Combining Composites to Make Better Bike Frames

Be sure to read the fine print before you buy your next carbon fiber bicycle. Why? Because the carbon fiber frame you think you’re getting may not be what it appears. It may have been manufactured with a combination of carbon fiber and other composite materials. That’s not a bad thing, by the way. Combining composites can make for better bike frames.

Rock West Composites, a Salt Lake City supplier of carbon fiber tubing and other composite products, says that the main advantage of carbon fiber as a bike frame material is weight savings. Carbon fiber is stronger than steel and aluminum while also being lighter, which is an excellent combination for cyclists in search of bikes that are more comfortable and easier to handle.

As great as carbon fiber is for bike frames, it does have inherent weakness: carbon fiber does not absorb shock very well. This creates a few problems that carbon fiber bike owners are all too familiar with.

Rider Comfort and Safety

The tensile strength of carbon fiber is clearly superior to both steel and aluminum. However, that same strength is carbon fiber’s biggest downfall. Because the material is so strong and rigid, its shock absorption properties suffer. Carbon fiber also does not flex very easily. That can make riding a carbon fiber bike a bit uncomfortable. Because the bike frame itself is not absorbing shock, the energy ends up traveling though the bike frame and right into the rider’s body.

The other problem is one of safety. When carbon fiber suffers minor damage, the damage is often difficult to see with the naked eye. Minor damage also doesn’t tend to be catastrophic right away. For example, tiny cracks can appear on the inner portions of a bike frame tube without the owner ever knowing. Over subsequent rides, the damage can grow until the bike frame eventually fails. More than one bike rider has been injured this way.

Combining different composite materials is the solution to both problems. One company recently profiled by Composites World magazine utilizes a second composite material made with polypropylene to make up for carbon fiber’s comfort and safety shortcomings.

A Different Kind of Prepreg

The company’s carbon fiber tubing is manufactured via a process that utilizes prepregs molded to create tubes. Polypropylene prepregs, created in much the same way as their carbon fiber counterparts, are also molded to create tubes for some of the most sensitive areas of the bike frame. Those tubes make up the inner portion of a few of the carbon fiber tubes.

Adding the polypropylene material in key locations makes a carbon fiber bike more flexible, more amenable to energy absorption, and less prone to catastrophic failures. The downside is that the polypropylene material adds weight. This forces the manufacturer to be judicious about where to use the polypropylene material and how much of it to use.

Making a Great Material Better

Combining polypropylene and carbon fiber prepregs to make bike frames may seem revolutionary, but it is just the next logical step in composite manufacturing. By definition, composites are blends of two or more materials. Creating a carbon fiber reinforced plastic (CFRP) requires combining carbon fibers woven into a fabric with an epoxy resin to create what will become a single piece once heat and pressure are applied.

Knowing this, it should be easy to understand that combining carbon fiber and polypropylene is really just creating a new composite. It is a composite composite, if you will. And it is making carbon fiber better – at least where manufacturing bike frames is concerned.

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