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Can someone explain how Watts = Exertion?

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Can someone explain how Watts = Exertion?

Old 04-10-16, 04:58 PM
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Sy Reene
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Can someone explain how Watts = Exertion?

I came back from a casual ride today. My Garmin uploads to Strava evidently, and I found various segment readings.

What I don't understand about the whole power thing...
I had one 1/2 mile segment, 12% grade. I thought I was going to die. Strave says my result equates to only a 115w power rating, going at roughly 10mph.

Meanwhile, during the same ride, there were other 'segments' with 200+ watt ratings, and I don't even recall them specifically as being difficult.

Can someone explain how this works?
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Old 04-10-16, 05:19 PM
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Originally Posted by Sy Reene View Post
I came back from a casual ride today. My Garmin uploads to Strava evidently, and I found various segment readings.

What I don't understand about the whole power thing...
I had one 1/2 mile segment, 12% grade. I thought I was going to die. Strave says my result equates to only a 115w power rating, going at roughly 10mph.

Meanwhile, during the same ride, there were other 'segments' with 200+ watt ratings, and I don't even recall them specifically as being difficult.

Can someone explain how this works?
Something doesn't compute.

Was that steady state (or average) speed of 10 MPH, and steady state of 12% grade? And only 100 W average? For how long?

Also make sure your weight and equipment are right.

Click on round ball, top right of screen.
Go to Settings.
Check your weight (pounds or kilos?)

My strava power jumps around a lot, but the average should be higher than 115W.

Strava power may not always be representative. I have one Strava segment that I regularly hit the bottom of the hill at 30+ mph, and Strava always over-estimates my power by quite a bit as I hit the first short slope up(I think).

Last edited by CliffordK; 04-10-16 at 05:41 PM.
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Old 04-10-16, 05:27 PM
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Strava power estimates are worthless. To go 10 mph at 115w on a 12% grade, you'd have to weigh about 35 pounds total with your bike.
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Old 04-10-16, 06:27 PM
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Strava is lying to you.
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Old 04-10-16, 06:32 PM
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Originally Posted by PepeM View Post
Strava is lying to you.
Or the OP is lying to Strava
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Old 04-10-16, 06:38 PM
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Originally Posted by CliffordK View Post
Or the OP is lying to Strava
Or both.
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Old 04-10-16, 06:44 PM
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Or it was raining and the rain affected the altitude readings which can really mess up the power numbers. Strava power will use the elevation data from your rideif you have a altimeter in your unit. Otherwise it will use strava elevation database which can be rather random.

Or it was really windy.
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Old 04-10-16, 07:06 PM
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Not sure what the skepticism is all about.. here's a screenshot. And, I'm not claiming, by any means, any achievement whatsoever. Strava indicates I rank about 2000th place out of roughly 3000 on this segment..

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Old 04-10-16, 07:13 PM
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Probably because if you look at the elevation vs. distance graph, the climb only looks like it's about 30 ft.
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Old 04-10-16, 07:18 PM
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Originally Posted by Xherion View Post
Probably because if you look at the elevation vs. distance graph, the climb only looks like it's about 30 ft.
30 ft of climbing over 2500 ft of distance....is a whopping 1.2%. So maybe the OP innocently got his decimal place in the wrong spot.

But for him to be huffing means he's out of shape.
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Old 04-10-16, 07:44 PM
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H8ters gotta h8..
Hey, I'm only referring to what Strava puts in the top left section of the screenshot I posted. I didn't run any math to see if it's right. To recap, says .5miles 336ft elevation gain, 12%. Maybe I'm just misreading Strava; don't know. Haven't really paid much attention until I decided to compare what I thought were difficult little segments to what I thought were relatively easy segments.
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Old 04-10-16, 08:06 PM
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Do you live in Death Valley?

https://www.strava.com/segments/1987057

Look at the starting elevation: -251 feet, ending elevation: +85 ft, total difference, 336 ft

Sometimes Strava goofs on start/ending points, and gives funky numbers.

Your screenshot probably has a more accurate elevation profile.

BTW:
If you look at the KOM's ride (michael cortazzo), it looks odd... He appears to be a strong rider, so it is hard to tell.

I'm going to call the #2 ride (Peter E) as fake. It appears as if he had a segment of hills/mountain biking followed by this extraordinary fast ride.

