Metallurgy for Cyclists IV: The Titanium Advantage
by Scot Nicol
This, the fourth part of our metallurgy series, is about that mysterious and expensive metal, titanium. The Titanium Development Association calls titanium "the material of choice," and there are many in the bike industry who would agree with them. Titanium combines light weight, super strength and fatigue life, a magical ride... and a hefty price tag. Let's find out about the physical characteristics that give titanium its lustre.
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 essential ingredients for a steel bike. You could argue that titanium will be with us for longer.
Like aluminum, titanium shines on density. Although its density is almost double that of aluminum, it's still only half that of steel.
Our second property is stiffness, or Young's Modulus. The titanium used in bicycle frames has a modulus around 15 million pounds per square inch -- once again, approximately half that of steel. This means that steel and titanium have comparable stiffness-to-weight ratios.
Because of titanium's combination of high strength, low density and moderate modulus, most fabricators choose tube diameters that provide a supple, shock-absorbing ride -- though there's nothing to stop an enterprising designer building a stiff titanium ride, of course. However, when titanium gets down into the realms of the super-light, its modulus becomes a problem. A two pound titanium frame is generally too flexible. Building an ultra-light frame is an awkward task in any material, and titanium is no exception.
Ti's Real Plus
So, in the first two properties we examined, titanium comes second to steel and aluminum. But when we look at property #3, elongation, titanium is miles ahead of either of its competitors.
Elongation is the property that tells you how far something will bend before it breaks, a kind of safety factor for framebuilders. Materials without much elongation are said to be brittle. Brittle frame failure is not a good thing.
The elongation numbers for titanium are often as high as 20 to 30%. For comparison, typical steels can be 10 to 15%, with the higher strength versions going down as low as 6%. Aluminum typically runs 6 to 12%.
Titanium also has excellent tensile strength. The cold-worked-stress-relieved yield strength of the 3/2.5 alloy usually found in bicycle frames is typically 100-130 KSI or more. This compares favorably with bicycle steels. Remember, too, that this yield strength is achieved with fantastic elongation numbers, and at a 50% weight saving against steel. And we haven't even talked about fracture toughness and endurance limits yet. Like steel but unlike all the other framebuilding materials discussed here, titanium has a fatigue limit. If that limit is respected, a titanium frame will never fail.
Now the Bad News...
There are some marks against titanium. First, it's expensive. The extraction costs are high, and its special processing requirements are cost-intensive as well.
It's also difficult -- or, at least, different -- to weld or machine. A Ti builder can't cut corners. Without meticulous procedures, he risks contaminated welds, and catastrophic failures. At the recent Cactus Cup race, I came around a corner on the course to find a guy whose titanium bike had just lost its 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 -- but it may be unsuited to mass production.
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 need to be so thin that beercanning will become a problem. There are ways around the buckling, however. Several manufacturers already have titanium bikes with internally and external butts, formed or swaged tubes, or combinations thereof. Watch for more developments in this area.
Will titanium become the industry's 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. 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 will probably remain in place, continuous improvements in tube forming and fabrication techniques will keep demand strong.
In the next issue, the "Heady Metal" series covers a non-metal: carbon fiber.