cravenju

I wish I would have started this thread a year ago, but I was busy building the car. It is currently in the garage with a low mile built engine, awaiting a new wiring harness and the rest of the turbo setup that I need to piece together.

Teaser photo of the "finished" state of the car:



On 3/28/2016, my colleague, Kyle, and I picked up a $600 miata with 208K miles on the clock, plenty of rust, and didn't make any power over 4k rpm. We planned to do something with it for our Undergrad Capstone project, but the path forward wasn't clear.




After tossing around ideas, we decided to build a "ghettocet", but with a little more engineering to support the design. We were inspired by the excocet, the OG ghettocet, locosts, and nickt93's build thread on his own eliminator. Our problem statement was accepted and I got us a sweet hookup on a high bay space at the schools lab building, where I worked.

Moving the unregistered, uninsured, door-less, and exhaust-less miata to her new home:



In order to ensure that our final product satisfied the community, we brought together a small sample size of miata owners to survey and gather data. Most of our design parameters were driven by this group of individuals. Through their input, we came to the conclusion that using the stock miata suspension, sub frames, and floor pan was the best route to go. Before we could start any sort of design stage, we had to cut the life out of the tub. Somewhere in this process, we added a third member, Morgan, to our group. The bad thing about Momo was that he wasn't a miata owner. The good thing about Momo was that he was Surprisingly, cutting the down to a workable shape is easier than it sounds. We wound up using maybe 20 reciprocating saw blades and a handful of cutting wheels. One thing I will note in this step is that precision doesn't need to happen in one pass. Make rough cuts and work your way down. This added a little time to do, but still only took 10-15 hours of actual work to complete.



Inb4 anybody gets mad about us "ruining" a perfectly good shell, take a look at the passenger side of the trunk. That hole went through the rear quarter, the trunk, and to the ground and was big enough to fit your arm in there...


Rough cuts are rough.



This was my favorite part of the "salvage" process. Removing the 60+lbs windshield and frame only makes room for a less space-limiting cage.



Wiring on these cars is extremely simple when you plan to delete everything that doesn't make the car go. No real sense in marking connections, because the whole thing is going to get chopped.



Detail shot of the rear shock/subframe mounting locations, which will be getting reinforced.



Second favorite part of this process: Discovering our rocker rust issue was minimal enough to just sad down and paint, later.



Nearly completed "floor pan". Morgan also spent 5-10 hours on removing seam sealer and paint from major areas. A painful job, that.

cravenju

Once the tub was cut down, we jumped straight into the design of the cage. We split our work up into three main parts: Main Cage(me), Front Core(Kyle), and Existing structures & Underbody Bracing(Momo). More on these to come.

Main Cage:
When you are designing a roll cage, it is extremely important to understand a few major guidelines.
- Manufacturing: The most important part of designing a cage for manufacturing is to understand the materials you are working with, the tube bender you'll be using, and the constraints that come with both of these. Your average draw bender will have it's limitations regarding the amount of tube that needs to precede any bend. This is because the beginning of the bend is always a set distance from the "lock" on the bending die. This limits you on the distance between two bends and must be considered before you waste hours designing. The material selection will also drive the choice of your die.
- Regulations: Surprisingly, there are only a handful of written rules for cage design out there. Depending on your sanctioning body, you could be severely limited on your creativity. For example, we were limited to 4 bends maximum on our main hoop. Our diagonal brace had to be co-planar to the main hoop and it had to terminate within 12 inches of the driver's corner of the main hoop. The worst scenario in cage building is that you show up to race and you don't pass inspection.
- Materials: With FEA software, it's simple to determine the best materials/dimensions that you should use. However, our sanctioning body requires a minimum of a DOM mild steel 1.5"OD x 0.095" wall for a car with a weight from 1500-2500lbs. Our goal was 1700lbs dry and 1900lbs with a driver and gear.

Base Cage designs with multiple additional members to test effectiveness.


7g Vertical load on driver’s corner for comparison to other designs.


Results of 7g Vertical load on driver’s corner.


Torsional load on middle and rear mounting points for comparison to other designs


Results of Torsional load on middle and rear mounting points

At this point, we decided to can upper X brace. An overall added weight of 19% and raised the center of gravity by a few inches. Also, the overhead clutter was just too much to bare.

Main Hoop Vertical Load (lbf)
𝐹=7.5∗𝑊
𝑊ℎ𝑒𝑟𝑒, 𝑊=𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐶𝑎𝑟+150𝑘𝑔
Max Stress = 32.6ksi


Rate of Deceleration (ft/s2)
𝑉_𝑓^2=𝑉_𝑖^2+2𝑎𝑥
15mph rear impact with tire wall – Max Stress = 31.8ksi

With a rollover - worst case - stress of 32.6ksi, we were happy to pick a 1020/1026 DOM material in a 1.5" x 0.095" size. 1020/1026 is rated to a Yield Strength of 60/70ksi and a Tensile Stength of 70/80ksi. That's a safety factor of nearly 2 and I am happy with that.

More to come...