1、Lesson 14 Suspension System Suspension is the term given to the system of springs,shock absorbers and linkages that connects a vehicle to its wheels.Suspension systems serve a dual purpose contributing to the cars roadholding/handling and braking for good active safety and driving pleasure,and keepi
2、ng vehicle occupants comfortable and reasonably well isolated from road noise,bumps,and vibrations,etc.These goals are generally at odds,so the tuning of suspensions involves finding the right compromise.It is important for the suspension to keep the road wheel in contact with the road surface as mu
3、ch as possible,because all the forces acting on the vehicle do so through the contact patches of the tyres.The suspension also protects the vehicle itself and any cargo or luggage from damage and wear.The design of front and rear suspension of a car may be different.In a typical suspension system fo
4、r a vehicle with front-engine and front-wheel drive(see illustration),the weight of the vehicle applies an initial compression to the coil springs.When the tires and wheels encounter irregularities in the road,the springs further compress or expand to absorb most of the shock.The suspension at the r
5、ear wheels is usually simpler than for the front wheels,which require multiple-point attachments so the wheels can move up and down while swinging from side to side for steering.Front-wheel-drive car with MacPherson-strut front suspension and strut-type independent rear suspension.(Saturn Corp.)A te
6、lescoping hydraulic damper,known as a shock absorber,is mounted separately or in the strut at each wheel to restrain spring movement and prevent prolonged spring oscillations.The shock absorber contains a piston that moves in a cylinder as the wheel moves up and down with respect to the vehicle body
7、 or frame.As the piston moves,it forces a fluid through an orifice,imposing a restraint on the spring.Spring-loaded valves open to permit quicker flow of the fluid if fluid pressure rises high enough,as it may when rapid wheel movements take place.Most automotive vehicles use gas-filled shock absorb
8、ers in which the air space above the fluid is filled with a pressurized gas such as nitrogen.The gas pressure on the fluid reduces the creation of air bubbles and foaming.Most automotive vehicles have independent front suspension,usually using coil springs as part of either a short-arm long-arm or a
9、 MacPherson-strut suspension system.A MacPherson-strut suspension(see illustration)combines a coil spring and shock absorber into a strut assembly that requires only a beam-type lower control arm.Some vehicles with short-arm long-arm front suspension use either longitudinal or transverse torsion bar
10、s for the front springs.One end of the torsion bar is attached to the lower control arm,and the other end is anchored to the vehicle body or frame.As the tire and wheel move up and down,the torsion bar provides springing action by twisting about its long axis.Most automobiles and many light trucks h
11、ave coil springs at the rear.These may mount on the rear drive axle,on struts,or on various types of control or suspension arms in an independent suspension system.Some rear-drive vehicles have leaf springs at the rear.Others use transverse torsion bars.Rear Suspension Systems Semi-elliptical leaf s
12、prings or coil springs are used in the rear suspension systems of all automobiles built in the United States.A leaf spring may consist of a single leaf of flat spring steel,or it may have a series of leaves.A multileaf spring includes a main leaf and three,four,or five secondary leaves that are prog
13、ressively shorter and have progressively smaller radii.The secondary leaves are placed underneath the main leaf,and the assembly is held together near the ends by alignment clips and,in the center,by two U-bolts and a plate.The U-bolt and plate assembly also fastens the differential and rear axle ho
14、using to,and aligns it with,the spring.Thin interliners are installed between the spring leaves to eliminate noise and to enable the leaves to move freely against each other.Rear suspension system parts The differential housing and rear axle housings move up and down as the wheels strike obstacles i
15、n the road.A flat base for the U-bolt plate is welded to the bottom of the rear axle housing near each end.Whenever power is transmitted to the rear wheels of a vehicle equipped with a Hotch-kiss drive system,the wheels attempt to force the differential and rear axle housing to turn in the direction
16、 opposite from wheel rotation.Leaf springs control this torque and prevent damage to the drive line and rear suspension system.The rear suspension systems of most vehicles in current production make use of coil springs.Two lower control arms,which are connected to the frame and rear axle housing,kee
17、p the housing from shifting too far toward the front or rear.A pair of upper control arms(torque arms)also connect the frame and rear axle housing.The upper and lower control arms prevent the differential and rear axle housing from rotating.Shock absorbers are used to absorb energy developed in the
18、springs or torsion bars when the wheels of an automobile move up and down.The energy is absorbed by means of hydraulic action.The lower end of a shock absorber is mounted on a control arm(or on an extension of the U-bolt plate if it is a rear shock absorber used with a leaf spring),and the upper end
19、 is mounted on the frame or one of its cross members.A front stabilizer bar is used to control the side roll of the body of a vehicle during cornering.Each end of the bar is connected to a lower control arm;and brackets,in which rubber bushings are installed,fasten the bar to the front cross member
20、of the frame.When the vehicle is turned,one side of the frame lifts,and the other side dips.This causes one end of the stabilizer bar to move up while the other moves down,and the center of the bar twists.Therefore,it functions as a torsion bar in counterbalancing some of the shift of body weight th
21、at occurs.The multileaf spring is also bolted to the carrier and,at the outer ends,to the control arms.A universal joint is placed at each end of each of the two axles.The camber adjustment for each wheel is made at the inner end of the strut on that side.Each rear wheel assembly is equipped with a
22、drive spindle that is held in a spindle support to which the disc brake caliper is bolted.The spindle turns in two roller bearings that are held in place by the spindle nut and the drive flange.Preload spacers prevent the spindle from shifting to either side.Unsprung weight transfer Unsprung weight
23、transfer is calculated based on the weight of the vehicles components that are not supported by the springs.This includes tires,wheels,brakes,spindles,half the control arms weight and other components.These components are then(for calculation purposes)assumed to be connected to a vehicle with zero s
24、prung weight.They are then put through the same dynamic loads.The weight transfer for cornering in the front would be equal to the total unsprung front weight times the G-Force times the front unsprung center of gravity height divided by the front track width.The same is true for the rear.Sprung wei
25、ght transfer Sprung Weight Transfer is the weight transferred by only the weight of the vehicle resting on the springs,not the total vehicle weight.Calculating this requires knowing the vehicles sprung weight(total weight less the unsprung weight),the front and rear roll center heights and the sprun
26、g center of gravity height(used to calculate the roll moment arm length).Calculating the front and rear sprung weight transfer will also require knowing the roll couple percentage.The roll axis is the line through the front and rear roll centers that the vehicle rolls around during cornering.The dis
27、tance from this axis to the sprung center of gravity height is the roll moment arm length.The total sprung weight transfer is equal to the G-force times the sprung weight times the roll moment arm length divided by the effective track width.The front sprung weight transfer is calculated by multiplyi
28、ng the roll couple percentage times the total sprung weight transfer.The rear is just the total minus the front transfer.Camber control Camber changes due to wheel travel,body roll and suspension system deflection or compliance.In general,a tire wears and brakes best at-1 to-2 degrees of camber from
29、 vertical.Depending on the tire and the road surface,it may hold the road best at a slightly different angle.Small changes in camber,front and rear,can be used to tune handling.Some race cars are tuned with-2-7 degree camber depending on the type of handling desired and the tire construction.Often t
30、imes,too much camber will result in the decrease of braking performance due to a reduced contact patch size through excessive camber variation in the suspension geometry.The amount of camber change in bump is determined by the instantaneous front view swing arm(FVSA)length of the suspension geometry,or in other words,the tendency of the tire to camber inward when compressed in bump.Thanks for your presence!
