For my roll cage dun know if i should do chromoly or mild steel
what i- think i know: Chromoly-More expensive, more dangerous because it so hard its brittle and will shatter on extreemly high impact. But its lighter.
Mild Steel-Heavier, but cheaper and safer because it will bend at extreemly high impact anyhow hold together.

 

Chromoly will collapse and bend before it shatters. I don't know why everyone thinks it's like a bag of potato chips.

If you do manage to shear a proper chromoly cage, you're probably already dead.

 

is it ok to use mild steel?

 

yes. NHRA wants min for rollbars to be 1-3/4 and cages 1-5/8. CHRMO is .083" wall and mild is .118" bare minimum even in bends.

1010 is a common tube material for mild. ive bought it from a company that makes hand rails.. pretty cheap in 24 foot lengths.

 

from what i have read (books like Carroll Smith "engineer to win") the problem with chromoly is that when welding, the weld often becomes brittle because of the difference in cooling temperatures of the chromoly tubing and your filler material. i've read that you can heat treat the tubing and eliminate this problem by using a heat putty of like 900 degree temperature, and placing in a few inches from the weld. use a rosebud tip on a oxy/acc setup, and heat until the putty lets you know you have reached the right temperature.

i have welded alot of chromoly without doing this and have had no problems with welds cracking. though i can't say i've had any parts placed under the type of impact that would happen flipping a car at 100 mph.
i think that sanctioning bodies however, fear that with such a high impact with a chromoly cage, the welds could easily give way if cage was not properly built and heat treated.

 

Both mild and chromoly are safe metals to use and equally strong if proper thickness and diameter is used. Mild is much easier to work with and cheaper. Chromoly must be tig welded and is quite a bit more than mild. Honestly, Mild is the way to go due to cost effectiveness. Chromoly would only save about 30% weight compared to mild steel. If you have a mild cage that weighed 200lbs, an equivalent chromoly cage would weigh about 140lbs. I don't know if the extra cost in welding and material would be worth just 60lbs. Both are safe materials to use if done correctly.

 

4130 was originally designed for building aircraft using gas welding (WWII). It is an air hardening alloy, rapid air cooling makes it brittle. It is also "hot short", moving it after welding while it is cooling can create cracks.
TIG tends to have a narrower HAZ (heat affected zone) and cools faster

TIG is the most widely used method nowadays though, and many sanctioning bodies only allow the use of this method for chromoly

 

Chromoly will not shatter because it is sent threw tempering many times before it is sold. it is a little more expensive but it is a lot stronger and better in the long run

 

chromoly shatter? the main concern with chromoly is the welds. that is going to be a possible weak point (brittle) ,depending on what type of wire you use.

chromoly itself is some heavy duty stuff. i mean look at bmx frames(bike frames) those don't shatter. i've had welds break on some bike frames, but the tubing has really stood up to HEAVY abuse, and that is thin double butted tubing.

 

Yea i have rode bmx since i was 7 and i havent had any bikes shatter out from under me, but yes if you are going to put a cage in i would recomend to welp it with an arc welder(stick welder) with either a e6013 or a e6011 rod( 60 thousand pounds tensile strenght) Hey iff you have any question on welding or anything just yell because i am a certified welder

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DOHCrx06 &raquo;</TD></TR><TR><TD CLASS="quote">Yea i have rode bmx since i was 7 and i havent had any bikes shatter out from under me, but yes if you are going to put a cage in i would recomend to welp it with an arc welder(stick welder) with either a e6013 or a e6011 rod( 60 thousand pounds tensile strenght) Hey iff you have any question on welding or anything just yell because i am a certified welder</TD></TR></TABLE>do not stick weld it! no way.... i do 90% of all my tube work out of moly. i never use erw (seamed tubing) if your going to build a cage but you have a buget and cant afford chromoly, use dom tubing which is mild steel seamless... dont stick weld it.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DOHCrx06 &raquo;</TD></TR><TR><TD CLASS="quote">i would recomend to welp it with an arc welder(stick welder) with either a e6013 or a e6011 rod( 60 thousand pounds tensile strenght)</TD></TR></TABLE>

I'd recommend not taking any of this guy's advice.

If you have the money and/or need to save weight use chromoly. Chromoly must be tig'd.

If you just need a cage use seamless mild steel. Mild steel can be mig'd or tig'd (using mig would save some hours worth of labor).

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by backpurge &raquo;</TD></TR><TR><TD CLASS="quote">

I'd recommend not taking any of this guy's advice.

If you have the money and/or need to save weight use chromoly. Chromoly must be tig'd.

If you just need a cage use seamless mild steel. Mild steel can be mig'd or tig'd (using mig would save some hours worth of labor).</TD></TR></TABLE>

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by silly4lude &raquo;</TD></TR><TR><TD CLASS="quote">


Please build yourself a roll cage in that manner and go crash your car</TD></TR></TABLE>

Very much not needed. Not in this forum, take it to GDD.

I am curious to hear about those saying you can't MIG 4130. This is something I wasn't aware of, and has a little story that goes along with my curiosity.

In the years before I joined Auburn Mini-Baja, from about 97ish-mid 03, all the frames built were MIGed 4130. I'm not sure what wire was used, I'm a TIG guy myself, but needless to say, it was done, and it worked. While I am a little hazy about 97-00, I can say that from 01-03, the only weld failure was a suspension failure. It was caused when an A-arm was constructed inappropriately, and the weld was actually in tension. All 3 of these cars experienced crashes (most during post season testing and run-out); 01 was flipped on end several times, 02 was flipped on it's side quite a few times, and 03 had the most significant crash, it was run into a solid aluminized 409 steel post with the rear tire, at full speed (Marc ran into a damn fence).

From these, all three frames experienced tube damage and bending, with 03's being the most extensive, but never a weld failure on the MIGed 4130 frame.

03's damage was actually spectacular, the entire rear section was twisted 10-15 degrees in a lozenge fashion, the top A-arm was pretzeled, and a Yamaha Grizzly rear knuckle, cast aluminum, was actually split completely in half. Through that, however, we never saw a single weld failure, nor a failure in the HAZ (we stress relieve our frames, so that helped in that area).

While I know that the forces we see would be quite smaller than those seen in a 1 ton car hitting a barrier at 110, it still makes me question why you can't MIG 4130.

An answer would be appreciated.

Just a note, thanks to extra capital investment after what was a succesful 03 year, the university bought us a Miller 180 SD, which I use everyday, and will only be used from now on, on our frames.

 

Mig welding 4130 generally leaves a bead without full penetration since filler is immediately fed into the puddle before the puddle can penetrate to the root. Also, with the fast pace of mig, the chromoly is heated up and cooled too fast, which can lead to cracking.

 

yeah sorry about that. taken care of.


Modified by silly4lude at 8:27 AM 3/19/2005

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by backpurge &raquo;</TD></TR><TR><TD CLASS="quote">

I'd recommend not taking any of this guy's advice.

If you have the money and/or need to save weight use chromoly. Chromoly must be tig'd.

If you just need a cage use seamless mild steel. Mild steel can be mig'd or tig'd (using mig would save some hours worth of labor).</TD></TR></TABLE>
Can someone verify that you are allowed to MIG weld a roll bar or cage. I know you can MIG weld DOM mild steel and it will hold fine but I don't think the NHRA will allow you to MIG weld any of it,It has to be TIG. Right???

 

Yea can MIG weld mild steel and get it to pass inspection, TIG is prefered. If using 4130 you have to TIG weld it to pass inspection. If you are a serious with performance then use 4130 it will save about 30% weight compared to mild. But on the down side 4130 is nearly 3x the price of mild. Any Q's about NHRA spec can be forund on their website and many others, try searching. For my cage I just went for mild steel for now, since I don't plan on runing 11's sec passes right now. If I do get to that point loosing some extra weight with 4130 could help.