DIY Miata Roll Bar?

I picked up a 1990 Miata a few weeks ago for $740 with the intention of running some HPDEs with it this summer. Picked up a torsen and some 1.8 brake parts at the local pull-a-part for another $100. Now I'm looking into roll bars. FYI, the plan is to run fixed-back racing seats (have a pair of momo composite seats for it) and 5- or 6-point harnesses. I'll probably drive it around town occasionally (not a ton, but I don't want a full cage).

I'm thinking about making my own roll bar since I'm a poor grad student and don't want to shell out $400 for an aftermarket one. Also, I have access to a nice hydraulic tubing bender, good tig and mig welders (which I am proficient at using), and have some spare cage tubing kicking around. I was planning to model the bar after one of the aftermarket designs and wanted to get some feedback from all you Miata guys with experience in this area. Here are some of the questions I have:

• Weld-in or bolt-in? I like the idea of weld-in since I expect that it will be faster and easier to fabricate (don't have to line up any bolts, cut out access areas under the car, etc), but if I want to change the design or go to a full cage later, will I regret this?

• Double diagonal, single diagonal, something else? What do you guys like best for HPDE; what about on the street?

• Harness bar? Any reason not to incorporate this into the main roll-bar structure (vs a separate bolt-on harness bar)?

• Main hoop mounting location? I think it's pretty well established that the best location is on the bulkhead behind the seats, but are there any issues with this?

• Anything else I should consider? Since this will be custom made I can change anything that the aftermarket bars do for economy, to facilitate bolt-in installation, or to make it a universal fit. Since this is a "blank slate" are there things that I can do better than the aftermarket units?

Thanks in advance - I really love having you guys as a sounding board for this kind of thing. Oh, and if you have pictures of things you like/don't like in certain bars it would be great to have them posted here. Thanks again.

My thinkin' has been bolt-in because if ya stuff the car ya can pretty much just unbolt the roll bar for the next car.

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^Any serious off is going to negate the value of the roll bar, much like a helmet.

OP, unless you have a Hard Dog or similar to copy, is a few hundred bucks really worth risking your safety?

(Coming from someone who wrecked a sport bike at the track and ended up with post-concussion syndrome for months and trashing a few thousand dollars in gear)

In reply to z31maniac:

I'd like the experience of making my own bar and I'm confident that I can produce something of similar quality and strength to an aftermarket unit at virtually no cost to myself. However, I want to make sure my time is spent making the best/safest bar for my application. Hence the questions.

Well, you can have my EGR rollbar for less than $400 but you'll have to pay shipping on top of that unless you want to do a quick cross-country drive to pick up the bar .

I'm not sure that building your own bar is such a good idea unless you have a known good bar you can use as a template. You don't really want to find out that the bar wasn't up to snuff while doing some upside down motoring.

In reply to JohnyHachi6:

As to the main hoop, many roll bars use the bulkhead behind the seats around the gas tank in addition to forward mounting point on the slope of the rear bulkhead itself. At this point in time, most regulations for a full cage require the main hoop to be mounted to the floor behind the seat, so if you wanted to turn this into a full cage, that might be a consideration.

The harness bar can be incorporated but sometimes the seatbelt towers are in the way. It depends upon how low you want to sit. If you mount the seat through the floor and you have a long torso you might be able to get away with changing the harness holes of the seat. Oh yeah, this may be the time you consider an aluminum seat with a back brace so that you can change the holes if you needed.

I suggest you mock up the roll bar with the seat. Integrating the harness bar can work but if the seat belt towers are in the way, a possibility, you may consider making a bolt in bar using the existing seat belt tower hardware like hard dog has done. Then again, if you have removed the soft top completely you don't even have to worry about that.

Cross brace or not? Whatever you feel like fabricating will work for a roll bar.

One more thing to remember, you can probably pick up a hard dog for $300 or so used. You can certainly replicate it for less with you skills but it is something to consider.

In reply to mattm:

That's a good point about any cage requirements that call for floor-mounted main hoops. I'll have to see if that's possible with the seats in the position I want.

I'll also check about the holes for the shoulder belts in the seats and see if an integrated harness bar that matches that level is possible.