Probably other rides like that... maybe a page or two of fake rides on that segment.

If you can discern fake rides, you can flag them. There is one local ride that I flagged a dozen riders that did the hill climb, then turned onto the freeway and headed off at 60 MPH.

Last edited by CliffordK; 04-10-16 at 08:18 PM.
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Old 04-10-16, 08:07 PM
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Based on the graph, it looks like the climb was 33.6 feet, not 336 feet. Don't know how Strava came up with 336. A half-mile of 12% downhill would be awesome. With a little push at the top I could top 45 mph easy!
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Old 04-10-16, 08:32 PM
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If you look at the 6th place guy, he recorded the segment with a power meter. He averaged 256 watts at an average speed of 26.4 mph. Given a typical CdA, that power and speed is very much inline with a flat road.

Strava bases elevation on public GPS data, which is often incorrect. The VAM on that segment should be a dead giveaway that the GPS data is bad on that segment.

Here's another example of bad elevation data. This segment is near my house and is pancake flat (if not slightly downhill). I soft pedaled this segment at under 100 watts and had a VAM of over 5000.

https://www.strava.com/segments/1105698

Last edited by Xherion; 04-10-16 at 09:18 PM.
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Old 04-10-16, 09:41 PM
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Strava is reporting 12% because the person who made the segment had crap elevation data. Your data shows a 1% grade.

There's a segment just like that near me, it seems to think that a short burst of moderate grade, which in reality has about 20 feet of gain, is the Koppenberg.
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Old 04-10-16, 10:45 PM
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Originally Posted by Xherion View Post
Here's another example of bad elevation data. This segment is near my house and is pancake flat (if not slightly downhill). I soft pedaled this segment at under 100 watts and had a VAM of over 5000.

https://www.strava.com/segments/1105698
I bumped into one of those sections a few weeks ago. Listed as a Cat 4 climb, and I couldn't remember the hill.

https://www.strava.com/segments/3244841

Oh, here is an interesting ride. Not me, but I've seen similar data before.

https://www.strava.com/activities/97869673/analysis

Somewhere between 6 miles and 10 miles, Strava is apparently paused, jumps across town, and somehow looses about 200 feet (below ground level?). Then at mile 18, there is another jump, this time gaining about 300 feet.

I would imagine putting one of those jumps in the middle of a segment could give some obscure effects.

Actually, that may be what happened here.

https://www.strava.com/segments/798116

If the person who defined the segment had Strava paused for 3/4 the segment, then the results would be unpredictable.
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Old 04-10-16, 11:34 PM
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For the OP.

If you want some hill climbs that peak at around 12% for comparison, try these:

Boat Basin to Sign at top of Alpine
Hillside Ave Climb

And, if you're lucky, the ride back down to Boat Basin, Strava will jump to: Alpine Boat Basin to Circle to make you look like a wicked fast climber.

http://www.bikeforums.net/road-cycli...avanomaly.html

Maybe hang your phone off of the end of a fishing pole to your left for the descent
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Old 04-11-16, 05:38 AM
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Originally Posted by Marcus_Ti View Post
So maybe the OP innocently got his decimal place in the wrong spot.
Heh. Tell me you've seen the film "Office Space."
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Old 04-11-16, 10:19 AM
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This thread is a pointed reminder [to me] why Strava and its related gizmos/gear are a huge waste of time vis-a-vis riding a bike...
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Old 04-11-16, 10:30 AM
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Originally Posted by Phil_gretz View Post
This thread is a pointed reminder [to me] why Strava and its related gizmos/gear are a huge waste of time vis-a-vis riding a bike...
Strava; often yes. A garmin with a barometer, no, not a waste of time at all.
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Old 04-11-16, 12:51 PM
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Originally Posted by Sy Reene View Post
I had one 1/2 mile segment, 12% grade. I thought I was going to die. Strave says my result equates to only a 115w power rating, going at roughly 10mph.
Can someone explain how this works?
Well, first of all, 12% is a REALLY tough slope. The Alpe d'Huez in the Tour de France averages 8.1 and maxes out at 13.1% And it is rated Hors Categories ("Difficulty beyond classification"). So you may wish to check the slope on a topo map or from some good digital source.

