Inliners International Forums General Discussion Tech Tips Acceptable thickness for 2X3 rectangular tubing?

I'm in the planning stages of building a light street rod with a smaller than normal engine (a mildly-modified Henry J 4-banger with a 3-speed OD transmission) and expect it to weigh around 1600 pounds. That's my goal, anyway. The local steel supplier has rectangular 2X3 and 2X4 tubing with thicknesses of .080" and .120". It seems to me that .120 would be over-kill for my use. It's been 40 years, or better, since I've built my last street rod frame and could use some advice from a pro.

Any advice will be very helpful. Dennis #2192

I think for drag racing, the minimum thickness allowed is .137 thick. But for street use, you actually need to go thicker, maybe 3/16". Street cars take a lot more twisting and flexing in day to day driving than a race car does, and that can fatigue tubing if its too thin. You aren't talking that much more weight either if its just for the main rails.

I'd never use tubing less than 1/8" for a hot rod frame. Even on all the old frames I rework and box, they're 1/8" from the factory, some are cracked, some are broken all to pieces, I usually end up adding more supports to the inside also to add some strength. and like CNC said it doesn't really add any weight to the main rails, only 36 lbs.

.120 Holds up most T buckets today even with a V-V-V-EIG-EIGH-EIGHT in them. .120 2x4 1500# with a henery j should be plenty strong in my opion. I usually over engineer everything and with a 1500# car for the street I wouldn't be affraid of .120 2x3. Then again thats just me. 1 OL REDNECK

.120 is plenty. most car & truck frames are a stamped "c" channel which isn't as strong as a piece of tubing anyway. i've been playing with AD chev pickups my whole life ( the 1/2 ton frames were 1/8. .005 thicker ) & have rarely seen 1 cracked or broken.
robert

This thread's a bit old but,

You can't make an accurate comparison by looking at thickness alone. High Strength Low Alloy(HSLA) steels are becoming more and more common in the automobile industry due to weight. Most steels bought in structural shapes for general use are 'A36 Hot Rolled Steel'; the 36 means it has a minimum yield strength of ~36ksi. The HSLA stuff that's out there now has a min. yield of 80ksi or greater.

IMHO actual strength (load before yield and failure) is not really a problem in the sizes and weights used.
It's more stiffness (resistance to bending), which is why frame rails are almost always much taller than they are deep - because the bending will be in a vertical plane. A 4" tall × 1" wide rail is very much stiffer than 1" tall 4" × wide (256 times).
Anything that adds height improves stiffness - such as a roll bar - because it raises the vertical dimension.
For stiffness, there is no practical difference between mild steel and 4130 - no advantage and no weight saving.

Wouldn't the greater problem be the joints and welds, rather than the longer runs of tubing?

The center of the span is by far the weakest point.

Also the majority of the stress in a frame is torsional not vertical, right?