We are mounting a subframe in the back of a mk2 Jetta. It looks like I have a choice between...

Cold rolled
Hot rolled
Alloy steel
Stainless

It will be two 3"/4" x 3"/4" square tubes welded between the stock framerails that run the length of the unibody to the rear bumper. They will be supporting an R32 rear subframe. I was curious anyone's experienced thoughts on the best type of steel to use without over-doing it. What is the stock framerail made of?

 

can you use chromoly?

 

STEEL-HOT ROLLED TUBE SQR STRUCTURAL
(4" x 4" x 0.188" STEEL-HOT ROLLED STRUCTURAL TUBE SQUARE)

or

STEEL-HOT ROLLED TUBE SQR STRUCTURAL
(3.5" x 3.5" x 0.25" STEEL-HOT ROLLED STRUCTURAL TUBE SQUARE)

Cold rolled didn't come in in anything larger than 2"x2", but the site states it is the strongest. I will be using base plates as per your suggestions. Do you think .25 plate is overkill?

I was told chrome-moly needed a special welding process and some type of annealing? We have a tig...The 4130 didn't come in anything larger than 2x2 as well...Thanks for the replies

 

First of all just cause you didnt see it in one manufactures list, doesn't mean that it doesn't exist, so keep looking. Secondly you want COLD rolled steel, not hot rolled. Cold rolled has a higher tensile strength and smaller elongation potential, also better overall strength. Stainless and chromoly is too expensive, plus a bit overkill.

Bending loads have no forces sustained in the middle section of a tube. All the forces are on the walls and the bigger the radius the stronger the bending stress a pipe can take. So in this case you'd want the thinner wall tube 4"x4", plus this a light low torque engine.

The list below will help you choose but I think 1020 or 1040 (.188 wall) mild cold rolled should be sufficient for that size tube you have listed above. BTW older car metal runs 1004 to 1010 depending on the panel, but the cold rolling Boron procedures in modern cars makes the steel have the strength of four times the regular steel.

In order for me to help you more you need to specify you load points along the tube (mounting points and lengths), and how you plan to secure them.