When speaking about the vehicle as a whole, the chassis is the first major component that is considered. The frame or unibody is the large structural unit that is the base of the vehicle’s assembly. The wheels, suspension, body, engine, and other vital parts of a vehicle all must attach to a base assembly; referred to collectively as the chassis.
There are differing opinions as to what is included in a rolling chassis. Some sources include an engine and drivetrain nestled within an otherwise bare vehicle frame. Others claim a rolling chassis is more of a complete vehicle without the drivetrain; which is not accurate as a rolling chassis would not necessarily include any coachwork or comfort items. Body-on-frame cars are easier to visualize as a rolling chassis; it’s the vehicle with the body removed. An engine need not be part of the package as long as the suspension arms, dampers, springs, and hubs are there, along with brakes and wheels.
Chassis versus Frame
To state a chassis as “the frame” may be an over-simplification of what a chassis is. The frame is the major component of the chassis; it acts like a skeleton that provides the backbone of the vehicle. To it, suspension arms with spindles, brakes and wheels are attached. As components are added, the frame becomes a chassis. To put it another way, the chassis is the pure functional component assembly without the body on it.
Chassis versus Shell
It becomes difficult to nail down where the chassis stops and the body starts when the vehicle has a uni-body construction. In these chassis configurations, the body shell serves as the frame. As such it is not practical to visualize the frame without the body. With the aid of auxiliary sub-frames that are affixed to the uni-body, suspension arms are attached to the body, to create a rolling chassis. Commonly, people will refer to this chassis as a shell or a rolling shell. While the body is present, the interior usually would not be. Again, the powertrain is “optional.”