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