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…?

[jd74914]

Welding in those square pipes would take a lot of effort. I think that if you want to foam the chassis it would take less time to cut open the longs and neutralize the rust (so that you can spray foam in them) than it would to weld tons of little tubes on.

As far as rigidity goes, that would be much better than a narrow layer. A thin layer like you suggest seems to me like it would have too little sheer strength in the given combination. I do not believe that suck a narrow laminate would give the torsional strength you are seeking. A roll cage decreases chassis twisting, such a narrow sandwich seems unlikely to help with twisting (at least anywhere near the degree of a cage).

You could do a FEA analysis on it to find out, for sure but in my opinion you are better off fixing inner long rust and spraying it in there.

[jamara]

Welding in those square pipes would take a lot of effort. I think that if you want to foam the chassis it would take less time to cut open the longs and neutralize the rust (so that you can spray foam in them) than it would to weld tons of little tubes on.

I should've mentioned my car is a shell right now. In that sense, the inner and outer longs are equivalently accessible. Actually, the inners are more so cause the outers are partly covered by the door sill.

You might be right about it taking some time to get the squares done, but when the outer longs are cut open and rewelded, the welds should probably be ground down. The 1/2" tubes would not require that. It would go fast, I think. Plus, on a chop saw they could be cut in 5 minutes. The skin is where the work would come in, but not more than the Engman kit. Fabing the skins would be the chore. You might be right about the overall time it would take, though. The foam in the longs might be the fastest way, after fixing any potential rust.

As far as rigidity goes, that would be much better than a narrow layer. A thin layer like you suggest seems to me like it would have too little sheer strength in the given combination. I do not believe that suck a narrow laminate would give the torsional strength you are seeking. A roll cage decreases chassis twisting, such a narrow sandwich seems unlikely to help with twisting (at least anywhere near the degree of a cage).

I agree that the urethane would not help torsional (twisting?) load on its own. That is what the square tube lattice is for. This would create tremendous shearing strength for the two skins, which is what ultimately causes torsion (right?, wrong? not sure?) But 8lb pour foam is pretty dense stuff and would only add to the shearing strength. Granted, all the components of their own are not very strong. It is their functioning together that really makes them strong, but I suspect you know that.

You could do a FEA analysis on it to find out, for sure but in my opinion you are better off fixing inner long rust and spraying it in there.

I thought about putting the foam in the longs for a while. But, aside from the rust, I don't know if I want that much material in there. It would take about 16-20 lbs of it at 8lb density. Maybe I should go with 4lbs. 2lbs is simply not enough gain for the trouble. I guess it wouldn't be that much weight compared to a cage, or even the Engman kit. Something to think about, anyway.

Thanks for the reply!

[davep]

8lb density pour foam? Are we talking polyurethane here?
We use 2lb density foam in our coolers for insulation and strength. It is injected under high pressure as a liquid mixture that reacts and rises to fill the void in the walls of the box. However the foaming fixtures are 1/4" plate steel with lots of reinforcing. I can only imagine what pressure would be created by 8lb density foam.

[jamara]

8lb density pour foam? Are we talking polyurethane here?
We use 2lb density foam in our coolers for insulation and strength. It is injected under high pressure as a liquid mixture that reacts and rises to fill the void in the walls of the box. However the foaming fixtures are 1/4" plate steel with lots of reinforcing. I can only imagine what pressure would be created by 8lb density foam.

Exactly, polyurethane, two-part foam. Mix it up and pour it in. During application, there would definitely need to be evacuation points for the foam as it expanded so it would not blow the welds. Also, measuring everything out with semi-precision would be good. In other words, X volume of premixed material makes X volume of expanded material, so don't over fill! This would prevent me from turning my teener into a big rolling marshmallow. (IMG:style_emoticons/default/biggrin.gif)

Also a two stage application might be good. First, do the vertical wall. Let it expand and ooze out of some predrilled holes toward the top of the long. Then, do the horizontal section, also allowing for expansion via other pre-drilled holes. I didn't think it was necessary that it be applied under pressure since it expands to fill the voids of its own accord. Yes, no?

All of this is, of course, only theory. So it could go down very differently if I actually tried it. I could end up making an expensive sculpture out of a 914.

[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

[drive-ability]

jamara,
I like the fact your thinking outside the box so to speak. I too would love to able to fill the longs with a polyurethane, hey maybe 95 shore Ha but rust is a problem anyway we tackle this issue. As soon as one part is hardened another starts to crack etc..
Keep working on it !!!

[byndbad914]

I would hack the inside of the longs and the floor completely out (but leave behind the center tunnel if you want stock shift mounting, etc.). Then box the car with 1.75 diam 090 chromoly or 2x4 120-wall DOM welded to the inside of the outer long steel. Run a couple 1" tubes across for seat supports, and then use 1/8" off-the-shelf laminate for the floor. Would be easier and work better.

