njiri said:
I am fairly new to time trialing and have been training with power for just over a year now. FTP of 340 and ride masters in South africa,and have been riding for just over 2 years now. I am really trying to do well in our upcoming national champs. Question I have is how does one go about measuring ones frontal area as per the figures given on Analyticalcycling. Im am 1.88 m tall wt 74kg. They give figures as typical as anywhere from 0.4 to 0.7 m2 and say to use 0.5 as a default. I have taken a series of photos with varying drops from 130mm to 180mm and then with a cad system worked out very accurately the area of my body at these various drops. For 130mm drop for example I get 0.24m2 way off what Analyticalcycling gets. This excludes the bike but that would add so little to it. In their explanation of frontal area they don,t say much but say it is very difficult to measure, why? If you have a pic and have the right computor programme then by drawing an outline around the object you can get the area exactly.
Any input on this would be much appreciated.
From a varirety of sources Cd for cyclists is in the 0.7 range (Heil, Padilla) so with an FA of 0.24 that would put you at 0.168 CdA.
Unless you're built like a torpedo that's highly, highly unlikely.
1. You should count the bike FA as well.
2. Reference area placed where along camera axis?
3. Camera depth of field?
4. How many crankarm positions per position? Four is normal IIRC.
I've done quite a few measurements over the years using the guidelines over on
www.biketechreview.com and came up consistently with FA's in the 0.37-0.40 range. Assuming a CdA of 0.7, those tie up quite nicely with CdA measured and those calculated from field tests. I believe several other posters there reported good results as well using those methods.
ACC.com: well what's important there is simply the product of FA and Cd so they suggest 0.5 and 0.5 for a guess of 0.25. It's not unreasonable as most folks will fall between 0.2 and 0.3 but it's really only a WAG for any individual.
In general you'll likely get good technical responses over on BTR - just stay away from physiology
edit: I also have a little spreadsheet with four equations estimating FA and Cda from height, mass, shoulder width, seat-tube angle and torso angle. PM me if you want a copy of that. It produced an avg. estimate of 0.39 FA for me which was quite close to what I measured.
