My frame rail solution
 

I was tired of looking at my beat up and worthless frame rails, and decided that I would do something about it. Instead of going with a frame rail cap like from Flyin Miata or V8 Roadsters, I thought I would make my own.

I wanted something that I could tie my roll cage into, and also use as a potential mounting point for a transmission cross member should I choose to do so. I decided that the cap method was not the way to go, but instead install new 2x2 inch square tubing. This is how I did it.

First I removed these square access holes, one in front of the seats and one to the back of the seats.
https://www.miataturbo.net/attachmen...ine=1360639368
https://www.miataturbo.net/attachmen...ine=1360639368

I measured the distance between the two, and added ½”. I cut a piece of 14 gauge square tubing which ended up at 37” long.

https://www.miataturbo.net/attachmen...ine=1360639368
https://www.miataturbo.net/attachmen...ine=1360639368

Next, I cut a strip out of the floor and did a rip cut on the lower section of the thin sheet metal portion of the rail. The floor cut should be a tad narrower than the tubing you are installing.

https://www.miataturbo.net/attachmen...ine=1360639368
https://www.miataturbo.net/attachmen...ine=1360639368
https://www.miataturbo.net/attachmen...ine=1360639368

The heavy portion will only get a slot cut out of the bottom so you can slide the tubing in place. You will then have to clearance a section of the rear sub frame so you can slide the new tubing in place without obstruction. Make sure it is not touching the rail to keep from having an annoying rattle.
https://www.miataturbo.net/attachmen...ine=1360639368

Slide the new rail in place. Note that the floor was cut just a tad narrower than the tubing, and that the tubing will overlap at the ends. This gives you a place to weld to with no gaps.
https://www.miataturbo.net/attachmen...ine=1360639368

You will also note that it fits nearly perfectly on the bottom of the old frame rail.
https://www.miataturbo.net/attachmen...ine=1360639368

Next I cut pieces of metal to fill the gaps at the access plugs that were removed earlier. I then welded the floor to the top of the new tubing. I then fully welded the bottom of the beefed up front and rear portions of the rail as well as stitch weld the old thin half to the new tubing.
https://www.miataturbo.net/attachmen...ine=1360639368
https://www.miataturbo.net/attachmen...ine=1360639368

I will say that welding under the car is a bit of a challenge. Having a lift sure would have been nice, but I got it done. I got better as I progressed through the project. I had to do a lot of clean up grinding to make it not look so bad, but I re-welded any areas that looked suspect. I finally wised up and went to a much finer wire (.023”) to do the thin stuff.

Clever!

You must have seen my build thread!

https://www.miataturbo.net/attachmen...ine=1350101644
https://www.miataturbo.net/attachmen...ine=1350101644
https://www.miataturbo.net/attachmen...ine=1350101644

Nice!

On my MG, I welded a second frame rail in beside the original. I needed the new ones (which extended into the engine bay to provide mounts for the new suspension) to be a bit further apart, so I laid the new square tube along beside the old and welded them together. I like the idea of the slot in the floor, that would have worked well.

I had an MGB in highschool. It was one of the reasons I got into Miatas.

Mine are so badly mangled/squished, they'd have to be cut out and replaced - like this. But another question - in a 6pt caged car, are they even necessary? Meaning do they reinforce the car so much that removing them would cause buckling/twisting? I really can't imagine mine giving any substanial re-inforcement the way they are now.

Nice clear pics

Very nice! Both of you!

Elegant solution! Nice work! Is the tubing wall thickness any greater than that of the stock rail?



My own frame rail work ended with very different results.

https://www.miataturbo.net/attachmen...ine=1360676570

https://www.miataturbo.net/attachmen...ine=1360676570

^ lol, you cut the wrong way :)

In both of our cases yes, the tubing is double the thickness of the stock rail.

Measure once, cut twice? I always get that confused.

That's a plus for rigidity. I was underwhelmed by the stock ones.

Absolutely! AAMOF you should tie them in to your cage mount points and then devise a brace to bolt in across the bottom (under the trans tunnel.) You'd be linking up the entire structure that way.

I think I know how I'm going to replace my flattened out frame rails now. Great idea. I was thinking of just having new rails formed up to replace the damaged sections, but I like this idea better.

I'm post a pic later of my rails... it'll make you wonder how I tracked the car as long as I did! Like I said - I've got to replace them - badly.

I do not think the Mazda engineers really meant for that thin sheet metal (which is literally as thick as the fenders) to be a crucial structural component. When you look at sixshooters pic, you can see that the real structure seems to be just under the door.

In saying that, it is known that beefing up that chintzy rail does stiffen the car significantly. I was hoping I could eliminate some of the excess tubing I so often see in roll cages in order to meet my power to weight goals.

Is it wrong that I enjoy watching Miatas get cut up? Seems a little morbid, but I can't look away...

Brian, sorry about your car. One of the great thing about Miatas is that when you wreck one you can remove the awesome parts, then put them on another one you bought for peanuts.

No worries. I bought the wreck from the POs insurance and used the drive train in my current '90 track car. I did tear up a little as I was producing the video last winter. The tub folded in the tunnel from the impact so there was no saving it. It even crumpled a bit behind the door so even the rear clip was suspect. The rest of the parts funded most of the build and everything useable went to good homes. Best of a bad situation... ;)

My understanding is that the frame rails are there to reduce the 'drumming' noise an otherwise flat bottom would incur. They're not meant to be strengthening devices.

Though as said overlaying them with stainless frame rail stiffeners does improve the rigidity markedly.

That is interesting, and does make more sense than to have a useless frame rail.

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