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That's a big deal. If you fabricate something with steel sheet that turns out to be .035", then the part as designed with .040" would be %66 percent stiffer. That's a hard lesson for an engineer. As homebuilders we can afford to design around typical values. If the piece truns out to be to weak we fix. That doesn't work in production though - you must design to the worst case specs. The maker of the steel sheet doesn't give the +- .005" spec because it makes them look good. If you design to .040" sooner or later you will get bit in production. This goes for things like electrical parts too, like timing on logic gates.



Those things burn great too. My high school friend got banned from school for a few days for setting a chunk of one on fire in metal shop with an arc welder...



Usually your trying not to mix so many things together. If you have structural skin, for example, then your doing that to not need the steel tubes. That's the attraction. In a Locost you land up having two things the shape of the car, the steel frame and the body - somewhere there is wasted weight. High quality fiber monocoque is expensive though. I think aluminum is doable, but it takes a higher skill level and more labor then the steel frame.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

From my perspective (bicycle frame building), the biggest problem with Ti other than $$$ is the hassle and cost of back purging. Coming up with proper fittings to run argon behind the welds would be a pain for the average builder and wouldn't be worth it.

Regarding cro-mo steel, I know I have read on here about post-weld processing of it. That might be the case in the automotive/race car world but I can guarantee that millions of bikes worldwide have been made with cromoly tubing and none of them have gone through any sort of heat treatment, annealing, etc. Just FWIW.

I'll second what Bbox said:



Again, my perspective is from bicycle frame design but this is precisely why an aluminum bike frame does not weigh 1/3 of that of a properly engineered steel frame. The one advantage that aluminum (and perhaps Ti) has over steel is when you need something that has to hold something else...for instance a bracket that holds a bearing and has to be welded to some tube. Sometimes you just need a certain amount of real estate to hold the bearing AND have enough area to weld to something else.

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-Keith

also, bicycle frames get their stiffness in torsion from tube diameter as well as tube thickness and material... and there is no way in hell you can get tubing diameters done in steel that are used on alu frames with wall thin enough and not get major buckling..

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Building a single seater middy BEC with GSXR 1000 power
build log: http://locostusa.com/forums/viewtopic.php?f=36&t=5899

day job: http://smg.photobucket.com/albums/v723/turbolimac/portfolio/

That's the thing. It's why they don't build rockets and airplanes from steel. You can even build a good wooden plane. The lighter materials allow larger sections to be used.

I'm not so sure race cars are designed to stiffness rather then lightness. That seems to be partly an artifact of things like minimum weight requirements. In the early sixties they were trying pretty hard to get F1 frames down to 50 lbs.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Well, actually they DO build airplanes from steel, at least when circumstances require it. Neglecting the tube-and-fabric construction aircraft, the B-70 Valkyrie was mostly stainless steel, because at Mach 3 much of the airframe was above the melting point of aluminum, and titanium didn't offer any advantages over steel in that particular application.

The Soviet Mig-25s were also steel. They not only operated in that speed range, they also had to sit on Arctic runways at 100F below zero too.


I know it's not really relevant to the point you were making, it's just a preprogrammed response from some aviation flamewars...

I may have stood on one of those !

http://forums.autosport.com/index.php?s ... 714&hl=MIG

Many more pictures here if you missed the link ..

http://s784.photobucket.com/albums/yy12 ... omilitary/

The fabric covered airplanes are a good point. The MIG-25 had as much aluminum as they could afford in it though. It had a pretty big range penalty from that steel too.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

There's an aircraft example for that sort of sitch too...
http://en.wikipedia.org/wiki/Tupolev_Tu-4

Quick notes, tried to copy the Superfortress, didn't come off quite right due to Russians working with metric sizes instead of the US Imperial sizes. They erred on the side of caution though, resulting in an overweight pig.

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traynee rockit serjun

DD#1 '95 Ford Escort Wagon 1.9 Auto, self PnPed stage 2-3ish head, 11ish CR, Airhog panel filter, awaiting headers and intake mods. ~110-120 BDHP
DD#2 '88 Plymouth Voyager, 3.0 Auto, shift kitted, alloys, walker exhaust, timing advance. Awaiting PnP heads. ~150 BDHP