Bicycle Frames Essay, Research Paper
Technology of Composite Bicycle Frames
December 18, 1998
Dec. 18, 1998
Technology of Composite Bicycle Frames
The definition of technology is systematic treatment/application of knowledge to produce something (Webster?s1496). I will take this definition and describe the technology of making a bicycle frame. Composites structures are defined as : ?…constructions utilizing admixture or blend or a compound of distinct parts or elements of various available materials to produce a resulting structure of the highest possible strength to weight ratio …? (Moynihan ). For this paper ?composite? will mean a plastic resin reinforced with structural fibers. In this paper I will be comparing composites against other methods of building bike frames.
First we will discuss the structural advantages of building a bicycle with composites. Using composites you are not limited to certain shapes as if you were using metal tubing. Most metal bike frames are tubes of standard sizes and standard wall thickness. With composites, you can differ the thickness of the wall of the frame. If you want more strength in one area, you just add more material in that area. By changing the wall thickness in composite layups, you will change the strength of your frame in a localized area. Metal tubing construction is mostly limited to constant thickness walls. Because of the constant thickness the metal bike frame will end up being heavier than it needs to be.
Composites do most of the things metals can do in a bike frame. Although, often the way you go about building with composites is different than if using metals. With composite construction a mold has to be built to form the required shape and then the fibers and resin are held in the mold to harden. With metal tubing construction, off the shelf tubes are cut to length and then fastened together to make the frame. Using composite construction you can build a frame with fewer joints than metal construction. Fewer joints give less chance of cracks starting.
Composite construction can end up being lighter than if you had made the part out of an equivalent metal. Weight savings of as much as half the equivalent metal are possible. The tensile strength of the composite frame can also be as much as twice the metal frame. It is also possible to have a composite frane that is both lighter and stronger than if you had built it out of metal.
Another point of interest is that composites are corrosion resistant. Metal frames must be painted to prevent corrosion. A chip in the paint on metal tubing will result in corrosion to the frame. Composite frames don?t have to be painted. If painted, flaking of paint is not a problem since the frame is naturally corrosion resistant.
Composites have a low thermal expansion rate compared to metals. Metals don?t maintain their dimensions as well as a composite over a certain temperature range. However some composites don?t hold up in low temperatures or in temperatures above 300 degrees F. Another negative if you use composite materials is the cost. The materials to create a composite are more expensive than most metals. Steel is $.23/pound while carbon fiber is $100/pound!
In order to benefit from the structural advantages of composites,you need to know the many different materials that you have to choose from to create a composite. To make a choice you need to know each of the materials strengths. Kevlar 49 is one of the reinforcing fibers. It has an ultimate strength of 525,000 psi. If you were to use pitch carbon fiber, it?s ultimate strength is 300,000psi. If you were to stop with these two choices, you would probably pick Kevlar 49 because it?s stronger. If you go on to look at more of the technical specifications you would see that carbon fiber has a modulus of elasticity of 55.0 in million psi, but Kevlar has a modulus of elasticity of 19.0 million psi. Modulus means the stiffness. You can obviously see that pitch carbon fiber crushes the Kevlar 49 but it all depends on what strength you are looking for.
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Another factor in making a bicycle from composites is that anybody can work with composite materials. The construction technology can be very simple. First, a mold is made. There are two kinds of molds: a female mold or a male mold. I will talk about building a female mold. To make a mold you need eight major components. First is a plan of what you want the bike to look like. Second is medium density fiber board (MDF) to build the mold. The third tool needed is a table saw to cut the mold material. Fourth, a miter saw to cut little pieces of the MDF. Fifth, sandpaper to round the edges of the mold. Sixth, glue to construct the mold. Seventh, filleting compound to create a fillited joint on the mold. Eighth, wax to wax the mold to keep the epoxy from glueing to the mold
Then you have to think about what kind of resin you want to use to bind the reinforcing fibers of the composite together within the mold. You have two main choices: epoxy or vinylester. Epoxy is a two part system. The two parts are resin and the hardener. A good thing about epoxy is that it is easy to work with. However, epoxy tends to be more expensive. Vinylester is a multipart system which includes: resin, promoter, accelerator and catalyst. Both epoxy and vinylester have a limited shelf life. Vinylester?s shelf life is shorter. Both of these resins are shipped with the ingredients apart. Small quantities of vinylester can not be created and thus a gallon or more must be made up at a time. Epoxy can be made up an ounce at a time. Vinylester is cheaper, so this is another thing to look at. With epoxy there are several things that you can buy to ease the process. One is buying an epoxy pump instead of measuring the amount of resin to hardener by hand. With the pump, it will give the correct ratio with great ease. Although the epoxy pump will cost upwards of $150, it?s well worth it. If you?re using Vinylester, you can?t use an epoxy pump because, of course, it?s only used for epoxy. To use Vinylester ?you?ll need one nasty little item?, (Wanttaja 219) a hypodermic syringe that has a capacity of 15cc but you will not need the needle. The syringe needs to have one-tenth cc increments marked.
Another tool that is needed are sharp scissors or a rotary blade cutter. One of these are needed because you will need to cut reinforcing fabric. For some of the reinforcing fibers you need diamond coated scissors!
Now you?re ready to think about your lay up schedule. A layup schedule is a description of the number of layers of reinforcement and the direction of the fibers. The number of layers and fiber direction determines the strength of the composite. Most people cut a piece of material to the right size and lay it in the mold, then saturate the cut piece with the choice of resin. ?Preperation greatly affects the quality of the final product.? (Wanttaja 232). When you are working with resin you need a warm room. This is for several reasons. One reason is because the resin cures faster in a warm room; the resin flows into the fibers quickly. This is good.
You need to make six decision to make the composite for the frame:
1) Fabric, you need to decide on what fabric you will be using,Kevlar 49 or carbon fiber, or other.
2) Resin, you need to know what resin you want, either epoxy or vinylester
3) Bleeder cloth, which is used for removing excess resin.
4) Breather batting , used to remove air and suck up excess resin.
5) Vaccum bag which is a bag that will hold a vaccum for an extended period of time.
6) Vaccum pump which pulls the vaccum which will draw all the excess resin out which will lighten the frame.
The third advantage to building a composite frame over a metal frame is up front cost. Up front cost means the amount of money spent at the beginning of the project for materials and equipment. Working with metals you could use welding machinery or brazing machinery. With brazing you need two tanks of gas, hoses and torches and mask, brazing rod, and a fireproof area to work. You would also need the tubing and a way to cut the tubing precisely. This could cost around $750.00 With welding you would need the electric welder and mask, special electric outlet, welding rod, fireproof work area, tubing and tubing cutter. This can cost much more than $2000.00. In addition to the equipment you need a great deal of skill, time and training. Using composites you need basic woodworking equipment, basic second grade paper mache? skills, resin and fiber. This can be done for a few hundred dollars.
In conclusion, composite bike frames can be stronger/lighter, easily constructed with basic knowledge, and easily constructed using basic wood working tooling. I know this because I built an all carbon fiber recumbent racing bicycle last year myself. I used the technology I described in this paper.
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5857 Wright Drive
Loveland, CO 80537
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