Automobile module iii
- 1. Steering and Suspension Systems Anoop P Department of Mechanical Engineering DepartmentofMechanical Engineering,MITSPuthencruz 1
- 2. FUNCTIONAND PRINCIPLE • The purpose of the steering system is to allow the driver to control the direction of the vehicle by turning the front wheels. • This is done by means of steering wheel, a steering column which transmits the rotation of the steering wheel to the steering gears, the steering gears which increase the rotational force of the steering wheel in order to transmit greater torque to the steering linkage, and the steering linkage which transmits the steering gear movement to the front wheels. • The steering system configuration depends on vehicle design (the drive train and suspension system used, whether it is a passenger car or a commercial vehicle, etc.). • At present, the rack-and-pinion type and the recirculation-ball types are in use. DepartmentofMechanical Engineering,MITSPuthencruz 2
- 3. Requirementsof Steering system • Excellent maneuverability • Proper steering effort • Smooth recovery • Minimum transmission of shock from road surface DepartmentofMechanical Engineering,MITSPuthencruz 3
- 4. Principle • The relative motion between the wheels of a vehicle at the road surface should be of a pure rolling type so that wear of the tires is minimum and uniform. • When the vehicle is moving on a curved path, the steering gear must be so designed that the paths of the points of contact of each wheel with the ground are concentric circular arcs. • For proper steering the axis of rotation of all the wheels should meet at G i.e. the instantaneous centre of rotation of the vehicle • To satisfy this inner wheels should be turned through a greater angle DepartmentofMechanical Engineering,MITSPuthencruz 4
- 5. DepartmentofMechanical Engineering,MITSPuthencruz To satisfy this inner wheels should be turned through a greater angle 5
- 7. STEERINGGEARMECHANISMS There are two types of steering gear mechanisms • Davis steering gear • Ackermann steering gear The main difference between the two steering gear mechanisms is that the Davis steering gear has sliding pairs, whereas the Ackermann steering gear has only turning pairs. DepartmentofMechanical Engineering,MITSPuthencruz 7
- 8. TYPESOF STEERINGGEARS • The steering gear converts the turning motion of the steering wheel into the to-and-fro motion of the link rod of the steering linkage. • Moreover, it also provides the necessary leverage so that the driver is able to steer the vehicle without fatigue. • There are many types and makes of steering gears in use for automobiles. DepartmentofMechanical Engineering,MITSPuthencruz 8
- 9. • The steering gear changes the rotary motion of the wheel into linear motion of the steering linkage. DepartmentofMechanical Engineering,MITSPuthencruz 9
- 10. Worm and wheel steeringgear DepartmentofMechanical Engineering,MITSPuthencruz 10
- 11. • The movement of the steering wheel turns the worm, which in turn drives the worm wheel. • Attached to the wheel spindle rigidly is drop arm, so that the rotation of the steering wheel corresponds to a linear motion of the drop arm end, which is connected to the link rod. DepartmentofMechanical Engineering,MITSPuthencruz 11
- 12. Worm and nut steering gear DepartmentofMechanical Engineering,MITSPuthencruz 12
- 13. • The steering wheel rotation rotates the worm which in turn moves the nut along its length. • This causes the drop arm end to move linearly, further moving the link rod and thus steering the wheels. DepartmentofMechanical Engineering,MITSPuthencruz 13
- 15. • It consists of a worm at the end of steering rod . • A nut is mounted on the worm with two sets of balls in the grooves of the worm, in between the nut and the worm. • The balls reduce the friction during the movement of the nut on the worm. • The nut has a number of teeth on the outside, which mesh with the teeth on a worm wheel sector, on which is further mounted the drop arm, which steers the road wheels through the link rod and the steering arms. DepartmentofMechanical Engineering,MITSPuthencruz 15
- 17. • This type of steering gear is used on light vehicles like cars and in power steering. • It is simple light and responsive. • It occupies very small space and uses lesser number of linkage components compared to the worm and wheel type of gear. • The rotary motion of the steering wheel is transmitted to the pinion of the steering gear through universal joints • The pinion is in mesh with a rack. • The circular motion of the pinion is transferred into the linear rack movement, which is further relayed through the ball joints and tie rods to the stub axles for the wheels to be steered. DepartmentofMechanical Engineering,MITSPuthencruz 17
- 18. Cam and Double Roller Steering gear • Similar to worm and wheel type steering gear DepartmentofMechanical Engineering,MITSPuthencruz 18
