# General Cycling Discussion - Frontal area determination

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View Full Version : Frontal area determination

CdCf
03-18-05, 07:17 PM
I just realised how projected frontal area of a rider, in any position, on a bike can be measured really accurately.

What you need:
a couple of guys helping you
a yardstick
a decent digital camera with a telephoto lens/zoom position (it doesn't have to be digital, but it sure speeds up the process! :D )
a normal printer
a very accurate set of scales, capable of measuring small weights

Here's how you do it:

Get on your bike and get someone to hold you upright, while still staying sufficiently clear of your projected area shape.
Have the same guy, or another one, hold the yardstick perfectly vertical right next to you, at roughly the same longitudinal position as the centre of your body.

Have a second, or third, person stand some distance away, perfectly in line (centred both sideways and vertically) in front of you, with a digital camera set to the longest focal length available, and take a picture of you, with yourself in the position you want to measure.
Make sure the image is absolutely sharp and focused in every way possible. Use tripod if necessary.

Next, get the image into your computer and print it on your printer. Colour or black/white doesn't matter.
Make the image fill the paper as much as you can, to maximise the area covered by you and your bike.
For maximum accuracy, use a great number of prints.
I'd probably go with 10. I'll explain why soon.

First, measure the yardstick in the image and divide its measured (in the image) length with the actual length.
It's vital here that you use the same unit throughout these calculations. Don't mix feet with inches, or metres with centimetres.
The more accurate your measurement, the more accurate the final result.

Ok, so let's assume your stick was 1 m and you measured it to be 0.143 m (14.3 cm) in the image.
That's a 1:7 scale.
Measure your paper and multiply the side lengths with each other. If we say it's A4 format, that's 0.21 m by 0.30 m. Which is 0.063 m^2.
Weigh the 10 sheets of paper together. Let's say that comes out as 50.5 g in this example. Divide the weight with the combined area of all the sheets. In this case, 50.5/0.630 = 80 g/m^2. So, now we know the weight/area ratio for our printed papers.

Then, carefully cut around the outline of you and your bike.
Make sure there's no "air" between arms and body or between the legs.
Repeat for all 10 sheets.
Weigh the 10 cut-outs together.
The reason for having more papers is to reduce the level of accuracy required from the measuring device.

Ok, a bit of a guess about what would come out of it here.
But let's make it 7.35 g.
To find the corresponding area, divide the found weight by the weight/area ratio found earlier. In my example, 80 g/m^2.
7.35/80 = 0.0918 m^2.
To get the area for just one paper cut-out, divide by 10. That's 0.0092 m^2.
So, the 1:7 scale area is 0.0092 m^2. Multiply by the scale squared (i.e. 7 * 7 in this example), and you get the total frontal area for yourself on your bike.
0.0092 * 7 * 7 = 0.45 m^2.

Of course, the procedure can easily be repeated for a number of rider positions, to find the best balance between comfort and speed.

- - -

I got the idea when I read a text saying how difficult it is to find the frontal area of a someone riding a bike.
The text then went into the method of coasting down a hill and working backwards to get an approximate frontal area...
Then I remembered something from when I visited the quality control lab in the local petrochemical refinery.
To measure the content of something over time, they took the linear plot and cut off the area above the graph line, and weighed the rest of the paper to get the area under the line. Since the paper weight was known, it produced a highly accurate reading.
That's what inspired me to this.

For those who are proficient with photo editing software such as PhotoShop, there's a much faster, but less precise, way of doing this, if you're willing to put some work into it.
It involves painting the area to be measured in the photo black, and everything else white, and then scaling the image down to 2x2 pixels.
Then add maximum possible Gaussian blur.
The resulting pixel colour should be the perfect average.
From that, it's possible to figure out the ratio of black to white, and use some of the methods above to find the scale and so on.
It's far less accurate, though...

BostonFixed
03-18-05, 07:30 PM
*Yawn*
Honestly, who really cares? Don't you have something better to do with your time?

CdCf
03-18-05, 07:37 PM
Nope. :D

miamijim
03-18-05, 10:32 PM
Why dont you take the picture then super impose it on piece of graph paper with appropriate scaling. Thats how they taught us in biology class.

Or better yet have a fat person ride in front of you.

operator
03-18-05, 11:06 PM
Better yet photoshop yourself with a bunch of girls.

Cloud Strife
03-18-05, 11:27 PM
*Yawn*
Honestly, who really cares? Don't you have something better to do with your time?

Well, my guess is that CdCf is trying to calculate the Cd of a bicycle and rider, right? Is that part of your handle?

F = (Cd*p*A*V^2)/2

Hmmm. At least I think that's the equation...it's been a while. I've done this experiment on various bodies several times.

Have fun! Only we geeks understand!

Agh! I logged in under Cloud Strife! I'm really eubi.

TheRCF
03-19-05, 12:15 AM
Well, I found it interesting.