need to say about Strava estimated power. Perhaps the number would be more useful if we ride in a vaccum with zero air resistance!


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## robdamanii (Feb 13, 2006)

duriel said:


> You guys better re-read that, I have no idea what your talking about.
> 
> We're talking Strava here.... On a windy day your power estimates go down. On a windless day your power estimates go up. Ergo.. my power on a windless day says I will rule the wed night crit training ride.
> 
> As per the real world... power on strava is virtually useless info. On a windy day, far more will be splintered off the back, on those days stay in the first 6 or be relegated to pack foder.


Strava takes no wind into account that I'm aware of.


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## Mr645 (Jun 14, 2013)

Fireform said:


> Strava can't tell whether you're powering solo into the wind at 25 mph or goofing along in a pack with a tailwind at 25. One case you're cranking out 500 watts, the other 100. Strava doesn't know the difference and assigns the same watts to each.



Exactly. Example on a 24 mile segment, I rode it a month ago and due to 50° temps and a solid 20 mph headwind I wore my self out with a 17.9 mph avg, but last week, the same segment I completed at 22.2 mph and hardly broke a sweat due to a nice tailwind and a tight 15 rider pace line that kept a 22-24 mph range


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## Dunbar (Aug 8, 2010)

I agree that it's worthless on flat segments. But I can't say I notice much difference with and without power in my average power for the entire ride on Strava on my out-and-back 2-3hr rides.


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## Cableguy (Jun 6, 2010)

I can average 532 watts for half an hour on Strava. I did a ride with a massive tailwind in a large group while drafting. I have a certificate arriving in the mail shortly to recognize my accomplishment. The certificate is for the biggest poser award, but it's still an award... and I feel both honored and distinguished to accept it.


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## Mr645 (Jun 14, 2013)

Ok, today had the chance to ride with someone actually using a power meter built into his pedals. 

we rode a 4.8 mile loop 4 times. totally flat except for one small bridge, 16' climb 

Riding north we held a steady 23 to 23 1/2 MPH pace, nice tail wind.

Strava says I was delivering 289 watts and the other riders meter said 184 avg for the segment

Against the wind we were in the 17-18 range, Strava 139 watts, meter 241

Now sure, pulling time was probably not the same for the riders, but this clearly shows that Strava is accurate + or - 50%


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## robdamanii (Feb 13, 2006)

Mr645 said:


> Ok, today had the chance to ride with someone actually using a power meter built into his pedals.
> 
> we rode a 4.8 mile loop 4 times. totally flat except for one small bridge, 16' climb
> 
> ...


Great. 

Most reliable power meters are +/- 2%.

F**k Strava's awful "power" numbers.


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## sandiegosteve (Mar 29, 2004)

Dunbar said:


> I agree that it's worthless on flat segments. But I can't say I notice much difference with and without power in my average power for the entire ride on Strava on my out-and-back 2-3hr rides.


This.

Strava is not going to tell you anything much about a segment, but total average isn't bad. If you want power for training, get a power meter. This is better than my Garmin's guestimate.

Wind - sometimes changes a bit during a ride, but 99% of the time I finish where I started, so I get both a push and a headwind. Same for the hills. Net elevation gain is 0 when I end up where I started.


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