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Sorry this is a long post with a ton of questions in it. Probably gonna start a **** storm with it, but I have questions and I'm not finding the answers and I have read a TON on this subject. This seems like a black art more than science.
Before we get into questions...Let's go over some background:
I'll be hiring someone to build this cage. I'm asking all of the questions so I'm more informed, know if I'm hiring someone who is doing the job right. (Note...everyone around here has recommend Robinson Racing to me, so most likely will be using them)
My car is only used for HPDE at the moment, but would like to make certain it can be used in both NASA and SCCA in the future if I go that direction or sell the car
I will have a drop floor installed at the same time.
Car is significantly faster than a stock Miata (~300 hp at wheels)
Passengers will ride in the car.
I want this car to be safe, and I'm willing to pay more for safety, but I'm also not looking for a baller cage that gets ogled at.
Let's assume we are starting with a basic 8 pt. Spec Miata cage and modifying from there. Since I would consider this the bare minimum for a cage. (NOTE: I stole a lot of pictures off the web for my questions. I apologize in advance if I'm breaking any rules here and will delete if needs be.) So this would be the most basic design that we are modifying from.
Thoughts / Questions:
Main Hoop -- Shelf or Floor attachment? I figured I'd leave this up to the fabricator / once we get my seat where it needs to be, but is one safer / preferred? I see a lot built on the shelf
Main Hoop Rear Braces - Single bar, X bar, or V bar? Guessing X is strongest (4 triangles vs 2 or 3).
I like this V bar that Moti built, it looks simpler then an X and I really like how the roof bars work well for a passenger and tie into this spot too.
Door Bars -- Let the insanity begin. This seems to be the one place where everyone has their own idea and no real consistency between any cage. I've seen some super crazy ****. I'm leaning towards X bar below, but open to suggestions on best for safety and getting in and out of the car. (My right knee doesn't bend as far as a normal knee anymore, so egress is a concern too.)
First off...I know I don't want an "S" Curve. But what about proper NASCAR bars?
This seems to be a common NASCAR bar set up for the Miata...with a basically straight lower bar and the top bar leaning out to help with a little crush factor. (Small pic sorry)
I like that this "X" door bar is in compression and that it ties into the rocker panel too. (Also like that it looks a bit easier to get in and out, but I could be wrong about this)
This looks safest of all, nascar door bars built properly, attached to the rocker and an x-bar behind it....but this looks a little bit like overkill? I also don't see it being easy to get in or out of the car with this setup.
Forward Hoops -- So I've seen a lot of different designs for these too. First off...where is best for these to come down in the car, closer to the door, or closer to the firewall? Or is this something that the door bars dictate? Additionally, I know the spec miata is just the bar, but I've seen extra support given to the bars in many different ways. Guessing this would be a wise thing to add. What direction is best here?
Here is a taco gusset added to the one curve. Also notice these forward hoops land around the door. This removes some of the egress
Here is a bar added for strength too. It extends down through the door bars
What about just connecting the bar to the A pillar? (although, the A pillar in a Miata is just barely stronger than a pool noodle, so not sure if that is much of a strength move? Also requires pulling out the windshield, but would leave more space for egress.) Vegas was done this way, and if you look closely it has smaller gussets added between the door bar and the down hoop bar.
Other items...
What else would you add to the cage to make it stronger / safer?
What else should I make certain is done while having the cage built?
Ensure hard top can still be bolted down or have that added to cage
Nets? Have that welded to cage at this time? (or just wait, that can be added later if needed?)
Main hoop position: Most Miata cage fabricators are moving towards putting the main hoop on the shelf to increase the room for the seat/driver. Putting the main hoop to the floor pan limits the room the seat can move back. Even MiataCage offers shelf top main hoops now.
On the door bars design, some of the decision depends how much weather the car will see. Door bars that tie directly into the sill isn't going to leave a lot of door skin and weather strip in place. My door bars are similar to what is shown in the white shell. I had the cage fabricator add a bar that runs inside door sill that the X support bar dies into. The white shell likely has the same thing, just not visible in the picture. Also keep in mind that NASCAR bars were designed for different types of impacts (generally more side-to-side) than road racing.
I would also suggest keeping the beam in the door. I had my fabricator push the X apex right to the door bar, so if there is an impact, the door bar will distribute some of the load before pushing into the X. Helps too for if you plan to go wheel-to-wheel and someone pushes in your door. Also sucks to close a door with no door beam.
Tie the a-pillar bars into the a-pillars. Miata door opening isnt large enough to really need the vertical support there. Land it as far forward as possible.
X for the door bars pushed into the door shell like the white NC pictured. Lightest. most room for the driver, easiest to get in and out.
