28 replies to this topic

[Protech Racing]

I am getting ready to build the 99. I have built/ serviced lots of tube and pan cars, cars in the past 25yrs..
One of the Miatas that we run/service, has crossed rear stays and I like the way it vectors the load and helps with preventing side deflection. This car has a note from SCCA stating that the crossed rear stays are fine and used in Miata/ Mini etc. Not a prblem.
I have only seen a few Miata and few more Minis built this way.
I see that NASA does not like the crossed stays/ hoop supports, .

Why is that? and how can I get it looked at more closely? The fit better, are stronger .
If some one has any data showing a fault with the crossed stay, I will build with straight stays. But , I am real sure that you will not.
Thanks, MM
This hoop will be a hybrid /tall boy cage. The driver side mounts on the upper pad and the pas side mounts on the floor/corner/sill .

[Jim Boemler]

I'm assuming you're talking about a single main hoop, right (as opposed to the two individual hoops that were sometimes used back in the 60's)? Now, when you talked about "crossed stays", are you talking about an X between two rearward supports, or are you talking about using the X instead of two rearward supports?

If it's the former, there's no reason not to do it, and it's perfectly legal. If it's the latter, it's not legal, because it's not strong.

[Protech Racing]

Crossed rear stays only. Just as strong as straight on any load test,( That I can find.) Can you find any other results?.
The total angle is about 42* off center. Not the same on both sides tho as I run one over the other instead of welding them together.
Thanks, MM

[Jim Boemler]

Consider what happens when a large force (car hitting the ground upside-down) is applied to one corner of the main hoop, in a fore-aft direction. With braces straight back, the force applied is resisted by a tube loaded in compression. With crossed braces (only), the center of the X becomes a fulcrum, around which the main hoop can pivot. In other words, strength in that configuration depends on bending rather than compression, making it weaker. Dunno about your "note" from SCCA, but the SCCA rule book specifically requires those side braces, just like NASA (which copied them from SCCA anyway). Why not just build your car to the rules?

[Bench Racer]

Mike, along with the SCCA rules Jim offered please see Jim's profile occupation.

I am in total agreement with Jim's analysis. Me being a retired..........................

[Keith Novak]

I agree with the geek and the retired geek.

You have to be a bit careful about how you look at structural tests. They are a bit of an idealized situation and what load case(s) were they testing? I could see it being stronger for some load cases but I also see enough potential structural problems with the design though that before I said it was as good or better than the traditional way, I’d want to investigate several different load cases to see how it compares in different directions and also pick tubes to remove to simulate cases where the cage gets damaged in an initial impact and then has more work to do.

[davew]

Please remember that if a roll cage does not pass tech, you do not get on the track, PERIOD.

There are some differences between SCCA and NASA, read both rulebooks very carefully. Do not interprit. Do not say "this is better". Follow the exact letter of the rule. Or you may be going home and doing it again.

Dave

[dstevens]

Crossed rear stays only. Just as strong as straight on any load test,( That I can find.) Can you find any other results?.
The total angle is about 42* off center. Not the same on both sides tho as I run one over the other instead of welding them together.
Thanks, MM

I'd like to see those tests as all the sanctions I've seen cage rules for or build or consulted (now I'm starting to do some off road work) require straight down tubes on the main hoop. Even using simple vector math you can see what a potential difference might be as explained in Jim's post above. I seriously doubt SCCA or NASA have done exhaustive tests on the configuration under controlled situations, but I know for a fact that NASCAR and NHRA have and I'm lead to believe that the FIA does as well. All require straight down tubes, usually with a minimum angle requirement on the main hoop.

But what the guys are saying is the bottom line. The car has to pass tech. It's cool to geek out on the engineering aspect but when the rubber hits the road (literally) it's what the sanction calls in the rules and not so much what the fabricator thinks. It's too bad that corner-carvers.com has closed registration for the forum to the public but there are still several guys with lots of fab experience in a variety of race machines.

[Glenn]

I'd like to see those tests as all the sanctions I've seen cage rules for or build or consulted (now I'm starting to do some off road work) require straight down tubes on the main hoop. Even using simple vector math you can see what a potential difference might be as explained in Jim's post above. I seriously doubt SCCA or NASA have done exhaustive tests on the configuration under controlled situations, but I know for a fact that NASCAR and NHRA have and I'm lead to believe that the FIA does as well. All require straight down tubes, usually with a minimum angle requirement on the main hoop.

But what the guys are saying is the bottom line. The car has to pass tech. It's cool to geek out on the engineering aspect but when the rubber hits the road (literally) it's what the sanction calls in the rules and not so much what the fabricator thinks. It's too bad that corner-carvers.com has closed registration for the forum to the public but there are still several guys with lots of fab experience in a variety of race machines.

Look at the video of Dave Pintaric's Viper @ RA last weekend. Thats the cage I want!

[rscottp]

I bought an aged Miata in January that had VIR and Summit Point logbooks for NASA in '05 and '06, and SCCA for subsequent years until '10. In April took it to a NASA tech shop and the cage was ruled noncompliant because of way to many bends in the main hoop and elsewhere. Then took a shot at the April NASA Mid-Ohio event and was turned down again, this time by two techs who found more issues than the first tech. Obviously, no racing. Any thoughts on this? How lax is tech on the east coast?

