Floor board strength is probably a major point for those rails. And a stiffer floor structure would impact the whole chassis stiffness.

I cut the rear seat mount braces out on my XP car. I was amazed at how much the passenger floor flexes when I step on it now. I left the front seat mounting point as I think that would make the floor way too flimsy. And this is with the FM frame rails. I rarely have a passenger seat installed but it is a nice option. Plus I end up in that area whenever I have to play with the ECU wires.

Frame rails are what both subframes bolt to and tie both ends into the tub. The rockers are definitely a strengthening member - focused on creating rigidity not found (lost) in the lack of a top.

https://www.miataturbo.net/attachmen...ine=1361215001

While I was installing frame rails one corner actually lifted off the jack stand. Kinda freaked me out as I was under the car at the time

On the cheap HF jackstands, my car tripoded after frame rails with one corner off by just one peg. My car was a stiffie!! :giggle::giggle:

I love these I have been thinking about doing this as my stock frame rails do not like having my trans cross member welded to them. Now that I have seen some pics of the floor and rails cut open I will be doing this in the near future!!!!

Tell us how it works out when you are finished.

Great idea! I ran across your post trying to find out if anyone had tried this. I'd gotten the idea from my Mustang days. Griggs Racing had a "through the floor" frame rail kit. Well done!
-Leland

I'm interested in doing this, but neither Lowes or Menards have 2" square metal tubing. Where did you get your materials?

Reasonably priced supplier which has everything, cuts to length, and ships fast and cheap:

Online Metal Store | Small Quantity Metal Orders | Metal Cutting, Sales & Shipping | Buy Steel, Aluminum, Copper, Brass, Stainless | Metal Product Guides at OnlineMetals.com

Google for a metal supply shop near you, or there are a few online:

Order Mild Steel A513 Square Tube in Small Quantities at OnlineMetals.com

(no idea if that's the right wall thickness for this application or not, but you get the idea)

--Ian

And Joe's ponies make him faster than me. :)

--Ian

Indeed. :giggle:

I've used that supplier for years. Always small orders, always had great service. Some stuff, like 3.5" diameter aluminum round rod, you just can't find locally.

+1 for online metals.

Not sure where you're located in Indiana, but Alro Steel in Louisville has damn near everything.

I've used online metals before, I just figured the shipping would be ridiculous. It's surprisingly reasonable. They don't carry 14 gauge 2" square steel, so much for being spoonfed exact instructions. :noob:

No metal shops in town, I just figured I'd see if a chain carried it.

???

It's right there on their website, under "A513 Hot-Rolled" https://www.onlinemetals.com/merchan...45&top_cat=849

I just googled 14 gauge metal thickness and the measurements didn't agree. Good to know.

There are different specs for different material-types, and if you're off by a few thou, it ain't gonna matter one bit where a welding process is concerned.

The 14ga spec is just a wag to begin with. Miatas are designed in Japan, and specified in metric. Any US measurement is going to be "close" at best.

Old thread is old-

But at the stage im at in the build, its time to consider frame rails of sorts. Although this looks like a very solid solution, would there be any benefit to virtually re creating the FM style frame rails but with thicket steel to reinforce the oem rail section vs cutting it all out and placing 2" tube steel in there as OP has done? Seems like doing it this way would yield almost the same result with less effort.

BOX is stiffer torsionally and in bending than a C