- 20. • The main objective of power steering is to reduce the driver’s effort in steering. • This system may employ electrical devices, pneumatic and hydraulic pressure. • Power steering is basically power assisted steering in which an arrangement to boost the steering wheel turning is provided. DepartmentofMechanical Engineering,MITSPuthencruz 20
- 21. Principlesof PowerSteering • It is type of hydraulic device for utilizing engine power as steering effort • Engine is used to drive a pump to develop fluid pressure • Pressure acts on the piston within the power cylinder so that the piston assists the rack effort • The amount of this assistance depends upon the extent of pressure acting on the piston • If more steering force is required the pressure must be raised DepartmentofMechanical Engineering,MITSPuthencruz 21
- 23. Electricalpowersteering • In this kind of power steering employs electric power instead of hydraulic power • The electric power is imparted to the rack which then drives the pinion Advantages • Vehicle can be steered even when the engine is inoperative • Energy is consumed only when the vehicle is being steered • The arrangement is neat and clean since the problem of leakage is out of question • Steering assembly is in the form of a compact unit DepartmentofMechanical Engineering,MITSPuthencruz 23
- 25. Front axle • The constructional features of front axle is quite different compared to a rear axle • Rear axles transmit power to rear wheels in rear wheel drive vehicles and front axles do not take part in power transfer. • They just obey the commands of steering linkages for swiveling the road wheels • Also known as dead axle DepartmentofMechanical Engineering,MITSPuthencruz 25
- 26. Front axle arrangements • Stub axle with rigid axle beam type • Used earlier on all types of vehicles, but now it is limited to medium and heavy commercial vehicles • Eg: trucks, buses etc. • Stub axle without rigid axle beam type • These are widely employed in cars and light commercial vehicles DepartmentofMechanical Engineering,MITSPuthencruz 26
- 27. RigidAxle Beam • It is a stationary structural member used to take up bending and torsional loads • Used to connect the stub axles • Bending is induced in the central region due to vehicle’s weight • Torsion occurs at its ends due to braking of wheels • Axle is made of I-section at the centre and circular portion at the ends so as to resist the loads efficiently. DepartmentofMechanical Engineering,MITSPuthencruz 27
- 29. Stub Axle • An inter--mediatory small link between the road wheels and the front axle. • It gets connected with the front axle by means of king pin or a ball joint • The ends of which are connected to the road wheels DepartmentofMechanical Engineering,MITSPuthencruz 29
- 30. Stub Axle- Types • Depending on the shape and style of connection stub axles can be of the following types: • Elliot type • Reversed Elliot type • Lamoine type • Reversed Lamoine type • Ball joint type DepartmentofMechanical Engineering,MITSPuthencruz 30
- 33. Wheel Alignment • It refers to the setting of front wheels and the steering mechanism that provides • an easier directional control of the vehicle • Minimum tyre wear • Stability to vehicle while negotiating a curve • Parallel rolling of front wheels in straight ahead position DepartmentofMechanical Engineering,MITSPuthencruz 33
- 34. Steering Geometry • It is the group of design variables outside the steering mechanism that affect the behavior Camber King pin inclination Caster Scrub Radius Included angle DepartmentofMechanical Engineering,MITSPuthencruz 34
- 35. Toe-in Toe-out • It is the initial setting of front wheels carried out in a garage • Here the front wheels are set closer at the front than at the rear – toe in • If the front of front wheels are are far off than rears – toe out DepartmentofMechanical Engineering,MITSPuthencruz 35
- 36. The purpose of providing toe in is to offset the tendency of wheel rolling: • On curves due to limitation of correct steering • Due to possible play in steering linkages • Due to camber effects The toe out is provided to counter the tendency of inward rolling of the wheels • Due to soil condition on agricultural land • On account of side thrusts and cross wind effects DepartmentofMechanical Engineering,MITSPuthencruz 36
- 37. DepartmentofMechanical Engineering,MITSPuthencruz 37 • The amount of toe in varies from 0-6mm on different vehicles • 2-4mm for Maruti 800 car
- 38. Camber/ Wheel Rake • The front wheels are tilted outwards at the top and inwards at the bottom when viewed from the front of the vehicle • The centre line of the tyre forms an angle with the vertical. • This is known as camber angle DepartmentofMechanical Engineering,MITSPuthencruz 38
- 39. • Some vehicles are provided with positive camber while others have negative camber • Most vehicles are provided with positive camber Camber is used to prevent inward titling at the top of the wheels caused due to • Excessive load • Play in king pins • Play in wheel bearings • Camber on both wheels must be equal • The amount of camber is generally kept between 0-1.50 DepartmentofMechanical Engineering,MITSPuthencruz 39
