so i’m reading the thread about equivalent 10k run/40k tt… and i see this picture posted:
and i got to thinking, how do i ‘easily’ determine my CdA? since i have been playing with some image programs for giggles and bits… do you think it would be possible to take a picture of yourself from square in front of you then create a scaled box around that picture and use some software (haven’t tried yet but i’m sure AutoCad or something could do this) to calculate the area left in the box, thus giving you your true CdA?
i realize all of this is theory at the moment since i don’t have a picture or all my programs here at work…but i like the thought behind this.
my way sure sounds easier… since i don’t live at sea level, have an option of ‘not much wind’, and i don’t know how to calculate “celsions” and i don’t have a speedometer since i only look at my power.
and i got to thinking, how do i ‘easily’ determine my CdA? since i have been playing with some image programs for giggles and bits… do you think it would be possible to take a picture of yourself from square in front of you then create a scaled box around that picture and use some software (haven’t tried yet but i’m sure AutoCad or something could do this) to calculate the area left in the box, thus giving you your true CdA?
That gives you A, not CdA.
People have been doing what you’re suggesting for a long time. Back at the turn of the century (no, not this one, the previous one) Dubois and Dubois calculated A by taking a photo of a person or an object, printing it on photographic paper, very carefully cutting out the outline of the object, and then weighing the cut-out on a precise scale. Now we just pop the image into Photoshop or GIMP.
and i got to thinking, how do i ‘easily’ determine my CdA? since i have been playing with some image programs for giggles and bits… do you think it would be possible to take a picture of yourself from square in front of you then create a scaled box around that picture and use some software (haven’t tried yet but i’m sure AutoCad or something could do this) to calculate the area left in the box, thus giving you your true CdA?
That gives you A, not CdA.
People have been doing what you’re suggesting for a long time. Back at the turn of the century (no, not this one, the previous one) Dubois and Dubois calculated A by taking a photo of a person or an object, printing it on photographic paper, very carefully cutting out the outline of the object, and then weighing the cut-out on a precise scale. Now we just pop the image into Photoshop or GIMP.
Using ‘A’ how close can we approximate CdA? My coach used this approach(using some sort of special software) before my first TT to approximate my CdA from a frontal picture.
The conditions for the race weren’t as ideal as the chart the OP posted, but using the approximated CdA number, I was able to predict my 40km TT time to within 30s using analyticcycling. Maybe it was just dumb luck, but it seemed like it worked about pretty well.
Using ‘A’ how close can we approximate CdA? My coach used this approach(using some sort of special software) before my first TT to approximate my CdA from a frontal picture.
Hmmm. Hand grenade close. How close do you need to be? If you want to know the difference in drag between small changes in position or equipment, you’re going to have to measure CdA, not just A. A rear disk (or wheel cover) has no effect on A. Those behind-the-seat water bottles? No effect on A. Loose, flapping number on your back? No effect on A.
I’ve been to the wind tunnel 3 or 4 times. My take-away has been that drag coefficient has become a bit of a “golden BB” metric around which enormous debate swirls- much of it rightly and much of it in lieu of some other single golden BB metric. Others have latched onto weight as the metric to chase. For some reason, few have grabbed fitness or aerobic threshold of the athlete as the relevant “speed metric”.
With drag as the go-to measure of “best-ness” there have been a host of interesting ideas on an improvisational way to quantify drag.
I’d be interested to see an insight on their individual/relative merits.
and i got to thinking, how do i ‘easily’ determine my CdA? since i have been playing with some image programs for giggles and bits… do you think it would be possible to take a picture of yourself from square in front of you then create a scaled box around that picture and use some software (haven’t tried yet but i’m sure AutoCad or something could do this) to calculate the area left in the box, thus giving you your true CdA?
i realize all of this is theory at the moment since i don’t have a picture or all my programs here at work…but i like the thought behind this.
May I suggest the obvious:
buy, beg, borrow, or steal a power meterdownload and install Golden Cheetah ( http://goldencheetah.org/download.html ) it’s free!!!
weigh yourself and your bike, and get an idea of air density on the day of your testchoose a windless test day – it will make the estimate much more accuratedo an* out-and-back* or a series of loops of your favorite (carless, smooth-roaded, slightly hilly) rideimport your power data into Golden Cheetahgo to the Aerolab tab and use the sliders to estimate CdAIf you need help, contact me (although Tom Anhalt and Robert Chung are much better at it than I am)
Full disclosure: I’m the Aerolab guy.
