Drag Racing Tech - Mig welding a chromoly cage? is it against nhra rules?

is it against nhra rules to run a chromoly cage that has been mig welded with special chromoly wire?

Its against NHRA rules. Even if you use Er70S MIG wire.

Can it be tacked with mig than completed tig welded later?

The mig tack would just be used to keep the members in place till final tig
welding.


Mike

That is a somewhat common practice (tack with a mig then tig weld 100%), not really right though.


You can mig a mild steel cage, and you can build a mild steel cage to 25.5 cert (7.50) but if you want to go any faster then that you will have to rip it all out and start over.

That is a somewhat common practice (tack with a mig then tig weld 100%), not really right though.


You can mig a mild steel cage, and you can build a mild steel cage to 25.5 cert (7.50) but if you want to go any faster then that you will have to rip it all out and start over.

Supposed to cut out any GMAW tacks before completing with GTAW.

You can do mild steel but look at the wall thickness differences. Lots of extra weight

That is cazy they you are supposed to cut out mig tacks even!!

Without a 4 page report why is mig welding chromoly so bad?

Supposed to cut out any GMAW tacks before completing with GTAW.

You can do mild steel but look at the wall thickness differences. Lots of extra weight

I've actually done the weight comparison on a piece of 1.75 chromoly vs 1.75mild steel, it's .1 lb difference per foot of tubing.

So, if you were doing a 6 pt rollbar with say 60 feet of total tubing it would be 6 lbs. The weight difference is pretty close to the same for 1 5/8 mild steel vs 1 5/8 chromoly.

So a 10 pt cage that probably has 120 feet, it's about 12 lbs, a 25.5 that probably has closer to 200 feet in it would be about 20 lbs.

Main reason I think that mild steel will not cert past 7.50 (madman correct me on this if I'm wrong) is that there's actually a schedule of black iron pipe that meets the requirements for mild steel... and there's no way that would be safe.


One other thing to think about, about 6 years ago, a regular 10 pt cage used to be good to 7.50, not 8.50. It was somewhat recently that all the spec's were updated for certain speed levels, etc.

IMO, which isn't worth much, I think they're off a good bit. You can go buy a brand new mustang or corvette and just by putting a drag radial on them go 10.x at 130 or so... so the rollbar rule, really needs to be changed, to 10.50 or faster from my perspective, 11.50 for a convertable, and those should be good to about 9.50 before you need a 8 pt cage in a car, and that 8 pt cage should be good to 8.0, once you get to 7.99 then the 25.5 makes sense, but that should be good to 7.0, etc.etc.


I know you'll get that old clunker someone put a bbc in that will go 9's and the car isn't safe to push down the road nevermind 140+ but no matter what you do someone will show up with something like that.

Just a perspective, looking at what you can buy today off a showroom vs the current rules, and the rules that were in place not too long ago.

That is cazy they you are supposed to cut out mig tacks even!!

Without a 4 page report why is mig welding chromoly so bad?

I can tell you that other industries will mig chromoly material, it's not allowed for nhra or scca, but it is an acceptable practice elsewhere... there's some science etc. involved with the materials and whatnot, I can't tell you exactly the reason.

I can tell you that I have mig'd chromoly threaded inserts to mild steel tubing for go karts and the like and it's held up with NO problems, and these kids beat the things up pretty hard. A simple hole thru the tubing to put a plug weld in, and then wld around the end and I haven't had one fail yet. I've had them bend he tubing after crashing into stuff at speeds that would have probably totalled a car LOL, but the weld joints, and material around the weld joint didn't fail.

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I was always told they don't like mig because anyone can slap something together, and it will look & test good.
With a tig, you need more skill & experience to get the proper heat & penetration to look right???
Now I know for a fact, that my Camaro had no business going 8.50's, and it was completely insane
they gave me a 7.50 sticker for all those years.
Yes it was legal, but I don't think sticking a cage on a 6" plate made it 7.50's safe!!

