Lightening Hole Question

FWIW, lightening holes today are generally scoffed at, an artifact of a time when car design lacked something in engineering analysis. The Italians, particularly, who drilled holes through everything but the driver and tires on their race cars, are disparaged for building cars that started off strong, became weak when they were filled with holes, and wound up having to be reinforced to make up the loss of strength.

However, if you're aware of this and you have other reasons for doing it, forget I said anything.

Ed Huntress

Placing 2" holes on 4" centers will definitely remove ALL strength in the tube! Draw a picture if it doesn't make sense.

Ed is 100% on the money. Putting holes in a tube member simply kills the strength. If you want a lighter frame, USE THINNER-WALL'D STOCK.

Regards, Hoyt McKagen

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Diameter, not radius.

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

I'm confused. 2" holes on 4" centers leaves a 2" web between the two inch holes, and an inch web top and bottom at the narrowest point. That should not remove "all" strength from the tube. What am I missing here?

Fitch

My picture has two inch holes with two inches of material in between. Gerry :-)} London, Canada

Perhaps *you* might want to draw that picture. :-)

Note that a 2 inch diameter hole has a radius of 1 inch. So two adjacent holes drilled on 4 inch centers would have 2 inches of material left between the holes.

Gary

If you changed the gauge to .083" which is more in keeping with auto chassis gauge, you would save approx 45% without any lightening holes as .120" weighs 5.610 per foot as against .083" at 3.857 lb per foot..see:

Tom

What you are attempting to do is to make a beam. (That's a good thing.)

You need to evaluate the strength of the beam as a function of hole size and hole placement. Holes make the beam more likely to buckle. They also reduce the weight of the structure thereby allowing you to make a more efficient structure overall.

In theory, you don't need nor want any material on the sides at all. Of course this won't work for obvious practical reasons, none-the-less that's what you are striving to accomplish. Given that you have a beam that is composed of two 3" wide plates separated by a 4" gap, what is the least amount of material that you must add back in between them in order to transfer the shear moment between the plates and to prevent out-of-plane buckling.

Really simple-minded description of a beam in bending:

A paper that tries to provide an answer to how thin a beam can be: Neat page loaded with graphics. Pay attention to "built up beams" because putting holes in the web is essentially creating a beam made of differing materials. Hope that helps, George.

IIRC, auto frames generally are strong enough when designed to have adequate torsion rigidity. Dan

I found this out the hard way when I had a BSA 441 fold up under me when I was hill climbing. The previous owner had "lightened" it up beyond repair.

Gunner

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If you're not competing in this thing, then you don't need the lightening. If you are competing, no scrutineer since the 1960's will pass a chassis with lightening holes drilled in it. Far too much chance of error, far too many accidents in the past.

I figure a 6x12 foot perimeter frame offers 72 of four inch face. With three holes per foot, that's 216 two inch circles of metal.

You could practically put that metal in the glove box. It removes nearly no weight, but ruins the tubing you paid for.

Only if loaded _strictly as a beam_ will your perimeter frame benefit from centroid thinning. And a perimeter frame is never loaded as a beam.

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