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How to make the element Unobtanium

So six wise and experienced bicycle fabricators were sitting in a bar discussing the best material for a bicycle frame.

"6061 aluminum because its half the strength of steel and one third the weight."

"7005 aluminum is better then 6061 aluminum because 7005 is stronger"

"Carbon Fiber is the best as it has the highest strength to weight ratio."

"Titanium because its the strongest metal known to man."

"High strength 4130 steel is the best because it has higher strength to weight then aluminum and it flexes better."

"Stainless Steel is nearly as strong as steel but it doesn't rust"

They all would have been right if they weren't all wrong.

Funny thing is without understanding the requirements of the structure choices on materials is a crapshoot. So lets begin with a high level basic review of structure.

There are 5 different types of stress that can change the shape of any solid material. Each type of stress may have its own unique value for where the material will fail.

  • Tensile - pull the material between 2 points like stretching a rubber band
  • Torsion - twist the material between two points like twisting a key
  • Compression - squish the material between two points like squeezing it in a vise
  • Shear - cut the material between two blades like paper and scissors
  • Bend - like place a load in the center of a bridge

There are a lot of assumptions made about materials in the bicycle business that are just plain wrong because the the wrong stress failure values are considered. The most common error is in assuming that a material has a high tensile strength value will also have a high bend strength value. It doesn't work that way.

Most frames are made from tubing. Just about every piece on a trike frame will see the highest stresses in bend loading. There isn't one part anywhere on a trike where tensile load is pushed anywhere close to the limits of the material. So lets drill down a bit to understand bend strength.

Bend strength is a factor of the Length, Shape & Elasticity of the material.

Length

  • If the tube is supported at both ends like the span of a bridge, the stresses applied the multiplier of stress is Length * Length
  • If the tube is supported at one end like a lightpole, the multiplier is Length * Length * Length

The simple rule of length is double the length and its gotta be 4 times stronger for a bridge, 8 times stronger for a lightpole.

Shape

  • Section Modulus is a formula used to determine the strength factor of a profile; say on a piece of tubing, the inner & outer diameters
  • As the outer diameter doubles in size the Section Modulus is squared in strength factor with the same wall thickness

The simple rule of shape is double the diameter and its four times stronger yet only two times heavier.

Elasticity

  • Modulus of Elasticity is generally the same across a range of alloys of the same base metal and tempers

If you were designing between two cliffs its not very likely that you would try to move the cliffs closer together to offset the Length stress factors. When it comes to length, it is what it is.

Shape is an easy one so long as you can get the material in the shape desired. The ideal strength would be a very large outer diameter with a paper thin wall. The trouble is no one makes it that way, except of course if it paper. Consider this as well; a larger diameter tube of aluminum will be stronger and lighter then steel or titanium. Whenever a structure is engineered for strength the shape is of most vital importance, much more so then the material.

Material selection is in part based on its properties along with availability, cost, ease of fabrication, and environmental considerations. If material was the most important asset, and other factors were of no concern, all trikes would be made from beryllium.

If you're still awake take a look at the animation below to illustrate how 6 different common used bike tubing materials behave when rider loads are applied. To give a fair comparison, the Outer/Inner Diameters of materials were sized based on a relatively equal amount of deflection among the ranges of sizes that are commercially available. Deflection rates were both manually calculated and checked with the Solidworks FEA Simulation package. Properties factors were source the material suppliers.

   

A few points about each material:

6061 Aluminum is pretty strong when its tempered to T6 rating. Most of the time it's sold already tempered T6 however once its been heated hot enough to weld, the region around the weld is no longer T6. You can either send it to a heat treating oven for a fast tempering to T6 or just bake it at 375° for 4 hours and let it cool slowly to reach a T3 temper - then let it set for 60 days or so and it naturally evolves to T6. Interestingly, most Powder Coat ovens reach close enough to temper a 6061 frame to T3, just let it set a while longer in the oven. and its there. By the time the rider has a few miles on the trike its probably reached T6.

6061 is cheap and plentiful. It comes in the widest range of sizes and shapes of all ally alloys. If vendor A has a shortage, of a particular size no problem, it will be back in stock in a few days or less. Or vendor B should have it.

7005 Aluminum is auspiciously confused with 7075 grade. The difference is huge. 7075 is the darling titan of aluminum, and its about as weldable as Carbon Fiber is. 7005 gains tensile strength in a fully tempered condition, but as you may recall, tensile strength is never an issue on a trike frame. As far as bend strength (Modulus of Elasticity) its about 4% stronger, but in tempered condition tensile is 50% higher. Tempering 7005 to T6 isn't as simple as 6061 T3 and aging to T6; it requires more much more heat which can warp welded frames so generally 7005 frames aren't heat treated.

7005 is about twice the cost of 6061, and there are often shortages. Foundries don't run 7005 quite as often so the availability of materials (and costs) will fluctuate in the market.

Carbon Fiber is almost legendary in terms of strength to weight. It takes a vastly different approach to design & fabrication. As we think of tube frame structures the animation was designed around carbon fiber tube. Using tube isn't really a fair evaluation as the ideal carbon fiber trike is molded in more of a monocoque fashion from carbon fiber cloth soaked with resin and shaped as required. Just the same while the strength to weight ratio is high, that's not because its super-strong, its because its super light. As a result to get the stiffness required took a large diameter tube. That could be an aerodynamic concern.

Carbon fiber not only takes special manufacturing techniques, its really difficult to predict failure. To be as strong as cheap old 6061 it still weighs about 66% as much, and that's not taking into account the extra weight required at every joint. Its cool stuff indeed but it comes at a cost. Since the powder coat oven temps will weaken Carbon Fiber resin it takes a soft paint to cover. Of course covering the beauty of carbon fiber may seem like a sin, leaving it exposed to the UV from the sun will deteriorate the fabric faster than most would like.

Titanium is the metal of the Gods. It makes great bolts as it has high tensile strength and great springs as it has exceptional fatigue resistance. But its only about 40% stiffer then aluminum at nearly twice the weight. Its weldable with steel filler and forms well once you get past the springback issues. Tough stuff to cut and drill.

Titanium is the 7th most abundant metal but refining it is pretty pricey. Its more difficult to find a consistent supply then 7075. It comes in shorter DOM tube lengths as its very tough to extrude in long lengths.

4130 Steel is the backbone of low cost high performance alloys. Abundant, cheap, not too difficult to work with. Welding makes it brittle so its very wise to anneal any welds; just get them red hot for a few minutes and let it cool no faster then about 100° an hour until its down to about 700°. Steel can use the smallest diameter tubes but even then it still weighs twice that of an equivalent strength of aluminum. There are one or two foundries that run really thin wall steel specifically for bikes but finding those in 2" OD might be a bit tough and costly as well. Besides if you're planning on tubing with the wall thickness of a soda can be careful not to dent it.

Steel frames are said to be the most resilient by the pros. Strange thing is you don't see many steel frames in Tour'd France. Although the outside will normally get powder coated the inside will rust if there is any possibility of water vapor intrusion.

303 Stainless (or 304, 316, 416) DOM tubing is pretty good stuff and doesn't need any coatings. That's probably a good thing as coatings don't stick nearly as well to SS. FWIW, it takes about an ounce of powder coat for the main frame tube alone. Its only slightly weaker then steel and a tad heavier depending on the grade of stainless. The 300's are pretty rust resistant, but the 400's will rust after washing with alkaline cleaners.

Stainless is a bitch to drill so buy some Cobalt drill bits and hole saws.