I'm also leaning towards the whole mild steel is junk at 7.50 rule being ridiculous!!
Make them tig it if you're worried about quality and penetration, but I think it's more then strong enough!!
Anyone see the nascar guys on the news??
Those are mild steel cages in those crashes!!


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I was always told they don't like mig because anyone can slap something together, and it will look & test good.
With a tig, you need more skill & experience to get the proper heat & penetration to look right???
Now I know for a fact, that my Camaro had no business going 8.50's, and it was completely insane
they gave me a 7.50 sticker for all those years.
Yes it was legal, but I don't think sticking a cage on a 6" plate made it 7.50's safe!!

I'm also leaning towards the whole mild steel is junk at 7.50 rule being ridiculous!!
Make them tig it if you're worried about quality and penetration, but I think it's more then strong enough!!
Anyone see the nascar guys on the news??
Those are mild steel cages in those crashes!!


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The reason why they would want the cage Tig welded is because 99% of Mig welding is done with globular transfer. You have to be a professionally trained welder to get full penetration with this process. Tig welding uses DC- polarity for this application and full penetration can be easily achieved. Tig welders as a rule have more training than a Mig welder.

GTAW or HeliArc is Tig welding

GMAW is Mig welding

If you grind all the Mig weld off no one will know if you Tig weld it after. Make sure you use the proper filler rod and I would back purge the pipe just to be safe. Do Not resell the cage.

Good read, and good to know. Thanks for posting up the question and thanks for the responses guys. I've been wondering this stuff for the last week since i went with MS for my 10point. I highley doubt ill ever make it faster then 7.50 so, works for me.

You'll need the 25.5 as soon as you get to 8.49, you can do that with MS so it's not a big deal to add on to what you have, and then you're good to 7.50...

To go faster then 7.50 takes CUBIC dollars... if you can afford that, then redoing the cage probably isn't going to kill you.

You'll need the 25.5 as soon as you get to 8.49, you can do that with MS so it's not a big deal to add on to what you have, and then you're good to 7.50...

To go faster then 7.50 takes CUBIC dollars... if you can afford that, then redoing the cage probably isn't going to kill you.

Lol, Works for me :) i dont have CUBIC dollars, and Wont for along time, lol

I never will.... and if I ever do come into it, I probably won't be spending it on this stuff. I'll make that money, make me more.

for less then 10lbs on a 6-8pt cage/bar, why spend the money for CM? I understand every pound counts..but your almost doubling costs. I mainly do arc welding at work, but i also do alot of mig welding when i dont feel like being showered with splatter and you would be amazed out how some awesome looking mig welds dont burn in for crap.

Thats another thing...whats the reasoning behind no arc welding for roll cages...I know it cant be a strength issue. Just something ive wondered, done a lil research but no concrete answer

I understand you can mig a weld and have it look nice, but not penetrate. When I was first learning how to Mig stuff it was easy to tell when there was no penetration at least to me... you could pull on the pieces and you'd get the weld to break off the surface of one side. Easy fix, higher power level stopped that at least for me and the welder I have (Lincoln 180)

Since I've gotten the hang of it, and know what looks right, anything I have tried to pull apart absolutely will NOT come apart. I had someone I know that's a certified welder take a couple sample weld's I did and go inspect them @ his work.. he came back with a thumbs up on the joints.. made me happy to know.

But, that being said.. I still won't weld a cage in a car. I don't feel confident enough to do that yet even a MS deal. Just not something I want to do, if something were to happen and it had to hold up to keep someone alive I don't want that in the back of my mind.

Exhaust, frame connectors, all that, yes. Cage, no I leave that to people that do it for a living.


As for not allowing arc weld's on a cage, I would guess the filler material isn't up to par maybe, or the amount of heat, or the porosity of the weld would be the reason.

One of the welding guru's in here will have the answer in technical terms I'm sure.

