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Running into tubing problems....


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Guest Anonymous

I've been getting my act together on the material aquisition end of the giant project I'm about to begin. I have the chassis design on paper and have done the math to make sure the thing won't collapse or warp. I figured out the quantity of tubing I am going to need and figured that I will be putting in around 125lbs of steel after all is said and done. This isn't really an accurate number because I will also be removing an unknown amount of metal before I begin welding in new stuff. Here is where the problem begins.

I was initially planning on using 4130N Chromoly tubing in the spaceframe but when I called the tech line at The Chassis Shop they said "MIG welding of chromoly isn't recommended". I asked why this was the case and the guy got all tight lipped and started talking down to me like I was some fool off the street. That pissed me off and I let him know it. Now I'm really not wanting to conduct any business with these guys because it is the third time that I have run into an attitude with their tech reps. Why can't I weld chromoly tubing to mild steel??? I have talked to a welding supply house in town and they said that with .023" high chrome wire and the right gas it shouldn't be a problem. Was the guy at The Chassis Shop just a ignorant fool or are there some real concerns?? My other option is to go with mild steel tubing. This would also work well and even though the strength isn't as high as the chromoly it would still be sufficient. It would also be easier to cut, form, and weld. Should I just go the mild steel route or should I go through the trouble of making the chromoly work??

 

SpencZ

rantin'like always....

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I'm no welding expert, but I'm working with a guy that is. He definitely recommends mild steel instead of chromoly for anything short of a pro race car. The Chassis Shop guy might be a pompous twit, but basically, he has a point.

 

Chromoly steel tends to brittle. The heating and (cooling down) inherent in the welding process tends to exacerbate the natural brittleness of the material, and may cause the formation of local microscale imperfections. These are the weak links in the chain. The result will still be strong in continuously applied loads, but if you whack the welded are with a hammer (or crash the car!), it might crack at a lower load intensity than the corresponding mild steel structure.

 

Your estimate of 125 lbs for the roll cage weight sounds about right, if you're building a full cage. With chromoly, you will save about 50 lbs. So, we can invoke the usual statement that it's easier and cheaper to add a little more power to the engine than to try to shave off the last bit of weight off the car.

 

If you have your heart set on chromoly, it might be worthwhile to first visit a local welding shop and talk to the pros.

 

"Chassis Shop" has a good deal on chromoly, but NOT on mild steel DOM tubing. You will have better luck at a local supplier.

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Guest Anonymous

...looks like I'm going to go with the mild steel tubing. I have heard that the welding process shocks the chromoly but didn't realize the consequences of such actions. What should I increase my wall thickness to if using steel? My weight estimation of 125lbs was not only for the roll cage but also my entire spaceframe. I calculated that weight for .058 wall (around 1/16") chromoly. The same wall thickness mild steel tubing weighs the same. should I go up a few thousanths in wall thickness for the mild steel? I will have to use .120" wall for the rollcage portion of the spaceframe but the rest definately won't have to be as thick. Any suggestions???

 

 

SpencZ

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Guest Anonymous

Hi,

 

Not defending Chassis Shop, but they hear this about welding CM all the time I'm sure. CM needs to be strss relieved to avoid cracking and this is beyond most home shops. He should have just told you that instead of giving you the product liability routine. There may be a way to weld CM without stress relief but I've not heard of it. I'm not a welder so maybe some of the others here can enlighten us. Also I think most shops TIG weld CM, not flame and rod weld it.

 

Richard

 

 

 

 

------------------

I can't tell the future but I can take a hint.-Webb Wilder

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I am a certified welder, although I haven't welded in over 10 years, and what Michaels friend says is true. CM tends to not recover well from the heat generated during the weld. Your best bet is to go with mild steel. Another issue to consider is if you ever decide to add on to your cage, say that shop goes out of business or you move...Everyone on the planet has mild steel in stock, CM is another story!

 

Mike

 

------------------

 

"I will not be a spectator in the sport of life!"

mjk

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  • 5 years later...

well i've done my fair share of research on welding 4130 and most people (air craft builders) that do tig and gas-weld 4130 don't stress relieve. keep in mind, if these welds break, they are in the air, and they will die. your best bet for research on welding 4130 is through a air plane hobbyist/fabricators forum. the reason why people don't recommend mig'ing chromemoly is it is so hard to manage heat; whereas tig you have your foot pedal and gas welding you can remove the heat source.

 

my question is: if one were to mig weld chrome-moly by doing a penetrative tack, quench it with water to let is cool down, and repeat until fully welded, would that generate less heat on the metal and therefore not make it weaker? just somethign i was pondering in the back of my head.

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my question is: if one were to mig weld chrome-moly by doing a penetrative tack, quench it with water to let is cool down, and repeat until fully welded, would that generate less heat on the metal and therefore not make it weaker? just somethign i was pondering in the back of my head.

