turbofan

iTrader: (1)

Glad the OP posted this. not because I was considering a cheap bar, but because now I understand yet another key difference -- the materials, and that DOM tubing must be pretty darned spendy.

thanks for all the info here guys. This is what makes this site the best damned site on the web.

emilio700

iTrader: (3)

+1
Would trust life with. Chrome is strong.

18psi

iTrader: (76)

because DOM never nark'd on nobody

freedomgli

I don't think ERW is as atrocious as some people make it out to be. It's less uniform in wall thickness than DOM and doesn't have the same strength because it wasn't cold worked like DOM. But it can still be strong enough for safe roll bars/cages. Rarely do you ever see a ERW tube split. Normally, roll bar/cage failures occur in the HAZ or due to design shortcomings like improper backing and gusseting, improper bends, improper load paths, etc.

Because ERW wall thickness varies, where it is allowed it generally must be larger diameter and/or thicker than DOM to ensure that even in the weakest location it's at least as capable as the thinner, lighter, more consistent DOM. What's most important is the design and quality of the fabrication, bends, tube fit, weld quality, backing plates, etc. All that being said, if it was my car I'd just get a good quality DOM roll bar/cage because I generally prefer to do it once and do it right. That way I would not have to worry about whether the car will pass tech inspection no matter where its run. But if I was building a LeMons/Chump car and I was sitting on a stack of free ERW tubing and was a skilled fabricator, I'd consider ERW.

Blackbird

Going through the amount of processing it takes to build a proper full cage and knowingly choosing to do it with ERW to save a few bucks is plain stupid.
The lion share of the cost of a good cage is the labor, not the material.
You'll end up with a cage that is much heavier for anything close to DOM cage strength, so your race car already starts off with a weight penalty, with much of it sitting far above the ground.
Remember that the cage is the one component in a race car build that you never want to redo.

The difference in stiffness between DOM and ERW is very evident when you're working with the material, I'll try to shoot a short video to show it at some point.
I assure you that you will NOT consider ERW an option for a cage if you saw it

freedomgli

I said if I was already sitting on a pile of ERW tubing I'd consider it for a cheap race car build because let's face it, for most guys cheap racing isn't about winning it's about participating and having fun. That being said, I can easily justify the small price increase of DOM over ERW if I was buying all new material for my own not-cheap race car build.

Everything racing costs money. And everyone has a limit. I would not begrudge a LeMons/Chump car for having an otherwise well built roll cage if it was made of ERW, especially if that materials cost savings allowed a team member to purchase a HANS-type device they otherwise might not have been able to afford. Everything is a compromise and people make different choices at different times for different reasons. Not every situation is the same.

The trade-offs are well understood. No one is saying ERW is better at anything other than being less costly (usually). Sometimes lowest-cost technically acceptable > best value. And different people value different things differently. So determining best value is a judgment call.

A typical LeMons/Chump car lasts 1-3 seasons. Very few go on to serve illustrious careers winning SCCA national championships so there is no redoing going on here. It's simply making the overall car last a few seasons of cheap fun.

No need. It's already been done.
Rather than type out my own opinion, I'll just repost what someone else already said about that video that I agree with

Blackbird

Hope you're not taking what I posted as if I'm referring to you personally, I might be replying to your post but it's all generally speaking.

Test done on a .120" wall tube, this would have taken even less time on .095" tubing.
Once you go past the point of plastic deformation it's kind of pointless..

JMHO, if the price difference between the tubing of a roll cage is the make or break budget component of going racing, one should consider whether going racing at all is a good idea.
Different state of mind, but if I'm going to be in a heavy crash I'd like to have the best stuff around me.. this isn't Xbox that you get a redo, it's your ONE opportunity of walking away.

rleete

iTrader: (21)

In the interest of preserving/improving the gene pool, we should encourage stupid people to buy style bars.

freedomgli

I hear what you're saying, but if you take that line of reasoning to its logical conclusion, then why don't you use CrMo instead of mild steel DOM? Why not build a DTM-style carbon safety cell around the driver that incorporates the energy absorbing seat mounts? Why race a Miata at all when other race cars are safer? Afterall, you only have ONE opportunity to survive a crash, right? The fact is motorsports is inherently dangerous and safety is relative. There is no such thing as absolute safety. You absolutely can build a "safe" Miata roll bar/cage using ERW, it's just going to be heavier than DOM or CrMo all else being equal. Everyone has a finite racing budget. The event organizers establish safety minimums. Many racers exceed the minimums in some respects but only meet them in others. Such are the decisions each racer makes when deciding how to best allocate their limited resources.