I hear ya on that. Ill do my own mild steel cage. Ive had others ask if i would do theres and that was a big no, i dont want that responsibility.

hopefully somebody will chime in on arc welding. I guess i like to defend it since i do it all day long, if done correctly its an exellent welding technique. I build steel containment tanks and when we do verticle storage tanks in the 20-30,000+gal range, we will never use a mig, always arc weld. may just be what were comfortable with

I'm pretty sure as others have said it's about the dumb asses pushing the wire. Anybody that owns a MIG welder thinks they are super welder. I'll give you a couple of examples and leave the pics big.
This is a mild steel sub-frame that came with a complete strut Chevy II clip that I purchased from an unnamed vendor that is popular with the Mustang crowd. When asked why the Chomoly suspension parts looked so much better than the sub frame it was explained to me that the mild steel was mig welded.

So then I sent the company owner pics of my rearend housing that I MIG welded and explained to him just because it was MIG welded didn't mean that some F*Tard had to lay down a dog shit weld on the joint! So they are now sending me a replacement chromoly subframe.

picture

That is one of the worst looking MIG welds I have seen on anything... and they sent you a part that looked like that? Jesus that's retarded.

Next time I'm in the garage I will take a picture of some of the recent stuff I've done. I'm not a pro by any means but it looks a hell of alot better then that snot pile. If you can, try to get a better picture of that part the thing is too small to really show the mess there.

The rear end housing you did looks good to me fwiw, that's about as good as I've been able to do up to this point and I know my work is sound.

for less then 10lbs on a 6-8pt cage/bar, why spend the money for CM? I understand every pound counts..but your almost doubling costs. I mainly do arc welding at work, but i also do alot of mig welding when i dont feel like being showered with splatter and you would be amazed out how some awesome looking mig welds dont burn in for crap.

Thats another thing...whats the reasoning behind no arc welding for roll cages...I know it cant be a strength issue. Just something ive wondered, done a lil research but no concrete answer



Slag entrapment and lack of knowledge would be my guess.

Depending on the material that the cage is made from. A mild steel cage could be easily and safely made from Arc welding, E-6010 and E-6011 are the 2 best rods for full penetration but they do not have a smooth pretty appearance because the are a Fast Freeze electrode. Fast Freeze rods are used for a Root pass on pipe to work a Key Hole and then a Iron based Fill Freeze rod like E-6013 or E-7018 are used for the cover passes. You could use 6013 for a cover pass on a cage.

Chromoly should be Tig welded because of the high risk of slag entrapment with an arc weld. If you are a professionally trained welder there is nothing wrong with Mig welding Chromoly tubing.

I don't see anything wrong with a cage made out of high strength Aluminum square tubing for cars 5 seconds or slower. Thickness and Diameter along with grade of Aluminum would have to be considered, but if its safe enough for an airplane it should be ok for those kind of speeds.

Don't kid yourself, if your going 5 seconds or faster in a quarter mile the only thing that's going to save your ass in a crash is being tight with Jesus.

The NRHA is probably thinking more about liability than safety that's why they have some of these rules.

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I know my car has only gone 6.50's, and will never get near 5 seconds,
but there is no way in hell I would get in an aluminum chassis car!!
I've seen way too many stress cracks & failures from vibrations & flexing to strap my ass in there!!

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Slag entrapment and lack of knowledge would be my guess.

Depending on the material that the cage is made from. A mild steel cage could be easily and safely made from Arc welding, E-6010 and E-6011 are the 2 best rods for full penetration but they do not have a smooth pretty appearance because the are a Fast Freeze electrode. Fast Freeze rods are used for a Root pass on pipe to work a Key Hole and then a Iron based Fill Freeze rod like E-6013 or E-7018 are used for the cover passes. You could use 6013 for a cover pass on a cage.

Chromoly should be Tig welded because of the high risk of slag entrapment with an arc weld. If you are a professionally trained welder there is nothing wrong with Mig welding Chromoly tubing.

I don't see anything wrong with a cage made out of high strength Aluminum square tubing for cars 5 seconds or slower. Thickness and Diameter along with grade of Aluminum would have to be considered, but if its safe enough for an airplane it should be ok for those kind of speeds.

Don't kid yourself, if your going 5 seconds or faster in a quarter mile the only thing that's going to save your ass in a crash is being tight with Jesus.
The NRHA is probably thinking more about liability than safety that's why they have some of these rules.

