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Material Assets
Titanium, Carbon Fiber, Aluminum or Steel - Which frame material is best for you?
 John Olsen, Bicycling Magazine

The stiffness, weight, and strength of a bike frame depend on many factors, only some of which involve the material.  The optimum frame design for one material will be different for another, because materials vary widely in strength, stiffness, and density (weight).

The best aluminum frames have fat, thin-wall tubes and don't deflect much side to side when you sprint.  The best steel frames have smaller-diameter, thin-wall tubes, and flex noticeably in a sprint.  Titanium and carbon frames fall in between. 

Experienced cyclists often divide into camps, with steel riders carping about the excess stiffness of aluminum and aluminum zealots decrying the spongy feel of light steel frames.   He we explain the advantages and disadvantages of the most common frame materials and compare in a graph how stiff they are when compared to steel.

How Stiff is your bike?
A comparison of stiffness (relative to steel) for various frame materials

wpe8.jpg (19433 bytes)

Steel is stiff but dense (heavy).  Light frames of adequate stiffness and strength are made with relatively small-diameter tubes, but steel isn't the right material for light frames or large strong riders.  Mild (inexpensive) steel frames need thick walls to be strong enough, and they're heavy.  Stronger steel allow thin tube walls, but then frame stiffness goes down.  Recent developments include "air-hardened" steels of very high strength, such as Reynolds 853.  (Unlike most other types, air-hardened steels gain rather than lose strength as they cool from welding.)  All steels have the same inherent stiffness, regardless of strength - 853 is no stiffer than 1010 (mild steel)
The Good
- Best steel alloys are very strong
- Best stiffness overall
- Long-lasting
- Air-hardened alloys make ultra-high strength affordable
The Bad
- Can be heavy - not the materials for big, light frames
- Rust-prone

Aluminum frames can be very stiff and light because the density is so low, but the tubes have to be much larger in diameter to compensate.  Still, these "fat-tube" frames are the prevalent design for quality bikes today.  Recent improvements include adding Scandium, an element that increases strength.  Overall, aluminum is a great material for stiff, light frames for riders of all sizes.  It is also one of two materials that is well suited to unconventional frame shapes.
The Good
- One-third the density of steel, allowing the use of big tubes
- Easily formed into aero shapes
- Even cheap frames can be light
- Makes a light frame for a big rider
- Doesn't rust!
The Bad
- One-third to one-half the strength of best steels and titanium (can break)
- One-third the stiffness of any steel, which requires larger diameter tubes
- Modest fatigue strength
- Not easily repaired or straightened
- Big, thin tubes means easy crash damage

Titanium has an excellent balance of properties for frame building, and gives the best combination of durability and weight.  Titanium alloys are half as stiff as steel, but also half as dense.  The strongest titanium alloys are comparable to the strongest steels.  Stiff titanium frames need larger-diameter tubes than comparable steel frames, but not as big as aluminum.  Titanium is very corrosion resistant, and very light frames can be made stiff enough and strong enough for bigger riders.  Most ti frames are the 3Al/2.5V alloy (3% aluminum/2.5% vanadium), though a stronger alloy, 6Al/4V (6% aluminum/4% vanadium) is increasingly used.
The Good
- Half as dense as steel, making the lightest most resilient frames
- As strong as most steels
- Wont rust - no paint needed
- Good fatigue strength
- Makes a light frame for a large rider
The Bad
- Half as stiff as steel (and known to be somewhat flexy)
- Difficult to repair
- Expensive

Carbon Fiber
Individual fibers of carbon are tremendously strong and stiff, but they are useless unless arranged in a strong pattern, and held together with a strong "glue" (usually epoxy).  Unlike metals, in which strength and stiffness properties are nearly the same in all directions, carbon fiber composites can be tuned to orient the strength where it's needed (for instance, stiff laterally and compliant vertically).  This is the ultimate frame material for unconventional frames and shapes, as it can be molded and tuned more than any metal.
The Good
- Readily molded into exotic shapes
- Excellent fatigue strength; no rust
- Strength and stiffness are controllable
- Low density and high strength make very light strong frames possible
The Bad
- Expensive raw material
- A bomb if poorly designed or made (too stiff or too flexible)
- Can be "notch sensitive" (prone to breakage)

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