For offroad vehicles, the emphasis is on lengthening the suspension travel and installing larger tires. Larger tires—with or without larger wheels—increase ground clearance, travel over rough terrain more smoothly, provide additional cushioning, and decrease ground pressure (which is important on soft surfaces). These suspension modifications are in contrast to lowriders, which use hydraulic or pneumatic suspensions. Lowriders use another type of suspension tuning in which the height of each individual wheel can be rapidly adjusted by a system of rams which, in some cases, makes it possible to "bounce" the wheels completely off of the ground. Body tuning Body tuning involves making modifications to the body of the car in order to alter the aesthetics of the car, improve performance, or both. Body tuning can also involve changing or replacing parts for better aerodynamic performance. Through downforce, cornering speeds and tire adhesion can be improved, often at the expense of increased drag. To lighten the vehicle, bodywork components such as hoods and rearview mirrors may be replaced with lighter-weight components. Often, body modifications are done mainly to improve a vehicle's appearance, as in the case of non-functioning scoops, wide arches or other aesthetic modification. Aftermarket spoilers or body kits rarely improve a car's performance. The majority, in fact, add weight and increase the drag coefficient of the vehicle, thus reducing its overall performance. Dating back to the 1940s, chopping and channeling was a popular method of modifying a car's aerodynamics. Increasing the wheel track width through spacers and wide body kits, or lowering the center of gravity via suspension modifications, can enhance the car's cornering ability. Often, suspension tuners unfamiliar with spring dynamics will cut stock springs, producing a harder, bouncy ride. It is also common to stance a car, lowering it beyond its optimal ride height purely for appearance. Competition cars may have lightweight windows, or the windows may be completely removed, as auto glass adds significant weight and detrimentally alters weight distribution. Plastic windows are much more vulnerable to scratches, which reduce service life. Tires Tires have large effects on a car's behavior and are replaced periodically; therefore, tire selection is a very cost-effective way to personalize an automobile. Choices include tires for various weather and road conditions, different sizes and various compromises between cost, grip, service life, rolling resistance, handling and ride comfort. Drivers also sometimes personalize tires for aesthetic reasons, for example, by adding tire lettering.