People will always build cages how they want, but as far as I'm concerned; get it as far away from the passengers as possible, and make it as light as possible. Anything more is wasted weight.
@Midtenn & @Oh4One4 Thank you both for the info! Have a cat. This was exactly the type of info I was hoping to receive!
You all have sold me on the Main Hoop to the Shelf for more space. I'll ask for that.
X for the door bars was the design I liked best after my reading, so glad that is what others think too. (FYI...Not that concerned about weather, but appreciate that heads up...putting a cage in pretty much makes this a race car only at that point & it lives in the garage) I hadn't thought about keeping the bar in the door to help distribute the load.
I like the a-pillar bars landing more forward...that gives me a bit more room to get out & wasn't thinking how that makes the bar longer / easier to crush, but agree, the door span isn't that long in a Miata. Was hopping to not have to pull the windshield, but I like the idea of connecting the a-pillars to the bars on that side for more strength & if it requires removing the windshield, then so be it. That area is out of the way and gives more room to get in and out.
Again, thank you both. Great information you all shared!
Might be a little late to the party, but would like to add my two cents.
I originally went with a spec-miata style cage, which had 8 attachment points to the chassis, "NASCAR style" door bars, and a straight roof bar. I was less than happy with that style of cage, and the chassis not nearly as stiff as I expected.
I then had the front half cut out, and switched over to a "FIA style" cage, which tied in the A-pillar, rockers, C-pillar and added various gussets. The difference was very noticeable, attaching the cage to the front half of the car, and the chassis was much more responsive and can only imagine that the safety factor has improved significantly. Seeing that you have 300hp you will be traveling at a much higher rate of speed comparatively to a SM, the cage should reflect that. Egress and ingress is quiet easy with the x door bars, but I have noticed that with the cross bar on the main hoop that it severely limits the height of the passenger and something to keep in mind.
+1 on the drop floor pan, and in my opinion is one the best modifications I have done from an ergonomic standpoint.
Final thoughts would be to check through the rulebooks on which class you would potentially enter the car in, many have rules limiting the number of attachment points to the chassis, and for potential resale value, ( although who are we kidding, modifying cars is giant waste of money).
Bryan makes a great point, If you plan to ever enter the car in a series, the cage needs to match. Formula Dunce for example, does not allow you to pass a bar through the firewall or tie into the door sill. It blows my mind. Chump car, charges points for anywhere you connect the cage other than the primary six mounting points. Good to keep in mind.
Also for what its worth, if a windshield is available for a car I'm building a cage for, that windshield is coming out. Easier to fit everything, and I can paint all the way around the tubes.
1. A full cage has no place in a street car.
Any full cage is going to remove stock harnesses, which work in conjunction w airbag. Unless you are willing to use a containment seat, 6 points, a HANS and helmet on the street?
2. The cage limitations for Spec Miata are outdated and need to be revised. Wrecks have shown a modern FIA cage better protects driver. The whole idea of not allowing cage to go fwd of firewall is outdated and limits cage integrity. The argument not to tie in the frame and suspension is just stupid.
3. Do everything possible to protect legs. Side impacts and debris intrusion have caused a bunch of injuries.
4. I have seen way too many crappy welding jobs with crummy capabilities. Using wrong gauge and alloy wire, too low amperage TIG penetration w wrong fill, crummy design tie in plates, ext. ext..... Find a good cage builder that welds tube regularly and has proper bending equipment..
5. Make sure seat drop pan is thick enough and reinforced to take a 30 G impact. Its integral to safety cell.
6. Remember how far harnesses and seat flex in a huge impact.
Another version of the "FIA" style A pillar in @Savington 's Rover. TCDesign in NorCal did the original SpecMiata minimum cage a decade ago and this round of upgrades a couple years ago.
I've driven this car and the extra windshield and doorsill attachments make a big difference in rigidity. It also has a triangle to the firewall gusset and my car only has a single bar continuing the top door bar to the firewall. I would do my next chassis like this, WITHOUT going through the firewall due to current classing penalties in NASA ST/TT.
The FIA bar does affect ingress/egress negatively, in a big way compared to my NASCAR bars.
A lot of experienced people have answered your questions, but I just wanted to throw out another option for your cage builder since I see you are in the Southeast: Alex Phelps of SharkCage Fabrication in North Carolina. He has been building Miata cages literally his whole life as his dad was the original owner/creator of Hard Dog. He does a ton of cages and is super chill and easy to talk to. You can find Shark Cage on Facebook and he will gladly answer any Miata-specific cage questions you have.
He build my cage and I personally went with the X style door bars with no FIA down bar. I do not currently compete in W2W, only TT, so I know I can go back and add on later if needed.
