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post #1 of (permalink) Old 06-21-2003, 09:15 AM
KenTheJeepGuy
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Roll Cage tips

I finally have some time to spare and some nice weather to kill it with.
I was going to start welding up my roll cage.
I have a 220 stick welder, and a 135a mig. between the two i think i should have the right tools.
I just am curious what steel should i use to fabricate a roll cage?
Do i need like CRO-MOLY, or will mild steel work?
what diameter and wall thickness do you all recommend for such a job?

And what about like shock towers and such? will mild steel work for this?

Is there anything mild steel isnt good for?
like when would you need to go with a special steel?
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post #2 of (permalink) Old 06-21-2003, 09:45 AM
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Re: Roll Cage tips

naw, mild steel will be fine. chromoly is used in race vehicles for weight reasons but is more brittle.

i dont think the 135a MIG will suffice, but if you are handy with the stick it should turn out just fine.
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post #3 of (permalink) Old 06-21-2003, 01:05 PM
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Re: Roll Cage tips

In order for a bar to be made with CM, it should be TIG welded, not MIG or stick. Using the wrong type will result in a weak weld.
The 135 may look like a good weld, but it's not strong enough to get decent penetration.

And - you can greatly improve the strengths of the joints by stubbing. Say you are using 1 3/4 x 1/8 tube bars. On all connections fishmouth the connector, mark where it goes, drill a 1 1/2 inch hole at that point, then slide a short piece of 1 1/2 x 1/8 tube inside it - extending into the holed tube, and back into the connector at least as long as the diameter of the tube.
It makes it mechanically strong even if the weld isn't great, and you can turn up the heat for good penetration without burn-through. The weld doesn't take the load that way, it just holds everything in place.

Remember, no shortcuts - someone's life may be at stake.
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post #4 of (permalink) Old 06-21-2003, 01:52 PM
 
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Re: Roll Cage tips

If you will be doing any serious off-roading I would not use mild steel on any critical components (like roll-cage or suspension), but rather DOM or cro-moly. Keep in mind that in order to do proper job with cro-moly you should TIG weld it and it should be heat treated (usually out of scope for average garage wrench). plus cro-moly is much more expensive them DOM tubing.
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post #5 of (permalink) Old 06-22-2003, 01:56 AM
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Re: Roll Cage tips

Isn't DOM still made from mild steel?
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post #6 of (permalink) Old 06-22-2003, 07:09 AM
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Re: Roll Cage tips

DOM is just plain old mild steel tube - electric welded unless you get the very expensive seamless DOM.
DOM means Drawn Over Mandrel. The process of pulling it through a mandrel makes it slightly rounder, insures the wall thickness is more constant, and it work hardens it some.
The main use is hydraulic rams and things that need the "roundness."
It is a little stiffer, it's harder to bend, and possibly slightly stronger, but since it's work hardened, it's also more brittle.

For roll cage construction, you have to decide - is bending or distorting "more better" than shattering or breaking?
The small difference gained, or lost, with DOM isn't near as important as structural design of the cage, the way the bends are bent or crushed and integrity of the joints. One brace in the right place can more than triple the strength.

Water pipe is designed to carry pressure from the inside, not take stresses from the outside. I know some people use water pipe for cages, but most professional racing associations ban it. Water pipe tends to shatter or split when impacted from the outside.

I've even seem PVC pipe used, covered with padding to "look" good.

Plan the cage on paper first, then try to imagine the streeses from an impact - do it from all directions. Then brace for it. Think about when/if a brace comes loose - does it become a spear to impale you?

And remember - it's not fun when your safety equipment hurts you.
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post #7 of (permalink) Old 06-22-2003, 11:52 AM
 
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Re: Roll Cage tips

mild steal tubing is hot drawn, while DOM and chromoly are cold drawn. For roll cage I would use 'seam-less' DOM. DOM tubing is about double the price of mild steal tubing, and price of chromoly is 2-3 times of DOM
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post #8 of (permalink) Old 06-22-2003, 11:54 AM
 
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Re: Roll Cage tips

A proper study done by a board memeber from here.. (sorry forgot your name) concluded that SCH40 pipe... 1,1/2" I/D.. about 1,3/4 outside... is equal if not stronger then the equivelant in everything EXCEPT CH-mo..

I was doubtfull so I emailed him and asked for qualifications. :P

Sure enough.. University major who did an experiment with the different kinds of steel and have scientific data and conclusions to support his argument..

Verdict..

Pipe weights about 20% more.. that's the only difference..

I've got pipe.. and out whole club uses pipe because of the significant cost reduction.. we've seen some pretty violent rollovers and flops.

my 0.02 CDN..

about $0.004 USD
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post #9 of (permalink) Old 06-22-2003, 12:12 PM
 
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Re: Roll Cage tips

If you want some tips in design here is a great post from the past with some tips from our resident tried and true expert H8MONDAY. [img]images/graemlins/burnout.gif[/img]

clicky here
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post #10 of (permalink) Old 06-22-2003, 12:35 PM
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Re: Roll Cage tips

I'm sure you'd have a tough time getting NASCAR, USAC, SCCA or other proffessional associations to use sprinkler pipe.

The study - by the same "educated?" people that say off-roading, roads in forests, mining, foresting, oil drilling, producing energy, and vehicles in general are endagering our environment?

Or are they same "educated" lawyers that think "Hot Coffee" shouldn't be served hot?

Water pipe will work if the structure design is correct. I just don't like the potential shattering or splitting. The difference in strengths can be made up with better design. But that means even more weight -- to save what, $10?

Put a small hunk of water pipe in a hydraulic press - push on it sideways, watch it split. Push on it lengthwise, splits again.
Now try that with ERW or DOM. They bend, buckle, deform, but don't fly apart.
Yes, I've done it.
Put a hunk of ERW in a manual bender, give it a good bend. Now do the same with DOM, about a 50% harder pull.
Now bend it back - the ERW bends back - harder than it was, but it does.
Bend back the DOM - even harder.
Now acid etch both - see the stress cracks on the DOM?
Since the DOM has already been cold bent (drawn) once it's ahead of the ERW in becoming crystalized.
Now bend the ERW again - stress cracks - but it's the 3rd bend, not the second.

But it's your life - or your kid's.
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