input needed: Why tubing and not pipe for rollbar/cage?

I've got a '30 mordoor Model A.
As most of you know, the body is basically a sheetmetal shell wrapped around a bunch of wood… like the chevy's of that era.
I am wanting to build a mostly street driven car that might go to local drag strips. (weight is not a major factor)
I want 4 seats for the family, all with racing seats and harness.
The plan is to build a semi-full cage and use it for protection AND to support the body sheetmetal.
My real dilemma is:
I've got a bud that can do great things with 1.25" pipe and a bow/arrow type bender.
1.25" pipe is close to the same OD as typical mild steel rollbar tubing but is quite a bit thicker. (moly is out of the question)
Pipe is also considerably cheaper and easier to get.
The cheaper pipe would be nice but safety comes before money.
Even though the pipe is thicker, it may be a lower quality steel that might fracture or have other problems if tested. (lower true strength)
I can only assume that the NHRA knows that tubing is safer. But for all I know, they don't use it because it is so thick and heavy…
It seems to me that it would provide as much strength and protection as lighter tubing but I need you guys to set me straight.

Thanks all... my sig says it all.

(I have indeed used the search but I stink at it)

 

It mostly has to do with weight. Can you imagine what a Pro Stocker would weigh if it had a PIPE cage?

But it is untested steel you would be using. I'm sure it would be safer than nothing, but if you take it to an NHRA track and a tech official sees what it is, he may kick you out. Or just laugh at you.

 

Pipe is for POOP!

 

Or Gas

 

pipe is made of all different qualities of steel, and there are alot of inpurities. This could easily cause weak spots in the pipe and this is why it is not used for such things as cages. Cant put a price on safety, and it will save alot of weight.

 

1 1/4" pipe is 1.66" O.D.(outside diameter), schedule 40 is .140" w.t.(wall thickness)
1 1/2" is 1.90" O.D. and .145" w.t. in sch. 40. This stuff will withstand thousands of pounds of pressure, where do you get your info that it's junk? I'm talking pipe used in chemical plants and oil pipelines, not the stuff under your sink or toilet.

 

Here's a couple of exchanges from a CNC machinist site:

http://www.CNCzone.com/forums/archive/index.php/t-18298.html

Are there any strength difference between the pipe I can get from the hardware store compared to DOM steel tubing? I'm not sure if I could use the pipe in place of DOM and still retain the strength?

There is a difference in strength between ordinary pipe and DOM tube; for the same OD and wall thickness DOM will be considerably stronger but if you are planning on using the material for the tracks on a router or something like that this is completely unimportant.

The important property for a router application is stiffness not strength and there is a very important distinction between these two properties of material. Strength is a measure of how much force needs to be applied to something before plastic deformation occurs; i.e. it is permanently bent and does not return to the original condition. Stiffness is how much elastic deformation occurs under a given load and when the deformation is elastic the material or structure returns exactly back to the original condition when the load is removed.

With any material under load elastic deformation occurs first followed by plastic deformation when what is called the yield strength is reached. With DOM tube a great deal more load can be applied and a great deal more elastic deformation will occur before the yield strength is reached than with pipe.

Ok I got a question, What is the material difference between black pipe and DOM,I know black pipe has a lacquered finish to minimize rusting, pipe is measured ID, DOM is measured OD, DOM wall thickness is .120 where as pipe is .113, ( BTW I am talking 1" DOM or 3/4 black pipe) whats the metalurgical difference. I want to use black pipe to build a motor cycle frame. I talked to a pro TIG welder and he mentioned that pipe does not weld as good as DOM? Any input.

Black pipe is not suitable for this application; I could say definitely, absolutely not suitable. Black pipe is very low carbon steel with a low yield strength and a low ultimate strength. DOM can be obtained in low and medium carbon steel and in some alloy steels. It has a much higher yield strength and ultimate strength.

Making a motor cycle frame is something that you need to get experienced advice on. Having a weld break or having the frame crumple while you are riding could make life interesting for a short while.

 

It's not junk. But it's also not the right stuff for the application.

Besides, being able to withstand high internal pressures and being able to withstand impacts without collapsing/deforming are two very different things.

Not to mention it doesn't bend very well, and those pipe bends look like shit.