I was planning on going with something like the hard dog bar design, but if I want to have a bar that can be made into a full cage later, maybe I should go with something like this:

I'll have to give that some more thought.

Torsen and 1.8 brakes for $100? That's a score.

JohnyHachi6 wrote: In reply to mattm: That's a good point about any cage requirements that call for floor-mounted main hoops. I'll have to see if that's possible with the seats in the position I want. I'll also check about the holes for the shoulder belts in the seats and see if an integrated harness bar that matches that level is possible. I was planning on going with something like the hard dog bar design, but if I want to have a bar that can be made into a full cage later, maybe I should go with something like this: I'll have to give that some more thought.

I have my seat mounted to the floor (rear humps removed) and use backing plates and washers for the seat mounts. The main hoop for my spec miata cage goes to the floor and even with my seat mounted towards the rear I still have plenty of room. However, you will of course need to position your seat etc etc. My older aluminum seat needed to have new harness holes cut due to belt angle for my harness. You can look online and see some seat belt towers that were "adjusted" in order to make room for the harness bar. That adjustment should not be necessary as long as the harness holes in your seat are positioned correctly. Remember that the angle can change with the addition of a HANS device or equivalent. Think about these issues before you commit anything to the welder.

The most important thing to remember is the angle of the harness. Make sure your seat position and harness angles are correct. Saving any money without correct angles is false economy and some HPDE instructor will catch it eventually.

That picture is good, as it is only some tubing and welding away from being a legal cage versus the use of the rear bulkhead as the floor for the cage. That design may be grandfathered with multiple organizations but will not be legal for very much longer.

Good luck.

What's changing in the rules that will make that design illegal? It looks a lot like the rear half of a Hard Dog, although the latter doesn't have the kink in the main hoop and is also tied to the seatbelt towers.

The only reason some Hard Dog bars use a bolt-in harness bar is for compatibility with a glass soft top window. The NA versions have a welded-in bar. Place it right and you're good.

The taller the bar is, the further forward it has to sit. This is the major compromise in Miata bar design.

In reply to JohnyHachi6:

If you have access to a Kinect, you can scan the area of the Miata and very easily mock up a bar in SolidWorks. You have a student license, right? If you have your final seat, you could get someone to scan you in the car to check for head clearances. FEA analysis is a cinch if you model it as a weldment. This could be a fun project, actually.

If you need any tips on how to do this, I'd love to be involved. I'm not too far away in Roswell, just a bit busy dealing with Exocet vendors and my other job.

I made my own sans harness bar, since I wasn't running a race seat. It's basically a copy of a Spec Miata back half.

Took a few hours, but I like it much more than many of the bolt-in options.

The tricky part is the two-part bend in the main hoop to bring it in and forward at the same time. That was fun to figure out.

Thanks for the replies guys.

Warren, after I get the seat brackets made up, maybe I could drive out to Roswell some time and you could show me how the Kinect scanner works. I've gone through some of the video demonstrations but haven't messed with one myself yet.

JohnyHachi6 wrote: Picked up a torsen and some 1.8 brake parts at the local pull-a-part for another $100.

Whoa Whoa Whoa..

Where on earth did you get a Miata Torsen and 1.8L brake parts for $100?

nocones wrote:

JohnyHachi6 wrote: Picked up a torsen and some 1.8 brake parts at the local pull-a-part for another $100.

Whoa Whoa Whoa.. Where on earth did you get a Miata Torsen and 1.8L brake parts for $100?

The local Pull-A-Part (it might be my favorite place evar).
Here's what was on my receipt:

P/N 149 - REAR END W/O DRUMS - $70.93 (including core charge)
P/N 207 - BRAKE PROPORTIONING VALVE - $7.14

They included the driveshaft (edit: and axles) with the rear end for some reason and they didn't charge me for the caliper brackets.

Don't worry though, I don't have any plans to make this a Challenge car, haha.

I also picked up a carrier from a Kia Sportage and swapped over the ring and pinion. Just the carrier was around $50. So, if you count that I spent closer to $150 after tax.

If you find another one of those I'll gladly pay for it, shipping, and a bonus for your troubles.

^ Haha, yeah I've been getting that a lot.

