klehner wrote:
tri_yoda wrote:
rosshm wrote:
What are the benefits of titanium as a frame material?
expensive, allows you to brag to your friends.
capable of having a brushed/polish finish which looks cool.
"superior" corrosion resistance. Which isn't really really much of an advantage, because corrosion isn't really a problem (even for steel bikes) if they are properly coated/painted and cared for.
the claimed benefit is "lighter" and more compliant than other metallic materials. The reality is that titanium is pretty "squishy" and a well designed steel bike built with a high quality, butted, alloy tube set (like Reynolds 753) is going to be a superior performing bike, same or lighter weight, stiffer where you need, compliant to the extent you need it.
Titanium is a "cool" material, but best left to applications of high temperature service, where corrosion or oxidation is a serious problem. For bikes, it really is a boutique material and if you did an objective comparison across a range of material properties against the functional performance requirements for a bicycle frame there is almost no way it would ever come out on top unless you just had some kind of emotional love for the material.
Where does this "compliance" or squishiness" or "stiffness" purportedly come from, in any material bike frame?
a combination of the mechanical (geometry, thickness, diameter of the tubes) design and the modulus of the material. The modulus is the underlying property of the material, basically how much does it deflect under a specific applied load. However, that's only half the equation, the other part is the design of the tubeset. The easiest example of design is look at an old cannondale vs an old steel bike. The old aluminum bikes had much bigger diameter tubes to get adequate tube stiffness out of a lower modulus material, older steel bikes could get away with smaller diameter tubes because the material had much higher modulus. There have been a lot advances in heat treatment, alloy composition (both for material properties and improving weldability) and tube shaping capability over the past 30 years, so these caricatures (skinny old steel tubesets and fat old aluminum tube sets) have changed a lot, but are useful for understanding how you approach the design differently depending on the material.
compliance refers to deflection in the vertical direction, "stiffness" usually refers to out of plane deflection. Again, the compliance and stiffness is due to a combination of the chosen materials and tube geometries (larger diameter tubes. flares in tubes). Old cannondales compensated for the low modulus of Al, by using larger diameter tubes (but with thinner walls to keep the overall weight down). But the aluminum alloys have gotten a lot better and cannondale has implemented a lot of innovations, they used to do a post weld heat treatment of the entire frame (although a lot of that was related to welding process, which has also been improved a lot, both process wise and by improving the materials), not sure if they still do.
It's hard to do an apples to apples comparison, unless you were to build identical bikes (tube diameters, thickness, shape) out of different materials. Because of differences in the underlying materials, you choose different tube diameters, thickness and shapes depending on material, which will tend to compensate for some of the differences in uderlying properties of the materials, so that at the end the performance of the different materials general converge (in the complete frame) to some extent. However, it has been a typical practice to build steel and titanium bikes with relatively similar tube shapes and diameters and in such a case the modulus of the as-built Ti frame is going to be less than the steel one (which some might describe as squishy). Again, there is a whole range of possible stiffness of a frame depending on how you build (regardless of the material), so there are no absolutes only relative generalities.
But kind of back to the original point of why there really aren't any advantages for titanium (aside from thermal stability and corrosion resistance), it is the most expensive material and since you can make a good bicycle out of steel, carbon fiber or aluminum by properly designing the frame and tube geometry, there is not good reason to choose a material that costs 2-3X as much on a per pound basis. Unless you just "like it" or have some emotional conception of "ride quality", which is often as much to do with the mechanical design of the frame.