Power Measurement during Spin Class
Phil Holman
for a couple of Spin classes. I noticed the solid disc wheels heat up to
a temperature depending on how much work was done. I got to thinking
that the point of temperature equilibrium occurs when energy input
equals heat loss (duh).
It must therefore be possible to calibrate temperature versus power for
one of these training bicycles. Ok, we probably need to correct for air
temperature and one or two other annoying variables (fan) but with most
of the heat loss occurring due to radiation (proportional to T^4) any
errors would be small by comparison. A rough check of P = kAeT^4 comes
out about right.
Heck, we could even set up a temperature display and compete to see who
could reach the highest sustainable temperature.
Phil (my wheel is hotter than yours) H.
Ron Ruff
Phil Holman wrote:
> Ok, we probably need to correct for air
> temperature and one or two other annoying variables (fan) but with most
> of the heat loss occurring due to radiation (proportional to T^4) any
> errors would be small by comparison. A rough check of P = kAeT^4 comes
> out about right.
Sounds like fun... but if neglecting convection "comes out about right"
then you probably have big errors. Having no idea what these things
look like, I can't say for sure... but I'd expect convection to be much
greater than radiation.
Phil Holman
"Ron Ruff" <rruff...@yahoo.com> wrote in message
news:1165108796.4...@79g2000cws.googlegroups.com...
I didn't check for convection/conduction but the distribution of losses
shouldn't make any difference. The wheel equilibrium temperature would
be calibrated for a known power input at a given air temperature and
rpm.
The bicycle wheel is a solid steel disk with a brake on the outer edge.
I estimate it to be about a 25cm radius and it gets too hot to touch for
more than a second (80C). For radiation alone, I accounted for approx
170 watts out of an expected 250 watts.
Phil H
dusto...@mac.com
Phil Holman wrote:
> Heck, we could even set up a temperature display and compete to see who
> could reach the highest sustainable temperature.
Done by one vet woman roadie at the local Y, only she took temps,
whispered to each spinner what their result was, and then told the
entire class what the range was. At least good for rough comparisons
but the great thing about Spin class, at least with the
non-instrumented machines we have is that no one knows how hard you're
pushing. IOW, it's easy to make it *look* like you're twisting the
tension knob up when you're not- or, in fact, turning it down.
--D-offseason-sky
Phil Holman
<dusto...@mac.com> wrote in message
news:1165160592....@73g2000cwn.googlegroups.com...
>
> Phil Holman wrote:
>> Heck, we could even set up a temperature display and compete to see
>> who
>> could reach the highest sustainable temperature.
>
> Done by one vet woman roadie at the local Y, only she took temps,
> whispered to each spinner what their result was, and then told the
> entire class what the range was.
Great, do you have any more details on this?
>At least good for rough comparisons
> but the great thing about Spin class, at least with the
> non-instrumented machines we have is that no one knows how hard you're
> pushing.
It may be a good thing for some but not for everyone. Having the option
to know how hard you are working sounds OK to me.
IOW, it's easy to make it *look* like you're twisting the
> tension knob up when you're not- or, in fact, turning it down.
> --D-offseason-sky
At my local Y, most spin at 100+ rpm so what they miss with low tension
is partially made up with, well, spinning.
Phil H
dusto...@mac.com
Phil Holman wrote:
> Great, do you have any more details on this?
She used an electronic thermometer, a "head" unit with a probe, which
took a surface reading, attached by a coil cord. Nothing beyond a rough
measurement of surface temp on the wheel. Some of those bikes are well
worn, there is a new one or two, and the brake (drag) action varies a
lot from machine to machine. Thinking, even if you could get an
accurate measurement of how much energy was being put into the system,
how would you use it, given the difficulty in duplicating resistance
settings-- and even rpm, since these have no "speedometer" or cadence
counter. --D-y
joseph.sa...@gmail.com
IR or laser type thermometers can be had for $70 or so. The instructor
could just point it at each wheel and try to get everyone to take it up
10deg or whatever. Sound slike fun.
Joseph
Donald Gillies
electronics? I can do 240 watts for 1 hour on our gym's upright
exercise bike. To my surprise I can't touch that level of output on
the recumbant bike, nor can I hit it while sitting upright to get my
pulse.
