Author Topic: The CdA thread  (Read 4000 times)

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Offline <gmt>

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The CdA thread
« on: August 09, 2013, 11:17:08 AM »
I've been using Aerolab to estimate my CdA on time trials.

Today was the first shakedown cruise of the Shiv (posted in the TT bike thread).  Here's the position:



During the one interval I did today, I maintained position for the duration (It's not an easy position but I think I can adapt to it by next season) and then did the analysis later.  CdA was .221.  That's assuming a middling Crr and doing a few estimates of total wind speed as well, but it seemed to correlate with what I calculated on bikecalculator.com (the numerical page)

That's with a normal kit, normal non-aero helmet and no shoe covers.  I left the sweatpants and loafers at home though.

Tomorrow's TT will see the full monty of aero: skinsuit, good helmet, new shoe covers.  I wonder what improvements that will bring.

P.S - even with the full monty, the best I was ever able to do with the Cannondale was .235
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Offline <gmt>

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Re: The CdA thread
« Reply #1 on: August 14, 2013, 07:31:22 AM »
two TTs in the bag now (both of them with power lower than normal which may indicate the position will require more adaptation)

both of them have had crosswinds, and aerolab doesn't deal with them well.

One of my calc sets indicated just over .21, but that was with an assumed value for air pressure.  I'd love to get a shot at this on a calm night.
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Offline <gmt>

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Re: The CdA thread
« Reply #2 on: June 06, 2014, 07:39:04 AM »
finally a solid data set with not a whole lot of wind and a 40k  flattish TT.

Since my power has dropped off the map I forced myself to be as faithful to the position as possible, although I stood up every 10-15 minutes, plus the turnaround.

Aerolab came up with a CdA of .203 which does seem quite possible.  Theoretically, once I regain strength, the speed improvements will be pretty astonishing, although it may be that the position robs me of some of that strength.

Last season, when I was still in relatively decent form, I was able to make .96 of measured FTP in this position.

So far this year, either my FTP has descended into the basement, or I am only able to make .90 of measured FTP.  I've been able to do a few shorter interval sets @FTP in this position, but for the long TT, it worked out to .90

One thing I have discovered is that a regular hamstring and calf stretching program helps, as it is what I am NOT doing this year.

Still, the speed is a wash, so any improvements in power should eventually mean PRs in time trials.  Just for fun, I calculated what I could "theoretically" do at last year's peak FTP and .2 CdA.  That came out to more than 28mph.  Here's hoping...

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Offline ygduf

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Re: The CdA thread
« Reply #3 on: June 06, 2014, 11:01:39 AM »
.2 is crazy. aerolab puts my TT bike at like .29 and my merckx at .33ish

Offline <gmt>

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Re: The CdA thread
« Reply #4 on: August 22, 2014, 08:56:50 AM »
The aerolab data from last night indicated a CdA below .2 for the first time ever.  It's not easy to estimate Crr, but I tend to err on the side of too much, choosing a number that is higher than what I measure on my rollers, but not so high that it puts things way out of proportion.  CdA worked out to be .1991

Additionally, I have to estimate transmission efficiency which I tend to keep around .965

The change?  Replacing the long Uvex with a Giro Selector has lowered the CdA by a measurable amount, so far as I can tell.
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Offline ygduf

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Re: The CdA thread
« Reply #5 on: August 22, 2014, 11:04:39 AM »
<2 is incredible.