Aim
To study and prepare report on the construction details, working principles and operation of the following automotive suspension systems.
a) Front suspension system
b) Rear suspension system
Theory
Labeled diagram, construction details, working principle and operation of the following suspension systems
a) Front suspension system
i) Double Wishbone Suspension
ii) Macpherson Strut Front Suspension
b) Rear suspension system
i) Leaf Spring Rear Suspension
ii) Coil Spring Rear Suspension
iii) Macpherson Strut Rear Suspension
Objects of Suspension
1 To prevent the road shocks from being transmitted to the vehicle components
2 To safeguard the occupants from road shocks
3 To preserve the stability of the vehicle in pitching or rolling, while in motion
Leaf Springs
Semi elliptic leaf springs are almost universally use for suspension in light and heavy commercial vehicles. For cars also, these are widely used for rear suspension.
Tapered Leaf Springs
English Steel Corporation Ltd. Of England has produced ‘Taperlite’ springs which have the following advantages over the conventional leaf springs due to which these are becoming increasingly popular compared to constant section conventional leaf springs.
1. Light weight – nearly 60% of the corresponding conventional spring
2. There is no interleaf friction in case of single taper leaf spring. Even in case of heavy vehicles where more leaves may be required, the number of such leaves is still less than in case of conventional springs. Further, these rub against each other only at the ends. Due to these reasons even in multi taper leaf springs relatively less inter leaf friction will be present.
3 Absence of squeaking
4. The stresses are lower and more uniform compared to the conventional springs, thus giving longer life.
5. They occupy less space.
6. In case of single taper leaf spring, there is no collection of moisture between the leaves and hence no fretting fatigue.
Coil Springs
The coil springs are used mainly with independent suspension, though they have also been used in the conventional rigid axle suspension as they can be well accommodated in restricted spaces. The energy stored per unit volume is almost double in the case of coil springs than the leaf springs.
Coil springs do not have noise problems nor do they have static friction causing harshness of ride as in case of leaf springs.
The spring takes the shear as well as bending stresses. The coil springs, however, cannot take torque reaction and side thrust, for which alternative arrangements have to be provided.
A helper coil spring is also sometimes used to provide progressive stiffness against increasing load.
Shock Absorbers
A springing device must be a compromise between flexibility and stiffness. If it is more rigid, it will not absorb road shocks efficiently and if it is more flexible, it will continue to vibrate even after the bump has passed. So we must have sufficient damping of the spring to prevent excessive flexing.
The friction between the leaves of a leaf spring provides this damping, but because of the uncertainly of the lubrication conditions, the amount of friction also varies and hence the damping characteristics do not remain constant. For this reason, the friction between the springs is reduced to minimum and additional damping is provided by means of devices called dampers or shock absorbers. In case of coil springs, the whole of damping is provided by the shock absorbers. The shock absorbers thus control the excessive spring vibrations.
In fact the name shock absorber is rather misleading since it is the spring and not the shock absorber that initially absorbes the shock. The shock absorber absorbs the energy of shock converted into vertical movement of the axle by providing damping and dissipating the same into heat. Thus, it merely serves to control the amplitude and frequency of spring vibrations. It can not support weight and has zero resilience. Therefore, ‘damper’ is a better term technically to describe the shock absorber.
The shock absorbers are basically of two types – the friction type and the hydraulic type. The friction type has almost become obsolete due to its non predictable damping characteristics. The principle of operation of a hydraulic shock absorber is that when a piston forces the fluid in a cylinder to pass through some hole a high resistance to the movement of piston is developed, which provides the damping effect. The hydraulic type has the additional advantage that the damping is proportional to the square of the speed. So for small vibrations, the damping is also small, while for larger ones the damping becomes automatically more.
Wishbone type suspension
Fig shows a diagrammatic sketch of wishbone type suspension with coil springs. The use of coil springs in the front axle suspension of cars is now almost universal.
It consists of upper and the lower wishbone arms pivoted to the frame member. The spring is placed in between the lower wishbone and the underside of the cross member. The vehicle weight is transmitted from the body and the cross member to the coil spring through which it goes to the lower wishbone member. A shock absorber is placed inside the coil spring and is attached to the cross member and to lower wishbone member.
The wishbone arms are like the chicken wishbone or letter V in shapem because of which the system is so called. Because of this V shape, the wishbones not only position the wheels and transmit the vehicle load to the springs but these also resist acceleration, braking and cornering (side) forces. The upper arms are shorter arms are shorter in length than the lower ones. This helps to keep the wheel track constant, thereby avoiding the tyre scrub thus minimizing tyre wear. However, small change in the camber angle does occur with such an arrangement.
The wishbone type is the most popular independent suspension system.
Mac Pherson Strut type of suspension
In this layout only lower wishbones are used. A strut containing shock absorber and the spring carries also the stub axle on which the wheel is mounted. The wishbone is hinged to the cross member and positions the wheel as well as resists accelerating, braking and side forces. This system is simpler than double wishbone type described above and is also lighter, keeping the unsprung wieht lower. Further, the camber also does not change when the wheel moves up and down. This type of suspension gives the maximum room in the engine compartment and is therefore commonly used on front wheel drive cars.
In India this system has been used in Maruti (Suzuki) 800 cars. This type of suspension with anti roll bar as employed in Volkswagen Jetta and Passat cars is shown in fig. this is claimed t provide increased road safety, improve ride comfort and light and self stabilizing steering which means that car continues along its chosen line of travel when the brakes are applied even though the road surface may vary.
Rear Wheel Independent Suspension
Though the rear wheels are not to be steered, yet there is a considerable difficulty in the rear wheel springing if the power has to be transmitted to the rear wheels. But even the rear wheel independent springing is coming into prominence because of its distinct advantages over the rigid axle type.
Fig shows one method of rear wheel independent suspension. Universal couplings A and B keep the wheel vertical, while the sliding coupling C is required to maintain the wheel track constant, thereby avoiding scrubbing of the tyres. This method has been used in the de Dion type of axle.
Reference Books
1) Automotive Mechanics – Crouse / Anglin
2) Automobile Engineering – Dr Kirpal Singh
Viva Questions
1 What are the objectives of a vehicle suspension
1 What do you understand by pitching and rolling of a vehicle
2 What is a function of a spring in automobile suspension’
3 What is up-sprung weight?
4 How does a side thrust countered in a vehicle
5 What is a function of a shackle with a leaf spring
6 Explain the function of a leaf spring and show how it is mounted on rear and front
7 What are the advantages of an independent suspension system over a rigid axle suspension?
8 What are the advantages of Mac Pherson strut suspension?
9 What is a function of a shock absorber
10 What are helper springs?
11 How is the fatigue strength of a leaf springs increased?
12 What is the effect of moisture on the leaf springs?
13 Describe the advantages of tapered leaf springs
14 Which type of independent suspension used in M-800 car
15 What is wishbone type suspension?
16 What forces are supported by a leaf springs
17 State the advantages and disadvantages of a torsion bar
18 What is torsion bar?
19 State the advantages and disadvantages of a coil springs