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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
Steel
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
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
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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