Hello all I am just about to start a BEC Locost build. I am in the process of getting all the tools together to start. I have taken a week of work and
had lessons in TIG welding by our on site fabricators/pipe fitters. So I have just acquired a TIG welding plant.
Now for the question. I have not seen mention of people fabricating their chassis from stainless steel is there any particular reason for this?
My initial thoughts are that I would like to build using 304 grade. I have just costed the price for the material and it would be in the range of £220
for the complete chassis excluding roll bars (this is with tube of 1.5mm wall thickness). So I don't think that the cost is prohibitive.
I know that stainless is stronger than mild so the slightly reduced wall thickness should not be a problem should it?
Are there any known problems of stress cracking in stainless chassis?
Any opinions info appreciated
G
Digger, the general concensus is don't do it....
No offence meant, but if you need to ask these sort of questions you aren't expert enough in welding stainless....I know I'm certainly not
expert enogh...yet I'll MIG mild steel all day without any concerns....
Stick with regular mild steel is my advice...
Some may disagree of course..........
Dr Hess (I think, its been a while) a brain surgeon no less 
is/has built a SS chassis for his locost. Can't remember whether he's from
the US or Aus but he used to post on the Yahoo list.
The Yahoo list is archived so have a search on there 
HTH
Mr Hess..(I refuse to call him doc on a car list...if he's examining me no prob, then he's doc....)...is in the US...Arkansas I
believe...pretty certan he's more of a GP than brain surgeon though...
His build is well documented IIRC...he posts on Locost NA...he swears by Stainless....
UncleFista...not a dig at you BTW....just a rant about misuse of titles that's all....
Cheers I will do a search to see what I can dig up.
As to "No offence meant, but if you need to ask these sort of questions you aren't expert enough in welding stainless....I know I'm
certainly not expert enogh...yet I'll MIG mild steel all day without any concerns.... "
I am not concerned about the quality of my welds, before my life as a chemical process engineer I was a toolsetter/fitter and I had to do a lot of
arc/mag welding. I got trained up on the TIG last week and we tested number of my welds and non failed before the material did.
The issue of stress cracking I was refering to was from possible chloride corrosion that affects the grain structure of stainless under certain
conditions. I have come across this condition a number of times on our plant, but in the real world of normal temperatures and pressures I do not know
how stainless performs.
The reason I want to use stainless is because I want the chasiss to be as strong as possible and completly unpainted without the fear of tin worm
setting in at some point in the future. Of coarse TIG welded stainless also looks great, nice smooth welds without any splatter.
So I was really looking to find out if there were any inherrent problems with using a less maliable materal, such as unusual handling characteristics
because of a higher youngs modulus of the steel.
G
[Edited on 11/11/03 by Digger Barnes]
Hi Digger,
I'm sure glad I said "no offence meant..."........it was based on your apparent novice status.......which is not the case.
Clearly, your expertise is not in question....however most people who say "can I use SS/Ali/cromoly etc?"...I would still say if you have to
ask then the answer is no.......
I realise now your question is somewhat different.....I don't have the expertise to say specifically why not SS......more a case of why mild
steel is simpler, cheaper and consequently most often used....
BTW, I agree that well tig'd SS is gorgeous...
Does SS actually have a significantly higher Youngs modulus?..I though all steels were pretty similar.....tensile strengths vary of course......a
lot..
[Edited on 11/11/03 by Alan B]
Digger, I suggest you have a look at the tensile strength of the stainless steel chassis in 304 and compare it to CDS tube. The stainless would be a
good idea however if you intend to "bond" the panels for the floor pan and panels with exotic materials like kevlar or carbon and say
Sikaflex. I have toyed with building a chassis in stainless too but opinion from automotive engineers is that the CDS is better, but hey how many crap
cars have u seen designed by so called experts
Shug.
I'm building a chassis at the moment and the materials so far, almost enough to complete it, have cost under £100. It's mig welded but would
tig without problems and will probably outlast me.
Why bother?
yours, Pete.
Hi Digger, Welcome to the site.
U2U me with your email.
Fellow chemical plant fabricator. ( Paul G )
Hi Digger,
I am building a chassis from 308 grade stainless box section, 1.5mm wall thickness, it may be more expensive, but properly TIG welded will not need
paint for protection and will be stronger. And of course it looks good.
Only problem I've encountered so far is due to its greater thermal expansion. I suggest you practice doing very quick tack welds, as if you take
your time it will deform as it cools. I haven't yet started fully welding the chassis up.......
I'm looking forward to panelling the sides and bottom with 0.9mm stainless sheet and "plug welding" instead of rivetting.
Stainless must be the choice for the process engineer!
JohnN
Rescued attachment ss chassis.jpg
Just checked my receipts, its 304 grade stainless, not 308, the filler rods I'll eventually use are 308.
BTW I'm not using stainless for the wishbones, as they are critical suspension items, they are CDS tube as per book. Just had them powder coated,
they look fantastic in black
JohnN
Hello all BTW no offence taken, my original post was a bit ambiguous.
I get a feeling that this forum may become my home over the next year or so. 
mmm interesting I had not considered stainless for the side panels. Is there a weight penalty for doing this (I have not looked Kg/m2 for these
thickness yet).
