The Titanium Development Association calls titanium “the material of choice,” and there are a lot of people in the bike industry who would agree. This, the fourth part of our metallurgy series, is about that mysterious and expensive metal, titanium. Its reputation within the industry is excellent: light weight, super strength and fatigue life, a magical ride … and a heavy price tag, to boot. So let’s find out what the physical characteristics are that give titanium such an enviable reputation.
Titanium is not as rare as you might guess – it’s actually the fourth most abundant metallic element in the earth, after aluminum, magnesium and iron. In fact, there’s a lot more titanium in the earth’s crust than there is chromium or molybdenum, two of the essential ingredients that accompany the iron used for steel bike tubing.
Density and Other Properties
As we learned last time, density is the giant feather in the property cap for aluminum. This is an area where titanium also shines, and although its density is almost double that of aluminum, it’s only 56 percent as dense as steel.
Our second property is stiffness, or Young’s modulus (E). The titanium that you find used in a majority of bicycle frames has an E of around 15 million pounds per square inch – approximately half that of steel. This means that steel and titanium are roughly comparable when it comes to the stiffness-to-weight ratio. Previously, we learned that the stiffness of a frame depends on design and the properties of the material used. The same goes for titanium – you can provide a flexible or a stiff ride, depending on execution. Because of the relationship between titanium’s high strength, low density and moderate modulus, most fabricators choose tube diameters that provide a supple, shock-absorbing ride. To push titanium down into the realm of the super light, the modulus becomes a problem, because then the frame gets too flexible. In this case, I’m talking about frames that weigh in the neighborhood of two pounds. Building ultra-light frames is not an easy task in any material … including titanium.
Ti’s Real Plus: Elongation and Tensile Strength
So titanium gets two second-place marks as compared to steel and aluminum in the first two properties we examined. But when we look at property No. 3, elongation, titanium is miles ahead of either material. This is the property that tells you how far something will bend before it breaks, a kind of safety factor for framebuilders.
Elongation numbers for titanium are often 20 to 30 percent. For comparison, typical steels can be 10 to 15 percent – the higher strength steels go down as low as 6 percent. Aluminum typically runs in the 6 to 12 percent range. Higher strength aluminums again creep into the low range of single digits, with warning bells ringing loudly. Things without much elongation are said to be brittle. Brittle frame failure is not a good thing.
The tensile strength of titanium is also excellent. The cold-worked-stress-relieved yield strength (see “Touring the Ancotech mill” to find out more on CWSR) of the 3/2.5 alloy (that’s the alloy usually found in bicycle frames) is typically 100-130 KSI or more. This compares favorably with many steels we find in bicycles. Remember, too, this is achieved with fantastic elongation numbers, and at almost half the weight. And we haven’t even talked about fracture toughness and endurance limit yet.
The fatigue strength is another property where titanium performs beautifully (By now, you may be asking: “Is he ever going to say anything bad about titanium?” ). As explained in the previous installments, there is not a definitive measurement of fatigue strength that will tell us how the material will last in a bicycle frame. Bicycles are subjected to forces of varying amounts in a random, cyclic fashion. As long as these loads are kept below a certain level, titanium and steel both have thresholds below which they will never fail. Almost none of the aluminum (including the metal matrix composites), magnesium and beryllium used in bicycle fabrication has a defined endurance limit, so you need to design around it, as was explained last time.
Now for the Bad News….
The negative sides of titanium are several, and they will keep titanium from becoming ubiquitous in the market. First, it’s expensive. Not only is the cost of energy used to extract the metal costly, but the processing requirements are cost intensive as well.
The other problems have to do with fabrication. You’ve certainly heard that titanium is hard to weld and machine. A more accurate statement is that it is different to weld or machine. What you can’t do is cut corners with titanium. Meticulous procedure is essential. Without it, you risk contaminated welds, which can result in catastrophic failure of the weld.
At the recent Cactus Cup race, I came around a corner on the course to find a guy with a titanium bike that had a freshly severed head tube. A quick inspection revealed my suspicion: a contaminated weld. Machining titanium is either a dream or a nightmare, depending on your procedure. If you use the proper speeds and feeds, and the right cutting tools, it will machine beautifully.
If steel is “density challenged” and aluminum is “strength challenged,” then what challenges face titanium? Modulus is the biggie. Even if we start building our bikes out of higher strength titanium like 6/4, the modulus will stay the same. As the walls get thinner and the diameters larger, stiffness goes up and weight goes down – but to enter the next generation of reduced-weight framesets using conventional tubes and methods, the walls will be so thin that buckling will be a problem. There are ways around the buckling, however. Several manufacturers already have titanium bikes that have internally butted, externally butted, formed or swaged tubes, or some combination thereof. Watch for more development in this area as a way to continue exploring the limits of lightweight, strong frame design with adequate stiffness.
Will titanium be considered the material of choice in the future? Its position and reputation as a magical metal probably won’t be seriously challenged for a while. But even so, look for some action from the aluminum fabricators, who are evolving their craft, and whose frames will get stronger, cheaper and lighter, giving the customer an excellent value. The titanium guys won’t stand idly by and just watch this happen, though. Litespeed is already pushing the price envelope to new lows with excellent road and mountain-bike frames in the $1000 range. Although the extremely low-price barrier probably won’t be broken, continuous improvements in tube forming and fabrication techniques will keep titanium’s demand and reputation strong.
In the next issue, the “Heady Metal” series covers a non-metal, carbon fiber.