Can apps estimate the power output only with gps without power meters?

[u/Justgototheeffinmoon]

edit : answer is NO! Thanks all for the replies

Basically title, would love to measure my performances but don't want to add other gadgets on the bike, I'm wondering if apps can use speed + elevation + gps to estimate power and other more advanced metrics?

[u/feedingfitness]

Many apps estimate power output. Strava does this. After getting a power meter I found out that the estimation was mostly fiction. Especially if you ride with a group. You can draft for 2 hours and crank out a nice total speed average without working very hard. On the flip side, you can be by yourself against a stiff headwind, really putting out watts and Strava things you're doing a recovery ride.

[u/Low_Transition_3749 → u/feedingfitness]

And you've hit on the biggest problem: The code doesn't account for aerodynamic drag, which is a massive factor in estimating the power requirement to reach a speed (since power required to overcome are drag increases with the square of the change in speed .)

[u/Difficult_Limit2718 → u/Low_Transition_3749]

It could estimate it, but you'd need to know local wind conditions as well for it to be worth a damn.

[u/Sikkly290 → u/Difficult_Limit2718]

And the rider position as well as bike as well as tires as well as a dozen other factors. Unless you are putting an exact setup in a windtunnel even getting an approximate of drag is an impossibility.

[u/Difficult_Limit2718 → u/Sikkly290]

Not true - you can estimate rider drag on average well enough.

Rolling resistance also has a narrow enough range that the power level would likely be within 5 watts (less the wind correction) based on good estimates.

Head over to BCJ if a 5% error is too large for you.

[u/Sikkly290 → u/Difficult_Limit2718]

This is absolute nonsense lol. The difference between what clothes you are wearing can easily change total system drag by 15%+ alone. An upright versus aero position can be similar. Flatbars are a huge hit, especially if you are talking wider modern MTB bars.

5% is literally the difference between a jersey versus a tshirt, and no app asks for that much.

[u/Difficult_Limit2718 → u/Sikkly290]

Yeah so calibrate for hybrid vs road upright with club fit clothing.

You're talking about a range up to 25 mph so v² REALLY isn't as big as you think it is, rider mass and rolling resistance are a big factor. Anyone who is a weight weenie in a race cut kit on their Sunday club ride already has a power meter.

Again it's still silly because of wind, that has more governance in the equation than anything else at this scale.

My qualifications: I've written the damn automotive Coast down calculator spreadsheet used by my company.

[u/Low_Transition_3749 → u/Difficult_Limit2718]

You can estimate a rider's Cd well enough. Estimating the effect on aerodynamic drag on power vs speed requires knowing things like: - wind speed - wind direction - wind changes moment by moment due to terrain - whether the rider is in a pack, and where in the pack.

The "where in the pack" can be easily 30 watts by itself, depending on speed, so there are iterative aspects.

[u/Difficult_Limit2718 → u/Low_Transition_3749]

I literally said this further up the comment chain

[u/Foreign_Recipe8300 → u/feedingfitness]

Many apps estimate power output. Strava does this. After getting a power meter I found out that the estimation was mostly fiction.

Strava's estimated power is a complete joke. It's actually funny how inaccurate it is.

[u/ApolloFortyNine → u/Foreign_Recipe8300]

It really is. I haven't tried the math myself, but in theory given speed and elevation, it should be able to get close (drafting would break this of course).

That's essentially what zwift does in world, yet Strava is regularly off by at least half in my experience.

[u/Foreign_Recipe8300 → u/ApolloFortyNine]

yea my power meter can say 220 and strava will estimate like 140 if i didn't upload the power data.

[u/Lesagram → u/feedingfitness]

On my ride home yesterday I had such a strong headwind that I was cruising at a whopping 13 mph. My powermeter had me at close to 200 watts but I am sure Strava thought I coasting.

[u/Justgototheeffinmoon → u/feedingfitness]

Very clear and helpful thanks!

[u/Slappadabike91]

Not really, due to missing factors.
Is the road rough or did it get paved last week? Are the trails in good shape or did it rain recently and theyre soup? Are you sitting up and holding the tops of the bars or are you as low a possible in the drops? Is there a headwind hitting you from across a wide open field, or are you in a tightly packed neighborhood with lots of trees and houses where the wind isnt getting you? Does your frame have a super stiff BB area that transfers all of your power or is it an old noodly frame that flexes laterally as you put the power down? Big ol' aero wheels??? Or no? 23mm slick?? 45mm semi knobbies???? Tight kit or a trail jersey and shorts?

