Power Profiling -- good stuff

In article , Andy
Coggan wrote:

"warren" wrote in message
...

I cheated by recalling an article in VeloNews from years back about two
guys who claimed they could predict TT times based on the riders'
wattage output and their aero drag. Whatever happened to those guys?

Maybe these guys? www.pkracing.com (I hope that URL is correct.)

Actually I remembered it as being you and Jim Martin. Guess not.

Last night I did a little test on a perfectly flat road, 26.5
mph, 1 minute, back and forth once each. There was a very slight wind
and my efa numbers according to his calculations were .75 and .90
(tailwind and headwind).

You *really* need to subscribe to the wattage list...

I've been lurking off the Topica website but I have to wade through
alot of talk about hardware issues and time trialing-neither of which
interest me that much. There are some pearls there though.

Bottom line: while *very careful* field testing can yield fairly precise
numbers for CdA (i.e., coefficient of variation of ~2%), it still really
isn't good enough to be of much use (e.g., for choosing equipment, or even
optimizing position).

I was just trying to get a ballpark for my frontal area and to compare
my Polar power readings with his caluclations.

That's why I'm going back to the wind tunnel this
winter...anybody want to join me?

Curious, what does that cost?

-WG

Ten tries upwind and down. Record each. The drag coefficient should
then be pretty predictable given the DIFFERENCE in times and the wind
speed.

"Robert Chung" wrote in message
...
warren wrote:

I cheated by recalling an article in VeloNews from years back
about two
guys who claimed they could predict TT times based on the riders'
wattage output and their aero drag. Whatever happened to those
guys?

I agree, you can do a good job predicting TT times based on wattage
and
aero drag; but what you wrote upthread was that if you know wattage
and
*weight* someone could predict TT times. Someone could -- I just
think the
prediction wouldn't be as good as wattage and aero drag.

At Analytic Cycling he uses "effective frontal area" measured in
square
meters for aero drag. I'm curious if you can calculate reasonably
well
this based on rider height and weight. He says typical values are
..4 to
.7 m2. Last night I did a little test on a perfectly flat road,
26.5
mph, 1 minute, back and forth once each. There was a very slight
wind
and my efa numbers according to his calculations were .75 and .90
(tailwind and headwind).

Estimating CdA from open air field tests is kinda difficult, though
Allen
Lim claims to be able to do it. I'm skeptical, though of course I'm
not
privy to his data. I went through a little exercise on the wattage
list a
few months ago with a guy who had some power data and was trying to
estimate CdA from it. Even though the data were moderately clean the
coefficient of variation was kinda high. Andy has done some stuff on
trying to calculate CdA from a rider in a velodrome (I *think* it
might
have been an outdoor velodrome rather than an indoor one).

"warren" wrote in message
...
In article , Robert Chung
wrote:

Andy Coggan wrote:
"Robert Chung" wrote in message
...

Estimating CdA from open air field tests is kinda difficult, though
Allen Lim claims to be able to do it. I'm skeptical, though of course
I'm not privy to his data.

He presented some data in an abstract at the American College of
Sports
Medicine meetings in early June. As predicted on the wattage list,
despite using a rather unique venue (i.e., a large aircraft hangar)
the
precision wasn't as good as what you can achieve in a wind tunnel, and
in fact was no better than what I found in my experiments (see below)

I know I shouldn't do this, but I can't help it. In case Warren thinks
Andy is exaggerating Allen Lim's claim about wind tunnels,

No I don't think that. My experience yesterday with a wind of maybe
2-3mph showed a range of about .75m2 to .9m2 so I can't see how one
could achieve +/- 2% accuracy with "no" wind.

A wind of 2-3 mph is actually quite significant in this context, as I tried
to relate. Be that as it may, your results may be better than you think.
With a reasonable sample size, the SD is usually about half the range, so
assuming that 0.75-0.90 m^2 reflects the limits, then you're already under
10%.
..
the exact quote
is, "it's our belief that the aerodynamic and rolling resistance values
we
calculate in the real world using the Power Tap are more accurate than a
wind tunnel."

Hard to believe somebody would even say that-especially when they're
relying on a PT.

If I had to trust the numbers from any power meter completely blindly, w/o
calibrating it myself, I'd pick a PowerTap over an SRM (esp. the Amateur) or
Polar. That's especially true in this context, since what you really want to
know if power delivered to the rear wheel/road, not the power the rider
actually produces.

Andy Coggan

"warren" wrote in message
...
In article , Andy
Coggan wrote:

"warren" wrote in message
...