- 40. King pininclination/Steering axisinclination • King pin is mounted in such a way that it remains inclined with respect to the vertical axis • The angle thus formed between the centre line of king pin and the vertical axis is called king pin angle (king pin rake) • In case of modern cars which employ ball joints instead of king pin, steering axis inclination is referred instead of king pin inclination • It is kept around 7-8 degrees DepartmentofMechanical Engineering,MITSPuthencruz 40
- 41. This inclination • Helps in a straight ahead recovery and provides directional stability • Reduces the wear and tear of the tyres • Tends to reduce the effect of road shock on the steering system Incorrect steering inclination leads to • Hard steering • Pulling of vehicle to a side • Wheel doesn’t return to straight ahead position DepartmentofMechanical Engineering,MITSPuthencruz 41
- 42. Caster DepartmentofMechanical Engineering,MITSPuthencruz 42 • It is the tilt of king pin centre line or steering axis towards the front or rear of the vehicle • It lies in the range of 2-8degrees • 3 degree is optimum • With positive caster the steering axis is ahead of road resistance to the tyre which aids in directional stability
- 43. • Excessive positive castor leads to • Increased road shocks on steering wheel • Requirement of greater steering effort • Wheel wobbling at low speed DepartmentofMechanical Engineering,MITSPuthencruz 43
- 44. IncludedAngle/Combinedangle • The angle formed in the vertical plane between the king pin center line and the wheel center line. • A correct measure of included angle contribute to steering ease. • Include angle is around 9-100 DepartmentofMechanical Engineering,MITSPuthencruz 44
- 45. ScrubRadius • The tractive force of the vehicle acts at a point where the steering axis meets the road at a point on the ground • The distance between the extended centre line of the steering axis and centre line of the tyre where the tread contacts the road is called scrub radius DepartmentofMechanical Engineering,MITSPuthencruz 45
- 47. Reversible,IrreversibleandSemi-reversibleSteering • When an effort (i/p) is applied on the steering wheel which causes the road wheels to swivel(o/p). • If this action is reversed by swiveling the road wheels (i/p), the steering wheel can be turned(o/p) then the system is known as reversible steering. • In an irreversible steering it is not possible to turn the steering wheel by swiveling the road wheels. • A compromise made between the short coming of the two systems results in semi reversible steering DepartmentofMechanical Engineering,MITSPuthencruz 47
- 48. Description Reversible Irreversible Semi irreversible Transmission of road shocks to steering Yes, fully No, nil no Control of steering Difficult Less difficult Better Fatigue to driver Greater Lesser Intermediate Feeling of road conditions Yes, fully No, nil Yes Load on joints Less More Fair DepartmentofMechanical Engineering,MITSPuthencruz 48
- 49. Slip angle, corneringforce & Self righting torque DepartmentofMechanical Engineering,MITSPuthencruz 49Cornering force produced per unit slip angle is defined as cornering power
- 50. • Various factors affecting cornering force and self righting torque DepartmentofMechanical Engineering,MITSPuthencruz 50
- 51. Oversteer and Understeer • It is evident from the names that the vehicle gets steered either more or less than the normal desired steering. • While negotiating a curve the vehicle should follow a well defined path under normal steering condition • When the vehicle exhibits a tendency to take either a more sharp turn than intended by the driver or a lesser turning. • The former situation is known as over steering and the latter under steering • Several factors contribute to this kind of steering to vehicle such as load, slip angle, cornering force and suspension systems etc. DepartmentofMechanical Engineering,MITSPuthencruz 51
- 52. Comparison Factor Under steer Over steer Stability of vehicle Increased Decreased Desirability More Less Path of travel Wider Narrower Driver’s effort More Less For correct turning of wheels Steering has to be rotated more Steering wheel has to be unwound Effect at higher speeds Increased un-stability Chances of accidents are high Means to minimise By increasing inflation pressure in front wheels By increasing inflation pressure in rear wheels DepartmentofMechanical Engineering,MITSPuthencruz 52
- 53. Steering Gear Ratioand overall steeringRatio SGR • The ratio of angle turned by steering wheel to the corresponding angle turned by the steering gear cross shaft. • Drop arm/pitman arm is attached to the cross shaft OSR • The ratio of angle turned by the steering wheel to the corresponding angle turned by the front road wheels. DepartmentofMechanical Engineering,MITSPuthencruz 53
- 54. Suspension Systems DepartmentofMechanical Engineering,MITSPuthencruz 54 The suspension system is made up of springs, shock absorbers and linkage so as to allow the wheels and tyres to follow the road roughness without excessively raising the frame and transmitting shock.
