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This is my first post. I put quite a bit of time into searching existing posts, and don't think this has been covered. Though my motivation is so I can use a classic Datsun body and not change it's California title, the body could be custom and the question would be the same.

As is obvious from a first look under a Miata, the unibody has what appears to be a ladder frame, but is actually a pair of really weak rails or U-sections under the unibody, that then continue as stronger frame rails to the front and back of the car. However, they have a pretty hefty size, or rather, they take up a lot of space, yet they are relatively simple to duplicate, including not just connection points to suspension as is common here using tube framing, but their entire length in a boxed cross section. Using ordinary and modern crossmember shapes, one could build a solid and perhaps quite heavy ladder frame. Crossmembers could be shaped like the X section in the rear of the Toyota Tundra, or a K shape to help with torsional rigidity. The car could then be built relying entirely on this frame for all but heavy torsional loads. (at least, that's what I'm thinking) Here's what got me to considering this:
http://en.wikipedia.org/wiki/Ladder_frame#Ladder_frame (see the mention of the Toyota Tundra's rear "X" section)

For my project, I would then weld on the old Datsun body (I'm thinking of Datsun 1200, 200SX, even B210) 1975 or older.
* Up Front: Gut the inner fenderwells and use Miata donor and/or custom light framing for the strut towers, potentially leaving the area completely open and use plastic junkyard hidden wheelwells to keep mud from splattering into the engine bay but otherwise using the open space for coolers and venting.
* Firewall to back of driver/passenger: floor of Datsun (minus structural members) would be maintained, for easier fabrication and the smaller transmission tunnel would be required for a narrower body, since the Miata's transmission tunnel is so large relative to car width.
* In the rear, use the Miata donor's floor behind the driver so that the Miata gas tank and all plumbing can be used, and finish out the rear (which sticks out further out than the Miata) with custom flooring, which might come from another Miata donor. A full sized spare and more could be mounted under there.

For a custom body, the only structural strength needed would be torsional rigidity and a safety cage for the occupants.

Has anyone built such a duplicate of the Miata rails as a fully structural frame?
What would be any snags that would cause this to not work out?
Would the fabrication of such a frame (plus floor work and just a rollbar in my case) be economical?

I considered making this next question another thread... but I'll bet it has been covered before:
Would extending the wheelbase make a significant difference, how much could I extend it, and what adjustments would compensate for the increase in low speed cornering like autocross?
(if that question has been answered before, please offer link(s) to the thread(s). I searched but did not find anything)

Thank you,
(name is Tongue-in-cheek, because I've been in the planning stages for far too long)
NeverFinished

Ok, it sounds like you wish to have an early 80s Datsun with a miata driveline and suspension.
It would be much simpler to just do an engine and transmission swap. The butchery to the Datsun would be less severe and time consuming. The end result will be heavier and may be less rigid than a miata, depending on how much weight you wish to add, but who cares? This is not a race. I'd swap in the miata engine and trans, then modify the Datsun suspension for any perceived problems, or just not drive it in such a way as to notice. It will still be interesting, where a slightly faster, better handling miata is still just a mass produced, common car.

The protruding rails on the bottom of a unibody such as the miata is only a portion of what actually makes the car rigid.
2x3 rails are adequate and commonly used for street rods, but would either cause the Datusn to sit higher off the ground or protrude into the passenger compartment.

I usually see a group of folks with similar Datsuns at the Mitty at Road Atlanta each year.

You can see in this photo the protruding "rails" under floor of the Miata are quite small as compared to the boxed sections that run under the doors.

_________________
Steve

While I do appreciate the two responses, something got lost in translation. If these two responses represent the consensus, then my idea has not been tried, or tried and failed. 80's Datsun? No, 1975 or older to be smog exempt in California. Miata engine? I have much better choices. Raise floor? Using Miata rail dimensions in relation to Miata subframes would drop the Datsun body significantly from stock. In fact, raising the floor relative to a Miata (to gain more vertical height and strength from the rails) isn't a bad idea at all, as the stock Datsun floor is several inches higher of the ground than a Miata.

