Filling body cavity voids w/ expandable structural foam
I was curious if anyone has ever done this on a 944 chassis? It was very popular for Miatas to add rigidity dampen noise and vibration. There seems to be some possible areas under the rear seats where a double pan exists. Other areas might be the A and B pillars, rockers. and cowl once all drains and wiring were sleeved well. I'm not talking about the foam insulation at Home Depot but the high density structuarl foams used in boat and aircraft building. There obviously are some problems that need to be addressed such as corrosion from blockage of the drain holes, fire concerns on track cars, and future removal of the foam if necessary.
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That's a really interesting idea, though I see a couple of problems.
1) real structural foams are not normally injectable
2) injectable foams really on benefit NVH issues. global stiffness - unrealistic, local stiffness - maybe.
3) the Miata is a convertible and will never be a stiff as a 944
So if you're looking to deaden the sound etc., great idea, if you're looking to stiffen it ... install a structure. I'd be interested in actually foaming some areas of the 944 for sound purposes. If that's what you're thinking of I'd be happy to collaborate.
A number of the SE-R guys have done it. Adds significant rigidity. Don't even entertain any though of ever removing it though (as you hinted at).
I might be able to put you in touch with some of those guys if you like.
It's funny you brought this up because it was something that ran through my mind regarding the 944 a couple of weeks ago. The 944 is an extremely rigid chassis as road cars go so you probably won't gain a lot. But, if I were building a track car (as opposed to a race car with rules to follow) I think I'd consider it. Frame rails, floor pan, rear seat pan, probably a few other places.
It might be a cool idea for stripped dual use cars to help with interior noise. I'd be careful with the floor pan though. It might buckle some, so weak is the 944 floor pan (although with enough escape holes it shouldn't be too bad).
I have been giving this some thought as well. I used "Great Stuff" on Eagle 1 about a week before I had to take it off the road. Therefore, I have no data to draw from.
I plan to do the same thing on the car I am building (Partial Eclipse) - unless I hear something that gives rise to it being a bad idea. As I see it, there may be corrosion concerns. For example, are there chemicals in the insulating foam that will increase the likelihood of rust propagation? There is no question there will be sound deadening improvements. And even a little structural improvment should make it worthwhile.
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I have a can of Great Stuff in the garage, I'm interested to know what it consists of. Since increasing rust progation is definitely the last thing I would want to do. Maybe I'll do some sort of test on some scrap metal as well. Should be interesting.
Well the content of the initial post sure didn't match up with my expectations based upon the great title to the post.....
You didn't really think anyone would open this thread if I worded any other way?
I recall that the density of the structural foams needed had to be above 8 lbs/cu.ft. This product was one of the injectable foams with very high strength/weight ratios as well as bonding strength. http://www.epoxysystems.com/4240.htm
The key to any structural / rigidity gains is to have an intimate bond between the expanding foam and adjacent steel. I've seen A pillars filled with structural foams that barely deflected in rollover tests compared to a pillar that would completely fold up and fail. A sealed chamber with well bonded epoxy foam is incredibly strong. For those of you that have stripped a 944 to bare body where are the double wall cavities or longitudinal tubular sections that you know of? I know that filling in the faux body frame rails on the floorpan will reduce flexure on a Miata in a very noticeable manner. I was hoping someone has tried this here.
In answer to the corrosion questions here's a few items to ponder. Our cars seldom rust from the outside in as they are well galvanized, it's the inner chambers where the galvanized dipping process is marginal such as double wall sections. Water and road salts get trapped in the rocker panels and panel crevices from winter driving. Chloride concentrations increase as these can not be effectively flushed out of the sealed areas and crevices. Most of the corrosion concerns can be avoided with a properly prepped structure such as hard drain lines from the sunroof, cowl area, and wiring placed in flexible plastic tubing. Do not confuse cheap canned foam insulation for residential thermal use for structural foam compounds. The typical Great Stuff is hydrophilic and will attract water, chemicals, and store it. Structural foams are hydrophobic and resist water penetration and breakdown of bond strength. Picture that rocker panel being an inert sealed chamber preventing all moisture intrusion forever. The cheap low-density foams will provide low bond strength values and crack under flexure creating a very bad corrosion scenario.
Thanks for the rehash of my construction materials class steve.... The conclusion that Great Stuff is hydrophillic is what I would have been hoping to discover during my testings along with other properties of the material. But since this information is already available whats the point. Understandably the foam reinforment must continue to be ductile throughout its entire servicelife. As for the strucutral grade epoxy based foam you referenced it seems like it would make a good candiate for our purposes. Since promotion of a strong bond between the structural steel and the foam is imperitive (and I havent had any experience using foam in this manner), is there a substance to use to help promote/improve bonding between the two materials?
Keep in mind that strength and stiffness are not the same thing. To actually increase torsional rigidity of an entire chassis (usually the "softest" mode) is quite difficult by simply having a core transfer minimal amounts of shear stress from one steel skin to the next through random areas of the car. Now strength, or more aptly, toughness or the chassis' ability to handle loads past failure can definitely be increased with the proper application of a structural foam. Which of these are you looking for?