THAT is funny. But thanks for the info....kinda what i was thinking was the reasoning behind it...

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I know my car has only gone 6.50's, and will never get near 5 seconds,
but there is no way in hell I would get in an aluminum chassis car!!
I've seen way too many stress cracks & failures from vibrations & flexing to strap my ass in there!!

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Just the cage, not the chassis.

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On a 5-6 second car, the cage is the chassis.
Look how many bars we have on these things, and my car is old.
I'm 5 seconds legal, and there is no way to mix & match aluminum with the moly frame rails??
I have X's everywhere, and don't forget on these cars, the trans comes out the door, the floor has no open area.

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On a 5-6 second car, the cage is the chassis.
Look how many bars we have on these things, and my car is old.
I'm 5 seconds legal, and there is no way to mix & match aluminum with the moly frame rails??
I have X's everywhere, and don't forget on these cars, the trans comes out the door, the floor has no open area.

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My mistake, I've never been around 5 second cars.

Any car with a separate cage say 7 seconds or slower would be safe then as long as the shop that makes it is a professional welding shop.

If the engineering is done right you can have a full chassis car made out of Aluminum that would be as safe as steel. Fighter jets and many other planes use high grade Aluminum for protection. It would cost a lot and the shop that does it would have to be a top notch welding operation.

Look at what the boys at Pfadt do with Aluminum suspensions. If they built a full chassis Aluminum car I would drive it.

To my knowledge air frames are meant to stay together flying, NOT crashing! I think if you KNOW you are going to crash an airplane it's highly recomended that you jump your ass out with a parachute straped to you. I'm not saying Aluminum isn't strong but comparing an air frame to a rollcage is apples to oranges. Now welding/brazing technics used to build air frames are more than good enough for roll cage construction IMO.

No hope I would suggest that you stay out of this conversation your lack of real world experience with anything involving race cars is very apparent.

Alum, is for engine blocks and heads. and other fabricated powertrain components..... NOT for safety equipment in a race car!

The parts that Pfadt make for suspension has nothing to do with rollcage safety either fwiw, there are plenty of alum chassis cars out there but they are NOT sfi cert'd for the speeds that we are talking about here...

Please pick up an NHRA handbook, and do some material strength reading.... it will probably enlighten you alot.

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Ditto.

That's why they use carbon fiber, titanium, aluminum more and more to save weight,
because the cage, chassis system is NOT where you want to dick around!!!
I sit inside a cage, inside a cage, and the rest of the car can crumble or break away,
but I still have a fighting chance surrounded by steel!!

I have the top style chassis, except my F/C cage is twice as deep, I sit way back with my butt
next to the rear tub.
I also have more bars then required as do most of the guys I race with.

As you can see, there is supports & reinforcements for every bar,
in a design that has many many years of testing & crashing to learn by!!


http://www.ihra.com/downloads/pdf/competition/SFI_25_1E_Kidney_Bar_Clarification.pdf


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No hope I would suggest that you stay out of this conversation your lack of real world experience with anything involving race cars is very apparent.

Alum, is for engine blocks and heads. and other fabricated powertrain components..... NOT for safety equipment in a race car!

The parts that Pfadt make for suspension has nothing to do with rollcage safety either fwiw, there are plenty of alum chassis cars out there but they are NOT sfi cert'd for the speeds that we are talking about here...

Please pick up an NHRA handbook, and do some material strength reading.... it will probably enlighten you alot.


Ok I'll stay out of the conversation because I do not have any experience building crash cages.

But I do have 25 years experience with welding, metallurgy and stress testing every alloy and metal there is so trust me its your lack of knowledge that leads you to think steel is stronger than some Aluminum alloys. Especially how thin the metal is in these crash cages.

The last 3 responses is your answer to why arc welding or anything else new is not acceptable. Closed minded to change.

I seem to remember the same type of responses when the Aluminum engines first came out.