I cheated by recalling an article in VeloNews from years back about
two
guys who claimed they could predict TT times based on the riders'
wattage output and their aero drag. Whatever happened to those guys?

Maybe these guys? www.pkracing.com (I hope that URL is correct.)

Actually I remembered it as being you and Jim Martin. Guess not.

Jim, John Cobb, Kevin McFadden (wind tunnel guru for GM), Doug Milliken
(race car suspension genius, HPV expert, and Moulton aficianado) and I
collaborated on a scientific study that was published in J. Appl. Biomech.
However, to the best of my knowledge that didn't result in a lay article in
VeloNews (unless perhaps Jim wrote one, and didn't mention it to me).


Last night I did a little test on a perfectly flat road, 26.5
mph, 1 minute, back and forth once each. There was a very slight wind
and my efa numbers according to his calculations were .75 and .90
(tailwind and headwind).

You *really* need to subscribe to the wattage list...

I've been lurking off the Topica website but I have to wade through
alot of talk about hardware issues and time trialing-neither of which
interest me that much. There are some pearls there though.

Bottom line: while *very careful* field testing can yield fairly precise
numbers for CdA (i.e., coefficient of variation of ~2%), it still really
isn't good enough to be of much use (e.g., for choosing equipment, or
even
optimizing position).

I was just trying to get a ballpark for my frontal area and to compare
my Polar power readings with his caluclations.

Ack - Polar?? ;-)

That's why I'm going back to the wind tunnel this
winter...anybody want to join me?

Curious, what does that cost?

Depends on who you are/who you know...but as I recently told somebody else,
you might end spending as much on travel, food, lodging as on actual tunnel
time.

Andy Coggan

FWIW, the contents of this post do not necessarily reflect the opinions or
thoughts of anyone other than myself (and that goes for anything you see me
write in any medium at anytime in the future or in the past) - if you can't
figure that one out, then, well, I reckon I can't be held responsible for
how you perceive my commentary.

I've done a bit on the whole cheap-ass aerodynamicist/garage science front.
I am not nearly as smart as Dr. Coggan, Dr. Chester Kyle, nor do I have the
cycling specific wind tunnel time of others in the bike related industry
(though, I have more than your average "joe"), but I think people can
objectively take a look at their position/frontal area for a small
investment of their time (yes, I realize that a wind tunnel is more than
likely the best way to definitively answer the "what position is best for
_me_?" question, but not everyone can afford/wants to go to Texas in the
winter, and the "right" methodology once you are there is still debatable!).
Field testing may be able to produce reliable results, but, IMHO, the time
investment necessary to produce a statiscally significant conclusion is
prohibitive for a large scale variable study - i.e, I can generate 100+
statistically significant datasets in a tunnel in one day - try doing that
with a field test...

I have documented some of my experiences (yes, the following links are
"free" to view) with mathematical modelling, rider position/frontal area
determination and whatnot on my website.

of particular interest are the comparative frontal area results (where my
body measures did not change, but my frontal area certainly did!): - forgive
the wordwrap - its either this, or get flamed for using tinyurl.com - so
live with it...

http://www.biketechreview.com/projec...talarea/kw.htm

a comparison between me and someone "important":

http://www.biketechreview.com/projec...comparison.htm

My freakishness has even been tolerated by a certain professional cycling
team:

http://www.biketechreview.com/projec...animations.htm

and concurrently documented by velonews.com:

http://users.adelphia.net/~kwillett/images/vn.gif

an analysis of the fitchburg TT and what type of power pacing strategy could
be most appropriate:

http://www.biketechreview.com/projec.../Fitchburg.htm

an analysis of the Redlands Rubidoux prologue TT and equipment selection
(the article titled:

6/5/2002 - Is Time-Trial Equipment Selection "Significant") on the page at:

http://www.biketechreview.com/projects/bikecomlinks.htm

I am still developing my methodology for determining frontal area, so for a
limited time (say, the first 5 people that email me offline) I would be
willing to document their frontal area for free if they agree to execute the
procedure according to a certain protocol and provide certain body
dimensions (heh, I wrote "body dimension").

Enjoy,

--
==================
Kraig Willett
www.biketechreview.com
==================

"Benjamin Weiner" wrote in message
...
Andy Coggan wrote:
"Robert Chung" wrote in message

So if CdA varies with m^0.33, then a 25% difference in mass gives a
7.5%
difference in CdA?

