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A body kit is a collection of parts fitted onto a vehicle that modifies a vehicle's appearance and performance. Body kits were first developed in the 70's to improve aerodynamics and to improve racetrack laptimes on race vehicles. Since then, body kits have spread out to the street car market and has become a popular modification for car enthusiasts and racers alike.
Body kit parts can be fitted all around the vehicle. Parts include the front bumper, rear bumper, bumper skirts, side skirts, hood/bonnet, boot/trunk lid, front and rear fenders, and spoiler. Body kit parts are designed with performance in mind, but the appearance of body kits also play a role in how they are designed. Many of these pieces come in a complete kit that match with all other parts alongside the vehicle, but there is a popular option of mixing and matching pieces from different kits onto a vehicle. Different types of combinations and/or sets yield different results due to each body kit part's specifications such as weight.
Commonly used materials for body kits include ABS plastic, fiberglass, polyurethane, and carbon fiber. Vehicles from the factory commonly come stock with ABS plastic front and rear bumpers and side skirts but can easily be swapped with an aftermarket body kit. The most common aftermarket material for body kits is fiberglass since it is relatively inexpensive, but the durability is low as it is prone to cracking. Polyurethane is more durable compared to fiberglass but is heavier, therefore limiting its use in the performance aspect. Carbon fiber is the least common but most popular choice for body kits as it is the strongest building material as well as the lightest, thus has the best weight to strength ratio. However, it is expensive to produce carbon fiber body kit parts which makes other materials more cost effective.
The effects of a body kit varies widely between vehicles, as some provide better performance advantages over the other. Vehicles, fitted with body kits, that are driven on tracks yield better performance boosts than when driven on public roads. Street vehicles with body kits will yield no performance increase because public roads do not allow for high speeds.
Body kit applications are most common on race vehicles when aerodynamics affect a vehicle's performance to improve laptimes by one second or more. A body kit's effectiveness depends on many outside variables like weather or air density. A common method of improving aerodynamics through body kits is the use of front ground skirt kits. A ground skirt kit fits under a vehicle's front bumper to provide a perception of a lowered vehicle stance and also to guide air away from the bottom of the vehicle and move frontward air over the top of the vehicle. For a street legal vehicle driven on everyday streets, there is little to no performance gain; however, for performance race cars, a body kit improves a wide range of performance aspects, such as streamlining and road grip.
Downforce, the downward force applied by air onto a vehicle during high speeds. A body kit assists on the creation of downforce by streamlining a vehicle to allow airflow to flow around and over without resistance. The downforce promotes greater traction by increasing air force on top of the vehicle which increases friction between the tires and the road. The enhanced friction between the tire and the road allows for a vehicle to turn a corner at higher speeds with more stability. Unfortunately, this causes the tires to wear down quicker. Although a body kit is meant to improve a vehicle's performance, the increased friction of the tire created by the downforce will inhibit acceleration at higher speeds. 
Body kits have heavily enriched modern day media. Video games like Tokyo Xtreme Racer 3 (2003), The Fast and the Furious: Tokyo Drift (2007), and Need for Speed: Shift (2009) were released with a large emphasis on vehicular modifications with a focus on body kits.