I'm out of this one...Bye

All the lab testing in the world won't give you real world experience on how parts, and cars stand up to disasters and the rigor's of racing. Don't take this the wrong way, but engineer's commonly suffer from this same issue, you have alot of knowledge no doubt, but you don't know how to apply it to something that's already proven and working, therefore want to redo the whole thing from scratch. I fight with engineer's on this stuff all the time, they always want to reinvent the wheel becuase they read somewhere that there's a better way to do it, but don't have the practical experience to understand why what's currently in practice and why it works.

You can read in a book all about material tensile strength and every other structural aspect of a material, but it's not going to tell you if you can or can not use the material. It just tells you the design has to take something into account.

I've worked in a metrology lab for several years, and know a thing or 2 about materials, and have a bit of aerospace experience as well... I'm well aware of how "strong" aluminum can be, but the weight of the required amount of material to stand up to the same type of blow is not even close to practical.

The design and what can be made from 4130 material as well as it's weight is why it is the chosen material for the sfi certification. SFI foundation has done thier homework, and what they're telling racers is the most practical, cost effective and safest thing that they can come up with.

The welding, as previously stated I think they chose to only accept TIG welding due to the level of experience someone has to have to do it properly. It prevents the back yard hack's from putting stuff together in a questionalbe manner and having it get thru tech, only to fail in a worst case scenario.

Sure an arc, Mig, Tig as well as a oxy-act. weld all can be a string way of joining 2 metals. But, one is very hard to hide bad material fitment, and requires a little more skill...

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All opinions are welcome in my book, and I learn new things everyday.
I've also seen some weird crap in this business.
Bars ripped apart, welds split, unbreakable this, and unbreakable that, BREAK??
Again, I'm not the fastest guy in the world, and even on my slow junk, I've had steel fail.
Hell, I get stress fractures in places that don't have any stress on them.
I'm sorry, but most of the guys here don't know about tire shake and 1.00 60' abuse on their cars.
I've had my bell rung when we had the car set up way off, and I didn't even make it 50'.
I just had my car in the shop getting stuff fixed that ripped apart.

So again, there is no frigging way I'm getting into anything with aluminum surrounding me!!
Besides, how can they stitch it together when some is aluminum and some is moly??
I've never been a fan of bolt in cages!!


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If you're going to bolt the cage in you mine as well not even bother putting it in IMO, the bolts used won't hold up to what the bar should hold up to in a bad crash I'm sure of that.


1.0 60 foot times can tear some serious stuff up, and damage just about anything.

And, in the same thinking that you're going by I did a 25.3 for a car that will probably only see bottom 8's when it's done, I may eventually try to push it to that 7.99 pass just so I can say I did it but honeslty as long as the car repeats on 8.50 I don't really care if it goes any faster.

100% overkill in all aspects of needed safety but I will feel good knowing if something happens I will probably walk away... and if I have to sell the car, it will be worth a ton more as well.

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I agree 100%, like I said, how does this guy think they can mix and match an aluminum cage with moly frame rails, bolts???

I have a heavy car, so be it, I like having an extra bar in the roof, I like having an extra bar in my F/C cage, more bars in the floor etc etc.
Yes I still bitch about being heavy, but in the long run, it's better to be safe then sorry!!!

Good luck on a 7.99, if you get close, and need just a little more, I'll let you draft off me :angel: :burn: :angel:


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All the lab testing in the world won't give you real world experience on how parts, and cars stand up to disasters and the rigor's of racing. Don't take this the wrong way, but engineer's commonly suffer from this same issue, you have alot of knowledge no doubt, but you don't know how to apply it to something that's already proven and working, therefore want to redo the whole thing from scratch. I fight with engineer's on this stuff all the time, they always want to reinvent the wheel becuase they read somewhere that there's a better way to do it, but don't have the practical experience to understand why what's currently in practice and why it works.

You can read in a book all about material tensile strength and every other structural aspect of a material, but it's not going to tell you if you can or can not use the material. It just tells you the design has to take something into account.

