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Power meter numbers, trainer versus road

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Power meter numbers, trainer versus road

Old 05-02-17, 02:53 PM
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Power meter numbers, trainer versus road

I'm thinking that power numbers may be less accurate on a trainer due to there perhaps being more flex or give versus the unforgiving surface of the road. I'm using the Stages crank arm power meter. Thoughts?
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Old 05-02-17, 03:10 PM
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same same
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Old 05-02-17, 03:24 PM
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I don't think the PM knows or cares about the road surface since calculates torque from strain gauges in the crank arm.
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Old 05-02-17, 04:56 PM
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Accuracy will be identical; the PM will measure the force you apply to the pedals equally. You may encounter different resistance and motivation on the road. For instance, being surprised and chased by a dog resulted in my first ever 1kw+ reading.
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Old 05-02-17, 06:39 PM
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Originally Posted by whitemax
I'm thinking that power numbers may be less accurate on a trainer due to there perhaps being more flex or give versus the unforgiving surface of the road. I'm using the Stages crank arm power meter. Thoughts?

In my case, im nearly 100% sure that my power meter (power2max) is reading lower indoor.

I would say about 20-30b watts aroung 250-300w and it its even worse at higher power. And Im not talking about loss of power due to heat soak. I can notice the lower readings within 3-4 pedal strokes.

The best example was last saturday when I did my first ride outside. It was so easy to do 500-600w seated vs indoor where its nearly not possible.


My guess its probably due to some drivetrain/tire loss.

Last edited by laf; 05-02-17 at 06:43 PM.
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Old 05-02-17, 07:16 PM
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Depends where the PM is. If it is on a trainer and at the tire (a roller) it could be lower by ~10W vs on the pedals.
I prefer the at the tire devices as it measures the net result including all losses.
Pedals measure you , which is good, but what hits the ground matters most.
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Old 05-02-17, 07:59 PM
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Originally Posted by Doge
Depends where the PM is. If it is on a trainer and at the tire (a roller) it could be lower by ~10W vs on the pedals.
I prefer the at the tire devices as it measures the net result including all losses.
Pedals measure you , which is good, but what hits the ground matters most.
Isnt power at the ground what matters least? What's the point of a power meter if you're not measuring just your physical work done on the pedals? Speed takes into account all losses...
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Old 05-02-17, 08:07 PM
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Originally Posted by Abe_Froman
Isnt power at the ground what matters least? What's the point of a power meter if you're not measuring just your physical work done on the pedals? Speed takes into account all losses...
Doesn't matter where it's measuredas long as it's consistent.

Speed is less than useless as a training metric.
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Old 05-02-17, 08:11 PM
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Originally Posted by reggieray
Doesn't matter where it's measuredas long as it's consistent.

Speed is less than useless as a training metric.
The only way that power meters differ from spedometers is that they eliminate Factors that throw off your personal energy expendeture.

Why would you purposely put back in varying factors by measuring at the back wheel? Crank friction, chain wear/lubrication, different cogs, whole drivetrain, hub friction loss.
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Old 05-02-17, 08:18 PM
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Originally Posted by Abe_Froman
Isnt power at the ground what matters least? What's the point of a power meter if you're not measuring just your physical work done on the pedals? Speed takes into account all losses...
I would say you are correct. A PM is for rider training.
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Old 05-02-17, 08:34 PM
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Originally Posted by reggieray
Doesn't matter where it's measuredas long as it's consistent.
Accurate is nice if you collect data over a period of years with different power meters.
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Old 05-02-17, 08:59 PM
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Originally Posted by Abe_Froman
The only way that power meters differ from spedometers is that they eliminate Factors that throw off your personal energy expendeture.

Why would you purposely put back in varying factors by measuring at the back wheel? Crank friction, chain wear/lubrication, different cogs, whole drivetrain, hub friction loss.
You think that's the only way those two devices differ? How many strain gauges are there in speedometers? That's like saying the only difference between a Ferrari and a Pinto is that the Ferrari goes faster.

