Automobile Transmission & Mechanism (3330202)

Practical-07

Practical-07

Demonstrate different types of Suspension Systems and Shock Absorbers.

Suspension system and Shock Absorber 1 SUSPENSION TYPES AND CHARACTERISTICS Suspensions can be roughly divided into two types, based on the differences in their construction. The characteristics of each type are as follows:

1.1 RIGID-AXLE SUSPENSION A rigid-axle suspension has the following characteristics: 1. The number of parts composing the suspension is small and the construction is simple. Therefore, maintenance is simple. 2. It is durable enough for heavy-duty use. 3. When turning, there is little tilting of the body. 4. There is little change in the alignment due to the up-and-down movements Of the wheels. Therefore, there is less lyre-wear. 5. Since the unstrung weight is great, riding comfort is poor. 6. Since the movements of the left and right wheels mutually influence One another, vibration and oscillation occur rather easily. There are various types of rigid axle suspension. Here we will explain the principal types of rigid axle suspension used at present in recent vehicles, and describe some of their characteristics. Parallel leaf spring type: This type of suspension is used for the front suspension of trucks and buses, etc. and for the rear suspension in commercial vehicles. Since the leaf spring also acts as a linkage for positioning the axle (holding it in place), separate linkages are generally unnecessary. Therefore, the construction of the suspension is simple but comparatively strong. Since the positioning of the axle is carried out by the leaf springs, it is difficult to use a very soft spring (a spring with a low spring constant). Therefore, this type of suspension is not very good with respect to riding comfort. Riding comfort suffers due to inter-leaf friction in the leaf springs. Page | 55

Automobile Transmission & Mechanism (3330202)

Practical-07

Acceleration and braking torque tend to cause wind-up and vibration, and wind-up in turn causes rear-end squat and nose-diving. Leading arm type with lateral rod, Trailing arm type with Lateral load: This type of suspension is used for the front and rear suspensions of the land cruiser. In this type, the positioning of the axle, which is accomplished by the leaf springs in the previously mentioned parallel leaf spring type, is instead accomplished by leading arms or trailing arms and a lateral control rod. It is superior to leaf springs in the following points: 1. Since the spring constant can be made smaller, riding comfort is good. 2. Since the trailing arm's rigidity is high, it is difficult for wind-up to occur.

Trailing arm type with Twist beam: The construction of this type is simple and it is compact, so it is used for the rear suspension of front-engine front wheel-drive (FF) cars.

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4-link type: This type is used for the rear suspension. It provides the best riding comfort of all the rigid-axle suspensions. Since axle positioning is accomplished by a linkage, Soft springs can be used and riding comfort is good. Due to the geometrical layout of the linkages, nose-diving during braking and rear-end squat during acceleration are prevented. Also, the up-and-down movement of the front end of the differential during bounding and rebounding can be minimized by making the upper control arms shorter than the lower control arms. This in turn means that the floor over the nose of the differential can be lowered, allowing more passenger compartment space. Use Of coil springs minimizes friction in the suspension; so minute shocks from the road Surface can be absorbed and riding comfort can be improved. Generally, when this suspension system is used in the rear, the linkage is located forward of the axle, and as a result, the capacity of the trunk can be made larger on passenger cars.

2 INDEPENDENT SUSPENSION

2.1 FRONT WHEEL (DEAD AXLE) INDEPENDENT SUSPENSION Independent suspension has become almost universal in case of front axle, due to the simplicity of construction of such a suspension system. Five types of independent suspension arc in use for front axle: 1. 2. 3. 4. 5.

Wishbone type or parallel link type. Mac Pherson strut type. Vertical guide type. Trailing link type. Swinging half-axle type.

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Automobile Transmission & Mechanism (3330202)

Practical-07

1. Wishbone type suspension Figure 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. These arms resemble letter 'A' of the Roman alphabet due to which these are also referred to as 'A-arms'. 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 shape, 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 in length then the lower ones. This helps to keep the wheel track constant, thereby avoiding the tyre scrub thus minimizing tyre wear. However, a small change in the camber angle does occur with such an arrangement. The wishbone type is the most popular independent suspension System. 2. 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 unstrung weight 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. 3. Vertical guide suspension. A suspension of this type is shown in Figure. The king pin is attached directly to the cross of the frame. It can slide up and down as shown, corresponding to the up and down motion of the wheel, thus compressing or elongating the springs. In this

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the track, wheel base and wheel attitude remain unchanged, but the system is having disadvantage of decreased stability.