Good to that there does not appear to be any real reason why SS should not be used.
I agree it is not worth building the suspension from SS, I was just considering getting them chromed after construction. But hey I think that is still
some way off in the mists of time yet. Although there are some stainless steels out there that have the same strength as high tensile steel, I am not
sure of the grade without looking but 430 grade rings a bell some where in the dark depths of my mind.
I'll just re-itterate what was said before. Fully weld quickly. Don't give the stainless time to move.
Then again if your chassis moves and is about 5mm out when finished, what does it matter. As long as the wheels are all at 4 corners of an imaginary
perfect oblong what does it matter. If the wheels are going in a straight line then the whole car will be.
Yep I have noticed that heat distortion can be a problem with stainless, especially with a slow hot weld like TIG.
So I suppose I will have to do a few time trials before I really get going at the beginning of next month.
I am a professional mechanical engineer.
My understanding of stainless steels is that they will work harden in use and be more prone to fractures in areas of high stress. The rate at which
this hardening occurs depends upon the particular alloy being used.
High stress points will occur at any attachment point and at points throughout the chassis as road loads are applied.
I wouldn't copy the book chassis and make it from stainless steel (or Aluminium).
I would re-design the chassis using a finite element stress analysis package and ensure the stress distribution was kept within the limits of the
stainless steel I was using. However this is outside of the capability of most of us.
One thing to bear in mind since you're building a BEC is that stainless is heavier (more dense) than mild and much heavier than ali. Granted, Jon
Ison used a reasonable ammount of stainless for his locost (side panels and wishbones I believe), and general opinion was that it certainly
didn't lack performance.
Another thing to consider is that there's potential for 304 to rust and given that some chassis members particularly at the rear are quite
exposed to all the crud the road can throw at them, 304 might prove to be no better than well painted mild.
Kingr
mmm I have got a good deal of FEM/DEM experience and there are 3D models out there on the net so it should not be to difficult to import one of these
into a package such as Ideas. But do you really think that this is necessary.
304 grade as you know is Austenitic, which is quite a ductile material compared with carbon steel. I agree work hardening is a particular problem
especially when trying to machine these materials. But given the strengths below then it would appear to be stronger than the normal steels used for
Locosts.
Carbon steel
Yield strength 165 MPa/in2
Tensile strength 415 MPa/in2
Elongation to failure 30% in a 2” test piece
304 grade stainless
Yield strength 241 MPa/in2
Tensile strength 565 Mpa/in2
Elongation to failure 60% in a 2” test piece
Is stainless much more dense than Carbon or mild steel?
Steel density @ 300K
Fe 7860Kg/m3
therefore assuming mild steel is to all intents pure Fe the density should be around 7860Kg/m3
304 grade stainless has a density 0f 7920 Kg/m3.
So yes stainless is more dense than mild, but only by 0.76% and the tube only comes in metric sizes i.e. 1.5mm wall thickness so the chassis should
actually be ever so slightly lighter than for mild of wall thickness 16 gauge.
As to corrosion of 304, it is true that 304 does rust under certain conditions. We use allot of 304 on site. Rain and water do not seem to have a
rusting effect, but chloride corrosion is defiantly a real problem. Oh well if I do decide to use this material then I will have to make sure that I
keep it clean and don’t constantly drive round on wet salted winter roads.
The information that people are giving me here is great it has really made me think and I have had to pull a few books off the shelf to check things
out. Great just the sort of input that helps me to come to a good final decision on the build materials.
P.S. one of the earlier posts mentioned not making suspension parts from stainless. On my trawls through the Perrys (the chemical engineers bible) I
noticed there are a few very high strength SS’s that are stronger than high tensile steel, such as Almar 362, AM350, W, PH15-7Mo, PH14-8M0, Custom 450
and Custom 455. Although I would imagine these materials are probably extremely expensive
I agree with Pete and kingr apart from the unanswered questions of the stress effect on stainless why make your life more complicated.Im sure most of
us agree that when your into the build proper you'll have more than enough problems to resolve without adding to em.
Cheers,
Bob
quote:
Originally posted by JAG
I am a professional mechanical engineer.
quote:
My understanding of stainless steels is that they will work harden in use and be more prone to fractures in areas of high stress. The rate at which this hardening occurs depends upon the particular alloy being used.
High stress points will occur at any attachment point and at points throughout the chassis as road loads are applied.
I wouldn't copy the book chassis and make it from stainless steel (or Aluminium).
quote:
I would re-design the chassis using a finite element stress analysis package and ensure the stress distribution was kept within the limits of the stainless steel I was using. However this is outside of the capability of most of us.
Alan B we sing from the same hymn sheet then
and I fully agree with you - I bought a chassis in steel from Stuart Taylor. Mainly because I hope he has jigs and better welders than I can
provide.
There are indeed plenty of other things to keep the old grey matter busy without re-designing the chassis to suit Stainless steel or Aluminium etc...
quote:
Originally posted by JAG
Alan B we sing from the same hymn sheet then
Har har har, methinks I will build the Second car in stainless steel to prove all u doubters wrong, using 316L, I build towing points for tugs and
workboats out of this stuff that tow upwards of 50 tons bollard pull and nuthins snapped yet in 20 years Anyone got any spare body armour to test
it with just in case
Shug.