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!