Getting Clever With Carbon Fiber
By Alex Vendier
Smart people might say that having carbon fiber parts on your car is a little bit like having a gold watch. Both are great and serve a purpose but are probably more style than substance. Other than for showing off, are you really getting anything out of using these rare commodities on your wrist or ride? Well, as the unofficial MotoIQ D.I.Y. mastermind I've been playing with some simple methods that allow a home user to gain the benefits of custom carbon fiber parts without having to pay the bling tax so often levied by the manufacturers. Also the D.I.Y. method gives one the ability to make parts that no one would offer in retail form. Like an undertray for a 1990 Geo Metro with midships mounted motorcycle engine for example.
As the development of the Geo Metro-Gnome has progressed we started looking to some simple aerodynamic aids to help improve grip at higher speeds. Since the car has a relatively modest 140 hp it tends to fall behind some of the more powerful LeMons cars on the faster sections of some tracks. The Geo's super light weight is less of an advantage as the speeds increase because the faster a car is going the more HP is used just to push it through the air. This means that in a low HP car, being able to stay on the gas in the faster sections is key because high speed acceleration is limited. Even though a wing or a splitter might add some overall drag, the straightaway speeds can be higher because the car enters the straight going faster. Making these parts on a no-budget is where I started with D.I.Y composites and now it's another part of the bag of tricks drawn on for all kinds of projects.
First off you have to get familiar with some of the materials that are used to make carbon fiber parts. As I am sure most of you already know carbon fiber is just that. A fibrous stranded material that has very impressive tensile strength. The art of this game is manipulating the material so that its amazing properties are put to good use. How one fashions composite parts is very important to their ultimate durability and usability. I'm going to start us off with a pretty basic project that shows a good survey of the techniques available to a composite fabricator and this is a composite sandwich structure.
Let's start by going to the store and buying an ice-cream sandwich (I.C.S.). Here we have a pretty good mock up of a composite structure system that's good to eat too. On the outside there are the chocolate cookie parts and on the inside there is the ice-cream core. If you take the sandwich and slowly break it in half you see that the top cookie is starting to tear apart while the bottom cookie of starting to buckle. This is a great example of how you can isolate the stresses on a material to make best use of its unique properties. In the I.C.S. the cookies are in both compression and tension depending on the direction of the forces. Don't forget the the ice-cream core though as it's in compression as well. Those cookies are pretty thin but they add a huge amount of strength to the I.C.S. because they are relatively strong in tension and compression. Without the cookies the ice-cream would just droop under its own weight if cantilevered any amount at all.
Now that we have a good way of looking at a composite structure let's make one that we can use with regular, home hero attainable tools. First off we need to learn a bit about what's available in the world of composites. There are several sources online that sell everything one needs to make nice composite parts at home but since none of the places I contacted took up my offer of helping me with costs in exchange for mention in this article I'll let you Google them yourself. Anyhow, the basics are resin and cloth. Everything else falls into the category of tools and expendable supplies. Let's take a look at what's what before we start.
Reinforcing cloth is really the basis for a composite part and the king of cloth is carbon fiber so that's what I am going to concentrate on here. There are a myriad of other options though including colored fiberglass, aluminized fiberglass, kevlar, and mixes of some of these too. If it's just a bling thing, take a look at black colored fiberglass and save your money for carbon when you actually need its strength. Looking onward, the first thing one needs to determine is what kind of carbon weave will be best for the part you are making. Any woven material starts with strands of fiber. These strands are gathered up to form the yarn that's woven into cloth. The thickness of these yarn strands are measured in the number of thousands of fibers per strand. The shorthand for this is “K”. You'll see 1K, 2K, 3K, xK, denoted cloth when shopping and oddly the thicker stranded weaves are actually cheaper due to production volumes worldwide. 3K is the most common and versatile strand thickness and it's what I have used so far. The next factor in cloth selection is the style of weave. Composite cloth is woven in two basic patterns and they are plain weave and twill weave. Plain weave is where the strands are woven in a standard over under pattern. This kind of weave is really good of complex curves but the trade off is that there is less density of material, particularly when it's worked into a complex shape. Twill weave, on the other hand is woven in a 2 (or more) over and under pattern and while it does not form to curves as well as plain weave it is more dense. There are also some some unidirectional weaves that have the majority of fibers arranged in one direction to maximize strength along one axis. The last, and most important, specification for a cloth is its weight per square yard. This is an important factor because it gives you a basic idea of how much resin you will need in order to saturate the cloth properly. The general convention on resin to cloth ratios state that one wants around a 50/50 ratio. That means that for a square yard of typical 5.7oz/yd cloth one would want to mix up about 5.7 oz of resin. This ratio changes with the technique used to make the parts. More on this later.
Speaking of resin, let's start speaking about resin. Back in the old days the most common resins used in composites were polyester resins. Cheap, easy to use and pretty strong they showed up in everything from surfboards and speedboats to Corvettes and lawn furniture. Polyester resins can be hardened at differing rates by using more or less catalyst in the mix. That's all great, but they have some significant shortfalls such as toxicity and most importantly the inability to bond to carbon fiber cloth that make them less popular currently. Now a days most composites are made using epoxy resins. Epoxy resins are significantly stronger and they bond very well to most composite cloth options. The only downsides of epoxy are a fixed cure rate and the requirement that the resin to hardener ratios be very carefully measured, usually by weight. There are many different brands and types of resins available including hi-temp resistant and optically clear versions. The most common epoxy resins are slightly gold or brown in color and are a bit U.V. sensitive tending to turn yellow with prolonged sun exposure. There are plenty of U.V. stable clear resins available for a small price premium though, so not to worry, your carbon hood can look perfect for the life of the part even if you live in a sunny place like So-Cal.