Lots of very good input in this thread. I'm not going to pretend to be a genius about the design of cages, but I do tech inspections for a national org and have a couple years of experience reviewing stuff that rolls through tech. From that perspective:
One of the most common cage issues that'll fail inspection is lack of proper padding. Anywhere a part of your body can hit the cage while strapped in must have SFI padding on it. In a small cockpit like the Miata this can become an issue if you go crazy with the number of bars. Don't just think about tube diameter, think about tube diameter plus padding thickness. And yes, it really must be SFI.
Consider the weight/speed of the other cars on track when deciding on tubing size. It's one thing to be dicing it up with other 2400lb Miatas all running 89whp. It's a whole different ball game to be running in an open class that has higher closing speeds from heavier cars, or heavy, slow cars with inexperienced drivers, etc. One of the fast TTU guys in my region loves to recount the woeful tale of the BMW that smacked into his stock car. BMW was trashed, stock car continued the weekend unfazed. All hail the immovable object.
Make sure that your builder actually knows how to weld. I'm glad you're not considering welding this yourself. There's one cage we keep rejecting because the owner had never welded before assembling his own cage. Colossal waste of time and materials. We occasionally find a missed weld or part of one, particularly where it's difficult to access the entire tube circumference. Experienced builders know how to use order of operations, non-structural holes, etc to turn an impossible weld into a completed weld. Your builder's incomplete weld will not pass tech just because they thought it was too hard to do the full 360.
Other than that, read the regulations for whichever rule set you intend to meet and make darn sure your cage checks all those boxes. After that, get a log book issued, whether you intend to go wheel-to-wheel or not. NASA will do a HPDE/TT-only log book. Make sure the cage number or log book number is clearly stamped on the cage and know where you stamped it. Keep the log book with the car and bring it to every event, especially when running with a new org. The log book tells the inspector things like tubing diameter and thickness for the primary bars, saving us the pain of having to determine it ourselves. I think we're now doing non-invasive measurements but the fallback is to drill holes in your cage. I personally hate doing this and it puts me in a foul mood when I have to drill into your cage on Saturday morning while you're breathing down my neck about making practice/qualifying. It's a lot easier for us to look at the measurements recorded in another org's log book, correlate those to what we see in person, and issue you our log book than for us to get out the measuring equipment and start at square one. You still have to do the inspection for that first log book, but having that available really speeds things up for subsequent inspections, even with other groups.
Thank you all for the info! Very informative and super helpful. Cats to everyone!
@Bryan Z. Thanks for the info and the photos. Huge help seeing your car with the two different versions / set ups. Your V2 is pretty close to what I'm picturing for my car. Thanks for the input!
@Oh4One4 & @Bryan Z. This is HPDE for now, but definitely don't want to build a cage that can't be used later. I think it's a bit silly that "over built" cages are not allowed in certain groups / classes, we are talking safety here, not performance. Anyway, the plan is just what @doward was stating he will do next...Build an FIA type cage and to NASA specs not past the firewall (which I believe will make the car legal for most groups).
@Blkbrd69 Thanks for the info! Car is strictly "race car" and will not be street driven. I will have a professional race chassis / cage builder building the cage with 30+ years experience with all the proper equipment / knows how to weld a tube together. He was recommended by multiple people in the ATL area (actually only person people recommended to me.) My welding is crap on a good day, and want this done right / safe... I.E. Buy once cry once type of item vs doing over.
@doward Thank you. I asked for the triangulated bars to the firewall earlier today. I hadn't seen / noticed that before, but saw it in @Bryan Z. photo and then you wrote about it too. I seam welded all around the door and was shocked at how much that stiffened up the windshield and the car in total, so I can definitely see how welding up to the A pillar would make a huge difference. I'm unsure about the A pillar support bar and ingress / egress. I'm thinking about asking for just gusset there like @Bryan Z. has on his car. I have no doubt the bar is stronger, but I am concerned about getting into and out of the car. FIA rules states you don't need that bar if you don't go more than 200mm (7.5") out past the top bend before going down. I'll ask about keeping that bar 7.5" or shorter and just have the gusset added.
@Quigs Thanks for the recommendation. Finding a cage builder is a bit harder than I thought it should be. I've already started working with Berry at Robinson Racing here in Atlanta area, and feel confident from the recommendations I'm in good hands, otherwise I would be calling up Alex.
@thebeerbaron I can't believe how often I see pool noodles or hot water tube insulation used as roll bar padding. I shake my head in amazement when I see that wondering what people are thinking...Not only is it too soft to do any good, in a fire that's going to drip right down on you...Ouch! I agree 100% with the proper SFI padding. That's what I currently have on my hard dog roll bar, and will be going on this cage when done. I like your comment about keeping tube diameter plus padding thickness in mind. Its hard to get the padding on and the hardtop to fit proper. My top doesn't actually seal at the bottom due to the padding, so I already see what you are talking about.
