Fundamentals of the Automobile and Chassis Layout
This domain covers what an automobile is made of and how the major assemblies are arranged: the chassis or frame, the power train, running gear, steering and electrical system. Exam questions usually ask you to name a layout from the position of the engine, drive axle and propeller shaft.
Major assemblies
A complete vehicle has the engine (power plant), transmission (clutch, gearbox, propeller shaft, final drive and differential), running gear (suspension, wheels, tyres, brakes), steering system, and electrical system. The chassis carries all of these. Its functions are to support the components and payload, to keep the assemblies in correct alignment, and to absorb road shocks without excessive twisting.
Frame types
A conventional ladder frame uses two channel-section side members joined by cross members, with cruciform or X-bracing for stiffness; it is used on trucks and many buses. A monocoque (unibody) design uses the pressed body shell itself as the load-carrying structure - there is no separate frame - and is typical of modern cars. Sub-frames may carry engine or suspension loads.
Drive layouts
Front engine, rear-wheel drive (conventional): engine at the front, power through a long propeller shaft to a live rear axle. Front engine, front-wheel drive: compact, no long propeller shaft, uses constant-velocity joints, with a transaxle. Rear engine, rear-wheel drive: used on some city buses and classic small cars. Four-wheel drive sends power to both axles through a transfer case.
Dimensions
Wheelbase is the distance between the centres of the front and rear axles. Track is the distance between the centre lines of the tyres on the same axle. Ground clearance is the height of the lowest point below the chassis. Overhang is the distance from an axle centre to the nearest end of the vehicle.
Worked example
A vehicle has its engine at the front, a propeller shaft running to the rear axle and no drive shafts at the front wheels. Reasoning: the engine drives the rear axle only, through a propeller shaft and live axle, so the layout is front engine, rear-wheel drive, conventional.
Traps that cost marks
Confusing wheelbase with track or with overall length; assuming every car has a separate chassis; forgetting that a monocoque body is the frame.