Break my neck or Crush my skull?

[steve121]

FWIW, at least one of the sanctioning bodies (can't remember which one) is no longer allowing thinner walls with chromoly. Apparently they don't feel that it is that much stronger to allow the thinner wall. Take that for what it is, IMO I would deal with the extra weight to stay away from the price of chromoly tubing, especially in an open track car that isn't trying to be as competitive with everyone else. I am going to do the calculations later to see what the difference in strength to weight ratios for 4130 and 1018 1-3/4" tubing are, but that'll have to wait until I don't have other stuff to do at work

Tom,
If I remember correctly the SCCA changed the roll cage requirements in 2007 based on weight. Any car over 3000# must use .120 wall tubing irregardless of whether it's D.O.M. or chromoly.

 

[BlackBolt9]

I watched an episode of Myth Busters, where they compared/tested, DOM tubing, chromoly, and aluminum. Aluminum came in last. The chromoly was so much stronger, even with a thinner wall, it was a clear winner for strength. The DOM was not even close.

From what I have read. SCCA no longer allows roll bars for competition, period. They do allow seamless, DOM and ERW mild steel, and chromoly. They also allow thinnner wall tubing for the chromoly because it is stronger.

NASA will allow, for the weight range most of us fall into, seamless DOM or chromoly. The size tubing at our weight rating is the same.

So if we are talking safety, why not spend the extra couple hundred bucks on the stronger/safer metal. If you are doing mostly SCCA events then your cage/bar will be lighter. If you are doing NASA events then it will be stronger.

If we are not competing in any racing events, as some have mentioned, then you can adjust the size tubing and material to fit your needs/budget. Also, why would you need to save a little money on safety for an Agent 47 or Griggs suspension. They will only add seconds on to your time, and you're not racing, so seconds should not matter.

From my MDS, it appears that 4130 (commonly known as chromoly), has between 44% and 51% more tensile strength than 1018 (typical cheap steel). What I still need to do is calculate the difference wall thickness makes on the strength of 1-3/4" tubing and determine how much of a weight difference there is as well. I probably won't get to that until I get home tonight though, as I already have a program written on my home computer that will do the work for me :-D

Tom,
If I remember correctly the SCCA changed the roll cage requirements in 2007 based on weight. Any car over 3000# must use .120 wall tubing irregardless of whether it's D.O.M. or chromoly.

That's what I was thinking of, thanks Steve:beer:

 

[TroyV]

After some thought, I find myself agreeing BlackBolt9's earlier post.

With a 4pt bar, harness cross member, and diagonal, and assuming you have bought into the system approach that also includes harnesses and good fixed back seats, I think the danger of smacking your head off of the bar in a street incident is minimal. I think that changes a little if you have a full cage and there is tubing connecting the "A" pillar location to the main bar adjacent to the "B" pillar.

I'm embarrassed to say that I have none of that stuff, mainly because I really want to buy it all as a system...including a HANS, which I really believe needs to be part of that system. That is a serious chunk of cash....but a necessary one.

I have a Schroth 4pt harness that I use for autocross, which obviously allows for submarining. I know it is inadequate for what I am using the car for....absolutely where the track is concerned.

What we do is inherently dangerous, and since most of us are not getting paid to do it, it becomes an exercise of acceptable risk vs. your budget constraints. You really do have to have a good measure of self discipline to follow the rules you set for yourself and your driving limits.

That is to say....are you REALLY driving 7/10 when you are at an event and nose to tail with a buddy of relatively equal or better skill for 10 laps, or are you allowing yourself to bend the rule?

I don't mean to go off on a tangent here, but I think it is relevant to talk about the risks posed to ones self just solely based on your attitude and approach on track.

 

[black-plasma]

I just added a custom 4 pt. installed in my car and when I asked about 4130 he responded with the following;

> Yes, I can do 4130.. in fact I'm doing one right now BUT the problem is
> the two different rule sets (NHRA vs SCCA's). If I recall correctly-and
> I'll check to be sure- NHRA allows a thinner wall when using 4130 -thus
> the lighter weight- while the SCCA no longer allows the thinner walled
> tube. So, you end up with the same weight as a DOM Mild Steel cage. 4130
> has a higher tensile strength but typically doubles the cost (in
> materials and weld time).

4130 will give you a weight break if you only plan on drag racing, which is why it is popular with that crowd.

