1020 Dom steel tubing & chromoly

[muffildy]

Hello,
I am working on designing a chassis for a car.
I was thinking that chromoly is a very strong tubing, but that the extra cost involved of having it heat treated after welding would make things too expensive.
So to get around this, i was thinking of using 1.75 in .095 thick 1020 Dom steel tube, i would make the chasis out of this, and to reinforce certain key areas i could insert 1.25 in .095 thick tube of chromoly using a rubber spacer on its bottom, and then filling in the extra space with an expanding urethane foam. Would this increase safety or would the chromoly/foam create points of high stress or be a liability in the Dom overall structure?

Would just using extra extra thick dom 1020 be better?

 

[salmon2]

Hard to comment the structural reliability as I don't know your application and load cases.

But with ERW/DOM, you have some limitation on the thickness, such as anything bigger than 10% of OD is very hard for ERW process.

 

[ohmdr]

What is the intended purpose of this chassis? Daily use, racing, etc.

Many drag racers (from your weekend warrior to professionals like John Force) use Chromoly tubing to build their chassis and some of them are putting their lives on the line at 300 mph with it. Chromoly must be tig welded to get good, strong weld. If you aren't experienced with tig welding, plan on having a professional welder do this, or have your work double checked.

I guess the question you really need to ask yourself is this:
How much "extra" is your, or someone elses, life worth?

I've debated this same question, on a project car that I am working on. The decision for me took a matter of minutes to decide. I will be using chromoly, and I will pay a professional welder at a nuclear power plant, has his own welding shop and is a drag racer, to weld up my chassis. When it is done, I will know that I can get in and drive it anywhere, anytime and I'll be as safe as I can possibly be.

 

[salmon2]

If quantity is low or really low, Chromoly tubing may be more available and cheaper than 1020. I have seen 1018 is more expensive than 4130.

 

[muffildy]

it is intended as a daily commute hybrid electric vehicle. a professional welder and the necessary heat treatment afterwards would take too much of the budget from the rest of the car. this problem of lots of extra cost is the driving factor in the problem of using chromoly vs DOM. So as a compromise i came up with the idea above so that i could have chromoly without needing the professional welder or the heat treatment. the only problem is alibre design doesnt seem to be capable of modeling how strong or weak doing something like that is.

 

[SnTMan]

I'm pretty sure many older lightplanes and homebuilts were built with Cr-Mo tubing and gas welded. No PWHT required. Might check it out.

Regards,

Mike

 

[bentwings1]

It will not be necessary to heat treat after welding. This has been gone over in the drag racing game by NHRA a number of times. If it was very thick sections then a post heat treat might be necessary. It shouldn't be necessary to have anyting over 1/4 in thick unless you into heavy trucks.

Welded properly 4130 tubing is very satisfactory as is. The tubing is commonly purchased in the annealed or normalized state. Don't chinze on the welding. I'd go to a hotrod shop for this. Most know what to do for cars.

Most of our streetrods are made from mild steel tubing with just the roll bar made from 4130. These holdup very well. Mine has over 15k miles on it and no repair to the chassis at all. With 550 hp and 2800 pounds it is far higher stressed than your EV will be.

99 Dodge CTD dually.