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How accurate are power meters?



 
 
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  #1  
Old December 11th 13, 09:22 PM posted to rec.bicycles.tech
James[_8_]
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Posts: 6,153
Default How accurate are power meters?

Or should I ask how inaccurate?

According to http://www.youtube.com/watch?v=5DRQwKREgvI it appears a 3%
increase in bike+rider mass results in a 14% increase in power to
maintain the same speed.

Unless I missed something, the rider+bike mass for the pro rider was
78.4kg, and at 16km/h on an 8% gradient the power meter recorded 279W
average.

When they added 2.6kg, to achieve 81kg total and approximately a 3%
increase in mass, the power required to maintain the same speed of
16km/h was 40W higher, at 319W. A 14% increase!

I thought the power increase would be about proportional to the mass
increase, i.e. 3%.

--
JS
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  #2  
Old December 11th 13, 11:21 PM posted to rec.bicycles.tech
[email protected]
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Posts: 6,374
Default How accurate are power meters?

nungh....4 thronproof 29er/2.2 tubes came in UPS...I hadda get a wheelbarrow...
  #3  
Old December 12th 13, 03:41 AM posted to rec.bicycles.tech
Frank Krygowski[_2_]
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Posts: 7,511
Default How accurate are power meters?

On Wednesday, December 11, 2013 4:22:05 PM UTC-5, James wrote:
Or should I ask how inaccurate?

According to http://www.youtube.com/watch?v=5DRQwKREgvI it appears a 3%
increase in bike+rider mass results in a 14% increase in power to
maintain the same speed.

Unless I missed something, the rider+bike mass for the pro rider was
78.4kg, and at 16km/h on an 8% gradient the power meter recorded 279W
average.

When they added 2.6kg, to achieve 81kg total and approximately a 3%
increase in mass, the power required to maintain the same speed of
16km/h was 40W higher, at 319W. A 14% increase!

I thought the power increase would be about proportional to the mass
increase, i.e. 3%.


I agree. A 3% mass increase should cause pretty close to 3% increase in power required for a given speed on a given grade.

If 3% mass difference yielded 14% power difference, then it seems that reducing one's bike+rider mass by 21% (not impossible with my bikes, especially if you change the rider!) should reduce one's power requirement to almost zero. That's assuming things are proportional, which is what the laws of physics claim.

I don't know the explanation for their findings, but they don't seem to make sense.

- Frank Krygowski
  #4  
Old December 12th 13, 01:06 PM posted to rec.bicycles.tech
[email protected]
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Posts: 6,374
Default How accurate are power meters?

EEE, the factors are not arithmetic.
  #5  
Old December 12th 13, 01:59 PM posted to rec.bicycles.tech
Duane[_3_]
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Posts: 1,900
Default How accurate are power meters?

On 12/12/2013 8:06 AM, wrote:
EEE, the factors are not arithmetic.


Everything can be explained with math. Well except Rob Ford and Steven
Harper but they defy logic.

Maybe this helps:
http://en.wikipedia.org/wiki/VAM_(bicycling)
  #7  
Old December 17th 13, 09:29 PM posted to rec.bicycles.tech
James[_8_]
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Posts: 6,153
Default How accurate are power meters?

On 18/12/13 04:00, Phil W Lee wrote:
Frank Krygowski considered Wed, 11 Dec 2013
19:41:02 -0800 (PST) the perfect time to write:

On Wednesday, December 11, 2013 4:22:05 PM UTC-5, James wrote:
Or should I ask how inaccurate?

According to http://www.youtube.com/watch?v=5DRQwKREgvI it appears a 3%
increase in bike+rider mass results in a 14% increase in power to
maintain the same speed.

Unless I missed something, the rider+bike mass for the pro rider was
78.4kg, and at 16km/h on an 8% gradient the power meter recorded 279W
average.

When they added 2.6kg, to achieve 81kg total and approximately a 3%
increase in mass, the power required to maintain the same speed of
16km/h was 40W higher, at 319W. A 14% increase!

I thought the power increase would be about proportional to the mass
increase, i.e. 3%.


I agree. A 3% mass increase should cause pretty close to 3% increase in power required for a given speed on a given grade.

If 3% mass difference yielded 14% power difference, then it seems that reducing one's bike+rider mass by 21% (not impossible with my bikes, especially if you change the rider!) should reduce one's power requirement to almost zero. That's assuming things are proportional, which is what the laws of physics claim.

I don't know the explanation for their findings, but they don't seem to make sense.

I think you need to distinguish between the different sources of drag
before you can even start to analyse it.
Aerodynamic drag isn't going to change at all if the speed remains the
same, unless the gradient requires you to stand up to maintain speed,
in which case it could increase quite a lot, but on an 8% gradient at
10mph the main source of drag will not be aerodynamic. Frictional
losses would increase pretty much in proportion with power use.
Climbing power is what is going to be affected by mass.

