So, you've decided that you're tired of looking at your stock car and you're going to take the plunge. You're going to modify the body in some way to personalize your car and make it stand out. Maybe you're going to start out slowly and add a spoiler or front air dam, or maybe you're going all the way with those parts plus side skirts and a hood. Whatever you decide, another decision you'll have to make is the type of material you want. Body kit parts come in fiberglass, polyurethane, ABS plastic, carbon fiber and composite. Some manufacturers specialize in one or two types, while others offer various kinds. All parts are not made in every type of material, and some materials are more suited to certain parts. Let's take a look at each type.
Fiberglass is the most common material used for body kit parts. Fiberglass parts are relatively easy to produce and inexpensive to buy, but their fit and finish as delivered is not as good as parts made from polyurethane or ABS plastic. They're lightweight and won't warp regardless of the temperature, and paint readily adheres to fiberglass. The downside to fiberglass is its rigidity. This lack of flexibility makes fiberglass more difficult to install and makes the finished product much more vulnerable. For example, if you're not careful going up and down driveways or around parking lot barriers you'll probably end up with a cracked or broken fiberglass front air dam, whereas a polyurethane part would just bend and flex. Because fiberglass parts can be rough coming out of the mold, they usually require more finishing than polyurethane parts for an acceptable end product.
And even when you do everything right and drive carefully, fiberglass is still susceptible to cracking. As stated earlier, fiberglass is inexpensive compared to other materials but there are also different grades of fiberglass. There are plenty of cheap fiberglass parts on the market that are flimsy and difficult to install; nothing more than a bunch of cracks waiting to happen. Parts from a reputable manufacturer will be more substantial, with better fit and finish; they're easier to install, and the final product will look good longer. Always buy fiberglass from a trustworthy manufacturer of quality parts.
Polyurethane, also known as just urethane, is another popular material for body kit parts. Polyurethane parts are more difficult and costly to produce; therefore they are more expensive to buy. However, the as-delivered fit and finish is usually superior to fiberglass. As mentioned earlier, polyurethane is much more flexible than fiberglass, making it easier to install and more durable. A polyurethane part can be "coaxed" into position during installation without fear of it cracking, and the end product will flex in response to a minor impact instead of cracking or breaking.
However, polyurethane parts are heavier than fiberglass, so they're generally not a good choice for a race car, where reducing weight may be a primary goal. And while the finish on polyurethane parts is generally better than fiberglass, getting paint to adhere to them can be more difficult. They require more specialized preparation to ensure a quality finished paint job, or the paint can flake or peel off. Polyurethane is also susceptible to warping from extreme heat, so your environment must be taken into consideration.
ABS plastic is a thermoplastic made by polymerizing styrene and acrylonitrile in the presence of polybutadiene. The result is a material that combines the strength and rigidity of the acrylonitrile and styrene polymers with the ruggedness of the polybutadiene rubber. ABS plastic is impact, chemical and heat resistant, and widely used by the Original Equipment automobile Manufacturers (OEMs) for body panels. Like polyurethane, ABS plastic is a high quality material with superior fit and finish, which is more expensive and heavier than fiberglass. It's somewhat less flexible than polyurethane but easier to paint.
Carbon Fiber, or more accurately carbon fiber reinforced polymer, is a polymer like epoxy reinforced with carbon fibers. Carbon fibers are constructed from carbon atoms bonded together in crystals that are more or less aligned parallel to the long axis of the fiber. This alignment makes the fiber very strong for its size. The fibers are woven into the fabric that gives carbon fiber its distinctive appearance. Carbon fiber parts are very strong and very light weight, making them ideal for race cars or anyone that needs strength without the weight penalty that comes with metal. However, for most average street cars, carbon fiber parts are installed more for their looks and performance image than any functional improvement. Carbon fiber parts usually come clearcoated and ready to install right out of the box. The most significant downside to carbon fiber parts is that they are very expensive compared to other materials. However, a cost effective way to go is to mix parts. For example, choose fiberglass or urethane for the lip spoiler, air dam and side skirts, and then spring for a carbon fiber hood.
As the term implies, a composite is a combination of materials. These materials are usually combined to create a product with benefits that could not be realized from the materials if they were used alone. Extreme Dimension's Duraflex product is perhaps the most well-known example. Duraflex is a combination of fiberglass, plastic and resins that result in a product that is more flexible and durable than ordinary fiberglass, yet remains light weight.
A final point to consider: Make sure that the shop you choose to install your parts has experience installing body kits. Just because a shop has years of experience in auto body repair it doesn't mean they're qualified. Installing body kits requires special expertise and it would probably be best to consult with whoever will be doing the work before you make a purchase. Regardless of the body kit material you choose, the end result will only be as good as the quality of the installation.