 

I come from a circle track background and it amazes me how many people still use pipe in their racecars. My racecar has 1.75" .095 wall tubing. The thicker wall of the pipe doesn't allow uniform radiuses to be bent. They tend to flatten out. You'll run into this more frequently with the type of bender you described. Unless all your joints are perfectly notched (preferably .125" gap or less) you might not get the propper penetration that is easily achievable with tubing and a good mig/tig welder. In case of an impact the pipe joints are more likely to shear than a properly welded tubing joint. A lot of that pipe is also galvanized which can make you very sick if you weld on it for a while. One more thing, my car has a crush zone in the front and back made of smaller diameter tubing. It's meant to give before my suspension pickup points so I don't ruin the chassis. If you have pipe with basically no give impact energy will be transfered to a weaker point. That stiff cage could find a weak point on your chassis and actually break off, taking whatever is mounted to it, i.e. seats, belts, driver, etc...I know pipe is cheap and tempting but in the name of safety for yourself and others, don't use it. I know somebody will say they used it for years and nothing happened, but there's a reason every sanctioning body in motorsports mandates rollcage tubing. Good luck.

 

i appreciate the input from all.

as always, the HAMB can even steer a redneck in the right direction!

 

I don't think the hardware store stuff is ASTM A106 seamless pipe. I mean REAL pipe, not cheap hardware store stuff. Anybody know?

 

I think you may have a point. Or several.

 

What about the 3x2x.120 wall tubing I bought to build my frame? That seems to be the popular material. If pipe is not suitable for a roll cage(I probably wouldn't use it myself), what about for a front crossmember/spreader bar, or bracing and trans/engine mounts?

 

I believe you are talking about structural steel tubing. Still, I would not use pipe! Proper wall thickness and good weld penetration will provide a satisfactory frame. Remember that with a beam front axle and an old style rear suspension, the frame will see some torsion loads so the fit of the pieces and the weld quality matters.
Other opinions?

 

Weight is part of it, but it has more to do with tensile and yield strength of the materials.

Tube is higher quality and has higher yield numbers.

Simply put, pipe of the same size and thickness as tube will not hold up to as much load as the tube would.

Now, most of the time pipe is heavier-walled than tube, so it's not an apple to apple comparison.

..but typically the tube still wins, but not by a big margin. Why? Now we get into things like section modulus and how a given load is applied to a given structure.. which ultimately gives you a "effective point load" of X psi which you can compare against the tensile strength of the tube or pipe.

FWIW, pipe is usually easier to come by, but they sell steel by the pound, and pound for pound tube is stronger than pipe (which is why racers use it.. stronger and less weight sounds like a winner.. and 4130 takes this another step further).

ERW/HREW 1010 grade tube is usually around the same price/lbs as Sch40 blackwall pipe. That means it should be cheaper (or at least, no more expensive) per foot than Sch40 pipe.

1020 DOM tube is stronger still, but double in price.

I get 2x.120 HREW for $2.20/ft, and 2x.120 DOM for $5/ft.

 

so in retrospect...

get yourself a basic 8 point kit for the application-it might set you back 150 bucks or so, but in the long run, you will have better welds, a stronger structure, the vast majority of the bends are already made, an official won't kick you off the track, and it has a better chance of saving your life.

not to mention that if you get around to painting it, it's not gonna kick your ass in prep work to make it smooth.

always a plus for those of us whom are "rustoleum challenged".

 

NHRA rule book calls for1.750x.118 mild steel tubing, or1.750x.083 chrome moly, so why ruin your chances of racing by using anything less.

 

im with lucky77 on this one. i have heard from many people that the pipe just doesnt work as a roll cage, it breaks instead of bends....just what i was told.

btw- good luck with the build and keep us posted. i'm startin to see more 4drs of all sorts being built. thats a good thing....now us broke guys can be cool too .

 

Greyone is right just get a copy of IHRA or NHRA rules and its end of discussion...

We run an NHRA approved RWD honda,pression turbo setup, and prior to that we ran an MNRA outlaw mustang on 10.5's and the tech inspectors in both sanctioning bodies check the tube frame closely...

As a point of interest we had a camera in the car while test and tuning last year when we hit the wall .....should have seen the flex in the tubing...never happen with pipe...

besides the guys are right ...would look like crap...

DO it right.

Robert BUllard Toronto

 

Here's a question as long as we're on the subject. I built a 8 pt cage with cm 1 3/4 .093 tubing, then to make it removable I basically cut it in half. Now for the joints couldn't find any thing in cm that would slip fit over the tubing so had to go with schedule 40 pipe, it slid over perfectly. Each sleeve is 8" long so 4" on each side of joint. Four 3/8 bolts 90 degrees apart bolt each one. Would this be approved method? or is there something better?

 

When you say "approved" method I'm assuming you are asking if it would pass NHRA or IHRA tech.? As far as I know, no bolted-in deals are legal. I think the roll bar or cage must be welded to the floor of unibody cars, along with a stout flat plate to reinforce the floor where it is attached. On a full frame car, the tubes are supposed to go through to the frame and get welded. That being siad you might get away with it if you were stealthy about how you did the work...no tech. inspector has ever gone under my car to scrutinize my cage to that degree-but I'll bet they do as you start to run faster. The other point I'd make to you is that I personally would not trust the sleeved and bolted setup in a hard crash. I would think there are all sorts of bending, compression and shearing forces on that cage as you are wadding up your car...and I would not want to think of the consequences of having it fail on you.