I know that's a rhetorical question, but the answer is yes: all else
(e.g.,
height, position) being equal, a 25% increase in mass theoretically
results
in only a 7.5% difference in CdA. I believe that makes sense if you
think
about it...for example, if I gained 37.5 lbs I'd be a true masters
fattie,
but I wouldn't be *that* much wider or higher (assuming I could still
get
down on the aerobars w/ a big belly in the way).

You guys probably know this, but you're talking about two
different relationships.

Actually, we're not (see below).

One is a relation between CdA and
mass across a population (all riders, elites, fatties, whatever).
I assume this is what was measured in the article.
The other is the relation between CdA and mass for a given rider.

I believe (or at least assume) that Robert is talking about the same thing I
am, which is the relationship in a population. I only tossed in the comment
about what would happen in an individual case as an example for those
unfamiliar with allometric scaling.

Andy Coggan

Kurgan Gringioni wrote:

I'd be surprised if you got very many to reply. Aerodynamics is such a
non-tactile, nebulous concept that almost all racers ignore their position
once they get one that sort of feels acceptable.

It's sort of amazing to me that people don't pay attention to it. They'll
spend $1000 to lose 2 lbs. from their bike, but that's only ~1% gain vs. the
effects of gravity and less than that when one takes friction and wind
resistance into account.

Even among physicists there is a tendency to stick to things that
are easily quantifiable - the desire to stick with an equation
that makes things easy to calculate, even if it's not really right.
So it doesn't really surprise me that Joe Bikeracer behaves the
same way - weights are all given in catalogs, but aero benefits
of position or equipment are harder to quantify.

Also, as you have alluded to, it's easier to do things that just
involve spending money, like shaving weight or getting more aero by
buying Zipps. Lots of people have Zipps, I dunno how many have
thought about their positioning. Trigeeks think about aero all the
time, but have they worked on their position beyond reading John Cobb
articles?

BTW, fluid dynamics is enough of a nonlinear pain in the ass that
most physicists are scared of it too.

On a flat TT course, OTOH, it's possible to make double digit gains, 10-20%.
Yet almost everyone ignores it.

How many other people in this country pay a lot of attention to aerodynamics
in the racing scene? You. John Cobb. who else?

OTOH, I'm not sure what Kraig is proposing to do. I'm not one to
talk, as I am too lazy to even put on my clip-on aerobars, because I
don't feel like redoing the bar tape (however I only do club TTs). My
major investment in this is doing crunches every morning so that I can
ride in the drops with comfort. I think it helps, but who knows.

"Benjamin Weiner" wrote in message
...

BTW, fluid dynamics is enough of a nonlinear pain in the ass that
most physicists are scared of it too.


As well they should be. I'm not talking about doing endless cascading
partial differential equations though. The real world Fred comparison test
on the windless hill is what I advocate.

In most cases, eyeballing will help. If a rider's torso is a big air scoop,
the fix is obvious.


On a flat TT course, OTOH, it's possible to make double digit gains,
10-20%.
Yet almost everyone ignores it.

How many other people in this country pay a lot of attention to
aerodynamics
in the racing scene? You. John Cobb. who else?

OTOH, I'm not sure what Kraig is proposing to do. I'm not one to
talk, as I am too lazy to even put on my clip-on aerobars, because I
don't feel like redoing the bar tape (however I only do club TTs). My
major investment in this is doing crunches every morning so that I can
ride in the drops with comfort. I think it helps, but who knows.


That would help in the TT position too as long as you go through a full
range of motion.

From what I've seen, the ones who are able to get into extremely aero
positions have very good hamstring and lower back flexibility, which allows
them to get low with the flat back and still make power. If the hamstrings
and lower back are short, then the rider has to sit more upright, with the
obvious consequences with respect to the amount of air they're going to
catch.

"Andy Coggan" wrote in
Jim, John Cobb, Kevin McFadden (wind tunnel guru for GM), Doug Milliken
(race car suspension genius, HPV expert, and Moulton aficianado) and I
collaborated on a scientific study that was published in J. Appl. Biomech.
However, to the best of my knowledge that didn't result in a lay article
in
VeloNews (unless perhaps Jim wrote one, and didn't mention it to me).

Wasn't me.


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Hi Jim,

Zinn wrote a little sidebar in the 5 February, 1996 edition of VeloNews that
attributed a formula to you. I saved that VeloNews since it was one of the
rare occassions that tunnel data was published (thanks for that, BTW) -
there was a big fallout over that entry and VN story if I remember
correctly, right?

FWIW, I may have the little sidebar "summarized" here for a brief period of
time:

http://www.biketechreview.com/images..._method_lo.gif

--
==================
Kraig Willett
www.biketechreview.com
==================