底盘即车辆的车底主框架。所有车辆的动力部分，包括引擎、车轴、变速箱、差动器等以及悬挂系统都安装于底盘上。部份车辆设计为底盘与车身分开，而其结构已实现汽车动力的基本功能，所以此种设计的车辆可以在没有车身的情况下行驶，大部份重型车都是此类设计。而另一部份底盘设计成与车身一体化，即车身与底盘为一完整结构，这种设计多出现于私家车上。在商用车市场，有的厂商甚至出售只包括底盘的货车和未有装配车身的客车底盘。专用车生产商在购买的底盘上再开发特殊用途的车辆，比如消防车、升降车等。

A chassis (/ˈtʃæsi/ or /ˈʃæsi/; plural: "chassis" /ˈtʃæsiz/ or /ˈʃæsiz/) consists of an internal vehicle frame that supports a manmade object in its construction and use. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame (on which the body is mounted). If the running gear such as wheels and transmission, and sometimes even the driver's seat, are included, then the assembly is described as a rolling chassis.