Crank friction is constant as long as bottom bracket bearings are serviceable, chain wear would have no effect on power, chain lubrication would have an effect only at an extremely low level of lube, I have no idea why you think different cogs would affect power measurement, and hub friction loss is only an issue after the hub has read the power.
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Old 05-02-17, 09:03 PM
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Originally Posted by Abe_Froman
Isnt power at the ground what matters least? What's the point of a power meter if you're not measuring just your physical work done on the pedals? Speed takes into account all losses...
I value speed over power as I'm not so interested in power output of the rider if it does not translate to speed and often it does not.

Speed also has aero loss, which I care about too.
But there is equipment loss and it is not totally trivial.
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Old 05-02-17, 10:30 PM
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Originally Posted by reggieray
...
Speed is less than useless as a training metric.
When training for TTs it is about the only thing we use. Some positions generate more power and are less aero and visa versa. It is the fastest setup that matters most and it is not always where the most power is being generated, nor the most aero - it is the fastest.
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Old 05-02-17, 11:03 PM
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Originally Posted by Doge
When training for TTs it is about the only thing we use. Some positions generate more power and are less aero and visa versa. It is the fastest setup that matters most and it is not always where the most power is being generated, nor the most aero - it is the fastest.
How are you controlling for the moving air during this TT training?
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Old 05-02-17, 11:08 PM
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Originally Posted by reggieray
How are you controlling for the moving air during this TT training?
Position.
It is not much a factor here, or I know when it is. 5mph ocean breeze off the right, then off the left are pretty predictable.
Then there is the indoor track.
Then there is Palomar (aero position not important).
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Old 05-03-17, 03:50 AM
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Originally Posted by laf
In my case, im nearly 100% sure that my power meter (power2max) is reading lower indoor.

I would say about 20-30b watts aroung 250-300w and it its even worse at higher power. And Im not talking about loss of power due to heat soak. I can notice the lower readings within 3-4 pedal strokes.

The best example was last saturday when I did my first ride outside. It was so easy to do 500-600w seated vs indoor where its nearly not possible.


My guess its probably due to some drivetrain/tire loss.
conditions outside? was power meter calibrated or zeroed?
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Old 05-03-17, 05:15 AM
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Originally Posted by reggieray
Doesn't matter where it's measuredas long as it's consistent.

Speed is less than useless as a training metric.
Improving your relative speed is ultimately all that matters.

Races are won by going the fastest at just the right times, regardless of how much power is put out.

Using power in training to figure out how to maximize speed is the opposite of useless.
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Old 05-03-17, 05:46 AM
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Originally Posted by rubiksoval
Improving your relative speed is ultimately all that matters.

Races are won by going the fastest at just the right times, regardless of how much power is put out.

Using power in training to figure out how to maximize speed is the opposite of useless.
what if one is trying to improve ones fitness rather than win races?
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Old 05-03-17, 06:14 AM
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Originally Posted by redfooj
conditions outside? was power meter calibrated or zeroed?
Yes, but the power2max zeroed itself anyway after not pedaling for three seconds.
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Old 05-03-17, 06:14 AM
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What do you consider 'fitness?'
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Old 05-03-17, 06:15 AM
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It is widely reported that it is harder to produce power indoors than outdoors. There are a few hypothesis for that, but I haven't heard of power meters somehow misreporting just because they happen to be inside until now.
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Old 05-03-17, 06:31 AM
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Originally Posted by redfooj
what if one is trying to improve ones fitness rather than win races?
Then it doesn't really matter. Simply riding more will help the vast majority.

Regardless, speed is far from useless and has been the de facto inducator of fitness for as long as people have tried to measure endurance progress.
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Old 05-03-17, 06:52 AM
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Originally Posted by rubiksoval
Then it doesn't really matter. Simply riding more will help the vast majority.

Regardless, speed is far from useless and has been the de facto inducator of fitness for as long as people have tried to measure endurance progress.
And if that speed is out on the open road with varying traffic and wind condition from ride to ride?
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Old 05-03-17, 07:05 AM
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Originally Posted by PepeM
...I haven't heard of power meters somehow misreporting just because they happen to be inside...
That's because they don't.
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