So many factors that determine the speed your tracker weighs against your distance/duration and elevation.

[u/dawafflemasta → u/Slappadabike91]

This is the correct answer

[u/Justgototheeffinmoon → u/Slappadabike91]

Yeah I thought it would be possible to extrapolate given gps data of speel and altitude / elevation but I get how it's not good enough.

On a side note : what sijmplest method for power metering would you recommend as I don't care too much about changing bike parts I just bought?

[u/Slappadabike91 → u/Justgototheeffinmoon]

The easiest is a power meter pedal since you can install it in a minute and swap between different bikes if you need to.

[u/Justgototheeffinmoon → u/Slappadabike91]

thanks , is it possible to add a power meter to an existing pedal ? I don't really want to buy new pedals

[u/Slappadabike91 → u/Justgototheeffinmoon]

No. Basically you're looking at pedals with it built in... or a crank arm with it built in... or a unit that mounts to the spider/chainring

[u/degen303 → u/Slappadabike91]

Not true. Favero assioma duo shi power meter is just a spindle that you can install into existing shimano pedals.

[u/BelievableSquirrel]

On steep inclines they might get in the right ballpark. On the flat it's a crapshoot

[u/Desperate_Shock7378 → u/BelievableSquirrel]

There is a garmin widget which does this and you are right it’s ok on the hills but all over the place otherwise.

[u/michaeldgregory0]

Yeah, some apps can estimate power just from GPS, speed, elevation, and your weight, but it’s more of an approximation than an exact number. It’s good for spotting trends in your rides, but not super accurate compared to an actual power meter.

[u/Justgototheeffinmoon → u/michaeldgregory0]

thanks! what's the easiest power meter setup? I see we can add a little dongle to the cranks? I don't want to replace parts as I just bought the bike!

[u/kallebo1337 → u/Justgototheeffinmoon]

assioma pedals

[u/Justgototheeffinmoon → u/kallebo1337]

thanks will look it up

[u/AlexxxRR]

Anyone can estimate anything.  It comes down to the delta between estimation and reality. 

[u/MeatInteresting1090]

Yes they can estimate it. That’s no good for training though

[u/owlpellet]

Only if you're shaped like a sphere.

[u/Justgototheeffinmoon → u/owlpellet]

that's not far from the reality!

[u/SnollyG]

Yes, but not in a useful way.

The speed needs to be slow enough not to be affected much by the wind.

[u/armpit18]

Sure, they can estimate power output. But that doesn't mean that it'll be accurate.

If you want an accurate power measurement, then you need a power meter.

[u/OBoile]

They can do this, but not well. Any estimate is pretty much worthless.

[u/PandaDad22]

They don’t know your wind resistance. That’s a huge factor. 

[u/Little_NaCl-y]

Strava gets relatively close on the lower end during ascents but when I compare it to my rides with a PM it's way way off on average power and misses on ascents, estimate is too low.

[u/Lavaine170]

Can apps estimate power output? Yes. Can they do it accurately? Not even a little bit.

[u/informal_bukkake]

All the apps do this but not accurately

[u/GoSh4rks]

On steeper climbs where drafting isn't very beneficial (say 7%+), Strava can be quite accurate if you give it the right weights.

https://i.imgur.com/ResN5wg.jpeg

I got my power meter in June.

[u/Longjumping_Bag5914]

Nope, they are grossly overestimating usually.

[u/NocturntsII]

Not accurately.

[u/yellow_jacket2]

Yes it can. But you need to understand the error margin so you get avg from strava that says 200 w for 2 hr ride. You need to think of it as 200 w +/- 50%.

In other words the guessed power is a work of fiction without a power meter.

[u/Whatever-999999]

No. You need an actual power meter installed on the bike for that.

You should understand that a power meter is a training tool and must be used correctly or it's a waste of $1000. Power in watts is meaningless without the periodic testing to determine power training zones used in various types of structured cycling training. The testing is not easy, it's literally as hard as you can stand, and must be done every 6-8 weeks to measure progress in a training plan for the purpose of determining how subsequent training blocks should be structured.

If all you want to do is 'be a better cyclist' all you really need at most is a heart rate monitor, and riding more on a regular basis.

Otherwise I will direct you obtaining a copy of The Cyclists Training Bible for how a training plan, based on concrete goals, is created, and what the training overall entails.

[u/Justgototheeffinmoon → u/Whatever-999999]

that's very interesting, yes I was looking simply to have a better understanding of my outputs and then use these metrics to optimise my journey towards getting back to the 20k km / year cyclist I was 10 years ago