* I live in California, and BAR and CARB laws (automotive laws) allow a car to change it's suspension and frame without inspection. A 1975 or older car is exempt from CA Smog Inspection (though has the possibility of an improbable random inspection). These biennial inspections are expensive, require using the correct engine and exact design of intake/exhaust/vacuum system/smog equipment used in the model year. 1975 and older can legally use any 1975 or older block from any manufacturer with any equipment including custom; but because of the improbability of a random road-side inspection (I've never seen one happen, much less had it happen to me), many use highly modified engines from newer years or from out of country and get away with it.
* I would not use a Miata engine or transmission. I might use a classic Datsun based modified engine, or a Nissan SR20DET which was never shipped to the USA, and cannot be used in CA but can go unnoticed in cars 1975 or older. These sleeved aluminum 4-bangers can be built to over 1000hp. I know that is hard to swallow, so here's an example of a Datsun 1200 with an SR20DET, getting a best 1/4 mile time of 6.916. I have neither the budget nor the interest to build that much power, nor 1/4 mile racing, but shows you I have zero interest in Mazda engines:
https://www.youtube.com/watch?v=68_XIMMIwwk
I have many engines to choose from, most likely Datsun/Nissan, but can put in anything.
* I would use a very light Datsun from 1975 or older. The model shown in the above video (Datsun 1200 [B110]) has a 30mm longer wheelbase than a Miata, is much narrower, and weighs 1600lbs stock. After swapping to a heavy frame and cage, and my choice of some heavier engine, but lighter suspension; the car might still weight under 2000lbs. Different donor selection (B210, 200SX) and choices in construction would increase that weight to 2000-2500lbs. Difference between that project and a kit car? Title, smog, and an enclosed coupe or hatchback body.

For the most part I just want to use the Miata subframes. The Miata framerails are so weak, that (as I've been told by a Miata racer, and could see for myself at the junkyard) if you jack the car up from the framerails, you smash them. I want custom framerails that are at least strong enough to jack up the car from any point, reinforce them so they become a ladder frame, deviate from the stock design sufficient to (effectively) push the front subframe forward as far as the body allows and customize the steering (like a Lotus 7 project) so I end up with a huge engine bay without pan interference, use the rear floor which is superior to the Datsun's rear floor, especially because it holds the Miata gastank ahead of the suspension and is then flat all the way to the back of the car, but use the Datsun's stock floor from the firewall to behind the front seats for easier fabrication, to preserve the stock interior proportions and the car would appear stock from every direction but underneath, and to use a Datsun transmission on the custom rails without the extra strength that the Miata relies on from it's structurally significant transmission tunnel and rocker panel box sections.

The purpose of this thread is to explore the option of building a [edit] full square box frame instead of full tube chassis, by keeping the strength of a unibody intact, as the body designer (in this case Datsun) intended; added additional strength from a subframe. A square box frame (strong relative to the frame rails on a Miata, but quite weak relative to a pickup truck) would tie all the components together with (I think) the least fabrication. BTW: The old Datsuns have framerails, too, they are structural in the engine compartment, but taper down to about 1/4 inch deep and then stop part way back under the floorpan; and I'd remove them. Using framerails (even without increasing it to a fully self-contained ladder frame) that are shaped based on a Miata but built heavier (square tubing or stronger steel U shaped section under the floor with a large doubler plate under the floor to distribute forces) would be a significant improvement over the stock classic Datsun. Adding square tubing frame out to the rocker panels, and tying them to a rollbar would increase torsional rigidity significantly over the stock unibody. Unibodies are already torsionally rigid, so if I don't damage it's passenger box section (unibodies can be thought of as two or three boxes: engine bay, passenger compartment, and often rear box which starts at the shock towers), I can avoid building a tube chassis.

Purpose of car:
Smog exempt CA car, 1975 or older
Datsun - for participation with a local classic Datsun club - different modifications are encouraged.
driving less than 5K miles per year
autocross, and driving that style all the time.
2-passenger with large cargo space behind seats, in an enclosed mass-produced stock body.
massive engine bay, to allow my choice of engines, most likely Datsun/Nissan
Beside the point: I'm aware that when using a different transmission and powerful engine, I'd change the differential to another LSD with gearing appropriate to my purpose.

