Harebrained idea about chassis stiffening, Just a thought for the engineering types Options

[jamara]

Well, I am not engineer, but I’ve got a harebrained idea for reinforcing the longs of the 914. I want to run it by you more experienced engineering types out there.

Theory:

1. Engman’s kit strategically places steel on the longs and lower firewall. Why? Because two pieces of sheet steel welded face to face are stronger than one by itself.

2. However, two pieces of sheet steel, separated and firmly affixed to a lighter weight material (fiber, honeycomb, thermopolymer, etc), is exponentially stronger than two pieces of steel welded together. Here is a site with more than you ever wanted to know about composite sandwich materials. The section at the bottom under "Core Materials for Sandwich Structures" is what you want. Pics at the bottom are a good illustration of the principle.

http://www.mdacomposites.org/mda/psgbridge..._materials.html

3. A similar technology is being used on modern cars during their production. They put polyurethane pour foam into the A and B pillars, as well as the rockers of new cars. It deadens sound and really makes the chassis rigid. Here is a link to a “tuner” car that this was done to. Look under “Foam filling the chassis”

http://www.sportcompactcarweb.com/projectc...rt_5/index.html

4. In terms of applying this to the 914 longs (putting pour foam in them), RUST is the deal killer. The longs on my car are “rust free” but still have some surface rust inside. With solid material in them to trap moisture, this could become a major problem. Also, the longs are fairly voluminous and so cost for foam and the resulting weight is another issue.

So, here is my proposed solution.

(IMG:http://www.fc.biola.edu/~james.calley/long_modification.jpg)

This would involve welding ½” x ½” ½” tube steel to the inner long in a lattice configuration. Then as with the engman kit, preformed sheet steel (18g) would be rosette welded to the ½” tube steel pieces, to the floor pan, and to the upper door sill seam where the inner and outer long meet. This would leave a ½” gap between the inner long and the “skin” The lower firewall would simply have steel added like the Engman kit, or perhaps this sandwich method could be used.

Now the good part….

8lb per cubic foot polyurethane foam could be mixed and poured into the ½” gap on the inner long. There would need to be evacuation points for the urethane foam so that when it expands it doesn't blow the welds.

Advantages:

1. Only 2.7 lbs extra weight in polyurethane given about 1/3rd cubic foot for both sides
2. Since we are dealing with the face of the inner long, it could be cleaned, phosphatized, Por-15’ed, gold plated, whatever, before everything is buttoned up. No rust issues.
3. Extreme rigidity since sandwich materials like this apparently behave like monolithic (solid) structures . In other words, it would be like having a huge 1/4" thick Engman kit with a fraction of the weight.
4. Only ½” invading the cabin area, as opposed to a roll cage.

Problems:

Relocation of the E-brake handle to the center console and possibly the back pad not fitting quite right. Also, the speaker grills in the front would need modifying. The ½” is not much but could cause problems here. Plus, my proposed design is to attach the top of the “skin” to the ½” lip at the top of the door sill where the inner and outer long are spot welded together. This is the area where the lower, inner door seal slips on and is held in place. Therefore, the trim piece that goes here would not fit without modification and the seal would need to be modified as well. Last thing, the seat belt bolt holes might need to be moved to the floor pan.

Anyway, this is just an idea, so don’t anyone start foaming at the mouth over it. (I will discretely NOT add a link to the 914world forum topic on the subject of pour foam in the longs, where someone started typing obscenities and colorful metaphors over the matter.) What do you think…?

[Katmanken]

As an engineer and a car repairer, the foam idea makes me want to (IMG:style_emoticons/default/barf.gif)

Ever notice that the areas in a teener and beetle that the germans used foam in rust from the inside out?

The other is merely increasing the height and width of the beam to increase the moment of inertia to stiffen it. Thats why a 6 inch diameter pipe is stiffer than a 1 inch diameter pipe.

Ken

[jd74914]

As an engineer and a car repairer, the foam idea makes me want to (IMG:style_emoticons/default/barf.gif)

Ever notice that the areas in a teener and beetle that the germans used foam in rust from the inside out?

