NEHA

Is it OK to cut and modify the 1-piece tube that makes the hoop behind the seats? The factory seat was squishy enough to fit next to this tube, but the racing seat's fiberglass shell isn't squishy. Basically, I need to move this one tube back about 3" and make a different mounting plate to attach it to the chassis.

My thought was to cut the hoop above/behind the drivers head, install a more shapely vertical bar, and combine this new joint with the new roof-perimeter-bar/halo and gusset the hell out of it.

Thoughts? (Walter & Don especially)


The seat really wants to mount so it's back almost touches the rear bulkhead.



I'd really like to fit the vertical tube into the corner where the wires currently run, but I remember some rule about certain cage tubes being one piece.

Spec miata cages-and MX-5 cup cars all mount the main hoop up on the ledge.

I was going to say...maybe that ledge. I dont know how the Miata unibody is made up. How does that surface with the wire harnesses running across it tie into the sills below it? If it doesnt look substantial you might consider going through it and tie the bottom of the main hoop into the floor and the sills behind that vertical surface between the "ledge" and the floor. You could plate some the vertical surface on the back side, weld a foot plate at the bottom, then weld the hoop to the plating and foot. Maybe even tie that into the existing foot and a plate on the sill above it to help spread the load into the chassis.

That would give you more room for the seat.

sdwarf36 wrote:Spec miata cages-and MX-5 cup cars all mount the main hoop up on the ledge.

Thanks Sherman. I think I have an example of why that's the most common method.

walterclark wrote:I was going to say...maybe that ledge. I dont know how the Miata unibody is made up. How does that surface with the wire harnesses running across it tie into the sills below it? If it doesnt look substantial you might consider going through it and tie the bottom of the main hoop into the floor and the sills behind that vertical surface between the "ledge" and the floor. You could plate some the vertical surface on the back side, weld a foot plate at the bottom, then weld the hoop to the plating and foot. Maybe even tie that into the existing foot and a plate on the sill above it to help spread the load into the chassis.

That would give you more room for the seat.

Walter

The vertical corner where the wires lay is the intersection of the door jamb and the rear bulkhead, so that area is about as beefy as a Miata gets.

My main question is about the acceptability of modifying the existing tube, making it 2 pieces. I'm hoping it's OK for NEHA and probably OK for future Mt. Washington events. Plan B is to cut out the whole hoop and replace it.

Also, I found that I can't use the Sparco seat brackets as they are, the floor is 3" too narrow. (Yes, I tried the brackets both ways.) ;)

Is it OK to cut off the interfering section of the bracket and weld it to the transmission tunnel? Also, how about cutting the L off of the left bracket and welding it to the sill, or a heavy load-spreading-plate welded to the sill.


All

Because of the curve of the racing-seat-back, I think the new tube will fit when nestled into the corner. I'm hoping to create another attachment point by adding a plate to connect the vertical hoop tube to the place where the hoop lands in Sherman's picture.

Paul

The rule states that it is "recommended" that the main hoop be continuous, but not required. I dont understand why you would need to cut then rejoin it. Is it too wide to move back?

"Ea. Recommendations for collision protection
1. Bend radii at least 3x tubing diameter, and continuous main hoop(s)"

For Mt Washington it must be continuous. Probably true of other sanctioning bodies too.

Thanks Walter. If I move the whole hoop back, it hits the rear window. What I really need to do is leave the top of the U right where it is and rotate it's legs back.

The more I think about it, it may be best to make a new roll-hoop/B-pillar. Any rules about back-stays being continuous?

Paul

Tube bending 101: start with your longest bar first-that way when you have to scrap it, you can cut it up into other usable pieces.
Don't worry-you're not the first person thats had to scrap some pipe + start over.

That, right there, am whatcha call a fact.

It was my intent, when I codified (writ down and put in some order) , the rollcage spec, that we acknowledge reality. Reality is, that cages are often altered, repaired, "re-railed", "clipped", and even swapped into different bodies. We permit this, to a limited extent, by allowing reinforcing sleeves/gussets on tube joins (necessitated by the "2x tube circumfrence" weld spec), and not requiring "continuous" roll hoops.

If you ever wish to compete with another sanction, though, be aware - as Walter said, this is a very common rule, and the continuous hoop rule is OLD, in sports car circles. Which means that some have internailzed this principle so thoroughly, that it may be applied without an actual requirement - or cause me to be called a liar,(which I SO love) in front of an entire Series meeting, by somebody who has evolved into one of the Series' more vocal detractors.

If you need an odd length of 1026 DOM, I have an online order source ("Metals for a Steel") who sells short lengths of tube priced individually, with no cut charge.

Oh, yeah - if you mount the hoop to the "shelf", figure it out so's you can get a weld all the way around it.

Something I saw when I was researching tubing specs, in the Howe Racing catalog, in their updated "road race"/GTA /GT2 cage - noticed some interesting and innovative uses for "taco" gussets
Note the front cage legs / framerail interface using a taco, with 1 side trimmed to fit flat. The other is the "t" brace in the middle of the A pillar truss - which looks like a neat way to strengthen the A pillar (And props to Paul Tinguad, who used these in the lunatic Audi which preceded the Super Chicken.)