Polyurethane Foam CROSSMEMBER D.I.Y
 

Just a thought...your feedback.

As we know Mazda have used "Polyurethane Foam" inside the front engine cross-member on Various RX-8 models over the years.

The Shinka, 40th Anniversary Model and now the Series II GT.

https://www.rx8club.com/attachment.p...1&d=1229055053

It is said to offer increased rigidity (possible handling improvements?), and less noise/vibration transfer.

Is there any reason why we (I) could not add Polyurethane Foam ourselves.

There is an after market kit available in Australia, I image you would have to TAPE over any open holes first before application.

What do you think?
An easy DIY?
Or Not worth the trouble.?

Here is an Australian Company who sells Polyurethane Foam Kits.

http://www.rigitech.com.au/touchNSeal.htm

Yes. Do it.
 

Wow, 13 years and nobody has even responded to this thread? Seems unusual, especially with such a valid, interesting question posed.

Well anyway, yes, it is worthwhile to do. It's incredibly cheap and easy & noticeably improves handling, reduces cabin noise/road vibration & enhances stereo acoustics (if done properly, and only after the required amount of curing time for your climate).

I did it to my '04 sport about 5 years ago and it's still holding up.

The results were good enough that I did the rear as well

I had this thought as well. Looks like it must be doable! What foam did you use? I would assume you'd want something especially rigid, and preferably available in smaller quantities. Did you do any prep? Without one to hand, I'm not sure what the rust situation would be. Seems like a winter project kind of thing where the car goes up on stands for a few months, the cross members are shipped out to be chemically stripped and coated to the bone, then fill, then paint and reinstall. Those are my thoughts at least... Still trying to find a good one.

I didn't get very complicated with things and tried to keep it low budget. I really liked the fact that I could spend less than a hundred bucks and have the work done in less than an hour.

No prep really, as my car is clean & rust free, but I did prepare enough 4 -6 inch strips of tape to cover the holes as I went along.

I used Great Stuff for large gaps (or Walmart equivalent) which pretty much sticks to anything....had to go out to get more twice. Kinda surprised how much it took; about 10 cans or so. (I also did the hollow reinforcements on the hood & trunk lid.

Finished by shaving off any overflow and since my 8 is black I just used a Sharpie to color in any visible foam.

Winter probably is not the best time to do this though, because most products will cure better with heat.

You probably want higher density structural foam rather than insulation like Great Stuff.

10 cans? Holy crap. I feel like I gap filled my whole house with 2.

It worked nicely although the product you're suggesting would undoubtedly yield even better results.

I was thinking a pourable Polyurethane ~60A durometer? Thoughts?

Did you get any pictures of the rear crossmember filled?

complete waste of time unless you’re trying to reduce NVH for the (very small) people tied up and gagged in your trunk.

structurally it does **nothing**
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Wrong.
E46 factory repair. Look it up. And many others both homegrown mods to factory spec.

But the image of people in the trunk is great!

Did you look it up? It seems not, because if you had, where the foam went and what it did isn’t at all relative to this thread as far as filling in large voids within an RX8 rear subframe.

It manifested itself in the sheet metal structure of the E46 chassis where the subframe attached to flexing, which the mounting surface flexing then led to cracks in the subframe attachment points. The foam was used to fill a gap between the two layers of chassis sheetmetal where the subframe mounted rather than the actual subframe itself. It was a weak design by the factory and a class action lawsuit forced BMW to come up with the cheapest cost fix possible.

https://bimmers.com/blog/e46-subfram...-metal-plates/

The RX8 chassis and subframe is much stronger. People have been running solid rear subframe bushings since the early days without any issue. I made an installed a set on my first white RX8 back in late 2009. if you want to stiffen the rear subframe then the weight to strength improvement benefit would be to weld on more sheetmetal imo. Similar to some of the Ford 8.8 etc. rear diff/axle kits do. They only do that to support the front of the diff housing though because the PPF is removed.
.

as a side note, take 1/4” plate and weld it on the entire bottom of the chassis and it will be stiffer. Does it benefit from the added weight is the question. People are just supposing the rear subframe is going to benefit from this mod based off that kind of thought process. The RX8 chassis exceeded many other cars in chassis stiffness. Mazda supposedly made some improvements in the S2, but again it added weight and the S2 has never proven to be the superior chassis for racing applications wrt the results.

It may have been stiffer, but is the benefit greater than the penalty is what matters.
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Well, no. You made a broad brush statement, "structurally it does nothing" . In the e46 example, the ONLY reason the foam was used, was for structural reinforcement. You can try to parse and qualify that, as you attempted, but the fact remains, it was done to reinforce, not to insulate.

I am surprised that you can't conceive that foam would reinforce and stiffen any hollow tube. It may not be enough, or in the wrong places, but it is not just NVH as you assert.


https://www.compositesworld.com/arti...reinforcements

And there are many more examples as I said, both factory and homegrown. You can search if you like.


Here's a fun experiment you can try at home:

Grab your refrigerator door and try to twist it. Try manfully. After you are exhausted, with zero result, please note to yourself that the door likely (99.99%) has no bracing whatsoever, and is a very inexpensively made tube of thin steel and plastic, and the only thing giving it that structural rigidity is very inexpensive (likely lowest bidder!) expanding foam. The stuff in the home stores is likely higher grade.

Now consider the hollow box section and round tubes in the subframes...