 

[black-plasma]

For you guys with the 4pt's and harness do you wear the harness when driving on the street or is it better to use the seat beat?

By the way this was not a trick question... a simply yes or no would suffice.

 

[steve121]

Just for reference on the weight of generic steel tubing

1.75 x .120 is 2.09 per foot
1.75 x .094 is 1.66 per foot

 

[WP64]

For you guys with the 4pt's and harness do you wear the harness when driving on the street or is it better to use the seat beat?

On the street I use the OE belt but only because it's easy and convenient.

 

[ac427cobra]

By the way this was not a trick question... a simply yes or no would suffice.

I was told a racing harness is NOT DOT approved! :read:

 

[BlackStripes]

I just added a custom 4 pt. installed in my car and when I asked about 4130 he responded with the following;

> Yes, I can do 4130.. in fact I'm doing one right now BUT the problem is
> the two different rule sets (NHRA vs SCCA's). If I recall correctly-and
> I'll check to be sure- NHRA allows a thinner wall when using 4130 -thus
> the lighter weight- while the SCCA no longer allows the thinner walled
> tube. So, you end up with the same weight as a DOM Mild Steel cage. 4130
> has a higher tensile strength but typically doubles the cost (in
> materials and weld time).

4130 will give you a weight break if you only plan on drag racing, which is why it is popular with that crowd.

Sounds about right, when i installed mine (6 point) my fabricator told me basically the same, you are gonna pay a lot more for 40 lbs or less....

 

[BlackBolt9]

I was told a racing harness is NOT DOT approved! :read:

Were you informed by the gentleman who was writing you the ticket?oke::lol:



BTW, I have to do some more studying on my material properties. I looked at the formula for calculating the force needed to bend a tube. It uses the modulus of elasticity of the material, NOT the tensile strength. From the one source I've looked at so far, the modulus of elasticity is the same between 1018 and 4130, which means they have no difference in bending strength:dw: I forgot my material property book at work though so I'll have to continue in the morning. I really need to consolidate where I keep my "tools":kaboom:

 

[sprint200]

I was told a racing harness is NOT DOT approved! :read:

+1

I do believe they are illegal for street use. I use the OE belts on the street.

Bruce,

To your earlier post regarding the autopower 4 pt bars. I was visiting with Jonathan Blevins at Mid America last year and he also told me that he had seen one of these bars punch through the floor, he said the car was airborn and landed on the roof and it just popped through.

This is what I have in my car right now. I would prefer a full custom cage, but that's not going to happen in the GT500. I think the car is "safer" now than it was before. But a dedicated track car is in the near future...........

 

[David Hester]

Roll bars are still legal in Track Trials in SCCA.
11.3. TRACK TRIALS (LEVEL 3)
A minimum of a roll bar meeting the requirements of this section of the TTR shall be required in all cars.....
11.3.1. Basic Design Considerations
A. The basic purpose of the roll bar is to protect the driver in case the11.3.2. Material
A. The roll bar hoop and braces must be seamless, *ERW or DOM mild steel tubing (SAE 1010, 1020, 1025) or equivalent, or chrome molybdenum alloy steel such as SAE 4125 or SAE 4130. It is recommended
that mild steel tubing be used as a chromium alloys present difficulties in welding and must be normalized to relieve stress. Proof of the use of alloy steel will be the responsibility of the entrant.
*Note: ERW tubing is not permitted in any GCR class car registered with SCCA after 1/1/2003.[/B]
SCCA - Sports Car Club of America
Harnesses are not DOT because of the afore mentioned, you can't lean over.
If you want to do hillclimb it depends on class.

11.4. HILLCLIMB (LEVEL 4)
Roll cages (as specified in the GCR) are required for the following classes: GT1, Specials, Super Production, all Formula classes, all Sports Racer classes, open GT, and open Production vehicles. In these vehicles, the roll cage structure must meet current GCR requirements for the specific class. If the vehicle does not fall into a Club Racing class, the cage should be prepared to the GCR equivalent or greater (for example, a tube-frame Special car should be compared to a GT class cage, while a street-driven car that ends up in Special because of odd modifications could be comparable
to a Production or IT cage).
All other classes at Hillclimb (Level 4) Time Trials events are required to have a minimum of a roll bar that meets the description in this section of the TTR.
Thing is, if you even THINK you may want to get serious, might as well do it right the 1st time.
Also note MOST Drag race type bars are NOT competition road course legal. Know your builder!

