mrdragster1970

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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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JL ws-6

iTrader: (50)

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.

5.3LJimmy

iTrader: (10)

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,

00Wildcat

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

JL ws-6

iTrader: (50)

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.

mrdragster1970

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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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kmracer

iTrader: (1)

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.

kmracer

iTrader: (1)

wow.

No Hope

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

No Hope

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.

kmracer

iTrader: (1)

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.

No Hope

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.

JL ws-6

iTrader: (50)

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.

mrdragster1970

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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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JL ws-6

iTrader: (50)

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)