Unit 54

Level: 3

Guided learning: 60

Unit introduction

Modern vehicles are highly developed machines that involve sophisticated and complex systems. Engines now provide more power, leading to higher torque and greater speeds than in the past, which drivers have to handle and control. This involves controlling power to the road wheels, stopping the vehicle when needed and directing it in a particular direction. In addition, the vehicle must be comfortable to travel in and be able to cope with the many forces that act on it.

This unit will introduce learners to the layout, function and operation of systems and components found in the main vehicle chassis systems. Learners will develop an understanding of the operating principles of a range of transmission, steering, suspension and braking systems.

The unit will also support further development of apprentice knowledge in areas such as fault diagnosis, specialised transmission systems, steering, suspension and brakes.

Note that the use of ‘e.g.’ in the content is to give an indication and illustration of the breadth and depth of the area or topic. As such, not all content that follows an ‘e.g.’ needs to be taught or assessed.

Learning outcomes

On completion of this unit a learner should:

Understand the layout and operation of a transmission system and its components
Understand the layout and operation of a steering system and its components
Understand the layout and operation of a suspension system and its components
Understand the layout and operation of a braking system and its

Unit content

Understand the layout and operation of a transmission system and its components

Transmission layout: drive method, e.g. front-wheel drive, rear-wheel drive, four-wheel drive; power path, e.g. flywheel, clutch, gearbox, drive/prop shafts

Transmission operation: function (clutch, gearbox, prop shaft, drive shaft, universal joint, final drive, differential); factors affecting torque transmitted by clutch, e.g. number of plates, diameter, friction; gearing arrangements,

e.g. ratios, simple and compound gear trains

Transmission components: clutch, e.g. single plate spring, diaphragm; release mechanisms, e.g. linkage, cable, hydraulic, pneumatic, electrical; gearbox (input shaft, lay shaft, main shaft, idler); types of gear (straight cut, helical); universal joints, e.g. Hooke’s type, constant velocity type; final drive–crown wheel and pinion (bevel, hypoid and helical gears), differential (sun and planet gears); drive shafts (hollow and solid); axles, e.g. semi, three quarter, fully floating; wheel hubs, e.g. taper, roller bearings; transmission lubricant,

e.g. hypoid, multi-grade

Understand the layout and operation of a steering system and its components

Steering system layout: steering method, e.g. rack and pinion, recirculating ball; position adjustment

Steering operation: Ackerman layout, toe out on turns, wheel alignment, camber, castor, swivel pin inclination, negative offset; oversteer and understeer behaviour; steering arrangement, e.g. two-wheel steering, four-wheel steering systems

Steering components: steering wheel and steering column (bearings, bushes, mounting); universal joint (mounting methods, gaiters); steering linkage and joints for single steer vehicles, e.g. track rod, drag link, drag link ends; steering arm, tie rod (bushes, joints); steering box (seals, bearing)

Understand the layout and operation of a suspension system and its components

Suspension systems layout: suspension method, e.g. beam axle, independent front suspension (IFS), independent rear suspension (IRS); vibration and damping methods, e.g. metal, rubber, hydraulic, hydro-pneumatic

Suspension operation: interaction of components, e.g. vehicle loaded/unloaded, cornering, ‘bump’ reaction

Suspension components: spring systems, e.g. leaf, coil, rubber, hydraulic; fittings and mounting, e.g. shackles, U-bolts, saddle, tie bar; hydraulic/ hydro-pneumatic systems, e.g. fluid supply, storage, actuation, control; suspension damping, e.g. oil, gas, friction; tyres, e.g. type (radial, cross-ply, markings) and impact on vehicle suspension system

Understand the layout and operation of a braking system and its components

Layout: braking methods, e.g. disc brakes, drum brakes; braking circuit,

e.g. hydraulic circuit, split braking circuits

Braking system operation: application of mechanical forces, e.g. pedal force, transmission of force (fluid pressure, piston sizes); brake shoes/pads; heat dissipation, e.g. mechanical to heat energy, vented brake arrangement; brake efficiency, e.g. vehicle testing, axle efficiencies, brake balance; leading and trailing brake shoe action

Components: hydraulic circuit components, e.g. single/tandem circuit, master/slave cylinders (machined surfaces, seals, pistons, springs), brake lines (fixed and flexible piping); brake servo, e.g. diaphragm, spring, valve; pressure limiting valve, e.g. seal, actuation; brake adjuster, e.g. manual, automatic; brake discs/drums e.g. machined surface, vented, solid; callipers/actuators,

e.g. piston(s), seals, mounting; brake shoes (leading, trailing, springs); brake drums (machined surface, mounting); handbrake mechanism (actuation),