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What material properties are important in choosing frame material? First, there are three types of material properties:
Physical - Density, color, electrical conductivity, magnetic permeability, and thermal expansion.
Mechanical - Elongation, fatigue limit, hardness, stiffness, shear strength, tensile strength, and toughness.
Chemical - Reactivity, corrosion resistance, electrochemical potential, irradiation resistance, resistance to acids,  resistance to alkalis, and solubility.

Let's start with an easy one. This is how much a material weighs for a given volume. For example, 6061 aluminum weighs 0.098 pounds per cubic inch. 4130 steel weighs 0.283 lb./in³, and titanium is 0.160 lb./in³.
This is an important and easy relationship to remember: Titanium is about half the density of steel, aluminum is about one-third the density of steel. Use that as a guideline, then start to look at other properties, like strength and stiffness.

Stiffness, Strength and Weight
Strength and stiffness are different properties that are often confused with one another. It is important to understand the difference, if you want to understand differences in frame materials.
  Imagine you clamp one end of a metal bar in a vise, and you hang a weight on the free end, causing the bar to flex temporarily. When you remove the weight, the bar snaps back to its original shape.
Different materials will flex different amounts for the same amount of force applied. This is stiffness.
Now imagine hanging a heavier weight on the bar, so heavy that it becomes permanently deformed. When you remove this weight, the bar does not snap back all the way to its original shape, but remains bent to some extent. When the metal changes shape permanently, it is said to "yield."
Different materials can withstand different amounts of force before yielding. This property is strength.




  Stiffness is determined by a property of the material called "elastic modulus" Elastic modulus is essentially independent of the quality or alloying elements in a given metal. All kinds of steel, for instance have basically the same elastic modulus.
  Stiffness is mainly related to the tubing diameter. Strength is mainly related to the wall thickness, though diameter also enters into it. Weight is affected both by diameter and wall thickness.
This modulus value shows that if you were to build identical frames from the 3 materials, using the same tubing diameters and wall thicknesses, the aluminum frame would be only 1/3 as stiff as a steel one, and the titanium frame only half as stiff.
The yield values show that the aluminum frame would be very much weaker, in the sense of being more easily damaged than either the steel or titanium frames.
The specific gravity values show that the aluminum frame would only weigh 1/3 what the steel frame weighs, while the titanium frame would be roughly half the weight of the steel one.
These generalities, however, are basically meaningless, because you wouldn't build frames out of the three different metals to the same tubing dimensions!
Comparing a one-inch tube and a two-inch tube of equal wall thickness., the 2" is going to be eight times as stiff as the 1" tube. And the weight will only double

Mild Steel - .125 wall mechanical tubing 
2.4 pounds per foot

Chromoly - .095 wall mechanical tubing
1.933 pounds per foot

Aluminum - 6063 T6 extruded tubing
.870 pounds per foot

Aluminum is .35 or 1/3 the weight of steel.

Flex-Frame™ technology.
Everything else is just a waste of tubing.
Material Modulus Yield Point   Specific Gravity
Aluminum 10-11
(4-22 annealed)
Steel 30 46-162 490
Titanium 15-16.5 40-120 280
Yeah.  Lets build that...
The tractor that's in your head.   
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