SUSPENSION SYSTEM Chapter 4: Suspension System I PRE-READING QUESTION How to maximize the friction between the tires of a car and the road surface? How to provide steering stability with good handling and to ensure the comfort of the passengers? How to absorb the vibration in moving of the cars? II READING Another major automotive sub-system is the suspension system, which is the mechanical assembly that connects each wheel to the car body The primary purpose of the suspension system is to isolate the car body from the vertical motion of the wheels as they travel over the rough road surface The suspension system can be understood with reference to Figure 4.1, which illustrates the major components Fig 4.1: Major components of suspension system SPRINGS Notice that the wheel assembly is connected through a movable assembly to the body The weight of the car is supported by SPRINGS Today's springing systems are based on one of four basic designs: 1.1 Coil springs: this is the most common type of spring and is, in essence, a heavy-duty torsion bar coiled around an axis Coil springs compress and expand to absorb the motion of the wheels (Fig 4.1) 1.2 Leaf springs: this type of spring consists of several layers of metal (called "leaves") bound together to act as a single unit Leaf springs were first used on horse-drawn carriages and were found on most American automobiles until 1985 They are still used today on most trucks and heavyduty vehicles (Fig 4.2) Fig 4.2: Leaf spring Technical English for AUTOMOTIVE ENGINEERING 21 SUSPENSION SYSTEM Fig 4.3: Leaf spring assembly 1.3 Torsion bars: torsion bars use the twisting properties of a steel bar to provide coil-spring-like performance The short torsion bar springs grip into the guide tubes and in the centre of the vehicle (Fig 4.4) Parts 2, and are jointly subjected to torsional stresses and so the torsional stiffness of the transverse tubes contributes to the spring rate On the outside, the cast trailing arms are welded to the transverse tubes, which (pushed into each other) support each other on the torsionally elastic bearings and This creates a sufficiently long bearing basis, which largely prevents camber and toe-in changes when forces are generated Fig 4.4: Torsion bars in suspension system The entire assembly is fixed by the brackets which permits better force transfer on the body side sill Guide tubes and are mounted in the brackets and can rotate, as well as the outer sides of the two torsion bars The two arms thus transfer all vertical forces plus the entire springing moment to the body The anti-roll bar is connected to the two trailing arms via two U-shaped tabs The two rubber bearings and located between the tubes and also contribute to the stabilizing effect The bump and rebound travel stops are fitted into the shock absorber 1.4 Air springs: The air-spring bellows are supplied by an electrically powered compressor The individual wheel adjustment permits the lowering or lifting of the vehicle as well as a constant vehicle height, regardless of – even onesided – loading It is also possible to counteract body tilt during cornering The damping properties of the shock absorbers are affected by spring bellow pressure depending on the load (Fig 4.5) Fig 4.5: Air springs in suspension system Technical English for AUTOMOTIVE ENGINEERING 22 SUSPENSION SYSTEM SHOCK ABSORBER (sometimes a strut) (Fig 4.6), which is in effect a viscous damping device There is a similar assembly at each wheel, although normally there are differences in the detailed configuration between front and rear wheels Enter the shock absorber, or snubber, a device that controls unwanted spring motion through a process known as dampening Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated through hydraulic fluid A shock absorber is basically an oil pump placed between the frame of the car and the wheels The upper mount of the shock connects Fig 4.6: Shock Absorber to the frame (i.e., the sprung weight), while the lower mount connects to the axle, near the wheel (i.e., the unsprung weight) In a twin-tube design, one of the most common types of shock absorbers, the upper mount is connected to a piston rod, which in turn is connected to a piston, which in turn sits in a tube filled with hydraulic fluid The inner tube is known as the pressure tube, and the outer tube is known as the reserve tube The reserve tube stores excess hydraulic fluid When the car wheel encounters a bump in the road and causes the spring to coil and uncoil, the energy of the spring is transferred to the shock absorber through the upper mount, down through the piston rod and into the piston Orifices perforate the piston and allow fluid to leak through as the piston moves up and down in the pressure tube Because the orifices are relatively tiny, only a small amount of fluid, under great pressure, passes through This slows down the piston, which in turn slows down the spring Shock absorbers work in two cycles the compression cycle and the extension cycle The compression cycle occurs as the piston moves downward, compressing the hydraulic fluid in the chamber below the piston The extension cycle occurs as the piston moves toward the top of the pressure tube, compressing the fluid in the chamber above the piston A typical car or light truck will have more resistance during its extension cycle than its compression cycle With that in mind, the compression cycle controls the motion of the vehicle's unsprung weight, while extension controls the heavier, sprung weight SUSPENSION TYPES 3.1 Dependent Suspensions 3.1.1 Dependent front suspensions: have a rigid front axle that connects the front wheels Basically, this looks like a solid bar under the front of the car, kept in place by leaf springs and shock absorbers Common on trucks, dependent front suspensions haven't been used in mainstream cars for years Fig 4.7: Dependent front suspensions Technical English for AUTOMOTIVE ENGINEERING 23 SUSPENSION SYSTEM 3.1.2 Dependent rear suspensions Fig 4.8: Solid-axle, leaf-spring Fig 4.9: Solid-axle, coil-spring Contrary to the front version of this system, many cars are still designed and built with dependant (linked) rear suspension systems Fig 4.10: Beam Axle 3.2 Independent Suspensions 3.2.1 Independent front Suspensions Fig 4.12: Coil Spring type Fig 4.11: Macpherson strut So-named because the front wheel's suspension systems are independant of each other (except where joined by an anti-roll bar) These came into existance around 1930 and have been in use in one Fig 4.13: Coil Spring type Technical English for AUTOMOTIVE ENGINEERING 24 SUSPENSION SYSTEM form or another pretty much ever since then Fig 4.14: Double Wishbone 3.2.2 Independent rear Suspensions Fig 4.15: Multi-link suspension Fig 4.16: The rear axle on Mitsubishi Pajero III NEW WORDS isolate spring in essence leaf spring twist stiffness counteract magnitude sprung weight hydraulic fluid orifice mainstream existance rough car frame heavy-duty bound grip transverse tube tilt vibratory axle reserve tube perforate contrary wishbone suspension system lower control arm torsion horse-drawn carriages guide tubes trailing arms strut kinetic energy unsprung weight excess dependent suspensions independent suspensions multi-link suspension Technical English for AUTOMOTIVE ENGINEERING shock absorber coil spring absorb torsion bars torsional stress air springs snubber dissipate twin-tube encounters rigid front axle so-named antiroll bar 25 SUSPENSION SYSTEM IV COMPREHENSION QUESTION What system connects each wheel to the car body? What is/are the primary purpose(s) of the suspension system? How to support the weight of the car? What are the four basic designs of the springing systems? How coil springs absorb the motion of the wheels? How to constitute the leaf springs? What is/are the property(ies) of the Torsion bars for supplying coil-spring-like performance? Are the air springs possible to counteract body tilt during cornering? Is it the same kind of shock absorber assembly at each wheel in a car? 10 How the shock absorbers slow down and reduce the magnitude of vibratory motions? V TRUE/FALSE Suspension system connects each wheel to the car body ( True False) The primary purpose of the suspension system is to isolate the car body from the horizontal motion of the wheels as they travel over the rough road surface ( True False) The springs in suspension system support the weight of the car ( True False) Coil springs compress and expand to absorb the motion of the wheels ( True False) Leaf springs consist of several layers of metal bound together to act as a single unit ( True False) Technical English for AUTOMOTIVE ENGINEERING 26 SUSPENSION SYSTEM Leaf springs were first used on bicycle ( True False) Leaf springs are used today on all trucks and heavy-duty vehicles ( True False) Torsion bars use the compressing properties of a steel bar to provide coil-spring-like performance ( True False) The air-spring suspension system are supplied by an electrically powered compressor ( True False) 10 Shock absorbers effect as a viscous damping device ( True False) 11 Shock absorbers control wanted spring motion through a process known as dampening ( True False) 12 Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into chemical energy ( True False) 13 A shock absorber is basically an oil source placed between the frame of the car and the wheels ( True False) 14 The sprung weight is the upper mount of the shock connects to the frame ( True False) 15 The unsprung weight is the lower mount connects to the axle, near the wheel ( True False) 16 Shock absorbers work in the compression cycle and the extension cycle ( True False) 17 Dependent Suspensions have a rigid axle that connects the front wheels ( True False) 18 Independent suspensions is named because the wheel's suspension systems are independant of each other ( True False) 19 Engine, clutch, transmission, drive shaft, differential and driven wheels are elements of drivetrain system ( True False) VI WORD(S) SELECTIONS Select ONE word(s) in the below box and fill in the gap in column B with its meaning word in column A a One type of spring is supplied by an electrically powered compressor b A device controls wanted spring motion through a process known as dampening No c One type of spring consists of several layers of metal bound together to act as a single unit e The upper mount of the g link shock connects to the frame i The lower mount connects to the axle, near the wheel h One type of f System spring uses twisting j frustrate d High connects each properties of a steel performance wheel to the car bar to provide coilbody spring-like performance Column A Suspension system Leaf spring Torsion bars The unsprung weight heavy-duty Column B No 10 Technical English for AUTOMOTIVE ENGINEERING Column A air-spring suspension Shock absorbers The sprung weight counteract bound Column B 27