 

[David Hester]

As far as AutoPower bars punching through.. The 4 point could use some more foot plate sq inches. SCCA ad I think NASA allow 10x10 inch square .
I saw a bolt in AutoPower cage in a CRXsi survive a high speed crash, several rolls, a couple of nose tail nose tail. Not a single straight panel left, but driver walked away. Owner just left the car in the lower paddock Sunday afternoon with a note that anyone that wanted it could have it. Cage was still in good shape.

 

[Maynor]

...Also, why would you need to save a little money on safety for an Agent 47 or Griggs suspension. They will only add seconds on to your time, and you're not racing, so seconds should not matter.

To reduce my tire budget costs of course. The 3 options have the potential to make better use of the entire tire minimizing edge wear.

 

[Ryan]

I was told a racing harness is NOT DOT approved! :read:

In my reading, someone at CC stated that I believe it was Schroth made a DOT approved belt....

 

[wheelhopper]

To reduce my tire budget costs of course. The 3 options have the potential to make better use of the entire tire minimizing edge wear.

Full suspension efficiency. :beer:


If I posted some mis information about the roll bar regs in SCCA I apalogize I did a search and found various items that stated roll bars are not allowed. I'll have to admit, it was hard finding guidelines for SCCA compared to NASA.

I would still use chromoly over DOM steel. Probably comes from my BMX racing days. The hot item to have was chromoly frames, forks, and handle bars. I used to race, jump, and ride half pipes with the same bike and never broke any of those parts. And it was light as hell. I think the whole bike weighed something like 13lbs. As a matter of fact, I just sold it about 2 years ago. It was 23 years old, and still rode like a champ.

 

[David Hester]

Wheelhopper,
no problem. I've been racing with SCCA 25 years. I've got a bookcaseful of old GCR's and SOLO rulebooks. ;^)
As mentioned, chromoly is light and strong (I raced AMA motorcycles back in the '70's-'80's and all our frames were chromoly, too), but you can't just weld it, it costs a bundle, and the weight of a cage helps meet minimum weight (I have 80 lbs of lead bolted above my rear axle in the a-sedan car and still am just 10 lbs over minimum with 1/2 tank of gas.)
Schroth made a DOT approved belt....
Yes, but it isn't road race legal with anyone, might get you into a track day.
Schroth Harness Guide and Harness Belt FAQ
Note, they require you order for your car (no universal fit) and don't make them for everything.

 

[wheelhopper]

Did I mention I would still use chromolyoke:

I don't think I would worry about using the harnesses on the street. If you get pulled over and a police officer sees you with a 5pt harness on, connected to roll bar/cage, I bet he would not write a seat belt ticket.

 

[Greensix]

In my reading, someone at CC stated that I believe it was Schroth made a DOT approved belt....

That is correct.


Also, I would never use chromoly as a structural material that is meant to resist impact forces. Just for the simple fact it absorbs no energy unless there is a total failure. I did several test this year with plain steel vs. alloy steel. Chromoly, while very strong, was so damn damn brittle during impacts. For the forces that a roll cage needs to endure, chromoly is out. The weight savings are starting to dwindle as most racing groups are requiring thicker and thicker tubing for chromoly.

 

[sprint200]

That is correct.


Also, I would never use chromoly as a structural material that is meant to resist impact forces. Just for the simple fact it absorbs no energy unless there is a total failure. I did several test this year with plain steel vs. alloy steel. Chromoly, while very strong, was so damn damn brittle during impacts. For the forces that a roll cage needs to endure, chromoly is out. The weight savings are starting to dwindle as most racing groups are requiring thicker and thicker tubing for chromoly.

+1

The physics of a crash are measured in fractions of seconds. Chromoly's plastic range is much smaller than mild steel (it doesn't bend as easy.........). What that means in a crash is that the time it takes for the overall mass of the vehicle to come to a complete stop is shorter with chromoly than it is with mild steel. Many thousandths of seconds are used deforming the mild steel as opposed to a more sudden failure of the stronger steel. That's energy dissipated on the cage instead of being transfered to your harness (and your body). Think of it like crumple zones in street cars or like the Indy and F1 cars designed to dissintegrate on impact thus shedding energy before reaching the driver.

What is good for the frame of a motocross bike is not necessarily good for a roll cage.......:beer: