I've heard people talk about how much lighter chrome moly is and the benefits of using chrome moly, but I've also heard people say, for example: "A chrome moly cage will only last about 4+- years before it starts getting brittle, where a mild steel cage may last 25 years". I've also heard that the "brittleness" comes from the weld area not the chrome moly itself.

What exactly does this mean? Surely everyone doesn't replace every chrome moly part, cage, k-member etc every 4+- years......

Discuss

 

Theres a special process to welding CM correctly. Main reason why its used is lighter weight cause you can run a thinner wall. I bought mild steel for mine, I only wish I had paid the extra and gotten DOM vs the ERS I got, but no biggie. I believe most drag racing sanctioning bodies also require a replacement or re-cert every few years with CM.

 

Thanks Mars.

Anymore info as to why a cage may become "brittle?" and need to be replaced or certified but a K-member, etc. wouldn't?

 

I used to build my own Kart racing chassis (not yard karts, real karts) and the industry had no problem MIG welding CM. Which I found odd since everybody says don't do it. However these harts were Mig'd and we never had a problem with them breaking at the welds. Occaisionally we would get a fracture at a bend, but you have to understand that these chassis are one big torsion bar, the size, thickness and type of material were chosen to provide flex in certain areas. Keep in mind, these thing went 90mph, cornerred at 1G, and held a 190 pound person for a total weight of 350 pounds (160 pound chassis/yamaha engine/clutch/tires/brakes). When I started building my own stuff I was careful about welding, but I just let it normalize in the jig. Some guys would TIG weld these chassis and had enormous jigs, they'd put them in an oven and gradually raise, then lower the temperature to normalize them,
In any case, after these experiences, I pretty much blew off most of these rumors about metal fatigue and improper welding technique. I'm certain it can be screwed up, but I wouldn't believe 90% of the horror stories. Yes it's true CM is brittle, but chosen for the proper application, you should have no problem.
I love CM strictly due to it's weight advantages, it's extremely sturdy, and also extremely light. (actually it probably is the same weight as mild steel, but since it is stronger you can use less thickness, I stated that because some nick picker will chime in about the weight properties) If you are using it for a rollcage in a stock chassis car, that has subframe connectors and all of the other chassis stiffening stuff I don't think that you will ever over stress a chromoly cage even though you'll be using it as a chassis stiffener. Again, the big deal with CM, especially in road racing, is that it takes weight out of the top of the car (roll bar/roll cage) and allows you to place it where it it will do the most good, usually down low and over the rear tires. Here's more info than you ever wanted about welding CM, welding CM to Mild, etc. etc.

Miller - Site Search Results

Don't be afraid, young Jedi

 

4130 Chromoly or ( Chromium Molybdenum ) has a 0.30% carbon content... thus you will have a higher tentancy for stress cracking or metal fatigue... Chromoly still is considered technically a "low carbon alloy steel." But 1020 steel ( Mild Steel ) is also a "low carbon steel" but it has a content that is 0.20%. The carbon makes the metal easier to weld, but the more carbon content makes it more brittle and thus lower the tensile strength of the metal. However, these metals both have a very high tensile strength.

There are several other factors that cause cracking and twisting and metal fatigue but that is a whole other post...

Every national recognized sactioning body recommends GTAW ( Gas Tungsten Arc Welding or better known as TIG ) on a Chromoly ( CM ) cage...

I.E. The NHRA rule book states in Section 16 (General Regulations), Sub-section 4:11 (Roll Cage), second sentence, "All 4130 chromoly tube welding must be done by approved TIG process; mild steel tube welding must be approved MIG wire feed or TIG process".

If you plan on actually using the cage... i.e. reducing torque/flex on the frame and body of the vehicle for a long period of time and don't plan to replace or fix it, go with MS (mild steel) the only way you actually save weight with CM is that you can use it in a thinner gauge. It actually weighs the same if it was the same thickness as the MS. TIG welding process has to be used with CM and . But it does make a slightly better weld. Due to the ability to have a greater tensile strength rod. However, that being said, welding with TIG requires greater skill and knowledge of metals. Which is why it normally cost more to get done properly. ( THIS IS AN AREA YOU DO NOT WANT TO CUT CORNERS ON ) And why would you want to... if it is done improperly you will not be happy with yourself. I know I would not want to trust my potential life on saving a few bucks...

MS can be welded a few ways, but mainly it can be welded in GMAW ( Gas Metal Arc Welding or better known as MIG ) which is cheaper.

Bottom line, if you are racing it 100% of the time, it is worth the cost and weight savings for Chromoly...

Otherwise save your pennies for ponies...

Hope this post helps everyone in the Ford community.


Sorry for the long post just wanted to make sure I was clear on everything and did not offend anyone.. Hope I was clear, but any more questions ask away... and I will try to answer them as best as I can... I am by no means an expert so, I may be off on somethings.. but I believe I am close to 100% on most of this topic...

 

I would agree with Mr. Smokeshow, the bottom line is, if done correctly CM can offer the same amount of protection/stiffening as other metals at a weight savings, there is nothing to fear.
The SCCA rulebook states approximately the same thing as NHRA, however, they get into undercutting welds and other technical stuff that you will find in the Miller links that I gave you.

 

Thanks for the info guys!

I appreciate it even if it is a long read!


Terry

 

Just wanted to make sure it was clear on the info...

 

1020 D.O.M mild steel roll cage $150.00 per point
4130 Cro-Moly $225.00 per point.

Cro-Moly has a stronger memoryi in relation to mild steel; if bent it wants to flex back to the original position, so for that reason Cro-Moly is more difficult to bend (takes more pressure) When welding cro-moly with a er70s2 filler rod the material is no longer 4130 cro-moly unless a cro-moly rod is used and the entire structure is heat treated.

 

Mild steel cages will only certify to go 8.50 in the 1/4 no matter how many bars you put in it, chromolly will certify for much faster depending on what bars are added to it to meet a certian rating..... This is according to NHRA rules.....

I have seen plenty of 10+ year old chromolly dragsters that are still holding up fine, and they flex way more then any door car.....

I had a chromolly cage put in my car for 2 reasons..... 1. I wanted to keep it as light as possible, and we put what was necessary to certify for 8.50..... 2. It is always upgradable later to a faster certification, just in case and if you ever decide to sell the car it's worth more because it can be upgraded to whatever down the road.....