Frame Material

[RussMcB]

I'm looking for suggestions for what type of metal I should get for my new rear subframe.

By "subframe", I mean the part that will have mounts for the rear suspension, differential and rear wing.

I'd like to use square tubing, since it's easier to work with.

I'm planning to build my new subframe as mostly bolt-on, using the five sturdy attachments at the back of my RF94 frame. Therefore, for the most part I will be building a stand alone frame, as opposed to welding it to the current chassis' main bulkhead tubes. I do expect to do some welding onto the existing chassis, probably when I start working on the engine braces.

I mention the welding-to-existing-frame aspect because I think it might make a difference if I need to be compatible with Van Diemen's materials.

[Brian]

If I was doing this, I would use 16 Ga mild steel tubing for main tubes, 1" square or round, with smaller tubes of lighter mild steel for triangulation/bracing members. I would tend to Tig weld the standalone subframe, and I would nickle-bronze braze anything that had to be added to the VD frame, as I believe they are completely nickle bronze welded/brazed.

I often tack weld with Mig if I am going to Tig later, it's kind of like a hot glue gun for metal...

Brian

[S Lathrop]

Like all things, every one has an opinion.

I would use 18 ga MS tubing. I tried using a lot of square on one chassis and decided that never again was too soon too repeat that mistake. Use square for the bottom if you must but it is way easier to mitter and cope round when things intersect at angles in more than a single plane. Also square adds a lot of weight with no gain in strenght.

I use tubes from 5/8 in to 1 1/8 in depending on requirements. 1 1/8 in is significantly stronger than 1 in square and weighs less. If 3/4 18 ga is not strong enough, you are better to go to 7/8 18 ga instead of 3/4 16 ga. Building from 16 ga will add a lot of weight unnecessairly. 1.125 dia 18 ga weighs .563 #/ft, 1" 18ga is .498, and 1" 16 ga is .58. Square tubing 1x1 18 ga weighs .634 and 16 ga weighs .827 #/ft.

[LJennings]

To make a decision you will need to identify the load path through out your subframe design.

One of the quickest ways to determine the strength of a "tube" is to calculate its moment of inertia. The higher the number usually the higher the load it can handle.

After determining you max stress level, and your comfort level (aka saftey factor), compare againist the materials yield stress and then your ready to build.

[RussMcB]

Here is a link to an interesting thread in the Sports Racer forum (Thanks, Mike D.):

http://p081.ezboard.com/mild-steel-f...icID=910.topic

[Bob Ramberg]

Is there any advantage to using 4130 for a chassis? The only advantage it has that I see is in tension. The compressive strengths are essentially the same?

[RussMcB]

What I've read in a few places (apexspeed & DSR forum, I believe) is that 4130 is not significantly better in compression or stretch, but it is better at not bending.

[Wren]

Since compression is a function of modulus of elasticity, no, 4130 will offer no real advantage. Moe does not vary with grade of steel. I would expect stretch improvements with 4130, but only near yielding.

[Stan Clayton]

There is a good discussion of frame material on the DSR Forum

The compressive properties and yield (tensile) strength of 4120 and mild steel (1020) are virtually identical. Only the ultimate tensile strength of 4130 is significantly better than mild. Mild is sure a lot easier to weld, though!

[Wright D]

T Mild is sure a lot easier to weld, though!

I haven't ever noticed a difference in weld ability, but then again I only TIG weld. I don’t even own a MIG welder.

And since we are on the topic of welding, a book that I found wonderfully useful was Performance Welding, by Richard Finch. My copy is well worn, and I think more then worth the few bucks it cost.