Calling all Roll cage experts!

I have a couple of sticks of 1 5/8 dom left over from my triumph cage that's been laying in my garage a few years and a tube bender if you want to rework some (all) of that. 1 5/8 x .125 is a little overkill for the weight of an exxie - but for a single hoop - that's perhaps a couple of pounds (1.93 lb/ft vs 1.62 lb/ft) - perhaps about 3 lbs difference for a single rear hoop.

there is your deal another GRM'r helping out. I have a couple Incomplete Cages here but with shipping you can go to the Steel Dealer Ps tubing is the easy part the bender is the catch .that and a Real welder(person) not the tool

Yup. Without steve and his tools and talent, our car would never have been caged.

Id suggest, before bending and welding gets started, looking at the rules for scca class above where you want to run, and nhra class step above. Whole lot easier to overbuild at this point.

In reply to Dusterbd13:

Well, in scca you can't step up from a-mod (unless they kick you out all together I guess), but quite a fair point.

Will add to my rules review.

Ok, updates!

first, I got the old top cage off. If this truly was built in 1978 or around that time like the date code on the fuel cell says, the fasteners were high quality. They all just unbolted.

The top piece weighs in at almost 40 lbs!

A little bit of careful reading of the rules makes it look like for both SCCA "A-mod" and NHRA "slower than 10 seconds" that a roll bar will be acceptable. Basic rules:

1. must be 3 inches taller than driver sitting in normal driving position
2. must be less than 6 inches behind driver's head
3. must be a single piece main hoop
4. needs 2 bars going back that attach less than 5 inches from the top of the bar and are at 30 deg or greater angle from vertical
5. needs 1 side bar going forward on the driver side, that passes the driver between shoulder and elbow height
6. there are rules for minimum thickness of attachment plates for welded in or bolted in, etc, and minimum size of tubes per vehicle weight
7. one sanctioning body requires horizontal bar, the other "recommends" a diagonal bar (really SCCA? WTF is a "recommended" rule?)
8. NHRA requires roll bar for cars between 10-11.5 et, and all convertibles faster than 13.5
9. SCCA requires roll bar for most mod and some prepared classes

TLDR - if you only need a roll bar, it really looks like if you are NHRA legal you are definitely SCCA legal. There are some technicalities, but that seems true for the most part. Note that NHRA requires SERIOUSLY bigger and thicker tube than SCCA, especially if your car is lighter than 2500 lbs.

NHRA:

So that is the plan. I will probably do the diagonal bar as well as the horizontal "A" for the added structure.

Question:

I will almost definitely do welded-in end plates. Is it better to terminate to the floor or to the "inner sill"? (In the picture below - mount to where the square tube mounts? or to the blue sill directly outboard of it?)

oldtin wrote: I have a couple of sticks of 1 5/8 dom left over from my triumph cage that's been laying in my garage a few years and a tube bender if you want to rework some (all) of that. 1 5/8 x .125 is a little overkill for the weight of an exxie - but for a single hoop - that's perhaps a couple of pounds (1.93 lb/ft vs 1.62 lb/ft) - perhaps about 3 lbs difference for a single rear hoop.

So, looks like NHRA requires 1 3/4" x .118". Which interestingly is almost EXACTLY the same weight per foot as the 1 5/8" x .125" tube you have.

Anyone know how the strength would compare for these two tubes? (Not that I expect NHRA to care, but it might be appealable for just the purposes of the challenge).

Robbie wrote: Anyone know how the strength would compare for these two tubes? (Not that I expect NHRA to care, but it might be appealable for just the purposes of the challenge).

I'm a software guy, not an ME, but AIUI the larger diameter the stronger it is for the same weight of material (well until you into tubes made out of foil or something stupid like that)

Just keep in mind for SCCA cages, the diagonal needs to go in first and be a continuous piece. The horizontals are the ones that install in two pieces. I would not worry about the NHRA rules. Car isn't going to be fast enough for them to scrutinize the cage. If you want to use larger tubing because the NHRA wants it, then fine, but SCCA will only want 1.5X.095 or X.120. Start with the SCCA diagrams and add NHRA. Its more sportscar than drag car. The advantage to smaller pipe is it takes up less room in an already small cockpit and you can make tighter bend radiuses. Minimum bend radius is 3 times the pipe diameter.

In reply to tr8todd:

I am ideally just thinking about a roll bar, not a full cage at this point.

I am also actually quite worried that for either sanctioning body if you have a roll bar, it must meet the rules, even if you are not fast enough/classed to require it. Is that not how NHRA works?

In reply to codrus:

Anecdotally, I agree, but I'd like to see a strength chart or something. I googled a bit last night unsuccessfully so far.

I may be completly wring, but IIRC larger diameter for the same wall thickness will be stiffer.

You can play wiht this calculator HERE I believe section modulus is the "stiffness"