Stainless Steel Frame Question

[Lew Ledyard]

You can definitely feel the difference. It's hard to describe - it's kind of like wearing your lucky underwear.

[Chilibit]

Does the aluminum body touch the frame and if so does stainless help there in terms of dissimilar metals interacting?

[rsimoes]

Originally Posted by Lew Ledyard

You can definitely feel the difference. It's hard to describe - it's kind of like wearing your lucky underwear.

What do you mean?

[JBo]

problem is a form of electrolosis. Only a problem in salt water areas or areas of heavy airborn salt and condensation. Frequent rinsing underside should prevent problem. Consider rubber isolators at body mounts as well.

[rsimoes]

Originally Posted by David Kirkham

No,

I am not that good of a driver! Maybe Morris, or Mario Andretti could tell.

David

I guess what I was trying to get at is when moving from one surface to another at a different angle, let's say a steep driveway, where you come in at a 45 degree angle to be sure that you won't scrape the front, so one wheel hits at a time. As the front wheel climbs the driveway, and the opposite side rear wheel is still on flat pavement, could you feel a difference in the frame torsion between the stainless and the steel?

I had a friend that I used to visit with a couple different Corvettes with a real steep driveway as described above. I remember that with my 1993 Vette, it was like jello, you felt the car's structure completely twist as it flexed going up that driveway, while the 2004 Z06 I had later would feel like it was carved from a chunk of steel. I think it would have picked up a wheel off the ground before it gave in this stress. The difference between these two generations of vettes is significant. . .

Furthermore, I assume the stainless is thinner than the steel in order to be lighter, since I believe SS is stronger than steel, is this assumption correct?

[Chilibit]

"I assume the stainless is thinner than the steel in order to be lighter, since I believe SS is stronger than steel, is this assumption correct?"



There are only about a billion variations, which one were you referring to?

The addition of atoms to combat corrosion typically does NOT improve strength, but it does corrode less.

I would happily yield to a metallurgist, but I believe (as opposed to know or think) that stainless alloys are softer and less strong depending on how and what you measure. But they can be specified to be strong enough for a given application.

It does not rust (not technically true, but practically so) and it looks great. Kinda what ERA said. I best get out of here before an SPF'er gets 'whooped' up on. ;-) Those ERAs look mighty fine.

Ciao,

[rsimoes]

Interesting stuff. . .

I am under the impression that SS is stronger, from my experience in working with fuel and brake lines, the SS is much harder to flare (also requires more flare angle) and more difficult to bend. . . .

Am I off my rocker?

[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

[Morris]

There is no difference that anybody can detect between mild steel and stainless steel.... Unless you did a Torsional chassis test of the frame..... then you would know....

And I'm not sure but I think the SS chassis are the same wall thickness..... as the mild steel....

Morris

[rsimoes]

Tom,

So which are you saying will have a shorter fatigue life? (in other words, which do you think will fatigue first)


Morris,
So for racing application, do you feel that mild steel is better?

[Morris]

They all work..... 4130...SS...mild steel....

[Tom Wells]

Hi rsimoes,

What I've been told (trying to keep the kit mfrs name out of this because I don't have their permission to use it) is that some SS chassis were made similar to mild steel in both dimensions and welding techniques, and were shown to develop fatigue cracks in commercial service much sooner than comparable mild steel ones.

Research showed this Canadian commercial vehicle mfr finally used an SS material different from 304L, 312L and the like, and over the last ten years their frames did not corrode in heavy winter salt usage, and did not develop fatigue problems as had their earlier efforts in SS.

For short-term use this would not really matter. For those of us who use the cars on the street for many years in all conditions and many tens of thousands of miles, it could be safer.

Tom

[rsimoes]

This is all good to know. Thanks for all of the input!

[4pipes]

In terms of deflection or stiffness, the Modulus of Elasticity are within 3% for SS and mild steel. So for similar dimensions and wall thickness, their differences would be negligible (the stainless would actually deflect slightly more).