Rolling Resistance
Measure your rolling resistance with a hanging scale at different speeds. Doesn’t have to be fast. Get a friend to pull you with the scale to measure the force.
Frontal area.
Get someone to take a photo of you from head on, in the aero position. Hold a ruler in your had for reference.
Print the picture out on a 8.5x11 sheet.
Weigh the printed sheet on an accurate scale. Your local bike shop or tobacco shop could do this for you.
cut out your outline of you & the bike.
weigh the cut out.
divide the cutout weight by the sheet weight. This gives you percentage of you verses sheet.
Using the ruler in your hand in the photo as a scale, calculate the total “scaled” area of the printout sheet. (sq.in.)
Multiply that area (sq.in.) by the percentage in step 6. This final calculation gives you the frontal area of your set up.
Once you know your frontal area and rolling resistsance, you can calculate your Cd by doing some coast down tests and ploting the data. You won’t get much usefull data under 20 mph, but we already know that.
Motor pace as fast as you can. (40 mph?)
2 slide out from behind the car into clean air, and hit the stop watch to start time.
at the 5 mph marks (40, 35, 30, 25, etc) hit the lap button.
4 Do this a few times for data.
Oh and before the prop heads blast me, this is a simplified engineering calculation. if you want 13 decimal places, go to a wind tunnel or sharpen your pencil. http://en.wikipedia.org/wiki/Drag_(physics)
My take-away has been that drag coefficient has become a bit of a “golden BB” metric around which enormous debate swirls- much of it rightly and much of it in lieu of some other single golden BB metric. Others have latched onto weight as the metric to chase. For some reason, few have grabbed fitness or aerobic threshold of the athlete as the relevant “speed metric”.
Really? Hmmm. I’d say your post is a straw man. See that plot in the first post of this thread? Look at the title. The point is that if you’re interested in speed, watts/CdA is a pretty important measure. Not CdA in isolation. The thing is, now that power meters are getting more common, riders are getting used to the numerator – but many still don’t have much of a handle on the denominator. Wind tunnels, field tests – and plots like that are helping to educate riders about the denominator so they can think about the entire ratio.
I’ve been to the wind tunnel 3 or 4 times. My take-away has been that drag coefficient has become a bit of a “golden BB” metric around which enormous debate swirls- much of it rightly and much of it in lieu of some other single golden BB metric. Others have latched onto weight as the metric to chase. For some reason, few have grabbed fitness or aerobic threshold of the athlete as the relevant “speed metric”.
.
How many times do we have to tell you (and every other person on this board that makes the same stupid comment) that the relevant metrics are both drag and fitness!! This ain’t frickin complicated to go fast you need both!
plug the power, weight, and drag options into analytic cycling
pick the combo that minimizes time
this isn’t philosophy, there are ANSWERS tom =)
Disclaimer: I’m not an aerodynamic expert.
I’ve been to the wind tunnel 3 or 4 times. My take-away has been that drag coefficient has become a bit of a “golden BB” metric around which enormous debate swirls- much of it rightly and much of it in lieu of some other single golden BB metric. Others have latched onto weight as the metric to chase. For some reason, few have grabbed fitness or aerobic threshold of the athlete as the relevant “speed metric”.
With drag as the go-to measure of “best-ness” there have been a host of interesting ideas on an improvisational way to quantify drag.
I’d be interested to see an insight on their individual/relative merits.
You can download an article that will walk you through the process here and a spreadsheet for making the calculations here. This is just one way to do it and you may also want to consider Dr. Chung’s virtual elevation method.
Cheers,
Jim
You can download an article that will walk you through the process here and a spreadsheet for making the calculations here. This is just one way to do it and you may also want to consider Dr. Chung’s virtual elevation method.
Embarrassingly I have not been on my tt bike in a long time and so have not tried your method. Its on my list and I now have a powermeter (one that I don’t have to snatch from the lab).