-Scott

 

You want seamless construction in cage material, pipe has welded seams. DOM tubing (drawn over mandrel) wont split and will conform well to bends.

 

I wouldn't want my head in that. If you are talking about something that bolts together at the top, I can't imagine it certifying regardless of what the sleeve was made of. As I understand NHRA rules, butt joints must be sleeved and welded with visible evidence of reinforcement (that means rosette welds on each side of the welded joint). However, I am not an inspector. I would get some serious advice on this one from someone who will be approving or not approving the chassis.

It is probably worth mentioning that the rules regarding roll bars and cages are based on elapsed times, speeds, and modifications to the car. Anyone planing to race needs to get the rule book of the sanctioning body for the track at which you plan to run. It is entirely possible that the car will not perform at a level that requires a roll bar or cage or will not be of a body style that it is required. At that point, you only have to worry about whether you want the protection or not.

That's my two cents worth.

 

I don't think NHRA specifies DOM tubing and I don't think the kits you buy from chassis builders are DOM.

Now a question. Isn't DOM welded and then DOM?

 

No. DOM steel tubing starts out as a round steel bar, is heated and then drawn through a die and pierced with a pointed mandrel. That way you have a continuous tube without a weld which is the point of the process.

A welded pipe starts out as a flat strip, is heated and then drawn through a round die to form the pipe and then welded along the seam. This weld produces a heat-effected zone on either side of the weld. This heat-effected zone is the weakest point in the pipe.

About 15 years ago an electric utility had a reheat steam line fail along a welded pipe seam that resulted in the death of four workers. The cause was erosion-corrosion of the heat-effected zone that thinned the wall down until it could no longer contain the internal pressure. The electric utilities nation-wide inspected the welded pipe in their steam plants and replaced the welded pipe where it showed evidence of erosion-corrosion. They all do annual inspections of welded seam pipe now.

You don't want to use welded seam pipe for your roll cage. Period. When you bend the pipe to get the shape you want, you add stresses to the pipe at the bend to get the plastic deformation. In other words you've reduced the load carrying capacity of the pipe. The size of the tubing required in roll cages takes into account the reduced capacity from bending.

When it comes to safety, especially of your family, I'd meet ALL the requirements of the NHRA rule book.

That's my nickle's worth.

 

A lot of that failure could be attributed to the alloy of the pipe and heat transfer. I work at a Nuke plant (Diablo Canyon) and this is something that is common on the secondary steam lines. This is going from memory because I don't deal with it but from what I remember the spec for the pipe alloy has .1%-.4% inconel (sp?). When you get two pieces of pipe that are still in the spec but at opposite ends (one is .1%, one is .4%) there is a change in heat flux at the weld. This causes more rapid erosion, even in a straight piece of pipe. You also get erosion at flow changes, like elbows, etc. We sonic check pipes constantly to schedule replacement at outages.

That being said, I think it has pretty much all been covered. Pipe is not meant for structural members. It is rated differently than tubing. Most pipe is electric-seam weld. Tubing in cages doesn't have to be DOM, HREW (hot-rolled electric weld) tubing isn't that expensive, go buy the right stuff. AZA-V8 covered it pretty well.

 

thanks all...

i feel informed. just what i wanted!

 

Sorry...but thats incorrect.
Regular DOM DOES start out with a seam weld and then is drawn to compress the material for strength and consistant wall thickness.

The pierced SEAMLESS tube you describe exists, but is extremely expensive and used in very high tech locations like Nuclear reactors etc, where failure is not an option.
I imagine it still gets "drawn Over Mandrel", but it isn't the DOM we use in automotive applications.

 

The above post is correct, DOM (drawn over mandrel) tubing is welded just like the electroweld tubing, except that DOM tubing is held to a higher tolerance as far as I.D. and O.D. go. Is is also made from a slightly better grade of steel. Most of the roll cage kits you buy will be DOM tubing from places like Art Morrison and Jeg's. I have installed hundreds of roll cages in my fab shop and the DOM will weld better than pipe, but there is no difference between the DOM and electroweld tubing as far as welding quality. I have built a few dozen IMCA modifieds and stock cars with the electroweld tubing and the cars have held up nicely. Back in the old days, my dad used to use schedule 40-1 1/4" pipe for his roll cages that were stick welded, they took some nasty flips and nothing ever broke. Stick with the electroweld or the DOM stuff and you will be good to go.

 

What he mentioned is seamless tubing. It is still technically drawn over a mandrel. Seamless tubing is used quite a bit (most decent quality bicycles are made of seamless tubing).