[Jim Boemler]

Pictures?

[dstevens]

I bought an aged Miata in January that had VIR and Summit Point logbooks for NASA in '05 and '06, and SCCA for subsequent years until '10. In April took it to a NASA tech shop and the cage was ruled noncompliant because of way to many bends in the main hoop and elsewhere. Then took a shot at the April NASA Mid-Ohio event and was turned down again, this time by two techs who found more issues than the first tech. Obviously, no racing. Any thoughts on this? How lax is tech on the east coast?

What were the other issues? You need to cut it out and have a compliant cage installed. Max number of bends for any piece of tubing particularly the hoop is common. Could be that when that cage was installed it met the spec and now it's past the grandfather date. The are several shops in that neck of the woods. Call them and talk to them.

[Jim Boemler]

I don't think the bend rules have changed during that period, but nothing says the original inspectors had their heads screwed on straight. If two inspectors have found problems, they're probably real. Post some pictures, and people can weigh in. I certainly wouldn't count on anybody being lax enough to let it go.

[Jason J Ball]

I bought an aged Miata in January that had VIR and Summit Point logbooks for NASA in '05 and '06, and SCCA for subsequent years until '10. In April took it to a NASA tech shop and the cage was ruled noncompliant because of way to many bends in the main hoop and elsewhere. Then took a shot at the April NASA Mid-Ohio event and was turned down again, this time by two techs who found more issues than the first tech. Obviously, no racing. Any thoughts on this? How lax is tech on the east coast?

If I was turned down by multiple tech inspections on any cage, I'd want it fixed before I ever got behind the wheel on a race track. What's the cost of fixing the cage compared to a race weekend, 4-5 weekends. What's more important than that is the cost of a remediations or replacement of that cage vs. your life??? Easy decision for me! IMHO Stop looking for a tech that will pass it and get it fixed! Then go racing

[Protech Racing]

I have run the data in simple vector mode and later in Solidworks. 30-42* angle on the rear cage stays are much better in any non direct vertical hit.
The best description for two parallel side braces . pushing over two bicycles with a diagonal brace between them.
The description for the 30*-40 angle stays; Pushing over two tri pods, with additional braces beween them.
The X bar bending considerations are valid to a point. The first failure point in all of these is the floor mount( my area of patent research).
The highest cage failure point is the floor pads.( punch through, happens with the small SCCA pads)
The next is lack of dash bar, and ripping out the driver side floor pad, injuring the driver.
Many cage failures are side to side ,ripping out the floor mounts. There are few tubing failures.
That is why my pads are "soft: crush boxes.They will deflect , taking some angle, loading the whole car before ripping the mounting welds. I add angles every where also. 90 degree joints are very weak, adding a few degrees make it a lot stiffer.
I will try to get a version of the solid works from my Son, that I can shoot a frame of.
SCCA, NASA have no data. . IMHO. If they did, they would have taken my engineering suggestion 8yrs ago.

[dstevens]

SCCA, NASA have no data. . IMHO. If they did, they would have taken my engineering suggestion 8yrs ago.

But the point remains, if they don't give you a book, you can't race. Send me a copy of your Solidworks file. Last I checked I still have a seat on our network for Solidworks.

[KentCarter]

The SCCA/NASA cage designs follow from work the FIA did some years back. You can still find the documents on the FIA website. In fact, in lieu of the 'standard designs', you can submit testing data that alternatives can withstand the specified forces in the prescribed planes.

I think you'd be really hard pressed to explain the physics of how a diagonal X bar as a support against rearward movement of the hoop after separation of the front mounting points is better than bars that that go straight back at the appropriate angle to the pan of the car.

[Keith Novak]

I have a pretty strong background in structures but it's been so long since I've done any modeling you'd have a hard time explaining it to me convincingly enough that I'd sign my name saying it's as safe or safer than a standard design. I'd need input from the guys who do nothing else, get paid well and take a lot of time before they approve anything.

When it comes down to brass tacks it's not only about how much you know that determines whether or not you pass tech. There's also the guy on the other end who may not know what the heck you're talking about or may doubt your methods. I've thought about how I could argue my preferred choice in socks are better protection than nomex and explain my background and patents in fireworthy materials but if they tell me I either change my socks or I can't race, I'm changing my socks.

[Protech Racing]

The Solid works wont give any valid data for the floor rip out values. Does anyone have any known good values?
I ran a bunch of real world testing for the floor crush boxes, showed that 360* welding of flat floor plates is very weak. 1in stich welding is much better.
How do I get a valid , typical floor plate rip out value?
The rear bars should be less than 42*, any more and they spread the hoop. Not all bad actually.
All of the cage builders that I have approached just use the common method without any engineering programs. Please speak up if you actually have a program !!

Last , does anyone have any data for the shelf mounted hoop? I want to mount the next cage on the shelf for the driver side.
Thanks,MM

[KentCarter]

...
I ran a bunch of real world testing for the floor crush boxes, showed that 360* welding of flat floor plates is very weak. 1in stich welding is much better.
...

How much you wanna bet that a combination of stitch welding and plug welding is stronger still? Can you test that while you are at it?