 

Well I am no welding expert, but cooling hot metal quickly will harden it making it brittle. Better to have the metal bend than crack. One way to do this is to cool the metal slowly and evenly. That is why some welding experts contend welding chrome moly with a torch is referrable to TIG because they believe it helps stress relieve the material.

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What does your spaceframe design look like and what wall thickness of steel are you using? I've pretty much finalized the design for my build, and mapped it out in a spaceframe design program to eliminate superfluous weight where there was minimal rigidity gain... Still looks like my spaceframe is going to tip the scales at about 300 pounds.

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What does your spaceframe design look like and what wall thickness of steel are you using? I've pretty much finalized the design for my build, and mapped it out in a spaceframe design program to eliminate superfluous weight where there was minimal rigidity gain... Still looks like my spaceframe is going to tip the scales at about 300 pounds.

 

 

where could i find a "spaceframe" program? i searched online but couldn't find none... sounds like somethign i should look into.

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Welding medium carbon steel (4130) requires heat management to keep the carbon from migrating together and creating a brittle part. But, there's a lot of misinformation because the processes are designed for thicker 4130 sections then what we normally deal with in the automotive world. So the pre-heat, interpass temp control, and post-heat numberas are irrelevant.

 

Our basic problem is having the part cool too quickly after welding, not too much heat input. Not matter what you're doing you're heeating the weld and a good part of the HAZ to 2,800+ degrees. What's important is to control the cool down and that's why time is spent reheating and "normaling" the weld area.

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Sleeper-Z

 

I am using a freeware program called GRAPE. It isn't really meant for automotive chassis design, but it fits the bill... and it is free! You can download it here... http://www.grapesoftware.mb.ca/

 

Here is a link on how to use the program. It is a little tricky... and I admittedly had an engineer friend help out with calculating the j-inertia and all that jazz.

 

http://corner-carvers.com/forums/sh...=grape+analysis

 

When all is said and done, my frame looks somewhat like this fellow's. He is the one who began the thread on using grape that I linked to above.

 

http://www.carter-engineering.com/preston67compcoupe.htm

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http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp has info on TIG welding 4130. www.metalforming.com also has interesting reading. None recommend MIG welding but neither indicated why. I have MIG welded some additional bracing on a 4130 Z car cage and it looks great but one can't tell easily without testing it by crashing, right? The Lincoln article listed slow cooling to reduce internal damage. I think this has to do with heating and cooling of 4130 so preheating before MIG welding should be better than not.
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Sleeper-Z

 

I am using a freeware program called GRAPE. It isn't really meant for automotive chassis design' date=' but it fits the bill... and it is free! You can download it here... http://www.grapesoftware.mb.ca/

 

Here is a link on how to use the program. It is a little tricky... and I admittedly had an engineer friend help out with calculating the j-inertia and all that jazz.

 

http://corner-carvers.com/forums/sh...=grape+analysis

 

When all is said and done, my frame looks somewhat like this fellow's. He is the one who began the thread on using grape that I linked to above.

 

http://www.carter-engineering.com/preston67compcoupe.htm

 

 

thanks, i'll be definately looking into that... how much are you planning to weigh? i'm trying to get as close to 1500 as i can with all fiberglass panels, possibly aluminum wheels and with a motor/trans setup about 100lbs less than stock.

 

i may look into doing a fiberglass main body (roof and rear side).

 

-steven m.

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  • 2 weeks later...

Bootlefoof, You're not going to meet your 2000# goal. Just not gonna happen... not with the words "creature comforts or street legal" in the project design.

 

I sponsored a car from northern California from 2000-2004. Ian and Steve and removed everything from the car except for the running gear (the L6 Met F-prepared specs for SCCA solo2), a windshield, no dash, race seats, NOTHING in the interior, plexi hatch glass, door skins without any frame work or hardware (They flapped in the breeze as the car went around the course) and a skin for a hood... They had no bumpers, no extras, not even horns or headlights. The headlight housings had covers over them.

 

They had a four point roll bar at the time, two race seats, and harnesses. They also had lightened suspension parts ie coil overs and control arms, and the shell had been scraped of all undercoating. The doors didn't even have regular hinges, but used cabinet hinges. :shock:

 

It weighed in at about 1980#s. This was a 72 model shell. No glass in it, except for the windshield, no heater motor or controls, this thing was a rolling SHELL of a 240Zcar.

 

You following me here? These guys owned the field when they raced, but typically did NOT drive the car on the street. So keep that in mind when you start to build your car. They eventually ended up with my old Marsh racing carbon/aluminum 3 piece wheels and installed a true cage, along with a fuel cell, and I think the car is still somewhere around 2000 pounds, or just under. But this car is a true track only car.

 

Mike

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