4. Trailing link suspension. In this type of suspension, shown in Figure, a coil spring is attached to the trailing link which itself is attached to the shaft carrying the wheel hub. When the wheel moves up and down, it winds and unwinds the spring. A torsion bar has also been used in certain designs in place of the coil spring. This system does maintain the camber and the wheel track constant. However, the distance between the front and the rear wheels does change. Difficulty to remedy this defect is the main reason for its very limited use in actual practice. 5. Swinging half axle suspension. In this wheels are mounted rigidly on the half axles, which are pivoted on their ends to the chassis member at the middle of the car. The main disadvantage of this system is that up and down movement of the wheel because the camber angle to vary.

2.2 REAR WHEEL (LIVE AXLE) 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.

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Automobile Transmission & Mechanism (3330202)

Practical-07

Figure 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.

Another method of rear Wheel independent suspension is the trailing link type. In this the trailing links are pivoted at right angles to the longitudinal axis of car and carry the rear wheels at their ends as shown in the figure. A's and B's are the universal joints to keep the wheel track and the camber constant with the up and down movement of the wheels. S's are the coil springs with shock absorbers mounted concentrically within them. The trailing links hold the wheels firmly and also sustain accelerating and braking forces. A trailing link or arm runs from a point forward on the floor pan back to the wheel assembly, while a leading link or arm is mounted to the floor pan behind the wheel. Although the camber remains constant with respect to the vehicle body, yet when the vehicle rolls into a comer, the trailing arm rolls for the same degree as the vehicle body, thus changing camber angle with respect to road surface. As both wheels now lean towards the outside-of the corner, it leads to under steer. Due to this reason, pure trailing arm is not being used. Further, when these arms are mounted at an angle to the longitudinal axis of the vehicle, these are known as semi trailing link or arm suspension.

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The trailing arms are generally inclined from 20 to 40 degrees to the longitudinal axis. Thus, the forces in the links may be resolved into the longitudinal and the transverse directions. The former causes under-steer as explained above, while the transverse component produces over-steer due to body roll. As a result, the two components cancel each other, giving almost a neutral steering response. However, the semi trailing links result in camber angle changes with the up and down movements of the wheel. Moreover, the increased unsprung weight due to the trailing arms gives rise to poor ride quality. That is why most modern sedans use multi-link or double wishbone suspension. A multi-link suspension is difficult to strictly describe since although theoretically any suspension having 3 control arms or more is a multi-link one, yet different designs would have different geometry and thus Characteristics. For instance, BMW's multi-link looks like letter 'Z' and offers very good handling, although occupying large space. Honda Accord's multi-link is only a double wishbone suspension with an additional control arm.

3 SHOCK ABSORBERS 3.1 DESCRIPTION

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When a car is subjected to shocks from the road surface, the suspension springs compress and expand to absorb those shocks. However, because springs have the characteristic of continuing to oscillate, and because it often takes a long time for this oscillation to stop, riding comfort will be poor unless some means is provided to damp this oscillation. This is the job of the shock absorber or ' 'shocks' '. The shock absorbers not only absorb the excess oscillation of the springs, thus improving riding comfort, but they also give the lyres better road-holding characteristics and improve steering stability.

3.2 PRINCIPLE OF OPERATION In automobiles, telescopic shock absorbers are used which employ a special fluid, called shock absorber fluid, as the working medium. In this type of shock absorber, damping force is generated by the flow resistance caused by the fluid being forced through an orifice (small hole) by the movement of a piston.

3.3 TYPES OF SHOCK ABSORBERS Shock absorbers are classified according to their structure and their operation as follows: (A) Classification by Operation: 1. Single-action shock absorber. Damping occurs only when the shock absorber is extended. No damping force is generated when it is compressed. 2. Multiple-action shock absorber. Damping occurs both when the shock absorber is extended and when it is compressed. Currently, most shock absorbers used on cars are of this type.

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(B) Classification Construction:

by

1. Twin-tube shockabsorber. The cylinder is divided by the pressure tube and the outer tube into a working chamber (inner cylinder) and a reservoir chamber (outer cylinder).

reservoir).

2. Mono-tube shock absorber. This is a shock absorber with only a single cylinder (i.e. „ without a

(C) Classification by working medium: 1. Hydraulic shock absorber. This is an ordinary shock absorber which uses only a fluid (shock absorber fluid) as a working medium. 2. Gas-filled shock absorber. This is hydraulic shock absorber which is charged with gas. The principal gas used in nitrogen, which is kept under either low pressure (10 — 15 kg/cm2) or high pressure (20 — 30 kg/cm2).

Of these types, shock absorbers which are used in current models have twintube or mono-tube construction and have multiple-action type operation. Most recently. Many gas filled shock absorbers of the above types have come into use.

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