How to calculate pounds of force on decklid spoiler

[California98Civic]

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?

[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?

[freebeard]

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.

[California98Civic]

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.

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.

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?

[Frank Lee]

When you get the steady-state forces all figured out a gust or a semi will come along and give it a good blast.

[freebeard]

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.

[ksa8907]

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.

[Frank Lee]

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.

[Vman455]

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.

[California98Civic]

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.