A Datsun 510. .. My brother in law had one of those. Pretty nice base to start from. Even came with an IRS and a Hitachi diff. ..
And I knew one with a Chebby small block in it too That one was a bit, well, scary.
Add a 10 point cage with full suspension mounts, add a "k" brace (looks like an export brace on a first generation mustang) and then weld up all the body seams and you'll be amazed at what you've created, those are some really tight cars.
To hit your stated weight goal (and meet tubing spec at the same time) you'll need to take a plasma cutter to the inner panels, strip out a bunch of stuff, etc, etc.
What I'm stumbling around saying is, why put it under the car when you have so much room available in the car?

Ummmmmmmm, you do realize that even 600 hp will be almost unstreetable. .. Definitely not a wet weather driver.
The rear tire width needed to control the happy pedal will be a severe detriment in the rain.

What part of California are you in? There's a bunch of us in the greater Bay Area and another cluster way down in the Southland.
(gee I don't sound prejudicial do I? )

_________________
"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

Does extending the wheelbase make a difference? Yes but. .. The value depends on the use.
Lengthening the wheelbase will make it "smoother" at the expense of an increased turning radius (compared to a stock Miata) unless you have a steeper steering angle which would require you to either widen the track or run a narrower tire to avoid hitting the (assumed stock Miata) subframe.

_________________
"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

The "8" is a typo. I meant "'70s". I attempted to put in a few sentences what you didn't say in several paragraphs; the point of all this. The topic is "custom ladder frame like truck", but now you are talking about not using a tube frame, just a subframe.

About apparent weakness, there are certain locations that are best for placing a jack and those locations are designed for it. The mfg has instructions in the owners manual for where to put the jack. The emergency jack is slotted to go over the pinch welded steel so no damage will occur and it is less likely to slip off when operated on the side of the road. The jacking points are also strong enough for a standard jack to be used without damage. They could have welded angle iron the length of the unibody, but they chose to provide instruction and make the car lighter, faster, better handling, and more fuel efficient. If a person puts a jack in the wrong place, the weakness is not in the unibody.

I'm not trying to sound sarcastic when I say this: It doesn't have to make sense to anybody else and you don't need to justify it. The classic "Win-Win" scenario. Good luck with your build.

(revised just to be clearer, same meaning, but I hope I explained myself better)

A 510 (at 2184lbs) can be built stock because of the attributes you mentioned, therefore not appropriate to this design idea. I'm referring to the really light bodies that are not sports-oriented from the factory, such as:
Datsun 1200 (B110) coupe at 1640lbs stock
http://en.wikipedia.org/wiki/Datsun_1200#B110_series
Datsun B210 fastback at 2000lbs stock
http://en.wikipedia.org/wiki/Datsun_B210#B210_series

I don't recall ever mentioning 600hp. While I may go with a SR20DET, the stock build begins at just over 200hp and 200lb ft of torque. I'd swap to an intercooled, modern fast spool turbo, but downsized depending on the weight of my car. I might end up with similar to stock maximum hp, or maybe 50% more. I'd also like to incorporate another idea I've considered for years, which is to prepare one body for two engines with easy swap between. I'd use the same transmission, motor mounts, and exhaust from the point past the SR20DET header. Which engine? Doesn't matter for this topic, perhaps a classic which may require that I keep the transmission and driveshaft as well. Might be used as a mere "spare" so that I can still drive while I make some major modification to the SR20DET. For that matter, I may simply choose a late model 150-200hp NA 4-banger and keep it simple.

Good point about the tires hitting the frame rails if attempting additional steering angle, I'll take that into account. Can the Miata suspension handle a wider track width (to match the ratio of wheelbase increase) simply by using wheels with the appropriate offset, and a change in the choice of coil-over set?

Back to frames:
I do want to use Miata subframes if I build an ultra-light Datsun.
I want to keep the unibody mostly intact, so the engineering of that unibody is preserved, at least from the firewall back.
However, I think an actual square tube frame could achieve my goals economically, rather than a fully custom tube frame. It need not be a strong frame on it's own, because it would still have the body welded to it with doubler plates and additional supports added for torsional rigidity. The Miata uses it's transmission tunnel and rocker panel setup to make a strong body, I would not be using those designs. The little Datsuns, being enclosed unibodies, use their outer skin and floor as a box for torsional rigidity. I will preserve and add to that.

