Stainless Steel Frame Question

[ItBites]

All steels have a modulus somewhere between 28 and 30 million psi. The stiffness of a frame with identical cross-sections will be effectively identical between stainless and mild steel. If the frame has different thicknesses or gusseting, etc... that will affect stiffness.

300 series stainless steels are considered soft (low yield strength), but so is the mild steel used in car frames. The yield strength of annealed 300 series stainless is usually somewhere around 35000 psi, while mild steel (A-36 or equivalent) is about 36000 psi. Note this is for annealed steels. If you cold work steels they will gain strength above the values presented here, but still the stiffness (modulus) does not increase.

Do not confuse strength with stiffness (modulus). All steels flex about the same amount for the same shape. High strength steels will flex much further before yielding (permanent deformation) or breaking. That is the only difference. Parts made from 4130 or 4340 (chrome-moly steels) will flex the same as those made from mild steel, up to the point where the mild steel part takes a permanent set. 4130 and 4340 are stronger, but not appreciably stiffer.

Identical design frames made from stainless and mild steel will have about the same stiffness (<5% variation).

[Tom Wells]

I've been told that one of the considerations between SS and mild steel is fatigue life. A chassis used for a show car may not face this boundary, but a street-driven / racetrack driven car may. As eloquently stated above relative stiffness should not be too much of a factor.

The considerations in fatigue resistance include metal composition (e.g., mild steel, 312L, 304L, 3CR12, etc.), corrosion resistance (yes, Virginia stainless corrodes too under the correct circumstances), welding materials and methods and skill of the welder, joint stress (design) and I'm sure I've left out some others

Just like other design considerations for our favorite conveyance there are lots of factors and compromises involved.

Not simple, but it can still be worked out in a practical fashion...

Tom

[Morris]

That was why we spent many months re-enforcing the chassis to make it stronger..... All over the chassis.....

In addition we added a 1/4" plate of Aluminum the length of the chassis to tie everything together ..... flat bottom aero and strength....

Much like a book case in your home that doesn't have a back strap or panel on it.....it's very unstable and will flex easily...... until you add the back strap or panel..... then the book case stiffens up very quickly......

The weakest part of these cars is the passenger compartment anyway.... they want to fold in half........ so we spent many hours welding in new stainless steel to make the chassis strong...... with additional bars under the floor and in the door area's....... tieing the main hoop to the front cowl hoop and frame....

All of this fabricating worked out very well..... as we now have a chassis that is Stainless Steel.....same as mild steel....but doesn't oxidize.... and we clean it with Brake cleen........and is strong...... as shown on the first time it went out at the race track.... Run n Gun....

Morris

[ENTDOC]

when I jack the front left of my car up ,the right goes up also with no sag. I suspect the non stainless frame will do the same. I also suspect that no one on this forum could tell the difference between the two by driving them. Which one is absolutly the stronger composition? I do not know,but it is likely very little difference and no practical difference except appearance and corrosion resistance ( how many of us actually let our cars get rusty). SS does look good. chuck