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?

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?

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.

https://ecomodder.com/forum/member-f...6-bedammed.jpg

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.

Turnbuckles are perfect.

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?

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

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.

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.

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.

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.

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.

Plastidip!

hAHAHA! Of course, right?!

magnets
 

Was noodling thoughts about the magnets.
The equivalent tensile holding power should approach what the common fasteners would have had.Say,that of 3,grade-8,10mm bolts,up into the cast-in-place riv-nuts on the spoiler's underside.
You'd have no protection against shearing forces though.If the foil had any resonance,she could try and walk right off the back of the car.(Frank Lee's 18-wheeler bow and stern blast).
You could have a lanyard on it like the Dodge Daytona Charger,so it couldn't get away from you.
Seems like the liability would outweigh the convenience of no body penetrations.
It scares me just thinking about it.I myself would chicken out.:p

An alternative might be finger clamps that grip the edges of the decklid (or hatch).

Does the spoiler have to be removable?

I like removability because I want to be able to test it. I want to be able to take it off and put it back on with ease for A-B-A-B testing and such. Also, built to be removable means I can change the mounting configuration at will, without worrying about holes in the decklid. Take a look in the design/build thread and you will see the current version. Holds well using neodymium magnets. Removable in under 10 seconds.

My take on the OP's problem:

1. Take your best guess, and then go 5x?
2. Strong enough magnets, no matter what protective layer you add, will likely scratch the paint by way of any dirt/sand/etc that collects on the, and then the strong magnet will nicely embed in the magnet cover. Of course keep the area COVID clean will reduce this issue when installing or removing.
3. And prudence IMO, would suggest a loose nylon? safety tether while testing. The forces to be calculated would have to as mention allow for odd wind gusts, from semis, etc, in addition, to rain reducing shear attachment, and road shock loads from roads aberrations/curbs/etc. So no matter what, you will have a big fudge factor, and the safety tether will help reduce that, IMO.

[citation needed] of course.

My paint is not good anymore, and I've been letting it slowly build patina for the most part. I think the safety tether is really important, and I've been considering how to do that.

I think I will make a new version this year & it can become the current [with citation] ... :)

safety tether
 

The nylon 'dental floss' aircraft electricians use to tie off aircraft electrical looms would have high tensile strength, and while flat, and 'small', if combined with a captured 'knot' ( a dog's soft rubber donut chew toy), run under the trunk lid weather stripping into the trunk, might be all that was needed, should the magnetic grip go south on you.

Check out Julians videos.

Calibrate your suspension travel with weights. Put X amount, and it compresses Y mm (or thou).

Then measure at desired speed.

You need means to measure distance to ground via an electronic sensor either by chassis to ground (linear) or suspension component (angle).