- Don Gillies
San Diego, CA
Dave Lehnen
By your calculations, how much power is being radiated at room
temperature? Using only the temperature of the wheel, and not
the temperature of the surroundings, is valid only if the wheel
is surrounded by absolute zero or something close like deep
space.
Dave Lehnen
Phil Holman
"Dave Lehnen" <dcle...@worldnet.att.net> wrote in message
news:WJVch.422085$QZ1.4...@bgtnsc04-news.ops.worldnet.att.net...
From the equation P = kAeT^4, e is emissitivity which can be anything
from 0 to 1. Black body radiation is 1.
I used 0.5 but the actual mechanism (convection, radiation, conduction)
for heat loss isn't important. I can calculate the heat capacity of the
wheel and also measure the temperature decay with zero input. From this
I can calculate the steady state power output of the rider to maintain a
constant wheel temperature.
Phil H
Dan Connelly
> Why don't you just use an exercise machine with 20th century
> electronics?
That would be 6 years out of date...
Dan
dusto...@mac.com
Dog years or electronic years?
But seriously, folks, some of the barcabikes have readouts. I've tried
two or three. Especially the old ones, I get the impression they're
just tossing a number up on the screen. Easy to "float" them with a few
rpm. --D-y
Johnny Sunset
The wrong vowel is the problem.
--
Tom Sherman - Post Free or Die!
Dave Lehnen
My quibble wasn't with the value you picked for emissivity, but with
assuming that radiant energy flow was one-directional, and only a
function of the wheel's temperature and emissivity, and not of the
temperature and emissivity of its surroundings. Using this method, a
wheel at room temperature is radiating a lot of energy. It is, but it
is absorbing an equal amount from its surroundings.
It was the statement about most of the heat loss being due to radiation
that got my attention. At the temperatures you're talking about, and
with a disk spinning in air, much more heat loss will be due to forced
convection than to radiation.
It's true that a hotter wheel will tend to indicate more power, but if
one rider spins very fast with light drag, and another spins slowly with
heavy drag, at the same power, the fast-spinning wheel will be cooler
due to more forced-convection cooling.
Dave Lehnen
Phil Holman
Probably true.
>
> It's true that a hotter wheel will tend to indicate more power, but if
> one rider spins very fast with light drag, and another spins slowly
> with
> heavy drag, at the same power, the fast-spinning wheel will be cooler
> due to more forced-convection cooling.
>
I did ponder that dilemma and concluded I should take into account rpm.
On the other hand, forced convection of rotational displacement is a lot
lower than translational displacement (the wheel still occupies the same
space). Rather than try to calculate the heat transfer with all its
complexities, it would be easier and more accurate to measure it with a
decay test at different rpms.
I'm expecting a decay along the lines of T = T(0)e^-.0005t. The wheel
weighs 21.3 kg and has a heat capacity of 9550 J/K so it would be easy
to time each degree drop from T(0) and plot a best fit regression. I
could do this at 60,70, 80 rpm etc. If accuracy comes out to be +/- 5%,
it's a lot better than nearly every exercise bike out there.
Phil H
dvt
> IR or laser type thermometers can be had for $70 or so. The instructor
> could just point it at each wheel and try to get everyone to take it up
> 10deg or whatever. Sound slike fun.
Quite a lot less than $70, actually.
<http://www.webbikeworld.com/r2/digital-infrared-thermometer/digital-infrared-thermometer.htm>
(link shamelessly stolen from an earlier r.b.tech post)
--
Dave
dvt at psu dot edu
Everyone confesses that exertion which brings out all the powers of body
and mind is the best thing for us; but most people do all they can to
get rid of it, and as a general rule nobody does much more than
circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and
novelist (1811-1896)
carl...@comcast.net
>joseph.sa...@gmail.com wrote:
>> IR or laser type thermometers can be had for $70 or so. The instructor
>> could just point it at each wheel and try to get everyone to take it up
>> 10deg or whatever. Sound slike fun.
>
>Quite a lot less than $70, actually.
>
><http://www.webbikeworld.com/r2/digital-infrared-thermometer/digital-infrared-thermometer.htm>
>
>(link shamelessly stolen from an earlier r.b.tech post)
Dear Dave,
And even less:
http://da.harborfreight.com/cpisearch/web/search.do?keyword=thermometer
Under $20 with this week's 15% single-item iscount coupon, available
only to carbon-based life-forms intelligent enough to pick up the
flier at the checkout counter.
Cheers,
Carl Fogel