EcoModder Forum How to calculate pounds of force on decklid spoiler

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 07-29-2019, 04:00 PM #1 (permalink) Cyborg ECU     Join Date: Mar 2011 Location: Coastal Southern California Posts: 6,297 Black and Green - '98 Honda Civic DX Coupe Team Honda 90 day: 66.42 mpg (US) Black and Red - '00 Nashbar Custom built eBike 90 day: 3671.43 mpg (US) Thanks: 2,373 Thanked 2,169 Times in 1,467 Posts How to calculate pounds of force on decklid spoiler For my flat decklid spoiler build I want to use countersunk neodymium magnets as the attachment mechanism. Flat fiberglass spoiler screwed into the magnets and magnetically attached to the decklid. How do I calculate the amount of aerodynmaic force I might encounter so I can acquire magnets with enough strength to hold, even at 70 mph in a windstorm? __________________ See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.
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 07-29-2019, 07:09 PM #2 (permalink) Moderator     Join Date: Feb 2012 Location: Urbana, IL Posts: 1,926 Pope Pious the Prius - '13 Toyota Prius Two Team ToyotaSUV 90 day: 51.62 mpg (US) Tycho the Truck - '91 Toyota Pickup DLX 4WD 90 day: 22.22 mpg (US) Thanks: 198 Thanked 1,782 Times in 928 Posts I'm not sure you'll be able to calculate that to any degree of certainty without some data. Do you have a Magnahelic or any other type of pressure gauge? __________________ UIUC Aerospace Engineering www.amateuraerodynamics.com
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 07-29-2019, 10:33 PM #3 (permalink) Master EcoModder     Join Date: Aug 2012 Location: northwest of normal Posts: 25,897 Thanks: 7,299 Thanked 8,120 Times in 6,660 Posts I was going to post in the other thread but you started this one five minutes later. If you like (itchy, sticky) fiberglass then 0kay, but personally I'd go with Polymetal [or equivalent] or 1/8" sheet ABS. Polymetal is stiffer, but ABS can be heat-formed and cemented. A Bonneville spoiler is subjected to low pressure on the bottom, just as a diverter is subjected to high pressure on the top. Use turnbuckles with magnets on the end. __________________ . .Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster ____________________ "The Flat Earth Society has members all around the globe" -- The Flat Earth Society
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Quote:
 Originally Posted by freebeard ... personally I'd go with Polymetal [or equivalent] or 1/8" sheet ABS. Polymetal is stiffer, but ABS can be heat-formed and cemented.
Polymetal! Of course. I had forgotten. I am going to start with just the spoiler, which has just one very mild curvature. Not making the side plate yet, probably, and its the part needing more bending. The rigidity of polymetal will be perfect.

Quote:
 Originally Posted by freebeard Bonneville spoiler is subjected to low pressure on the bottom, just as a diverter is subjected to high pressure on the top. Use turnbuckles with magnets on the end. ...
Turnbuckles are perfect.

Quote:
 Originally Posted by Vman455 I'm not sure you'll be able to calculate that to any degree of certainty without some data. Do you have a Magnahelic or any other type of pressure gauge?
No gauges. There is no formula for an reasonable estimate? This aint a race car... seems like an estimate should be good enough if I build something twice as strong as estimated need and be fine, no?
__________________
See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