I would like to duplicate the shape of the frame-rails on the Miata. In fact, it may be most economical to use actual Miata frame-rails, and cut them at exactly floor level, and replace that bottom (weak) rail that goes under the passenger compartment with a very solid rectangular steel box, slightly longer than the Miata one (for changing the wheelbase), seam welded to the original front and back sections of the Miata frame, and then add doubler plates to increase the connection between them. The stripped Miata donor (maybe flipped upside down on the shop floor!) would provide a good jig to keep everything aligned. One measurement I can't find so will actually have to measure in a parking lot, is how far the body panels & actual floorboards are from the ground, stock for each car. Assuming the Miata body & floorboards are significantly lower than the stock Datsun, that allows increasing the frame-rail vertical height which significantly increases strength. With all the books and documents I have available to me, I'm only going to figure this out by measuring each car in a parking lot. Depending on practicality, I might put the Datsun 1-3" lower than it's stock position as that is fairly normal.

Then (and this is where it becomes a ladder frame) add crossmembers to tie the two rails together.
The crossmembers can be a K or X section at the front and rear, a box frame to match (or cut into) the firewall at the Datsun bellhousing to go around the transmission, with a bolt-in transmission mount that not only (obviously) mounts the transmission, but also completes a straight crossmember when bolted in. At about the position of the Miata gas-tank which I would like to use in the Datsun and eliminate everything stock under the Datsun from behind the front two seats, a rollbar would be tied into the custom frame. Another cross member (probably K shape, but also could be a straight tube) would sit just behind the suspension where it can clear. In fact, a knee bar might be used where there usually is one in the dash, and tied to the outer square frame vertically and at an angle straight to the shock towers.
Likewise, square tubing can go out to the very weak rocker panels of the Datsun (discard the stock jacking points) and in toward the transmission tunnel, but as in the previous member described, only becomes a real crossmember when a section is bolted under the driveshaft. That section under the driveshaft and transmission body need not be brackets but a sheetmetal plate (with doublers at the bolts) that ties the floor together all the way across. There are many variations to this theme, I've only described one here. It is hard to put an image in a paragraph, but I hope this gives the image.

As to jacking points, if my custom frame rails are of thicker square tube steel, they would be the strongest points on the car, for either jacks or jackstands. As for slipping, that's a good point, I'd use a matching set, maybe a jack with a dowel, and a matching hole in the frame rail near each corner. Another way would be to design such a point in the square tubing that goes closest to the wheel, reinforced appropriately. In fact, old Datsuns often are crushed slightly where the jack is intended to go, and in the rails where one would put jackstands.

As to the unibody, let's consider this one three boxes.
* The front box would be destroyed, I'd cut out the Datsun original completely including the frame rails all the way under the floor, and use what might be better called a snout, tie in the front grill support, the stock towers, the subframe obviously, the fenders, and provide torsional support through bars to the firewall by using a sheetmetal box from the framerail to the firewall.
* The center box (passenger compartment) would be kept entirely intact, from firewall to behind the door supports. At it's front the firewall would be tied to the front snout as just mentioned, and would require some additional support. At it's rear it would be tied to the rollbar all the way around, making the rollbar completely structural (a "bulkhead" in chassis engineering books).
* The rear box would be intact like stock except that the bottom would be replaced with a flat floor, right on top of the complete subframe I'm attempting to describe, and welded to the quarterpanels to make a box. Wheelwells/shock-towers would be eliminated or minimal, because they would be positioned like the Miata, and these have narrower bodies. Fender flares, even giant ones, would be used.
*** Torsion (from opposite corners) is intended to travel through the unibody and all the extra members described.

So the intent is a minimalist square tube frame that duplicates (or is based on) the Miata rails (though would be adjusted to the selected wheelbase and to tie into the body of choice) and provides the majority of the structure, with everything else (including center box of unibody) tied to it as the primary component. I'm avoiding a switch to tube framing, which of course could be done, eliminating the need of the Miata rail-based frame I'm describing - at a higher fabrication cost (I think).