I've worked in a metrology lab for several years, and know a thing or 2 about materials, and have a bit of aerospace experience as well... I'm well aware of how "strong" aluminum can be, but the weight of the required amount of material to stand up to the same type of blow is not even close to practical.

The design and what can be made from 4130 material as well as it's weight is why it is the chosen material for the sfi certification. SFI foundation has done thier homework, and what they're telling racers is the most practical, cost effective and safest thing that they can come up with.

The welding, as previously stated I think they chose to only accept TIG welding due to the level of experience someone has to have to do it properly. It prevents the back yard hack's from putting stuff together in a questionalbe manner and having it get thru tech, only to fail in a worst case scenario.

Sure an arc, Mig, Tig as well as a oxy-act. weld all can be a string way of joining 2 metals. But, one is very hard to hide bad material fitment, and requires a little more skill...


I posted on this thread because the OP wanted to know why the NHRA didn't allow Mig welded cages. I think I answered that, I never pretended to know anything about the cages or the stresses they are subjected to. But if you feel safe in a cage because its made from a material your familiar with then stick to it.

If the conversation is going to go in a direction for a lighter better cage then your going to have to "reinvent the wheel". Chromoly is kind of Barney Rubble technology, think 6061-T6 or 7075-T6 Aluminum in a Honeycomb welded support system.

I might be way out of line with this one so please don't jump all over me for it, but why would you want to be in a cage that is welded to a frame that is out of control and probably on fire? I have much more experience with boat racing than I do with car racing and we have had break away cockpits for years. The boat is smashed into little pieces but your safe in the cockpit because it separates from the boat at impact. I have been in one those crashes myself and other than underwear full of poop and 2 broken collar bones I was fine.

Shear bolts on a cage with a parachute that deploys on impact with a drag car could work the same way. Just a thought. Sometimes fresh eyes can look at things in a different way.

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Actually they have looked into capsules, but the problem is there is a giant 8000 HP fire breathing monster between the driver and the track.
On a short wheel base car, I don't see how it can work with the layout of a body, drivetrain, giant slicks etc etc.
Like I said, the cage is the chassis, so if you remove all the strength, rigidness, now the car either tears itself apart, or won't handle the HP.

Guys have tried new designs. Including new materials, titanium, carbon fiber, and probably aluminum, and they keep coming back to moly for some reason??

Remember, huge difference in real world application, and what should work on paper.

This is me with my small motor, 1150-1200 HP and 1.00's 60' on the back wheels for about 55'.
I also have a 2000 plus HP combination, that is a torque monster.
Multiple that by 50-100 runs per year and you'll understand why I want STEEL!!!

http://i52.photobucket.com/albums/g8/mrdragster1970/_MG_0223.jpg



Now look at guys like this, I will bet the farm, they will never trust anything else either??

http://www.robinson-race-cars.co.uk/Peter%20Wacker%20wheelie%20webpic%202.jpg


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Lol you are clear for take off!!!!

Broken wheelie bar! Have you guys seen Mike Christiansen (not of spelling) roll his red Camaro about 100' off the line because of a broken wheelie bar tab? As far as crashes go it was pretty funny

Wouldn't chromoly be even lighter when used in a 10point cage being as a 10pt only requires 1 5/8 .083 wall where an 8 point or less requires 1 3/4 .083 wall. DOM mild steel requires 1 3/4 .120 wall at 8 point or less and 1 5/8 .120 at 10point or more. Regular mild steel requires the .134 wall in 8 point or less?

So wouldn't the weight differences be substantially more when talking about a moly 10pt vs a mild steel.

Seriously trying to figure this out. S&W advertises a 38% difference between .83 wall moly and mild steel.

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Last number I heard anyone throw around was well under 38%.
Don't remember the number exactly, but I know it was definitely not 38%??


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Difference from a 1 foot section of 1 5/8 moly to 1 5/8 mild steel I'm 99% sure is .1 of a lb.

