Combined Carbon/Titanium Bikes: Bikefanclub.com

Here is a perspective I have. Every part of a frame performs a different function. If one material can perform that function better than another material, then engineers tend to use it. I don't see any reason why a frame has to be made entirely of one material since this is only past practise. We have modern glues, epoxies, manufacturing techniques and computer-aided design and finite element analysis making designs more well-engineered rather than based on emperiacal incremental improvements.

Aluminum, for example, is great where the frame wants to uber-stiff. Where the frame wants to have some flex to soften rough pavement, then carbon fiber, steel and titanium are used to good effect. Another design aproach is the use the expensive material, such as titanium where the properties of it make the most sense and use a cheaper material, such as carbon fiber where it's properties can be used to good effect, in essence making a cheaper frame than one made from just titanium.

A casual look at a bicycle frame cannot give anyone a good idea on just how much engineering goes into it. What we want in a frame is torsional stiffness (that's the flex we see when we pedal that many feel robs energy when pedaling hard). On the other hand, we do not want vertical stiffness since that leads to a harsh ride. The main diamond in a frame is very stiff vertically, it's a triangle, after all. What we want to have sufficient flex is the front fork. The fork wants to flex vertically but we don't want any torsional flex in the fork. Carbon fiber is being used more and more in forks because it has excellent properties for that function.

Chainstays want to be torsionally stiff and have some vertical compliance. They work with the seatstays in that function. Steel, aluminim, titanium and carbon fiber can be engineered to work well in that area of the bike.

Butting of tubes in steel, aluminum and titatnium are used to help stiffen the tube where necessary and shedding mass where stresses are less. Carbon fiber layups perform the same function.

Some designers don't want to go whole-hog into using a completely different material than what they have their expertise in. Titanium frame makers therefore may use carbon fiber tubing in the areas where they are confident they will do the better job with the least liability and risk. I don't believe mixing materials in a frame is a fashion. I believe it is the attempt of engineers to use what materials they have to make the optimum frame they can.

Material science is constantly advancing. At the beginning of the industrial revolution men began making self-propelled vehicles. Steel became dominant because the properties of steel made it the most suitable for making a light, stiff, yet compliant frame. When aluminum became cheaper and designers began to understand the properties better, aluminum frames became dominant because an aluminum frame could be made lighter than a steel frame without costing more.

Where titanium has properties that rival steel and is lighter, it's downside is that it is a very expensive metal. You can't make a cheap frame from titanium, hence you only see high-end bikes made with the material. Carbon-composites have such potentially great properties that more and more designers are becoming profficient in designing frames with the material making them the lightest frames possible that have adequate stiffness and strength that no metal can rival. There are cons to using carbon-composites just as there are cons to using any material, nothing being perfect.

People will often mention that steel frames are best because they bend when they fail. Many forget, though, that high-end steel frames use alloys of steel that are brittle and will fracture when they fail, rather than bend. Carbon-composites have rather poor abrasion resistance. A chain suck at high speed that throws the chain can dig through the bottom bracket or chainstay area of bike in short order. In areas where abrasion resistance is highly desireable, carbon fiber is not the optimum choice. That said, a bike frame can be engineered from many materials to be a bullet-proof frame. The tradoffs will be in weight-penalties. Steel is a dense material so it's not easy to design a steel bike to compete in the weight weenie category.

No matter what materials go into a bike, a poor design will ruin your day, if not threatening your life. There are too many variables to generalize that one material is superior to another in all areas of a bicycle. I happen to be attracted to carbon fiber, not for any other reason that it appeals to my sense of style. What type of bike you ride depends on your style. For what it's worth, we will see manufacturers constantly experimenting with mixing materials in a frame.