I saw this posted up on the junkyard's website near closing time on a Friday and got up at 6:00 AM on Saturday to go pull the parts. Out of all the Pull-A-Part yards I check in Atlanta, I've only seen 3 miatas come in over the 5 years I've lived here and other than this one, they've all been essentially tubs by the time I got to them. This was just crazy luck.

JohnyHachi6 wrote: Thanks for the replies guys. Warren, after I get the seat brackets made up, maybe I could drive out to Roswell some time and you could show me how the Kinect scanner works. I've gone through some of the video demonstrations but haven't messed with one myself yet.

If you can find me a fountain of goddamn $70 Torsens, and I'll design, FEA test, and fab up your damn rollbar for free. :)

Come on up and get scanned! Don't expect a grand tour; we're still waiting on our lasered steel to build our jig table and first run of chassis, so it feels a little empty and spartan right now. Gimme a heads-up on Facebook or something, I live right by the shop.

I'd love to show someone the process, I'm getting some really, really cool results. Haven't had any time to clean or parameterize them up, but here are some of the scans I've done. The measurements check out, you can import this into SolidWorks with little to no overall distortion! It doesn't spit out data as nice as the NextEngine scanner that the Invention Studio has, but it can do large areas in one scan.

This was done in very dim light, lots of noise, but still solid packaging/visualization data.

This is wiggling the sensor inside an empty engine bay. Very little light, tons of noise.

This is when I could control the lighting.

And a FaceTube video: http://www.youtube.com/watch?v=QtgrcV77iOo

I think I want to use the Atlanta! Meme on you for offering Free 3d scanning to people and being so far away.. I would love to get some things 3d scanned for future use but that is just to far to go. Also would like to visit your operation once it's up an running.

nocones wrote: I think I want to use the Atlanta! Meme on you for offering Free 3d scanning to people and being so far away.. I would love to get some things 3d scanned for future use but that is just to far to go. Also would like to visit your operation once it's up an running.

It's not magic or even difficult, it's just a little fine tuning with a few different Xbox Kinect scanning programs. I learned how to do it on YouTube and just playing around with it. All you need is a Kinect and a computer.

Demo with software info: http://www.youtube.com/watch?v=ZZrmPLas66E

So I installed one of my Momo seats in the car today and spent a couple hours taking some measurements and fabricating a mock up bar out of electrical conduit. No matter how much I tried, I could not get a setup that would clear the soft top and pass the broomstick test by any reasonable margin. If I slouched a little in the seat it was possible to stay low enough with a helmet on, but in the position that I would realistically drive in it wasn't happening. I am 6 ft tall, but didn't think it would be that tough considering the bare-bones composite seat which sits so close to the floor. I guess I could pick up another inch maybe by getting rid of the slider, but the plan is to have the GF drive some HPDEs in this car too, so I'm not super keen on that (she's quite a bit shorter and wouldn't be able to reach the pedals).

Anyone have suggestions? Is there any more room under a hard top for a taller bar? (I've read a little about this a little bit on the Miata forums and it doesn't sound like there is.)

I too have recently figured this out, the same issues as you have. My daughter and I have a Harddog M2 bar in our 99 Miata. Just installed Corbeau FX1 seats on double locking sliders, I'm 5'11" and she's 5'5" and we plan on co-driving together. Almost seems that with the Corbeau's fixed back we actually sit taller in the car and my head is just below the convertible top. Haven't tried it with a helmet on yet but pretty sure the helmet will touch the top. Hardtop isn't in the picture right now. I have seen taller rollbars in Miata's but not with the top up. New a local guy that had a tall bar he had to drop the top then install the bar. He removed the bar for daily driving with the top up and raced with the bar bolted in and top down.

Don't think there is any more room with a hardtop, just that the hardtop is more rigid than a soft top and doesn't collapse the same.

JohnyHachi6 wrote: I guess I could pick up another inch maybe by getting rid of the slider, but the plan is to have the GF drive some HPDEs in this car too, so I'm not super keen on that (she's quite a bit shorter and wouldn't be able to reach the pedals). Anyone have suggestions? Is there any more room under a hard top for a taller bar? (I've read a little about this a little bit on the Miata forums and it doesn't sound like there is.)

find a taller gf

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fasted58 wrote: find a taller gf .02

Ha ha. She'll like that one.