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TITANIUM ALLOY

Bicycle frames made of Titanium are actually made of a titanium alloy. The two most common titanium alloys used in the cycling industry are 6/4 and 3/2.5. The numerator in each fraction refers to the percentage of aluminum and the denominator refers to the percentage of vanadium in the alloy. These two different alloys are both high strength titanium and are both fairly common in the industry. 3/2.5 titanium alloy is by far the most commonly used for tubing in titanium frames while the 6/4 alloy is actually the stronger of the two.

Many people assume that since 6/4 titanium alloy is stronger, all Titanium frames should use it, but it is not that simple. Because 6/4 is stronger than 3/2.5, the mills who draw titanium tubing have a very difficult time working the 6/4 alloy. In addition, strength of these alloys is not the issue that many people think it is. Quite simply, the high quality 3/2.5 bicycle frames on the market simply do not break. Therefore, making a frame from 6/4 alloy simply doesn't make sense (it just costs more money). Please note that there are 6/4 frames on the market but most of them have seamed 6/4 tubing. The fact that their 6/4 tubing is seamed is very significant. There are three problems with seamed 6-4 titanium. First, in order to make 6-4 plate into tubing it needs to be annealed, lowering the strength by about 20%. Second, the grain structure of the plate, when rolled into the tube shape is altered and no longer appropriate for optimal alloy strength. Indeed, it lowers the tube's strength considerably. Third, the weld area that runs down the length of the tube has a completely different grain structure from the rest of the tube has surface irregularities that lower the fatigue strength of the tube. While these tubes can have well finished external welds, the inside surface of the welds are not finished and the grain structure and surface irregularities present inside the tubes create stress risers that can lead to premature failure.

In addition to the strength of the alloys, the modulus (or measurement of stiffness) is also important. 6/4 and 3/2.5 have effectively the same modulus of elasticity. In simple terms, the two alloys have the same effective stiffness. This in turn means that the ride between frames made out of the two alloys will be the same assuming all other frame specifications are the same.

The Merlin Extralight and Spectrum Super are made out of double butted 3/2.5 custom drawn tubing from Haines International. The tubing specifications Spectrum and Merlin use are the most stringent in the industry (in any industry come to think of it). Even the tubing shipped to Boeing for their passenger jets are not manufactured to as tight tolerances as Merlin/Spectrum's. You can actually see some of the result by looking at the surface finish on a Spectrum or Merlin and comparing it to other manufacturer's frames.

When talking about metals, in this case titanium alloys, there are some basic considerations to be discussed. Tensile strength, yield strength, modulus of elasticity, and notch hardness are all characteristics of metal alloys that effect real world performance.

Tensile strength refers to the greatest longitudinal stress a substance can bear without tearing apart. While this can be a factor in the overall strength of a metal alloy, in a bicycle, it's not the only factor. This is due to the fact that bicycle frames rarely fail by being pulled apart. They fail from either yielding (bending) or from fatigue. Tensile strength alone does not directly quantify how a frame will hold up to these types of failure. In fact, tensile strength (above a certain point) is essentially irrelevant to the strength of a bicycle frame. As long the tensile strength is high enough to resist all TENSILE failures, that is enough.

Like tensile strength, yield strength is also a contributing factor in the overall strength of a frame. Yield strength comes into play as tubing walls gets thinner and tube diameter increases. Thinner tubing tends to buckle or "beer can" more easily. Higher yield strength will help the tubing resist this kind of stress and thus resist "beer can" failure. Because lighter steel and titanium frames do have larger diameter tubing and thinner tubing walls, there can be yield problems especially in head-on crashes. Some tubing is so thin even simple handlebar hits can cause ripples and dents that could lead to failure. Thankfully, larger diameter thin-wall titanium tubing is still much tougher than the ELOS type steel tubing on the market by an average of about 75%. i.e.. it takes about a 75% increase in force to do the same thing to a Spectrum Super than it does to an ELOS type steel frame and you can chalk that strength up to the amazing yield strength of titanium.

Another factor to consider is the modulus (or stiffness) of titanium. Though a fairly technical concept, I'll do my best to explain it. In measuring modulus of a titanium tube, one has to measure it in both bending and torsion to fully evaluate it. Of the two titanium alloys used in bicycle tubing, 6-4 and 3-2.5, the 6-4 is clearly stronger over-all. However, top shelf 3-2.5 and 6-4 tubes compare in some interesting ways.

Although 6-4 sounds great at a glance (and indeed is pretty neat stuff) it is not necessarily better than other titanium alloys in all situations. As expected the stronger 6-4 tubing has a slightly higher modulus in bending compared to the 3-2.5 tubing. However, the 3-2.5 alloy has a higher modulus in torsion. Moreover, if you average the two modulus strengths of the alloys, you end up with a surprisingly even match. Its seems then that to build the most rigid frame with the lowest weight, a builder should use a mixture of both 6-4 and 3-2.5. Of course there is more to it than that though. One must not forget notch hardness.

Now to notch hardness or fatigue strength. Notch hardness refers to a way of quantifying how well the crystal structure of an alloy will hold together under repeated cyclical stresses. This is where 3-2.5 alloy starts to really make sense when compared to the seemingly better 6-4 stuff. Not only does 6-4 have a lower notch hardness than 3-2.5, but the way most 6-4 tubing is manufactured causes additional negative outcomes related to tube strength. The seamless 6-4 titanium tubing on the market is quite well made and finished. It still has the notch hardness problems that all 6-4 has, but its seamless manufacturing eliminates some of problems of seamed 6-4 tubing. Currently, seamless 6-4 tubing is available in very few diameters and gauges, again reducing its usefulness.

Please note that steel tubing, unlike 6-4 titanium tubing, can be rolled and seamed without the same weaknesses. That is because steel tubing can be drawn to smooth the weld, then annealed and heat treated to recover its strength. 6-4 cannot be processed the same way. If you were to heat treat it the way steel is heat-treated, it would make it too brittle for use. If you were to anneal titanium, strength would decrease and for practical purposes, it cannot be cold worked enough to make any significant difference.

If any of that seems to technical, give me a call. I love talking about metal!

In short, I'll say this, 6-4 titanium alloy has no advantage (other than for marketing) over 3-2.5 alloy when it comes to bicycle tubing applications.