SUSPENSION SYSTEM IN AUTOMOBILES

Written By T. SIVA KUMAR

                                                                    Asst.proff: Sai Sakthi Engineering College(ssec)

                                                  E-MAIL:talursivakumar@gmail.com

INTRODUCTION           

Suspension system is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels . It is basically cushion for passengers protects the luggage or any cargo and also itself from damage and wear.             The automobile chassis is mounted on the axles, not direct but through some form of springs. This is done to isolate the vehicle body from the road shocks which may be in the form of bounce, pitch, roll or sway. These tendencies give rise to an uncomfortable ride and also cause additional stress in the automobile frame and body. All the parts which perform the function of isolating the automobile from the road shocks are collectively called a suspension system. It includes the springing device used and various mountings for the same.

Figure.1

             

PRINCIPLES  OF  SUSPESION  SYSTEM

             When a tire hits an obstruction, there is a reaction force. The size of this reaction force depends on the un sprung mass at each wheel assembly. In general, the larger the ratio of sprung weight to un sprung weight, the less the body and vehicle occupants are affected by bumps, dips, and other surface imperfections such as small bridges. A large sprung weight to un sprung weight ratio can also impact vehicle control. 

ROLE OF  SUSPENSION  SYSTEM
The main role of suspension system are as follows:

            It supports the weight of vehicle .Provides smoother ride for the driver and passengers i.e. acts as cushion. Protects your vehicle from damage and wear .It also plays a critical role in maintaining self driving conditions. It also keeps the wheels pressed firmly to the ground for traction . It isolates the body from road shocks and vibrations which would otherwise be transferred to the passengers and load. 

DEFINITIONS OF SPRING & UN-SPRING MASS

Spring mass Spring mass (weight) refers to vehicle parts supported on the suspension system, such as the body, frame, engine, the internal components, passengers, and cargo.

 Fig no: 2.1 

Un spring mass

                Un sprung mass refers to the components that followthe road contours, such as wheels, tires ,brake assemblies, and any part of the steering and suspension not supported by the springs. 

WORKING OF SUSPENSION SYSTEM

                   No road is perfectly flat i.e. without irregularities. Even a freshly paved highways have subtle imperfections that can be interact with vehicle wheels. These are the imperfections that apply forces on wheels.  According to Newton Ëœs law of motion all forces have both magnitude and direction. A bump in the road causes the wheel to move up and down perpendicular to the road surface. The magnitude of course ,depends on whether the wheel is striking a giant bump or a tiny speck. Thus, either the wheel experiences a vertical acceleration as it passes over an imperfection. 

Basic suspension movements Bouncing : The vertical  movement of the complete body. Fig no: Pitching:  The rotating  movement of  all  the  parts  between  the spring  and  road  and  the portion  of  spring  weight  itself.  Fig no: 3.2 

Rolling: The movement  about  longitudinal axis produced  by  the  centrifugal  force  during cornering.

 Objective of Suspension System: The following are the objectives of suspension system,To safeguard the occupants against road shocks and provides riding comfort.To isolate the structure of vehicle from shock loading and vibration due to irregularities of road surface without affecting its stability.To minimize the rolling and pitching tendency.To keep vehicle body in perfect level while travelling over rough roads.To support the weight of the vehicle.

PARTS OF TYPICAL SUSPENSION :The suspension of a car is actually part of the chassis, which comprises all of the important systems located beneath the car's body. These system include :         

1.   Frame         

 2.   Suspension system         

3.   Steering system         

4.   Tires or Wheels          

Need of Shock Absorber
    If the suspension springs are rigid enough, they will not absorb road shocks efficiently, and if they are flexible enough, they will continue to vibrate for longer time even after the bump has passed.Therefore, the springing device must be compromise flexibility and stiffness a shock absorber needed in Automobile Suspension system.