Wouldn't chromoly be even lighter when used in a 10point cage being as a 10pt only requires 1 5/8 .083 wall where an 8 point or less requires 1 3/4 .083 wall. DOM mild steel requires 1 3/4 .120 wall at 8 point or less and 1 5/8 .120 at 10point or more. Regular mild steel requires the .134 wall in 8 point or less?

So wouldn't the weight differences be substantially more when talking about a moly 10pt vs a mild steel.

Seriously trying to figure this out. S&W advertises a 38% difference between .83 wall moly and mild steel.

Difference from a 1 foot section of 1 5/8 moly to 1 5/8 mild steel I'm 99% sure is .1 of a lb.

For a roll cage mild steel is required to be 1 5/8x.134 and Cromoly can be 1 5/8x.083". Wolfe lists its mild steel cage at 130-135lbs and the moly cage at 90-95lbs. Thats about 30% lighter. I don't have the material here to compare but I believe the cromoly is heavier than mild steel given the same size and wall thickness. The fact that you can use thinner wall tubing if you use cromoly is where the weight is saved,

If you are set on installing a cage yourself and only have means to mig weld it, you are limited to running 8.50 or slower. This is pretty cut and dry cage construction, well documented on the web. If you think you will ever run quicker than that , SFI 25.5 to 7.50 or SFI 25.3 to 6.50 if you have a stock steel floor pan there's not much difference between those 2. Anything under 8.50 Must be TIG welded the 4130 chromoly tubing, PERIOD. If you are going quicker that 8.50 invest $30 on SFIspec book, well worth the investment

For a roll cage mild steel is required to be 1 5/8x.134 and Cromoly can be 1 5/8x.083". Wolfe lists its mild steel cage at 130-135lbs and the moly cage at 90-95lbs. Thats about 30% lighter. I don't have the material here to compare but I believe the cromoly is heavier than mild steel given the same size and wall thickness. The fact that you can use thinner wall tubing if you use cromoly is where the weight is saved,

the 2 pieces I weighed were the same diameter and wall thickness, the .1 of a lb was material only.

The wall thickness also gives additional savings for sure.

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I really must be getting my numbers confused??
I would have sworn it wasn't much I keep hearing??
Like double the cost and only 20% weight saving??
If it's actually closer to 30-38% then double the cost looks much more reasonable??

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The reason why they would want the cage Tig welded is because 99% of Mig welding is done with globular transfer. You have to be a professionally trained welder to get full penetration with this process. Tig welding uses DC- polarity for this application and full penetration can be easily achieved. Tig welders as a rule have more training than a Mig welder.


incorrect there dude. most of mig welding in chassis and light fab stuff is going to be short circuit transfer.

all things equal, with proper pre-heat and post-heat treatment, a gmaw weld will be just as strong as a gtaw weld on chromoly steel.

and truth be told, the fact that someone can lay a nice tig bead is no indication of their knowledge of welding or metalurgy.

Just the cage, not the chassis.
wow. :bomb:

incorrect there dude. most of mig welding in chassis and light fab stuff is going to be short circuit transfer.

all things equal, with proper pre-heat and post-heat treatment, a gmaw weld will be just as strong as a gtaw weld on chromoly steel.

and truth be told, the fact that someone can lay a nice tig bead is no indication of their knowledge of welding or metalurgy.

The welding process for chassis SHOULD be short circuit transfer. The logical reason the NHRA wants chassis Tig welded is most little shops have a cheap little Mig welder that uses globular transfer and don't know to use short circuit transfer.

That's very true about a Tig welder, just because they can lay a pretty bead doesn't mean they have the education to determine the proper filler rod for the base metal. The wrong filler rod with a Tig weld can be much worse than a Mig weld.

You can get good penetration with Mig welding if you use the proper angle to the puddle or using the 3 T technique. There is no need to post heat light gauge Chromoly steel

wow. :bomb:

Hey cut me some slack, I stated I didn't know anything about the chassis or cages. I just know the welding processes that was in question. That's what the OP was wondering about.

The logical reason the NHRA wants chassis Tig welded is most little shops have a cheap little Mig welder that uses globular transfer and don't know to use short circuit transfer.


