Which Frame Material is Better?

There are many materials from which bicycle frames are made and many wonder which is the best. The truth is that there is no "best" material; they all come with their respective pros and cons. Personally, I prefer steel; others prefer aluminum. There are many material properties that affect the quality of the ride as well as the economic feasibility of the material.

Alloying

Any pure metal on its own is extremely weak. The reason for this is due to the atomic structure of metal. A metal can be described as a bunch of positively charged nuclei floating around in a sea of electrons. The electrons from one atom jump freely from one nuclei to the next so that there is no directional bonding; this is called metallic bonding. The mechanical ductility, electrical and thermal conductivity derive from the electron mobility.

The atoms aren't just ordered randomly though, they pack themselves in certain crystal structures (dependent on the type of metal or alloy) Its sort of like when you have a whole bunch of pingpong balls in a box, they tend to fall into a certain pattern. There are a few crystal structures but there is no need to mention them.

So, since these electrons are free to move where ever they please, the metal is much easier to bend and shape (and break) because the nuclei of the metal just slide around (imagine a squadron of soldiers in a square formation, they can move forward, sideways to make the square into any shape). To make a metal stronger (and harder) all we have to do is stop the nuclei from sliding around. Alloying is one way of doing this. Pretty much, we just add different metals as a solute (in small amounts) to the solvent metal. The alloying atoms impose attactive and repulsive strains on the solute atoms so shifting becomes harder (one of the soldiers didn't shower, so no one wants to be around him or walk around him. Maybe one of the soldiers is an attractive female, nobody wants to walk away from her; the net result is that the soldiers don't want to move around). A resulting alloy can be around 2 to 3 times stronger than the pure metal.

The Properties

Each material has physical, mechanical and chemical properties. These include density, yield strength, elasticity, Ductility, toughness, fatigue limit and corrosion resistance.

Density is an obvious property. Density = mass/volume, in essence, density is a way of determining the weight of the frame. Some materials are very dense (like a brick), others are very light (like foam). The materials from which bicycles are made of also vary in density.

All metals are like springs. You bend them and they spring right back into the same shape. As long as this happens, they are still in their elastic region. Once the force you apply exceeds this elastic region, the metal bends permanently (or is plastically deformed) and stays in the bent shape. The point at which the bending stays permanent is called the yield strength; When you bend/dent a frame, you have just exceeded the material's yield strength. Ultimate tensile strength (UTS) is another thing, it describes the maximum stress a material can take in tension. If this maximal stress is applied, failure will ensue. UTS is usually higher than the yield strength. In general, the yield strength can be used to determine the strength of a material, or how much force it can take with a certain shape before failure.

Elasticity, or Young's Modulus or the modulus of elasticity describes how stiff and springy a material is. Play dough has an almost 0 modulus of elasticity. A metal ruler has a much higher modulus of elasticity. A glass rod has a super high modulus of elasticity. The higher the modulus of elasticity, the stiffer the material (the more force it takes to make it elastically deform). In application to bicycle frames, a stiff material makes a stiff frame! (one that bends less and transfers vibrations very well) We'll see later on that geometry plays a roll as well.

Ductility is a measure of the degree of plastic deformation that has been sustained at fracture. A material that experiences very little plastic deformation (bending, stretching) before fracture is brittle (like glass, you don't see glass bend and twist before it snaps) A material that experiences more plastic deformation before fracture is ductile (like putty, you can stretch it out, bend it before it snaps or breaks into two pieces) Ductility is expressed as percent elongation, which is the percent of how much a material stretched before fracture in comparison to its original length. In application to bicycle frames, ductility determines whether the frame breaks by way of bending or by way of cracking.

Toughness takes the yield strength, ductility and elastic modulus into account. Toughness is a measure of the ability of a material to absorb energy up to fracture. A tough material takes a lot of energy to break. This can be because it is very stiff (like glass) or because it is very ductile (like putty). If you have a material that has a high elastic modulus (like glass) but is very ductile past the elastic region (like putty) then you have a very tough material that takes a lot of energy to break! Fracture toughness, is a measure of a material's resistance to brittle fracture when a crack is present.

Some materials have what is called a fatigue limit. As long as the stress applied to the material is below this limit, the material will never fail. Some materials do not bave a fagiue limit, these materials will get weaker no matter how little a force is applied to them. This generally dictates the lifespan of a frame.

Finally, Corrosion resistance. Most metals react with oxygen in some way, its called oxidization. Iron rusts and aluminum produces aluminum odixe. Titanium and gold do nothing. This too can dictate the lifespan and aesthetic outlook of a frame.

A Chart for Comparison

alloy density
(g/cm3)		 yield strength
(MPa)		 Elasticity
(GPa)		 Ductility
(%EL)
Aluminum 2024 T3 2.77 345 72 18
Aluminum 6061 T6 2.7 276 69 17
Aluminum 7075 T6 2.8 500 71 11
Steel 4130 (Quenched and tempered) 7.85 1600 200 11.5
Titanium 5Al-2.5Sn Annealed 4.48 760 110 16
Titanium 6Al-4V Solution heat treated & aged 4.43 1100 114 10
High Modulus Carbon Fibre-epoxy matrix (longitudinal direction) 1.7 760 (tensile strength) 220 0.3

Strengthening techniques

We've seen that alloying is one way to increase the strength of a material by up to 2 times. Another way to make a material stronger is simply to bash it! Technically, its called cold working. Cold working means that the metal is shaped and plastically deformed at a relatively low temperature (compared to melting temp). This can include rolling (squishing material through two rollers), tube extrusion, forging etc. Most tubes are extruded which is good. The rolled and welded tubes seen in cheaper bicycles are not cold worked. Cold working can increase the strength of a material another 9 fold.