Types of Shock Absorbers

 They are mainly two types Mechanical. Hydraulic                     

a) Van type Piston                    

i )Single Acting                     ii). Double Acting

Telescopic type:    Construction and Working of Single acting Telescopic hydraulic shock absorbers. The telescopic shock absorber consists of a cylinder to which a head is welded to screwed to the outer tube. The space between outer and inner tube is called reservoir. A pressed steel cap and axle eye by means of which cylinder is screwed to the axle are welded to the outer tube. A piston slides inside the cylinder and screwed to the piston rod at which its upper end of chassis eye, it is attached to the frame of the vehicle. The part of the piston rod that is outside of the cylinder is protected by a cover which is welded to the chassis eye. A piston rod gland packing prevent the leakage, when the piston passes through the head and any fluid is trapped by it is supplied to the reservoir through drain  hole Working:    If the axle eye moves upwards then the fluid must be displaced from the bottom. Top side of the bottom side fluid through the outer ring of the piston by lifting the non return valve. But  since the increase in the volume of upper end of the cylinder is less than the volume of the lower end. Fluid will also displaced through the inner ring of holes of non return valve of foot valve, and the level at the fluid will raise in the reservoir. The pressure setup will depend on the size of hole in the piston and Foot valve and the squire of the speed of which the cylinder is moved.  For downward motion of the cylinder the fluid will be displaced form the upper end of the piston. In the leaver end through the inner ring of hole of non return valve in the piston. The fluid will also be draw in the lower end of the cylinder from the reservoir to the outer ring of hole of non return valve of the foot valve.  

Stabilizer Bar:  Stabilizer is necessarily used in all independent front suspension.  It reduces tendency of the vehicle to roll on either  side when taking a turn. It is simply a bar of alloy steel with arms at each end connected to the lower wish bone of the independent suspension system. It is supported in bush bearings fixed the frame and is parallel to cross member. When both the wheels deflect up or down by the same amount, the stabilizer bar simple turns in the bearings.  When only one wheel deflects, then only one end of the stabilizer moves, thus it acts as a spring between two sides  of the independent front suspension.

Torsion Bar:     Torsion bar suspension is used in independent suspension system. It is a rod acting in torsion and taking shear stresses only. It is made up of heat treated alloy spring steel. This bar stores amount of energy nearly as same as coil spring.  The bar is fixed at one end to the frame, while the other end is fixed to the end of the wheel arm and supported in the bearing. The other end of the wheel arm connected to the wheel hub. When the wheel strikes a bump, it starts vibrating up and down, thus exerting torque on the torsion bar which acts as a spring.   

COMPONENTS OF  SUSPENSION: There are three fundamental components of any suspension system .            

1.  Springs               

2.   Dampers               

3.  Anti sway bars                                                         

Anti-sway Bars:- Anti-sway bars (also known as anti-roll bars) are used along with shock absorbers or struts to give a moving automobile additional stability. An anti-sway bar is a metal rod that spans the entire axle and effectively joins each side of the suspension together.

TYPES OF SUSPENSION SPRINGS
**Steel Springs   :         

a.  Leaf Springs                                       

b. Coil Springs                                                

c. Torsion bar Springs                                                       

** Rubber Springs:         

a. Compression Springs 
b. Progressive Springs                                                                 

**Plastic Springs                                                                

**Air Springs                

a. Bellow Type                                                          

b. Pestoon  Type                                                                  

Hydraulic Type springs                                                   

TYPES OF LEAF SPRINGS

There are five types of leaf springs

• Full  – elliptic type
• Semi – elliptic type
• Three Quarter – elliptic type
• Transverse Spring type
• Helper Spring type

 Full elliptic :  The advantage of this type is the elimination  of shackle and spring. The lubrication and wear frequently which are on of the main draw back of this type of springs.                                    Semi – elliptic: This type is more popular for rear suspension are used in 75% of cars.                          Three – Quarter – elliptic type: This type is rarely used in now-a-days. It gives resistance, but occupies more space than other types.       

Transverse Type: This  type of  spring  is  arrange transversely  across  the  car  instead  of  longitudinal  direction. The transverse  spring  for front axle which is bolted rigidly to the frame at the center and attached to the axle by means of shackle at  both ends.                                                     Helper Springs: The helper springs are used in heavy vehicles for rear suspension. When vehicle fully loaded the main spring as well as helper spring to come in action and absorb the road shocks. When the load of the vehicle is less the helper spring will not act and the main spring only absorb  the road shocks.                                                                     

 ADVANTAGES: 

    The main advantages of suspension system are Comfort to passengers Good handling Shields the vehicle from damage Increases life of vehicle Keeps the tires pressed firmly to ground It provides softer suspension.It reduces wheel wobbling .It reduces the tendency of fitting of the wheel on one side due to road vibration.  It provides more space for engine accommodation.It promotes under steer which results low un sprung weight.The movement of the spring on one wheel is not transmitted to the other wheel. 

 Reference     
1. Suspension system of analysis  by  Barbara and David P.Mikkelson
2 Formazin reference of suspension system by    HACH LANGE S.R.L.
3. Memoir on Suspension Bridges  by orme,brown
4. suspesion  system  by Drewry, Charles Stewart

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