Roll Cage Chromoly Questions

Anyone here have experience with chromoly roll cages/bars? I'd like to know what's required to properly build one, specifically the differences between CM and mild steel and the required heat treating/stress relief after welding.

I've read on Miller's website that CM with a wall thickness of 0.125" or less should not need stress relief after welding, but thicker materials should be heated to 1100 F with an O/A torch.

You guys have any advice on this?

Application is a miata roll bar/cage, material is 1.75" x 0.120" CM.

Thanks!

That sounds like a lotta cage for that car. What sanction? SCCA GCR says:

F. TUBING 1. Seamless or DOM mild steel tubing (SAE 1020 or 1025 recommended) or alloy steel tubing (SAE 4130) must be used for all roll cage structures.

Vehicle Weight/ Minimum Tubing Size (inches) /outer diameter x wall thickness) Up to 1700 lbs: 1.375 x .080

1701 - 2699 lbs: 1.500 x .095 1.625 x .080

2700 lbs and up 1.50 x .120 1.750 x .095 2.00 x 0.80

DOM would be a lot cheaper, equivalent weight-wise to CM and you could MIG it.

In reply to fasted58:

...and for the record... 1.75 x .095 DOM is a little lighter than 1.5 x .120 so it applies equally well to the middle weight range if tube diameter isn't a clearance issue.

My Dad built one for the Super Stock car. You need a TIG, and he didn't do any stress relieving. It's lighter than steel, but the old wives' tales say that it work hardens over a few seasons - I have no idea as to the truth of that, though.

We've got a line on some very cheap CM and I've run the numbers - it will be a little more than 24% lighter than mild steel, which is why I'd want to use it. The tube dimensions are the minimum allowed by NASA's CCR for roll bars in open top cars. (It won't be a full cage). We have a TIG, so welding won't be a problem.

Mostly, I'm curious about heat treating and stress relief requirements.

For reference here's the NASA rules about tube dimensions for roll bars:

If using Chromoly: (1501 - 2500 lbs vehicle weight) 1.625" x .095"

If using Mild Steel: (2001 - 3500 lbs vehicle weight) 1.75" x .120"

That mild steel will weigh about 2.08 lbs per liner foot, while the CM weighs about 1.67 lbs per linear foot.

Unless you are spending some big bucks, like a professional racing team, NOBODY in the drag racing world is stress reliving CM.

If I was looking at stress reliving CM, I would bolt the whole car to a MetaLax table and vibrate it silly.

If I may ask... You have the tools, fabrication skills to build it yourself - and are going to thru the time and effort to research rules... why are you not doing an entire cage? A roll bar for a Miata is an off-the-shelf item that isn't terribly heavy or expensive to begin with... but a full chromoly cage would be nice piece of kit.

JohnyHachi6 wrote: If using Chromoly: (1501 - 2500 lbs vehicle weight) 1.625" x .095" If using Mild Steel: (2001 - 3500 lbs vehicle weight) 1.75" x .120" That mild steel will weigh about 2.08 lbs per liner foot, while the CM weighs about 1.67 lbs per linear foot.

OP said 1.75 X .120 CM

If you're looking to do a Miata cage I have a new weld-in Autopower kit. .120 DOM, door bars. Let me know if you want pics, etc.

Chrome moly Prostock cars generally get discarded after a couple of seasons, because nobody has an oven big enough to bake them. Theory is they develop stress cracks, which is why you will often see a pressure gauge screwed into the chassis tubes of a drag car.

Giant Purple Snorklewacker wrote: If I may ask... You have the tools, fabrication skills to build it yourself - and are going to thru the time and effort to research rules... why are you not doing an entire cage? A roll bar for a Miata is an off-the-shelf item that isn't terribly heavy or expensive to begin with... but a full chromoly cage would be nice piece of kit.

Well, we're making one of these:

It's not really an off-the-shelf setup and it doesn't technically qualify as a full cage.

We also already have an NHRA 10-second legal cage but it's really big and will have more wind resistance and weight than we'd like for track racing.

After doing some more reading about this, I think we'll probably go ahead and stress relieve the joints with an A/O torch as recommended by Miller, event though our wall thicknesses are less than the minimum thickness that requires this. Additionally, I think we'll probably gusset the major joints to reduce fatigue stress.

If you guys have any more thoughts on this, or any more experience with chromoly cages, heat treating, or stress relief, let me know.

Thanks!

Definitely gusset the cage. One of the issues that I have seen with chromoly is that it will end to shear at attachment points rather than bend as mild steel will do. As a general rule gusseting will great increase the strength and rigidity of your cage. Are you building it for EP?

Chromoly gets what's called under-bead cracking when not heat-treated correctly. The cage can look totally fine from the outside, but underneath the weld it can develop microscopic cracks that can fail catastrophically.

There was an article in the American Welding Society magazine a few years back about a top fuel team and how they build tough reliable chromoly cages. The do pre- and post-heating. The most important part of the heating process is not how HOT you get the metal but how LONG you take to both heat and cool the chromoly.

Filler metal is also very important. Use the wrong stuff and all that spendy tubing and time spent fabricating will be totally worthless.

For most applications, chromoly is overkill compared to a properly designed, fitted, gusseted, and welded DOM-tubed cage. The top fuel team for instance only used chromoly on the back half of the 7000hp dragster, just to hold the driveline.

I will see if AWS has a copy of the article online.

If you are interested in the subject of cm welding and roll cages as it applies to Top Fuel and Funny Cars, this is a good read... tubular tales

Can't find the original Lincoln info, but most of it is in post 5 here:

You probably can't get an accurate post weld stress relief waving a torch around on a joint (i.e Can you get all sides of every joint up to a known temp for a known time?), but the above says it's unnecessary anyhoo.

Also says regular er70 filler is adequate and er80 approximates the strength of CM. Some people use stainless.

-James (has tig welded a CM cage with no post-op )

Awesome - thanks all.

Jamscal: I ran in to that same write up a little earlier (maybe Sunday night) and I think we'll just follow that advice.

We have an IR thermometer, so I think we'll give the stress relief with a torch a try. Just gonna try to heat up all the joints and tubing in close proximity to welds to 1100 F for 10 or 15 minutes, then let it cool gradually. Sounds like we don't really need to, but it won't hurt right?

throw welding blankets over the treated areas while they cool, keep out of cool air or drafts

JohnyHachi6 wrote:

Giant Purple Snorklewacker wrote: If I may ask... You have the tools, fabrication skills to build it yourself - and are going to thru the time and effort to research rules... why are you not doing an entire cage? A roll bar for a Miata is an off-the-shelf item that isn't terribly heavy or expensive to begin with... but a full chromoly cage would be nice piece of kit.

Well, we're making one of these: It's not really an off-the-shelf setup and it doesn't technically qualify as a full cage.

I still don't understand these, there is one at my local autocross and I never understood why he didn't just buy an open wheeled car?

No offense it looks the business, I'm just curious.

Joshua wrote: I still don't understand these, there is one at my local autocross and I never understood why he didn't just buy an open wheeled car? No offense it looks the business, I'm just curious.

None taken - the car is a GRM Challenge car so it can't be tube-framed unless we want to run in exhibition class. We're making the new cage to take it to the UTCC this year because we want to reduce weight and aero drag from the NHRA cage we have in there now, and also change the seat location a little.