I wonder if the accuracy of the power meter could be affected by
standing on the pedals to climb, as that would have a tendency to put
more weight on the pedals throughout their rotation.
That is, of course, only relevant if they are the pedal type (and may
indicate a superiority of the hub type).


Did you watch the video, Phil? The power meter was a "power tap" rear
wheel hub. It appears to have a wireless connection to a display unit
that measures average power over some time interval.

--
JS
  #8  
Old December 17th 13, 11:04 PM posted to rec.bicycles.tech
Frank Krygowski[_2_]
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Posts: 7,511
Default How accurate are power meters?

On Tuesday, December 17, 2013 4:29:23 PM UTC-5, James wrote:

Did you watch the video, Phil? The power meter was a "power tap" rear
wheel hub. It appears to have a wireless connection to a display unit
that measures average power over some time interval.


I was wondering about the details of the hub's technology, and the algorithm for averaging the constantly varying input. Does anyone here use one of those hubs?

- Frank Krygowski
  #9  
Old December 17th 13, 11:54 PM posted to rec.bicycles.tech
James[_8_]
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Posts: 6,153
Default How accurate are power meters?

On 18/12/13 10:04, Frank Krygowski wrote:
On Tuesday, December 17, 2013 4:29:23 PM UTC-5, James wrote:

Did you watch the video, Phil? The power meter was a "power tap"
rear wheel hub. It appears to have a wireless connection to a
display unit that measures average power over some time interval.


I was wondering about the details of the hub's technology, and the
algorithm for averaging the constantly varying input. Does anyone
here use one of those hubs?


I've been wondering the same.

--
JS
  #10  
Old December 18th 13, 03:16 AM posted to rec.bicycles.tech
James[_8_]
external usenet poster
 
Posts: 6,153
Default How accurate are power meters?

On 18/12/13 13:45, Phil W Lee wrote:
James considered Wed, 18 Dec 2013 08:29:23
+1100 the perfect time to write:

On 18/12/13 04:00, Phil W Lee wrote:
Frank Krygowski considered Wed, 11 Dec 2013
19:41:02 -0800 (PST) the perfect time to write:

On Wednesday, December 11, 2013 4:22:05 PM UTC-5, James wrote:
Or should I ask how inaccurate?

According to http://www.youtube.com/watch?v=5DRQwKREgvI it appears a 3%
increase in bike+rider mass results in a 14% increase in power to
maintain the same speed.

Unless I missed something, the rider+bike mass for the pro rider was
78.4kg, and at 16km/h on an 8% gradient the power meter recorded 279W
average.

When they added 2.6kg, to achieve 81kg total and
approximately a 3% increase in mass, the power required to
maintain the same speed of 16km/h was 40W higher, at 319W. A
14% increase!

I thought the power increase would be about proportional to
the mass increase, i.e. 3%.

I agree. A 3% mass increase should cause pretty close to 3%
increase in power required for a given speed on a given grade.

If 3% mass difference yielded 14% power difference, then it
seems
that reducing one's bike+rider mass by 21% (not impossible with my
bikes, especially if you change the rider!) should reduce one's power
requirement to almost zero. That's assuming things are proportional,
which is what the laws of physics claim.

I don't know the explanation for their findings, but they
don't seem to make sense.

I think you need to distinguish between the different sources of drag
before you can even start to analyse it.
Aerodynamic drag isn't going to change at all if the speed remains the
same, unless the gradient requires you to stand up to maintain speed,
in which case it could increase quite a lot, but on an 8% gradient at
10mph the main source of drag will not be aerodynamic. Frictional
losses would increase pretty much in proportion with power use.
Climbing power is what is going to be affected by mass.

I wonder if the accuracy of the power meter could be affected by
standing on the pedals to climb, as that would have a tendency to put
more weight on the pedals throughout their rotation.
That is, of course, only relevant if they are the pedal type (and may
indicate a superiority of the hub type).


Did you watch the video, Phil? The power meter was a "power tap" rear
wheel hub. It appears to have a wireless connection to a display unit
that measures average power over some time interval.


No, I didn't - I was watching something on TV at the time.
Doesn't mean I can't speculate as to why displayed readings don't
match theory though.


"don't match"? They're not even close.

Theory predicts a 3% increase, or a little over 8W. They measured a 14%
increase, or 40W.

Another possibility has occurred to me though.
On a gradient, is there more of a tendency for the speed to be less
consistent - constantly accelerating and slowing (maybe even in time
with cadence) would increase average power use, and speed would bleed
off far more easily up a gradient, meaning it would be more difficult
to keep to a consistent speed.
The extreme case (obviously not this one) has a rider almost stalling
as the cranks are vertical, then reaching peak acceleration as the
cranks are horizontal. It would take much of that component to have a
serious effect on efficiency.


In the extreme case on the road where the maximum speed causes a
significantly higher wind resistance than the wind resistance at the
average speed, yes.

But this test was done indoors on a treadmill for bicycles, inclined at
8% gradient. There was no significant change in speed and the rider
remained at pretty much the same location on the treadmill the whole time.

--
JS
 




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