That foam core makes a great filler but I question its bond strength in shear. Additionally, the strong laminate stuff has aluminum core with varying cell size and wall thickness, not foam. I know there is foam core stuff being used in home construction and so forth, but the light and really strong stuff is not going to have a foam core.

[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.

[Borderline]

My idea would be to work on the outside similar to the engman kit: Sandblast the long to get a good clean somewhat rough surface and then laminate layers of carbon fiber using epoxy resin for good bonding properties. carbon is stiffer and lighter than steel and would be relatively easy. just a thought, FWIW

[Aaron Cox]

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.

hah! i am missing out...

we never used ay foam in statics, dynamics or structure analysis! (IMG:style_emoticons/default/laugh.gif)

[andys]

I would hack the inside of the longs and the floor completely out (but leave behind the center tunnel if you want stock shift mounting, etc.). Then box the car with 1.75 diam 090 chromoly or 2x4 120-wall DOM welded to the inside of the outer long steel. Run a couple 1" tubes across for seat supports, and then use 1/8" off-the-shelf laminate for the floor. Would be easier and work better.

That would sure stiffen it up!!!!

I saw a similar idea implemented on a V8 car I saw a few years ago. They used channel iron on each side outside of the longs with round tubing bridging between them similar to what you describe. The channel hung about 1" below the long with the tubing below the pan. The execution was a bit rough, but the owner claimed it made a huge difference. Not how I would have done it, but that's the idea.

Andys

[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

[davep]

Free rise density of the foam is less than 1 lb per cubic foot. To get the 8lb density you are talking about means containing it in a very strong fixture until it cures. Even then, if the foam is not well mixed (impingement under about 1000 psi, think two opposed nozzles in a very wide \/ configuration), then you can have unreacted material that may react even years later and grow again. We had problems like that with coolers up to five years old that just burst the casings with 1/2" wide cracks. Using foam for structural purposes is a tricky prospect.

[jd74914]

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.

FWIW

I should have been clearer. The point of his example revolved around helping reduce buckling in columns.

And Aaron, you definitely missed out (IMG:style_emoticons/default/laugh.gif) . . . I think adding some "homemade" problems in was his way of breaking up the monotony... fun (of course this is coming from a guy who has custom caution tape that says "DRAW FREE BODY DIAGRAMS" and a parrot which repeats the same.) (IMG:style_emoticons/default/laugh.gif)

[jamara]

Free rise density of the foam is less than 1 lb per cubic foot. To get the 8lb density you are talking about means containing it in a very strong fixture until it cures. Even then, if the foam is not well mixed (impingement under about 1000 psi, think two opposed nozzles in a very wide \/ configuration), then you can have unreacted material that may react even years later and grow again. We had problems like that with coolers up to five years old that just burst the casings with 1/2" wide cracks. Using foam for structural purposes is a tricky prospect.

This is very good info to know. I have heard this sort of thing must be mixed well, but I didn't know it required this degree of processing. The products I have found do not seem to require it. Also, they are marketed as 8lb foam and are designed to be mixed with a power drill and attachment, then poured in place. The density is determined by their chemical composition, not the pressure under which they cure. So, we may be talking about two different polyurethane foams. I wonder if I'm thinking of the right stuff. Something like this, in a small quantity, is what I had in mind.

http://www.polyfoam.cc/products/foam/mold.html


The site says it is all hand mixable and the higher densities are for structural applications using sandwich composites.

[davep]

You guys might be correct that some of the foams may get that density in free rise. Be sure that it is a free rise density though, and then be very sure you get the quantity bang-on for the volume you are filling. You want to be sure that it fills the entire cavity and does not trap bubbles of air. Those bubbles would reduce the filled volume. Excess volume of foam will always create high pressures. We can often find voids in complicated parts where the foam was too stiff to flow in narrow areas. It gets tricky.

[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.

[Katmanken]

I've built aluminum and foam core structures for military applications. The foam was used to stiffen a thin aluminum floor so yer foot wouldn't punch a dent in it and to stiffen the outer skin for the same dent reasons. Aluminum tubing stringers are used for the strength and load carrying capabilities as the skin wouldn't handle the load. Lightweight structure, but my analysis showed the foam wasn't capable of proper load carying abilities- even with aircraft structual epoxy.

Kinda like wood 2x4's in your house carry the real loads, the drywall and foam sheet insulation don't.

As you may surmise, I fricking hate ruining a good german car with spray foam. VW sprayed the crap into my Superbeetle and after 20 years, the rust came through the steel from the inside out in areas that never see moisture. I had to cut welded steel to get in there and the rust was only under the foam and not in metal right next to it which leads me to believe the foam is corrosive or becomes so over time.

And why the heck you would look at a torsion load is beyond me. Rockers get bending and compressive loads, and not a lot of torsion.

Ken

[Brando]

I second the carbon fiber idea.

The only way I could see foam injection working properly, as intended, without causing damage to the material of the car, is in a carbon fiber chassis.

Can someone crank out a teener body, primed in white, made of carbon fiber real quick? Thanks.