There is no need to post heat light gauge Chromoly steel
actually, there is. AWS doesnt have standards for no reason, and 4130 is considered a medium carbon steel. its carbon equivelant is .7%, which is relatively high, and regaurdless of thickness the material SHOULD be pre-heated and post heat TREATED. you can look up the chart if you're interested. the heat affected zone is going to be very brittle (super super hard and tough, but very little ductillity) without either of those steps.i've done testing on welds that're identicle other than proper pre/post heat and the differance is shocking. the problem is (obviously) its pretty damn difficult to post heat treat a chassis, and its really not cost effective. at my work (we build trophy trucks) all the suspension components are post weld heat treated. while it all may sound like BS and no one does it, doing things the right way reduces chances of weld failure greatly. patience + attention to detail = quality.

the reason that i've always heard was what was mentioned a few pages back. nhra doesnt want joe schmoe welding up 6 second cars. the whole transfer method has nothing to do with anything.

actually, there is. AWS doesnt have standards for no reason, and 4130 is considered a medium carbon steel. its carbon equivelant is .7%, which is relatively high, and regaurdless of thickness the material SHOULD be pre-heated and post heat TREATED. you can look up the chart if you're interested. the heat affected zone is going to be very brittle (super super hard and tough, but very little ductillity) without either of those steps.i've done testing on welds that're identicle other than proper pre/post heat and the differance is shocking. the problem is (obviously) its pretty damn difficult to post heat treat a chassis, and its really not cost effective. at my work (we build trophy trucks) all the suspension components are post weld heat treated. while it all may sound like BS and no one does it, doing things the right way reduces chances of weld failure greatly. patience + attention to detail = quality.

the reason that i've always heard was what was mentioned a few pages back. nhra doesnt want joe schmoe welding up 6 second cars. the whole transfer method has nothing to do with anything.


That AWS recommendation is for 1/8" thickness or more. The do not recommend it nor do I for thin tubing. The Carbon contend for 4130 is between .28% and .33% that is the low side of medium carbon steel. Its the Chrome that gives you fits. Even at 1% like in 4130 it just doesn't get along with Carbon. Add Molybdenum to the mix and it can get brittle in the heat effected zone fast.

Make sure the weld area is clean and I mean squeaky clean. Use emery cloth then scotch brite when cleaning the weld area. Use as little amperage as needed to get a good weld then wrap the weld in fire resistant cloth or submerge it in oil dry if you can.

They have come a long way with filler rods ER70S is the best rod for this kind of application. Back in the day they would only use OxyAcetylene welding to weld Chromoly steel and that was if'y.

I know everyone on this page is sold on Chromoly but for me I would use something else. I don't know if kmracer will agree with that, but he knows like I do that so much depends on the person doing the welding with Chromoly that I wouldn't get in that car unless I welded it myself. I would never trust that the guy welding it is qualified. You don't save that much weight with it so I don't think the risk is worth the weight savings. The NHRA would be better off making a rule that a chassis or cage has to be built by a qualified shop like the one kmracer works at than a welding process.

Any Joe Schmoe can Tig weld so that is not the reason. I have trained mechanics in 6 weeks that could lay a bead like a 10 year pro.

If the NHRA called a degree'd Welding Engineer like myself and asked what kind of weld failures would be caused by Mig welding thin wall Chomoly my answers would be #1 wrong machine (globular vs short circuit) #2 wrong filler wire and #3 wrong shielding gas. The question is about a welding process not if the person doing the welding is qualified. An unqualified Tig welder would have the same effect and result with the Tig welding process.

That's one of the resons that they have to be certified, the inspectors have the tools nessassary to test the welds (at least as much as you can at the point that a chassis is without cutting anything up) and also to verify material thickness.....


The inspectors, at least the one in this area, when he is looking at something that's getting an sfi cert does a pretty good job of looking it all over, takes him a while to do a car I know that.

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They also require a builders tag.
That way they at least have an idea if the guy is a professional.

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Not only will they know who built it but they will know who inspected it, or at least have an easy way to look into that (only so many inspectors per region)