Imagine taking a Mustang for a canyon cruise. Come back for coffee and have your ace mechanic glue styrofoam to the rear axle housing. Wow, the handling must be really good now ....

Imagine going into mammoth cave with 4 tour bus fulls of kids on energy drink. Call up urethane express to go, and have them spray the inside of the cave with foam. Is it stronger? Nope. Is it quieter? yep!

If there's a benefit, it's less NVH. Any handling differences would be indistinguishable.

Thimk.

 

I'm sorry, but this is a gross oversimplification with obviously no regard for automotive engineering principles.

Think (spelled properly).

 

Which specific principles am I not understanding? Foam is foam and has little strength in compression, tension, or torsion. What it does do is dampen vibration, i.e. convert motion into heat. "Thimk" was spelled as I wished it, based on a widely-circulated series of posters representing engineering disasters. Humor, I thimk it's called.

 

If you're that desperate, then drill a hole and fill 'em yourself. Make sure you use a high density polyurethane, not the stuff you find at Wally World.

I am planning on doing this, and going much farther than just a crossmember.

For those who are curious, the foam isn't for noise damping. Filling a crossmember wouldn't do that. The foam does make the crossmember stiffer.

If you take any solid piece of material (let's say steel) and measure stress as it is flexed you will find that 90% of the stress is being placed on the outside of the material. The inner material doesn't really see much stress from bending. All the inner material is doing is keeping the outer material from collapsing in on itself. A prime example is a steel I-beam. They are nearly as strong as a solid steel beam, but weigh much less. The load is taken up by the top and bottom of the I-beam, and the center is just there to keep the top and bottom from collapsing. The forces trying to collapse the top and bottom towards each other are much less than the forces the outer layers see as the I-beam flexes, thus the center piece doesn't need to be as strong.

This is what Formula One chassis builders do when they build a car from carbon fiber. You think those things are solid CF? Nope! They lay it over foam. Why? Because the stresses exerted during a race are placed on the outer layers. All the inner core needs to do is keep those layers from collapsing towards each other. Again, the forces that this foam encounters are far less than the outer layers see, thus it doesn't need to be as strong as the outside walls. A CF board with a foam core is nearly as strong as a solid piece of CF, yet it weighs significantly less. A car that weighs less is faster.

This same principle applies to our crossmembers! Since the average crossmember is not filled, there is nothing to stop the outer walls from collapsing towards each other as it flexes. However, the crossmember on the R3 is filled with a high density foam and should be much stronger because there is something keeping the walls of the crossmember from collapsing as it flexes. This is simple physics.

Now, is a single, stiffer crossmember going to make that much of a difference? Probably not, since there are other parts of the car that flex a good bit and need damping. However, comparing the two side-by-side, there is a significant difference.

P.S. "Dampening" is getting something wet, like a towel. "Damping" is removing energy from a system, like a shock absorber or the foam filling.

 

"In Principal" sure. In practice, not so much. For the foam filling to have much effect in this way, it needs to be incompressible, which the standard sort of foam is not. The main effect of foam coring fiberglass structures is to seperate the layers of fiberglass in much the same way the web seperates two bands of steel from each other in an I beam. (The stiffness of an I beam goes up as roughly the 5th power of the web depth.) Structures are hollow in the first place because most of the strength of a piece comes from the "skin", not the "filling", so making a piece slightly larger more than compensates for the removal of the material in the middle with a large net savings in weight. If one was to take advantage of "refilling" a structure with something, it would best be done during the design process.

NVH for cars is "easy" because the car is coupled to an essentially immovable object called Earth. With airplanes, this coupling is much weaker so there's an increased awareness that vibration results in noise results in destruction of vital pieces if the vibration is not dampened (turned to heat) in a safe way. An efficient way to control vibration of a piece is to bring it into close mechanical contact with something of very different resonant characteristics. A good example is our aluminum hood meshed with the 1/2" or so sound blanket that Mazda provides on its undersurface. I suspect that will be the main effect of filling the chassis crossmembers with foam. It probably won't hurt anything at least. I suspect that the primary use for the foam in this case is to justify a big bump in the MSRP over the non-shinka GT.

 

The foam dampens vibration, plain and simple, which augments the GT ride quality. The magnitude of same is a feel of the beholder item.

btw: I had some experimental True Temper golf shafts a number of years back that were urethane filled. They really did do an exceptional job of damping vibration.

 

You got bad information.

 

That is such a gross misuse of the word "costed" that I just have to point it out... couldn't help myself. Now, back to the regularly scheduled misunderstanding of Shinka parts.

 

The only thing on the Shinka that makes it handles better and go faster is the paint. It's a proven fact that Copper Red is aerodynamically and better suited for track use.

 

whatever ya'll can attribute this to....

i've had no les than 4 people tell me my shinka was much smoother on the road than another stock 8. only two of them knew enough about rx-8's to know mine was a shinka, and only one of those knew about the crossmember and shocks. the other three all asked what i had done in suspension mods to make it so much nicer....

then again, i cannot tell the difference between my car and any other 8...?

 

Not to beat what looks to be a dead horse, but would foam crossmember make any noticeable difference in general ride quality?