Different ways to measure/calculate frontal area (A)

[Christ]

Now, note the curve of the road relative to flat... roads are always tapered down from the center, so it should be slanted down on the left of the pic, but it should also be either nearly flat, or curved upward toward the car, not concave.

[TestDrive]

You're right, Christ.
My guess-timated lens correction is obviously incorrect.

[Christ]

I wouldn't say it's incorrect, I'd just say that it brought focus to another problem... if you correct one thing, does it make the perceived fish-eye effect worse to other parts of the photograph? Should you then correct more for everything at once? Man... setting CRT monitors up is sooo hard sometimes.

[tech 87]

I haven't tried it, just read this page and thought about it, but wouldn't it be possible to do a pixle count of an image of a car's silhouette, like say by putting a stretched white sheet in front of the car between it and the camera. If you do it at night and use bright enough flood lights far enough behind the car to get an outline underneath and all the way around, and you keep the camera and the lights both at the centerlines of the car's height and width, it should be the same as the cutout, zero distortion, right?

[Istas]

After much searching I couldn't really find a relevant frontal area number for my 2005 Accord, so I found technical drawings on the-blueprints.com that lunarhighway mentioned above. They're not the exact same trim (I don't have the foglight recesses or the side mirror reflector/indicators), but it looks close enough.

Put it into Photoshop, selected the area of the car, then looked at the histogram (in CS5.1, Windows -> Histogram) for the number of pixels. Oddly, the number of pixels reported in the histogram is significantly less than the actual number of pixels you get when you multiply the dimensions of the image, however after adjusting for that, I got 23.8 square feet as a frontal area, which isn't far out enough to be obviously wrong at least.

Removing the side mirrors apparently lowers this by 2.5%. Neat.

[aerohead]

In C&D's June,2014 article about the Cd comparisons they did,Don Sherman commented that they photographed the cars from a distance of 150-feet,with a 200mm camera lens,then imported the digital image into Siemens Solid Edge CAD software to ascertain the frontal area.

[freebeard]

MetroMPG had to crank the handle on this page for a whole page before it took off...

EDIT: ideally you would use the sun at sunrise/set, but good luck finding such a wall/unobstructed sightline.

I like this best. A turn-out on Highway 101 at sunset, and a 4x8 sheet of white coroplast (folded in half for transportation). Use an East-West shadow to judge exactly when the light is horizontal. Trace 1/2 on one side and the other on the obverse.


Then fill it in with a Sharpie, photograph it, put it in the GIMP, crop it to a bounding box, blur it to infinity, sample the resulting gray, convert the RGB to a percentage and take that area of the bounding box. Done.

[wmjinman]

When I was campaigning my salt flats race car, I wanted to know it's frontal area, so I used the shadow method. At the time, I didn't have a way to drive it (not street legal and/or engine not in it at the time), so I had it on my flatbed car trailer.

Then I took a sheet of particleboard and built a simple brace so I could stand it up at the back of the car trailer behind the car. The board and brace was held up and held together by c-clamps and "quick grip" clamps. Then I went to a place I could park it facing the setting sun and jacked up the hitch until flatbed of the trailer was aiming at the right angle up (the sun was still a few degrees up).

I used the symmetry of shadow on the board to help get the aiming right. Once it was right, I quickly traced the outline of the shadow on the board with a fine tip marker.

Then I drew a grid on the board, spacing the horizontal lines one inch apart. Then I just carefully measured the lengths of each of those lines, wrote them on one side of the shadow outline, and added up the numbers. The result is what I used as the frontal area of the car in square inches.

....To get square feet, just divide by 144.