Titanium has about half the density of steel and double the density of aluminium. Also, it has half the strength of steel and double the strength of aluminium. Most people think it is strong, harder, tougher etc etc than steel. It is not. It is stronger, harder, tougher than aluminium. Yes. Stay with me. Titanium work-hardens wildly. In other words, as you manufacture (work) it, be it drilling, milling, bending or extruding, it becomes harder. This makes it more difficult (read expensive) to manufacture than either steel or aluminium. In order to make a very light strong titanium frame, you have to extrude it paper-thin into big tubes (think Cannondale). This is very, very difficult. Hence, they extrude it into thicker, smaller-diameter tubes as in classic Litespeed. This automatically makes the frame heavier than an equally-strong aluminium one. Aluminium is easy to extrude into paper thin tubes without losing strength or maleability. That's just on the raw tube manufacturing side. Now for the mining/sourcing side: Titanium oxide is abundant right here on earth. It is a white powder readily surface-mined. The problem comes in driving the oxygen off the powder so that the metal remains. This requires a huge amount of energy, many times that required to obtain steel or even aluminium. This makes it expensive and gives it in modern parlance, a high carbon footprint. Coming back to manufacuturing. This process requires welding, bending, milling, mitering and forming. All if which is easy with aluminium, difficult with titanium. Difficult and expensive. Further, the welding actually weakens the titanium structure and to counter this, it requires precise heat-treatment. Alu also requires heat treatment but the process is well understood commercially, and quite successful. On titanium it is exactly the opposite, hence all the cracking we see on ti frames. Titanium is not all negative. It has high PR value. It was after all glorified as the one metal that became available after the cold war. During the cold war all ti was locked up in missile hulls and jealous gaurding of sources and methods. After the cold war Ronald Reagan and Michael Gorbachov (f?) famously teed of with drivers made from molten-down missiles. This sent the world on a titanium frenzy. Citizen (watches) bought a huge amount of the stuff and produced a titanium watch. Titanium became available for medical use and it became a household name for what has been known as Unobtanium up to now. Russian companies entered the free market in the west with titanium nuts and bolts and other forbidden fruit. It went wild. Even brushed stainless or alu became know as "titanium finish". The latter word only added by honest merchants. Another nice thing about titanium is that is is practically inert. What that means to you and I is that it wont rust. Those are its two prime properties - brag value and corrosion-resistance. An alu frame can be made lighter, stronger, more durable and much much cheaper than a titanium frame. What more is there to want from a frame? Since titanium is twice as hard as aluminium, some parts can successfully be made from it. Bolts - sprockets etc. Insofar its use in bolts goes, it is a poor substitute for steel. In that application it only saves half the weight of the bolt, but bolts weigh nothing to start off with, looking at the overall bike. As a sprocket it is even poorer. It wears like candle wax. It is a totally inappropriate material for bicycles. Smallprint: I wildly avoid mentioning alloys of all three metals discussed since all alu, iron or aluminium used in bicycles is alloyed in anyway. It only complicates the issue. The core assumptions remain true.
