Rollcage questions

[millermania]

I've started looking around for someone to build a cage for my racecar and I wanted to get some advice from folks here. There's a shop that a lot of local racers use to have cages built. They all claim his work to be top notch, but they don't know a whole lot about welding so I thought I'd go over to the shop and take a closer look. After speaking with the cage builder, a few things bothered me (after seeing some awesome welds on Andy's cages). The first thing that concerned me was that on all the joints, he stitch welded them (as evidenced by a row of dimpled MIG beads). The next thing that bothered me is that he said when you weld baseplates to the floor, you have to MIG everything because the chassis sheet metal contains contaminants that only MIG can deal with (galvanizing, other weird chemicals from the car manufacturer). Oh and he's using mild steel tubing.

The guy is a pretty good engineer and his designs are sound, but should I be wary of his welding ability?

[MAC702]

Generally speaking, slag-producing processes (FCAW and SMAW) deal much better with contaminants.

[Scho]

I TIG weld all bars and MIG weld floor plates and such that are connected to the chassis.

I would not have my cage stitch welded at all. You get poor penetration and what you are after is complete fusion of all joints for maximum strength.

Good luck.

[JMFabrications]

I also MIG weld floor plates and TIG the bars, easiest way to do it. I wouldnt trust the stitch welding either.... Couldnt you ask them to TIG weld all the joints?

[Speed Raycer]

Like the others, I use a MIG on all of my pads because of the time factor involved. Not welding time.. prep and tungsten grinding time There's just too much residue from seam sealer/undercoating/gal coat paint etc to deal with TIG. If it was a "body in white" or a tube frame application... TIG all the way, but in a production car it's not an easy process.

I TIG all tube to tube joints. Occasionally I'll have to break out the MIG for a joint that I just cant get to with the TIG torch, or where I'm welding looking into a mirror

Millermania... you're not in St. Louis are you? If so, I have a feeling I know who your talking about...

[Gary]

I have found the stitch weave welding technique to work very nice to help remove or burn off coatings like galv. I personally think it can work better than a large side to side weave. I think the welds can be very strong as well as have great bead appearence and good wetting at the toes if applied correctly. This is a matter of opinion and what welding technigue that each of us has found to work the best for us. I also would not argue with the guys who prefer to tig welds these criticle joints. Again its what they feel comfortable with.
Gary

[Speedddy]

Rollcage

I would never use mild steel.I would use chrome molly, harder steel.You will have to pre heat the chrome before welding.I would fully weld with a 3/32 root and filler, cap.I would take the extra time your life is riding on the cage.Mig is fine if you use gas 75% argon ,25 % co2 not flux core wire.Tig would be better. But it will take more time skeel.I would leave 2/32 hole drilled in the legs at the bottom for air vents. If not the pipes will rust from the inside from condensation.If you look under hand rails you will see alot of the time it will have air vents.

[Scho]

I would never use mild steel.I would use chrome molly, harder steel..

Why do you say this? Chromoly has a tensile strength closer to its yield strength which means it will not give as much in impact before failing. I am curious as to why you recommend Chromoly as opposed to mild steel??

[ross]

Why do you say this? Chromoly has a tensile strength closer to its yield strength which means it will not give as much in impact before failing. I am curious as to why you recommend Chromoly as opposed to mild steel??

Chromoly weighs less.

[Compchassis]

Moly does not actually weigh less than mild steel. Both steels in the same diameter and wall thickness weigh exactly the same. The weight savings comes from the fact that NHRA and IHRA allow you to use a thinner wall.

Both steels have thier uses. Unless you are running a car in a class that requires moly, or trying to save every last ounce, I dont recommend moly on customer cars. 90% of the cars out there dont need moly and most people dont have the ability to weld on them, so every little bracket that they might want to add means the car has to be taken back to the chassis shop.

Mild steel is plenty strong. Look at NASCAR. Those cars are built of 1 3/4" .090 wall mild steel, weigh 3500 lbs and pound the wall pretty regularly at 200 mph.

[TheRealSpinner]

I vote no to cromoly

I have a friend that has been doing some research on regulations for cages. Most say that mild steel of (whatever) diameter and thickness is fine. I also believe that this could be tig'd of mig'd. I am quite sure that everything I have seen says that cro-moly MUST be TIG welded.

I also feel that mild steel would be better because cars, in general, are built to crumple so that there is less stress on the driver. The faster something stops, the more force is exerted on the driver. Mild steel will bend significantly before it starts to tear. Cromoly, on the other hand, is much harder. It will hold its shape longer in an impact, but once it reaches its stress point, it will crumble (tear) apart.

Because cromoly holds its shape longer, the cage will retain its shape in an impact, but the driver will feel the impact more. If built right, the mild steel cage will still protect you and will also give you a more comfortable wreck.

Also, cromoly is MUCH more expensive than mild steel, even if you factor in the difference in wall thickness needed.

[NorthernLights]

Chromoly may offer a weight savings because it is stronger other steels, thus can utilize a thinner wall to provide the same strength. There are a wide range of specs out there for chromoly and high strength steels, so I am using some average numbers. 4130 chromoly will have a yield strength of about 85K vs. 65K of 1018 steel (BTW, standard mild steel is about 36K). The chromoly tube should be about 30% lighter because it can use a thinner wall thickness to achieve the same strength tube for tubes of the same O.D.

