Stainless Steel Frame Question

[Morris]

Hi Gary......

The Stainless frame is nice because of the "no rust factor".....

And we'll be doing some interesting things to stiffen up the chassis..... I'm sure when we get done it should work just fine....

The design of the chassis is not the best and flexes to much in the torsional way...... the longitudal or long way seems to be OK..... so we are going to spend some time preventing the twist....and making the whole chassis much stronger....

Morris

[David Kirkham]

I can't wait to see these new chassis mods!!!

BUT!

DANG YOU MORRIS!!!

I stayed up all night last night when my brother emailed me the pictures of your new clutch set up! I drooled on my computer so much I thought I was going to short it out.

As I was telling you today...I am having a fantasy right now about making a custom, super light weight, billet bellhousing with a reverse starter in it on a tiny custom flywheel with that 3 disk clutch pack in it. I LOVE a fast reving motor! Now if I could only figure out how to get the fantasy out of my head on into the CNC mill!

David

[ToyCollector]

Nice of Tom to mail the pics. Where did they originate from on the internet?

[Morris]

TC

They are the photo's I did of the trick clutch we are using in KMP 259......

Morris

[David Kirkham]

Morris,

I have been thinking about that small diameter flywheel a lot lately. I ordered a complete set up like yours a couple of days ago and it should arrive shortly. I will keep you updated on my progress and experiment when it arrives.

David

[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).