The amount of mechanical work done is normal force * distance. If we just look at the work to climb the vertical distance, this is easily calculated. We'll ignore the friction of tires and (at 10mph) the aerodynamic drag. The force required to lift you is much greater than either, as you noted when you went from level ground (probably faster than 10mph) to the slope at 10mph.

1/2 mile is 5280ft/2 = 2640 ft. You have 12% slope, so (roughly) the height you climbed was 0.12 * 2640ft = 316.8ft. A standard human weight used for anthropomorphic calcs is 70kg. You can scale that later, but for now let's use that. 70kg = 70/.454kg/lb = 154 lbs. So you are lifting 154 lbs a distance of 316.8 ft, and the amount of work done is W=f*d= 154lb * 316.8ft = 48,846 pound feet.

If you did 1/2 mile at 10mph, the time you took was 1/2 mile/(10 mile per hour) = 0.05 hour. Which is 0.05 hr * (60min/hr) * (60sec/min) = 180 seconds.

Power is work per time, so P = 48,846 pound feed/180 seconds, or 271.4 pound ft/second. The conversion factor is 1 watt = 0.7375621492783 pound-foot/second, so

P = 271.4 pound ft /sec /(0.73756 pound ft/second per Watt) = 368 W. There are 745 Watts in a horsepower, so you were putting out about 1 half horsepower. You can adjust this by multiplying it by the ratio of your weight to 70kg, or 154lb. This is getting into the range that is not physically doable for most folks, so its no wonder you were breathing hard. Most folks would have to slow down to make it up a 12% grade of any great duration. Again, you calculate the energy used for just the climbing part of the energy by multiplying 1/2 hp by your speed divided by 10mph.

So, unless you weigh only 50 pounds, there is no way you were only putting 115 watts. Does Strava take into account the slope? If the altitude change is wrong (for example, if the Strava thinks this is level ground, or if it has the wrong weight for you, the power will be way off). If you can, look at the altitude recorded by Strava at the beginning and end of the tough interval. If it's not a bit more than 300 ft/100m, their map is wrong. The Garmin app has something that you select to improve the altitude calibration. I've tried it. Not sure I noticed any difference when I've tried that.

Hope this explains it well enough.

Last edited by WizardOfBoz; 04-11-16 at 01:13 PM.
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Old 04-11-16, 12:58 PM
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Originally Posted by corrado33 View Post
Strava; often yes. A garmin with a barometer, no, not a waste of time at all.
I think that the Garmin Connect calculates work using speed, weight, and essentially a digital topo map. I think that mine is pretty accurate. And again, you can click "Elevation Correction" on the web page. In any case, I agree with Corrado33: The Garmin units work pretty good. I've gotten very good customer service, too.

Here's my elevation change from a ride:
Attached Images
File Type: jpg
Elevation.JPG (19.1 KB, 8 views)
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Old 04-11-16, 12:58 PM
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Prefer measuring in Joules Instead?


It is equal to the energy transferred (or work done) to an object
when a force of one newton acts on that object in the direction of its motion through a distance of one meter
(1 newton metre or Nm).

Or

It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.

Last edited by fietsbob; 04-11-16 at 01:03 PM.
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Old 04-11-16, 01:08 PM
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Originally Posted by fietsbob View Post
Prefer measuring in Joules Instead?
Well, first, a Joule is one Watt-second. That is 1 W = 1 J/s.

There's a Neurolinguistic Programming technique that suggests that you respond to someone in language that they use. If someone says "How does the plan sound?", it's best to respond "I'm hearing that...", rather than "What I see is..." or "I'm feeling...", or even "Smells bad...".

I'm doing the same with units. The OP asked a question concerning Watts, mph, and miles. The answer was in those units or related customary ones.

In my day job I write math model of disease, so I have to use SI most of the time. Really weird ones, too, like receptors per square nanometer. But I went to engineering school when ft-lb-in were used more. So I know really obscure conversions like a gallon is about 231 cubic inches. Sigh. True story: tthe chicks go wild over guys that can convert units. In fact, if I go into a bar and let it be known that I can do units conversion, I can't buy a drink and the gals won't leave me alone. (ok, I missed April Fools day by a week or so).
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Old 04-11-16, 01:14 PM
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Electronic things Its that language.. Watts is a electrical measure .. so the device can deal with that

People may prefer Verbs .. how Knackered were you?
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