 07-29-2019, 11:40 PM #5 (permalink) (:     Join Date: Jan 2008 Location: up north Posts: 12,762 Blue - '93 Ford Tempo Last 3: 27.29 mpg (US) F150 - '94 Ford F150 XLT 4x4 90 day: 18.5 mpg (US) Sport Coupe - '92 Ford Tempo GL Last 3: 69.62 mpg (US) ShWing! - '82 honda gold wing Interstate 90 day: 33.65 mpg (US) Moon Unit - '98 Mercury Sable LX Wagon 90 day: 21.24 mpg (US) Thanks: 1,585 Thanked 3,553 Times in 2,217 Posts When you get the steady-state forces all figured out a gust or a semi will come along and give it a good blast. __________________
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 07-29-2019, 11:57 PM #6 (permalink) Master EcoModder     Join Date: Aug 2012 Location: northwest of normal Posts: 25,897 Thanks: 7,299 Thanked 8,120 Times in 6,660 Posts The Arfon brothers bent the metal for their land speed record car around a tree trunk. Consider a full size top piece with a underhung strip riveted to the back edge. Turn a flat plate into an airfoil, plus the mechanical advantage of the fasteners not being in a straight line. A Magnahelic gauge can be simulated by two pieces of surgical tubing, a double-stoppered flask and a webcam. Technically, a downward-pointed U-bend in a single tube marked off in inches. __________________ . .Without freedom of speech we wouldn't know who all the idiots are. -- anonymous poster ____________________ "The Flat Earth Society has members all around the globe" -- The Flat Earth Society
 The Following User Says Thank You to freebeard For This Useful Post: California98Civic (07-30-2019)
 07-30-2019, 07:34 AM #7 (permalink) Master EcoModder   Join Date: May 2012 Location: Tampa, FL Posts: 1,745 Volt, gas only - '12 Chevrolet Volt Premium 90 day: 38.02 mpg (US) Volt, electric only - '12 Chevrolet Volt Premium 90 day: 132.26 mpg (US) Yukon Denali Hybrid - '12 GMC Yukon Denali Hybrid 90 day: 21.48 mpg (US) Thanks: 206 Thanked 420 Times in 302 Posts I will chime in with some mildly annoying clarification. Magnehelic is a model/brand of gauge which usually measures inches of water column. A more generic name would simply be inches gauge, low pressure gauge, or monometer. To the OP, I would not attempt with magnets. I assume the idea is to not damage the vehicle but I would wager that neodymium magnets strong enough to do what you're asking may also lead to scratches in the paint. Mounting it to somewhat disposable features like a tail lamp housing might be an option. __________________
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 07-30-2019, 10:22 AM #8 (permalink) (:     Join Date: Jan 2008 Location: up north Posts: 12,762 Blue - '93 Ford Tempo Last 3: 27.29 mpg (US) F150 - '94 Ford F150 XLT 4x4 90 day: 18.5 mpg (US) Sport Coupe - '92 Ford Tempo GL Last 3: 69.62 mpg (US) ShWing! - '82 honda gold wing Interstate 90 day: 33.65 mpg (US) Moon Unit - '98 Mercury Sable LX Wagon 90 day: 21.24 mpg (US) Thanks: 1,585 Thanked 3,553 Times in 2,217 Posts Good point. Perhaps for testing where changes are likely- or simply if you're uncertain about the permanency of it- a cheap scrapyard lid could be procured? Or take the nice lid off completely and fab a lid with integral spoiler. Re: magnets: If you put tape down first to protect the paint from scratches I doubt the holding power would be seriously affected. __________________
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Quote:
 Originally Posted by California98Civic No gauges. There is no formula for an reasonable estimate? This aint a race car... seems like an estimate should be good enough if I build something twice as strong as estimated need and be fine, no?
I don't know how one would be able to come up with a formula for estimation since this will vary on every car due to shape and lift coefficient, and won't be constant on the car anyway (there might be different pressure at the outside edges of the trunk lid than the middle, for instance).

As freebeard said, you can make a cheap pressure gauge like we used to use at one of the organ building shops I worked in (but only one--the others, we used Magnahelics which are much easier to get an accurate reading from). Take a length of clear vinyl tubing, bend it into a U, zip tie it to a ruler, fill the U with some water. Keep one end in the cabin for atmospheric pressure, put the other end at a right angle to the flow in the area of interest. The difference between the height of the water in each arm of the U is your pressure differential.
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 The Following 2 Users Say Thank You to Vman455 For This Useful Post: aerohead (08-21-2019), California98Civic (07-30-2019)
 07-30-2019, 06:33 PM #10 (permalink) Cyborg ECU     Join Date: Mar 2011 Location: Coastal Southern California Posts: 6,297 Black and Green - '98 Honda Civic DX Coupe Team Honda 90 day: 66.42 mpg (US) Black and Red - '00 Nashbar Custom built eBike 90 day: 3671.43 mpg (US) Thanks: 2,373 Thanked 2,169 Times in 1,467 Posts All great points about the gauges and measuring/calculating pressure. Thanks. Re: magnets on the body paint surface... I was imagining an insulator of some sort, probably thin rubber or soft plastic, which would make removal of the magnet easier without scratching. Now, realize that my paint is, well, "mature" and not "show room" spec... I just don't try to deliberately scratch it up. __________________ See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.