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 Post subject: Re: Titanium frame?
PostPosted: September 28, 2011, 11:25 am 
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The voice of reason
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a 0.005" variation on 0.040" steel is 12% but only 6% on 0.080" aluminium maintaining a higher safety margin and QC.


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.

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VW Beetles had their blocks made from mag


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...

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Titanium bulkhead/subframes, connected by steel tubes, wrapped in a structural carbon fiber skin.


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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 Post subject: Re: Titanium frame?
PostPosted: September 28, 2011, 1:24 pm 
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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:

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All other things being equal, stiffness is a linear function of Young's modulus (called "E"). The E value of Aluminum is 10 million (no units), E for titanium is 20 million and E for steel is 30 million. NOW HERE'S THE INTERESTING BIT: Titanium weighs twice as much as aluminum and steel weighs three times as much as aluminum. Their E values are in exactly the same ratios as their weight!!!

So what's the bottom line? In a properly designed chassis, the weight for a specific stiffness will WEIGH THE SAME NO MATTER WHAT THE METAL USED. That is why most spaceframe chassis, like the Super 7, were made from carbon steels, not chrome/moly, titanium or aluminum.


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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 Post subject: Re: Titanium frame?
PostPosted: September 28, 2011, 10:16 pm 
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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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 Post subject: Re: Titanium frame?
PostPosted: September 28, 2011, 11:28 pm 
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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..


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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 Post subject: Re: Titanium frame?
PostPosted: September 29, 2011, 7:12 am 
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horizenjob wrote:
It's why they don't build rockets and airplanes from steel.


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...


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 Post subject: Re: Titanium frame?
PostPosted: September 29, 2011, 12:03 pm 
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TRX wrote:

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 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/


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 Post subject: Re: Titanium frame?
PostPosted: September 29, 2011, 2:20 pm 
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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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Car9 Roadster information - models, drawings, resources etc.


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 Post subject: Re: Titanium frame?
PostPosted: October 9, 2011, 8:58 pm 
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horizenjob wrote:
Quote:
a 0.005" variation on 0.040" steel is 12% but only 6% on 0.080" aluminium maintaining a higher safety margin and QC.


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.

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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