Landracing Forum
Tech Information => Technical Discussion => Topic started by: 116ciHemi on December 13, 2010, 10:53:10 PM
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Does anyone have any more accurate methods for calculating frontal area other than pictures and graph paper?
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Slightly more accurate would be to paste the picture to something like Masonite and cut it out and weigh it. Compare that to a known square measured piece. Sorry, it still requires a picture but would cut down significantly on the ciphering. :-D Wayno
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If you do use a picture be real careful to avoid being tricked by distortion caused by wide angle or telephoto lens.
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simple answer is no, easy way.
I have had luck taking a picture and importing into acad.
Then you can draw the outline of the car and calculate the area.
John
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You could frame the car with 4 pieces of wood and calculate the area and estimate or measure the voids . For accuracy you could make graph paper templates of the voids and count the squares .
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This is the method i came up with and use,,,, get yourself a dressmakers tape (take the wifes), go measure the outer extremities of the vehicle as viewed from the frount, you may have to move fore and arft a little to obtain the "total girth length" (in inches),, divide this total by 4, gives you one side of the box, (box= you are converting the frontal shape into a square,) now square this size to give you total square inches,, divide by 144 and you will have your frontal area in square feet :-),,
If its a motorcycle you will have the rider sit on in full kit, and tuck position, then measure,,
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Not trying to be a smart %ss, but depending on what it is, you could google the vechicle or try this site, they have gobbs of tech info and links.
http://www.purplesagetradingpost.com/sumner/bvillecar/bvillelinks.htm#Aerodynamics (http://www.purplesagetradingpost.com/sumner/bvillecar/bvillelinks.htm#Aerodynamics)
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This is the method i came up with and use,,,, get yourself a dressmakers tape (take the wifes), go measure the outer extremities of the vehicle as viewed from the frount, you may have to move fore and arft a little to obtain the "total girth length" (in inches),, divide this total by 4, gives you one side of the box, (box= you are converting the frontal shape into a square,) now square this size to give you total square inches,, divide by 144 and you will have your frontal area in square feet :-),,
If its a motorcycle you will have the rider sit on in full kit, and tuck position, then measure,,
Let me clarify one thing i have said above,,, that is, "measure the total distance "around" the outer extremities as viewed from the frount",, :-),
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That would work on a square car but not on a round one. The math is different for circles. Wayno
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Using the circumference will not only be off, but the amount off will vary greatly by how irregular the shape is in X-section in addition to the general curvature.
What I don't like about graph paper is that the usual method (haven't found anything else) is as follows:
1. all lines inside a grid are included
2. all lines outside the grid are excluded
3. all lines through a grid are included at 50%. This is based on the average of 99% in and 1% in, duh.
My problem is that unless you have a 40 foot photo, the grid lines will have far too many lines running through them, and 50% isn't close enough. To get good resolution, you need something the size of a dining room table, like 1/3 scale.
My method:
1. good front photo, high-res
2. re-size it to a multiple of the actual size (width across headlights is easy), 1" = 10 pixels, blah
3. convert to a .bmp
4. fill in the entire outline with a solid color
5. graphics program gives the area of the colored mass in same units as your original measurements
This also allows you to predict area from an actual photo of the real car (not your car necessarily), then lowered, then sectioned, then chopped 6", etc. without doing the work.
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Using the circumference will not only be off, but the amount off will vary greatly by how irregular the shape is in X-section in addition to the general curvature.
What I don't like about graph paper is that the usual method (haven't found anything else) is as follows:
1. all lines inside a grid are included
2. all lines outside the grid are excluded
3. all lines through a grid are included at 50%. This is based on the average of 99% in and 1% in, duh.
My problem is that unless you have a 40 foot photo, the grid lines will have far too many lines running through them, and 50% isn't close enough. To get good resolution, you need something the size of a dining room table, like 1/3 scale.
My method:
1. good front photo, high-res
2. re-size it to a multiple of the actual size (width across headlights is easy), 1" = 10 pixels, blah
3. convert to a .bmp
4. fill in the entire outline with a solid color
5. graphics program gives the area of the colored mass in same units as your original measurements
This also allows you to predict area from an actual photo of the real car (not your car necessarily), then lowered, then sectioned, then chopped 6", etc. without doing the work.
Excellent idea. Thanks, Don
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What vehicle are you trying to calc. the area of?
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What vehicle are you trying to calc. the area of?
I have a lakester on the back burner, but will be running a bike [RZ350] in the near future providing I get our house finished.
Don
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You'll; be running -- will Gus be the rider?
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You'll; be running -- will Gus be the rider?
Probably not yet.
Due to his age it would have to be at the Bubs meet and football gets in the way.
Rest assured it will happen at some point. Hey :-D
Merry Christmas, Don
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The most accurate methods are John’s (reply no. 3) and Panic’s (reply no. 9). They are both quite easy. Panic’s method is the best if you do not have a CAD program. Thank you both for posting.
Something super easy, fast, and almost as accurate, follows. However, note Panic’s reservations above.
Park the front of your vehicle in the garage doorway. You now have a perfect physical measurement.
(1)Take photo from as far away as possible.
(2) Measure height and width of the doorway.
(3) Using an architect’s or engineers scale transfer those precise dimensions from the doorway to the photo and draw a grid pattern.
A small discrepancy in area may not be a big deal. Your Cd probably will be less precise than the area you calculate. Cd numbers should be suspect unless they have been calculated by a real coast-down test or a wind tunnel. A small .01 or .02 error in Cd may result in greater drag error than your physical measurement of area. Since HP increases as the 3rd power of speed, I am not minimizing precision as a goal. However, traction on the salt varies by the hour and it may become a greater variable than absolute precision in calculating CdA.