I don't know if this can be understood, as I would do much better to draw it in 3D... that's a hard task with or without software. There may come a stage where I build a scale model using balsa wood.

B210 hatchback. .. I had a "HoneyBee" for a couple of years
Had an A14 powered sedan when I lived in Sonoma County, the first wife loved the fuel economy, it made really nice commuter car after I put in a fatter main jet and swapped in a 5 speed.
I also had a swimming pool business for several years in Yolo County but that's another tale.
A couple of comments;
The Miata gets a lot of its capabilities from it's ladder within a ladder construction. That's what the PPF does.
The frame rails on a Miata are in fact a major component of the chassis, big thin wall tubes will have a higher torsional rigidity than small thickwalled tubes for a given weight.
The floor pan and seat rails function as gussets so that it doesn't go parallelogram on you.
You would have a much easier time adding (and tying together) front and rear sub frames to the existing Datsun frame rail locations to carry the suspension components of your choice.
Not that it couldn't be done or isn't worth doing but it sure would be easier to build something to suit existing dimensional limitations.
Take a look at the VFR powered Honda 600 on this site, he diced and sliced Miata sub frames for his suspension.
Designing them takes some effort but isn't really all that difficult if you're willing to take the time.
You'll also need full anti intrusion packaging to run it in a sanctioned race. .. If your going to pay the weight penalty then you might as well put it to good use as part of the chassis.

_________________
"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

Try http://www.sketchup.com/ for a free starter drafting program. ..

_________________
"There are times when a broken tool is better than a sound one, or a twisted personality more useful than a whole one.
For instance, a whole beer bottle isn't half the weapon that half a beer bottle is ..." Randall Garrett

I haven't gotten any further, besides research. CA Smog Exemption for foreign cars appears to be 1967 model year and older, with 1975 being only exempt from inspection unless they pop your hood randomly. One race-car fabricator built two Miata based projects (not in CA), one being a wagon's complete sheetmetal & windows with Miata undercarriage from the front to the back of the passenger floor, plus the rear subframe - successful but claims it was a $70K build and was not helpful when I asked him about it. His previous was a 60's Datsun pickup, welding the Miata front subframe into the truck frame, building an extension to hold the Miata rear subframe, but he did a lot else. Both of these projects are well documented online, with photos of the changes.

Of all his projects, the only part that interests me is the proof that the front Miata subframe can be cut, the front pickup frame can be cut, and they can be welded together for a really strong structure. As for the rear, the Miata subframe can be used with a structure from the frame, but so could the original leaf spring pickup rear axle, or any rear upgrade including 4-link. This would allow much higher horsepower than a Miata rear. I've even figured the frame could be set up for both:
* An extension that holds the Miata rear subframe at the Miata wheelbase or a bit longer to smooth it out, for autocross
* Connectors to hold the leaf spring solid axle (even the original ones left intact) further back, for 1/4 mile
Switch between them with about six bolts, brake lines, and a driveshaft swap (both would be custom shafts)

Here's my question (or opening for dialogue?):
Do you have any examples of similar projects, that use a light pickup frame with Miata subframes (front one as a proper structural crossmember, the rear mounted to extensions that either bolt or weld to the rear subframe)?

The rectangular section truck ladder frame would technically be strong enough already, but undoubtedly reinforcement would be wise, the body would not have to be structural. While this might take the shape of a Jeepster or panel wagon in my case, this would be a good platform for a T-Bucket design, where the driver and passenger are just forward of the rear subframe, the engine is in the middle, and the front subframe is quite a distance from the the firewall.

Likewise, this might be a way to build a Lotus 7 shape, dragster, or a fiberglass bodied funny car (where the body lifts up out of the way). The tall ladder frame might be enough support, though additional hoops and lateral members would make it approximate a dual ladder frame or space frame, with the pickup frame serving as the main structural member (and maintains alignment) during construction.

One major shortcoming is the extreme length of the steering column, but I'm sure that has been worked out many times in many ways. Would a two piece column be available from a donor car, with U-joint in the middle, so most of it runs along the frame, attaches securely to the frame, and the section up to the steering column stays clear of all equipment (header or intake, turbo if installed, etc)?

Thanks for any input and examples.
Also, is this a reasonable approach to explore?