Finally, we have a technique called precipitation hardening. Its also called quenching and tempering, aging or heat treating. In precipitation hardening, the material is heated to a certain level, it is cooled very quickly (quenching) and then it is allowed to age at room or higher (artifical aging) temperature. Precipitation hardening can increase the strength of an alloy 30 fold! Good frames are heat treated because the heat from welding usually weakens the metal around the welding joint. Heat treatment of the frame brings the entire frame back to the same strength.

Increasing strength is a good thing, but one has to keep in mind that good things come at a price. By increasing the strength of the material, the ductility is decreased and the resulting metal is more brittle.

Geometrical Aspects

If you looked at the chart above, you'll probably be asking yourself, Why is aluminum's elastic modulus only 1/2 of steel's when aluminum frames are much stiffer? Well, we haven't taken the geometry of the tubing into account yet.

The cross-sectional area of a tube dictates how stiff the tube is. The greater the cross-sectional area, the stiffer the tube.

Another affecting factor is the diameter of the tube. The greater the diameter of the tube, the stiffer the tube. The diameter greatly affects stiffness, much more so than the cross-sectional area.

Steel

cromoly Steel is cheap and strong. The only disadvantages are the weight and lack of corrosion resistance. Heat treated steel has the highest yield strength of all the materials used for frame construction. As a result, it is possible to use less material to make a sufficiently strong frame. It is possible to construct steel frames in the weight range of 3.5 to 4 pounds but these frames can be excessively flexible. Usually, steel frames weigh in at 4.5 to 5 pounds.

The tubes are usually very thin walled and thin diametered. The elasticity of steel is actually the highest but the geometrical limitations make a steel frame not as stiff. Mnay regard this as a benefit as a less-rigid frame is better able to absorb and release the vibrations of the trail rather than transferring the vibrations directly to the rider. Steel also has a fatigue limit under which it will never fail. That means that a steel frame is very durable and may outlast you!

Rust. Many abhor it, thinking it a weakness. Rust in the most part is only surface oxidization and only weakens the frame in its aesthetic sense. The only way rust can significantly affect the structural soundness of a frame is if the frame is kept under water for months and months. As long as a frame is left to dry after a wet ride, and no water is trapped inside the frame, rust will not be a factor.

Aluminum

The majority of mountain bikes are made from aluminum now. Aluminum has become increasingly popular due to its weight advantage over steel. The density of aluminum is very low and allows for frames to be build weighing a mere 2.9 pounds. The majority of aluminum frames weight from 3 to 4 pounds, depending on the specific alloy.

Since aluminum has a yield strength much lower than steel, much more material needs to be used. Although the tubes are thicker, the weight in general stays lower than steel. The increased thickness also increases the rigidity of the frame, something that many complain about. Unlike steel and titanium, aluminum does not have a fatigue limit. This means that any strain on the frame slightly weakens the frame. As time progresses, the frame will become weaker and weaker until it breaks. Most manufacturers only warranty their aluminum frames for 3 to 5 years. To combat this, aluminum tubes are made with a greater diameter to reduce the amount of strain on the frame. This, in conjunction with the greater tube thickness make an aluminum frame a very harsh ride.

Rust. Aluminum does not do it. Aluminum does oxidize however, but it is not visible. Once the surface is oxidized, it forms a protective coating so that the aluminum underneath does not oxidize. This is one major difference between steel and aluminum.

Titanium

Many claim titanium to be the best material to construct bicycles from. In fact, with its relatively low density, corrosion resistance and high strength, titanium frames can be made to be only 2 to 2.5 pounds. There is of course a drawback, this drawback is money. Titanium frames are very very expensive due to the strict manufacturing and welding environment requirements.

Titanium frames are usually constructed with thin walled but larger diameter tubes than steel. As a result, some claim that titanium frames flex too much. Then again, others say that they are more "resilient" because of this. Titanium frames are incredibly light and have the same elastic properties of a steel frame.

Titanium frames are completely free of corrosion. In fact, most titanium frames are not painted but are only clear coated. Like steel, titanium has a fatigue limit as well, under which infinite life can be enjoyed. If one can afford titanium, you'll be getting exactly what you pay for.

Carbon Fibre

New on the market, carbon fibre is a composite of carbon fibres in an epoxy matrix. The advantage of carbon fibre is that any shaped frame can be produced. Extra reinforcement can also be laid anywhere in the frame. The direction of the fibres affects the stiffness and flexibility of the frame as well so a frame can truly be fine tuned for optimal performance. Carbon fibre has a comparably high yield strength, the lowest density and the highest modulus of elasticity. This means that an extremely light frame (~2 lb) can be produced that is not excessively flexible and is strong enough to take the beatings of mountain biking.

Although carbon fibre may be stronger than steel, the limiting factor is its ductility which 0.3% (compared to 11% of 7075 al). This means that when carbon fibre fails, it does so with flying colours. A catastrophic failure is never a good thing and is one of the major downfalls of this material. Corrosion is a factor as well since carbon fibre is more reactive with acids and bases. Carbon fibre is more expensive than aluminum and steel (and some cases titanium). Is it worth it to buy a carbon fibre frame? It depends on how badly you want to save those grams.

May 7, 2004

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