[Billet Benny]

What in the world has happened to this board?? Saying chromoly is "harder" than mild steel is not necessarily the best term. And it's not necesarily true. Hardness refers to a steels ability to resist penetration and this aspect doesn't really make one cage better than another. And suggesting to weld a thin wall (in the range of .120" and under) cage with an open root pass, a fill pass, and a cap pass?? Oh my. You fellas need to read a book and check out the real world. Also, a chromoly cage will not just explode when it fails and you won't be able to "feel" the difference when you cream a wall. Cage design has more to do with how the structure reacts to a wreck than material. I posted this on pirate4x4.com a few weeks ago in a Roll Cage thread. I think it may help here too. Enjoy.


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I just read this and am glad to see it. However I want to add a few things myself.

Mild steel tubing is made from sheet that is rolled and welded. The alloy is typically 1010 or higher. This material is not as strong as the others, but is totally acceptable with a proper design. It is even preferred by some for its tendency to bend before breaking.

Mild steel tubing is NOT always made from rolled and welded strips. You can get "seamless" mild steel tubing. More clarification later.

DOM steel tubing is manufactured the same way as mild, including the welding. The alloy is typically 1018 up to 1026, the higher the number, the higher the carbon content. DOM means Drawn Over Mandrel, which “trues” the tube and hides the weld, giving it more accurate dimensions. DOM is about twice the cost of mild, and almost as much as 4130.

DOM is manufactured the same as SOME other tubing, but not like all mild steel tubing. The drawing process also helps to add a little strength due to the cold working of the metal as well as tighten dimensions. Prices and differences are going to be regional. More to follow.

4130 chromoly steel tubing is a true seamless tube, with chromium and molybdenum added for strength and light weight. 4130 is very expensive and is used most often in big budget builds. It requires heat treating after welding to achieve maximum strength. Some say that if a 4130 chassis is not heat treated, it is no stronger than the other steels. 4130 suspension components should definitely be heat treated.

Ok, 4130 is NOT always seamless tube. 4130 can be DOM though it's not extremely common.

There two main factors to what makes a tube and that is the material (for example 1010, 1020, 1144, 4130, 4140, etc. etc.) and the process in which it goes through to become a tube.

The process and material have no correlation. 4130 can be made via all kinds of processes as can any grade of mild steel.

The processes of manufacturing we are concerned with are going to be A512, A513, and A519. These are PROCESSES and they can be ANY grade (aka material)

Definintions:
A512 - rolled tubing that is BUTT welded at the seam. The least desireable process of the 3 we are talking about. Its benefit is cost.

A513 - this is rolled tubing that is electric resistance welded down the seam. This is where DOM, ERW, and the like come from. Within this spec there are different types. These types explain further processes or lack thereof about the tube. A513 Type 5 is your DOM. Type 5 means it has been drawn over a mandrel after being rolled and resistance welded. When you refer to DOM you are saying A513 Type 5. A513 Type 1 would be as welded hot rolled and A513 Type 2 would be as welded cold rolled. A513 type 5 can be made from 4130 steel and therefore would not be a seamless tube, yet it's still 4130. Even beyond all these specs the tube can then have other processes done to it such as annealing or stress relieving.

A519 - this is your "seamless" tube. You get a rod pierced and extruded into a tube. It is strong though not as dimensionally accurate as DOM. Just like every other tube, it can go through stages of heat treating before being sold. A519 can be any grade of carbon steel, alloy steel, or chromoly. Most of the 4130 tube that is used in our case (and aircraft) is A519 in the normalized condition.

In my shop I have 20 feet of "seamless" tube that is 1020 mild steel. It is A519 1.5" od .095" wall 1020 stress relieved. It is actual seamless and it is mild steel. To boot it was stress relieved.

As for this heat treating 4130 jazz. 4130 is used in the normalized condition for airplanes and cages because it is both workable in that condition, weldable if done properly in that condition, and strong in that condition. It is not as strong as 4130 can be but it is still extremely strong and almost as workable as mild steel. I do not believe all 4130 light wall tubing welds need post weld heat treatment. I am not the only one. Look at nascar suspension parts and other planes and cages being built by knowledgeable builders. IMO, heating with a torch cannot be controlled in a shop properly enough to do the job that's trying to be done. There are methods to properly weld this stuff without PWHT work. There is an AWS spec in the works for light 4130 work for use in the aircraft and racing industries. It will specify proven and tested techniques for welding 4130 for these applications. It will not include PWHTing the part. Hopefully it will be in print soon. I believe 4130 is an experienced builder's tool and should not be used and welded by those who don't understand it's metallurgical properties or advanced welding techniques. 1020 is by far the best option for the serious wheelers who are hobbiest builders. That's my opinion.

[Billet Benny]

Chromoly may offer a weight savings because it is stronger other steels, thus can utilize a thinner wall to provide the same strength. There are a wide range of specs out there for chromoly and high strength steels, so I am using some average numbers. 4130 chromoly will have a yield strength of about 85K vs. 65K of 1018 steel (BTW, standard mild steel is about 36K). The chromoly tube should be about 30% lighter because it can use a thinner wall thickness to achieve the same strength tube for tubes of the same O.D.

1018 is a form of "standard mild steel". Standard mild steel covers a very broad range of materials.

[90blackcrx]

I will comment on the stich welding.

A lot of cage builders beleive stich welding is better, because if you ever do roll over and a weld rips off, well with stich welding that would just happen, one weld would rip off.

But if you welded it all around and connected it, if one side pulled up, more then likely the whole plate would come up to.

Make sense at all ?