The other is merely increasing the height and width of the beam to increase the moment of inertia to stiffen it. Thats why a 6 inch diameter pipe is stiffer than a 1 inch diameter pipe.

Ken

The foam does work as a stiffener. That's proven; we've actually used it in statics problems. (IMG:style_emoticons/default/laugh.gif) The problem with it is rust.

[byndbad914]

The foam does work as a stiffener. That's proven; we've actually used it in statics problems. (IMG:style_emoticons/default/laugh.gif) The problem with it is rust.

Be careful as to what you mean by "stiffer". that is a bogus term overused in engineering. What was stiffer?

If you have two facesheets and you throw some foam between them, but the foam doesn't BOND to the facesheets, then it may be good for filling a void and reduce localized creasing and buckling and supply stiffness in compression, but put the two facesheets in shear with each other and you slide them apart. So how well the foam sticks to the metal is the important question.

If you did a "statics" problem with a composite stiffness calculation, be careful what your assumptions are. Most basic problems assume stiffness is perfectly transferred thru all the materials and therefore assumes the components are perfectly bonded. However, there is a true shear strength to the bond joint and tho' mathematically the joint is "stiff" the bond sheared a long time before and the joint in reality is junk.

FWIW

[jamara]

The foam does work as a stiffener. That's proven; we've actually used it in statics problems. (IMG:style_emoticons/default/laugh.gif) The problem with it is rust.

Be careful as to what you mean by "stiffer". that is a bogus term overused in engineering. What was stiffer?

If you have two facesheets and you throw some foam between them, but the foam doesn't BOND to the facesheets, then it may be good for filling a void and reduce localized creasing and buckling and supply stiffness in compression, but put the two facesheets in shear with each other and you slide them apart. So how well the foam sticks to the metal is the important question.

You are absolutely right about the adhesion of the materials being critical. This is my understanding as well. However, everything I am reading about this polyurethane foam says if it gets on something and you don't want it there, you are SOL. It doesn't come off anything but smooth plastics, apparently. This is why it works as a sandwich material in structural applications!

If the surfaces are properly abraded, then I would think this stuff would stick like superglue. Also, I added the 1/2" square tubing in V formation to address the shearing issue, as well as making the wall thickness consistent. There would be little chance for shearing with the two facesheets joined in this way... the 1/2" tubes welded to the longs and then rosette welded to the skins would be like the Engman kit, only much, much stronger. The foam, in that sense, may be an unnecessary after thought except that it does fill the voids between the tubing and provides additional shearing and buckling strength. I don't know, but my intuition and research says it would act like a piece of 1/4" angle iron, only 1/2" thick and much, much lighter. Is this not what the Engman kit does, only 18 gauge (1/20")?

All that said, I am leaning more toward the tubing idea as you have suggested, but I think I might not completely remove the inner long, just supplement it somehow by welding the tube steel inside the long as you suggested and then putting the inner long back on. Who knows...

I am going to do something though! I want to create an elegant yet highly effective solution while incorporating some modern design techniques with the caveat that it be cost effective (I'm no trust fund hippie) and do-able in my garage.

If you did a "statics" problem with a composite stiffness calculation, be careful what your assumptions are. Most basic problems assume stiffness is perfectly transferred thru all the materials and therefore assumes the components are perfectly bonded. However, there is a true shear strength to the bond joint and tho' mathematically the joint is "stiff" the bond sheared a long time before and the joint in reality is junk.

FWIW

Very true. But... The polyurethane foam in cavity technique has been proven in real situations. Not to say it is as strong as theoretically possible or that it will work indefinitely for all load types, but it has been made to work for certain apps. davep has just said his company uses it for insulation and strength. Some of the tuner guys are supposedly getting success from it (never know about some of those ego maniacs, though). It is used in modern production cars.

Ultimately, it may not be worth it but it could be interesting to at least do some small model testing. Hmmm... card board and Home Depot expanding foam, anyone...??? Put a twisting load on it, before and after, see what happens... ??? hmmm... Materials are too different, I'm afraid. Maybe I'll try it anyway, for kicks.