Savington

iTrader: (31)

Because it's not needed. Both are tested for weld quality and uniform wall thickness. ERW is not.

freedomgli

DOM isn't strictly "needed" either unless the rules specifically call for it. CrMo is stronger than DOM is stronger than ERW. Both ERW and DOM are available in a variety of different quality and certifications. How good is your steel supply chain?

afm

iTrader: (3)

CrMo is more prone to HAZ problems without post-weld treatment. Therefore, it's not strictly better without greater process control.

DOM is better than ERW in every relevant way except cost. Seems like a nice happy medium.

freedomgli

I'm with you. Each has its pros/cons and I agree DOM is the happy middle ground for most applications in our corner of the motorsports sphere. But like I said earlier, I don't think ERW is as bad as some make it out to be. If you choose ERW you just have to work around its comparative weaknesses.

18psi

iTrader: (76)

y'all going in circles now. just sayin

Dunning Kruger Affect

For someone who has maintained that DOM is the way to go, you're really going out of your way to defend ERW.

freedomgli

DOM is the way to go for most Miata applications and most builders. That much is obvious. But I also think it's important to provide a counterpoint to the absolutely ridiculous blanket statements made by others that derided ERW as wholly unfit for roll structure. That's simply not true at all. It can adequate for some builders in some applications depending on all the other factors that go into making a "safe" roll structure. Just like a Sparco Sprint seat may be acceptable vs. Racetech RT4009HR for some racers in some applications. As always, read the MFRB and follow sound engineering principles and do your best.

Blackbird

This post reads like a troll bait, but I'll answer anyway because there's a chance to learn something.Because of two reasons -
1. CrMo requires post welding heat treatment. Since it's practically impossible to heat treat an entire chassis there's no way to insure the stress in the structure is relieved which can lead to cracks.
Great material to use for smaller components like suspension arms and subframes, those can be heat treated without any issues.
2. The organizations that I run with do not allow in their rules any break in wall thickness when CrMo is used, like some other racing bodies do.
3. Nevertheless, I always look to improve upon where I can and have been doing some reading on Docol R8 lately.
I'd like to, but my budget is missing a bunch of zeros.
Oooh, got me there, now I'm starting to talk about budget in this pointless argument... however DTM tubs cost a fortune to build, a FAR cry from the ~$250-300 difference between building an ERW cage or a DOM cage.
You do realize DTM cars still use a steel cage, right?
It's obvious that this is very important to you, not sure why though, the difference in cost of tubing being a miniscule fraction of anyone's racing budget.
The people you describe are the ones I turn away when they come in to my shop, you do whatever makes you happy.
At the end of the day, the question in the OP was "is it cheap because it's terrible" to which the answer to is yes.

freedomgli

Yes, DTM use monocoque, steel cage and carbon safety cell. They're very neat cars, but yes, very $$$$$$. There are lots of CrMo roll cages: pre-fabricated bolt-in, pre-fabricated weld-in and also custom professional-level weld-in. The post-weld stress relief and heat treatment is not a huge issue but is something to be aware of. Much depends on the specifics of the metal, cage design, welder, filler material, fabricator skill, etc. Blanket statements from Carrol Smith should be taken with a grain of salt. He was a very smart man and his advice is generally good to follow but there are always exceptions to the rule. Thanks for taking the time to contribute to the discussion for the sake of learning. We're all on a journey.....

afm

iTrader: (3)

All those 4130 cages are illegal for NASA, just as all new ERW cages are illegal for both SCCA and NASA.

freedomgli

Are you sure about that? I understand that the prefabricated OMP and Sparco CrMo roll cages would be subject to extra scrutiny because, despite their FIA homologation, they use metric tubing that does not conform to the NASA CCR. But there is still an allowance for them to run at least temporarily. As for other 4130 roll cages, it's right there in the rules. Which is what I've said before: RTMFRB.

According to the NASA CCR 2016.8 Section 15.6.18 "Roll Cage Tubing Sizes"
Understood. I'm not suggesting you build a new car for NASA or SCCA competition using ERW. I've said all along: RTMFRB and you may use ERW where allowed if you prefer. My previous example of LeMons/Chump car and already sitting on a pile of ERW are a perfect example of where it might make sense.