You also stated to consider the speed / weight of other cars when deciding upon tube size. Funny enough, that's one of the reasons for the cage. I know car on car crime is rare in HPDE, but I have seen a Camaro run over a Miata before. Camaro driver walked away, Miata driver spent a month in the hospital.
Log book info is really informative. Never thought about that. Good info & Thank you!
All, Thanks very much for the info. Dropping the car off to have cage & drop floor done next Friday. (got to gut the doors & pull the windshield first.) I spoke today to the builder and he was good with all the items I asked about / wanted. He understood that I'm looking for something stronger than a Spec Miata cage and agreed with the FIA cage people have recommend. Thank you all for the info, it helped confirm some changes that I was thinking about and also informed me of others that I needed. Appreciate you all taking the time to respond!
Definitely attach main hoop to shelf, not floor.
I chose the FIA style A pillar reinforcement and X door bars for egress. In my particular case being very tall, my elbows hit traditional style NASCAR door bars. The X puts them a tad lower where my elbow swings out.
At 300whp, I'd insist on extending cage to shock towers. The Miata was designed for a maybe single 50mph frontal impact with all the OEM bumper beam and crash structure in place. Think about a 90mph hit with all that stuff missing.
If extending cage to shock towers, its' anchored to firewall section of cage which then transmits loads into A pillars and door. So making the firewall and door hoop area beefier makes sense. That junction is the drivers primary protection against intrusion. Note that the extensions on Vegas are each triangulated.
For passenger side, we built our harness bar straight and just shifted the whole main hoop backwards. Most cages have a 2 piece harness bar thats set back for the driver but far forward on the pax side. That forces the seat forward which reduces safety buffer for big passengers. I wanted the pax seat to be as far back as possible. Short passengers might not be able to see out bug big guys will be safer.
As far as attachment points go, go crazy. The more the better. In an FIA style cage, the cage is far stiffer and crumple resistant than the tub itself.
Regarding weight, ignore that factor. No one will care that you saved 20 lbs when you're getting an expensive helicopter ride from the track. Bullet had 400whp , big aero and essentially a good Spec Miata style 6pt cage built by Renderos Racing. I could still feel that car flex at full power in esses. Understeering when the boost hit and you could feel the steering column move laterally. Vegas' cage probably weighs 50 lbs more and the car feels like a chunk of billet. Enough stiffer to require slightly more rear spring to make it turn. That's a good thing.
I like your comment about keeping tube diameter plus padding thickness in mind. Its hard to get the padding on and the hardtop to fit proper. My top doesn't actually seal at the bottom due to the padding, so I already see what you are talking about.
The padding/top interference is a uniquely streetable-roll-bar quirk. Since the cage doesn't have to work with stock seats, the soft top, or the hard top side latches, the main hoop is positioned differently. It's a totally different animal than even a high-quality street bar.
I should state that the primary places people run out of room for padding has to do with the arms/hands and feet. Yes, you need roll bar padding where your feet could hit the cage. And you need it where your hands are (likely) to bash into the cage should you go for a ride. The other one is if one of your overhead tubes comes near your helmet - if you go crazy with the tubes and you don't take driver height/position into consideration, you're going to have a bad time. Your drop floor should help here, but think about where your head goes.
There is "low profile" SFI padding that can get your out of a tight spot, pun fully intended. Also, make sure the padding is in good condition (ie, don't reuse from your current roll bar). Like most things, it deteriorates over time and loses effectiveness. I'm not saying this for you really, but for anyone else who's reading and might think "oh yeah, my padding is starting to look a little rough".
NASA GCR for all classes: Gridlife and Champcar are similar if not default to .120" wall.
1501 - 2500 lbs 1.500” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only)
2501 - 3000 lbs 1.500” x 0.120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) 1.750” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only)
Swapped and turbo miatas with oil coolers very easily reach that 2500lb breakpoint with driver. The GCR rules are technically quoted as car only, no driver, but that's not a technicality worth gambling on.
If I was building today, I'd go straight to 1.5x .120" tube. My current cage is .095" and lacks a roof diagonal, only single bar footwell/firewall attachments and no doorsill or A pillar attachment. Times have changed since I commissioned my cage, for the better.
Fitting 1.75" tubing into a Miata is not fun. Ask my cage builder. Currently fitting a way to big seat w/ roll bar padding and everything is getting tight fast. I don't have a floor drop in my car, but it maybe in my future.
06-28-2022, 12:30 AM
Roll Cage Questions
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