- 55. Objectives • To safe guard passengers and goods against road shocks • To preserve the stability of vehicles while in motion • To provide the wheels always in contact with road while driving cornering and braking • To maintain proper steering geometry • To provide suitable riding and cushioning properties DepartmentofMechanical Engineering,MITSPuthencruz 55 The suspension system connects the body of vehicle with the wheels and restricts the direct impact of wheels to the chassis and body.
- 56. Components • Springs, which neutralize the shocks from the road surface • Dampers (shock absorbers), which act to improve comfort by limiting the free oscillation of the springs. • Stabilizer (sway bar or anti-roll bar), which prevents lateral swaying of the car. • A linkage system, which acts to hold the above components in place and the control the longitudinal and lateral movements of the wheels. DepartmentofMechanical Engineering,MITSPuthencruz 56
- 57. Types of Suspension System DepartmentofMechanical Engineering,MITSPuthencruz 57 Non-independent/Rigid suspension has both right and left wheel attached to the same solid axle. When one wheel hits a bump in the road, its upward movement causes a slight tilt of the other wheel. Independent suspension allows one wheel to move up and down with minimal effect to the other.
- 59. DepartmentofMechanical Engineering,MITSPuthencruz 59 Coil Spring used in cars and even in modern vehicles Leaf spring is seldom used in rear part of the cars
- 60. Suspension springs • Metallic springs • Leaf springs • Coil Springs • Torsion bar • Non- metallic springs • Pneumatic(air) springs • Rubber springs DepartmentofMechanical Engineering,MITSPuthencruz 60
- 61. Leaf Springs • It is a stock of spring steel strips/ laminations called leaves DepartmentofMechanical Engineering,MITSPuthencruz 61 These are also known as semi-elliptical leaf springs since they are in bent form
- 62. • Leaves/ laminations are comprised of leaf springs that are curved and are bound together to act as a single unit. • Stack of leaves is fastened together at the center with a central bolt/rivet • The longest leaf in the assembly is called master leaf. The assembly acts as a flexible beam due to different lengths of each spring. • The curvature of leaf spring is called nip. The overall curvature of leaf spring is called camber • Inter leaf friction reduces riding comfort, so to reduce this silencer pads are inserted in between the leaf springs, also leaves are lubricated with special graphite rich oil, • Helper springs is a special type of leaf spring with no eye at ends. It is placed above the master leaf spring. When the load exceeds a prescribed value both the main and helper springs comes into operation DepartmentofMechanical Engineering,MITSPuthencruz 62
- 63. Types of Leaf Springs DepartmentofMechanical Engineering,MITSPuthencruz 63 • Quarter elliptic was used in earlier models • Transverse leaf spring increases the tendency of vehicle to roll while negotiating sharp turns
- 64. Coil springs • It is a heavy duty strip of spring steel formed into shape of helix • These are ideal for absorbing vertical energy and can take up bending and shear stress. • There is no inter leaf friction as with leaf springs. • No control of oscillations by the spring itself • Helper springs provide progressive stiffness against increasing load DepartmentofMechanical Engineering,MITSPuthencruz 64
- 66. Torsion Bar • It is a circular rod made of spring steel. • One end is anchored to the frame and other end is attached to the control arm of vehicle, which is subjected to torsional load DepartmentofMechanical Engineering,MITSPuthencruz 66
- 67. • The energy absorption per unit weight is high • Suspension system gets lighter • Provides a compact design • Can’t take driving/braking thrust • No friction force and no inherent damping to control the vibrations produced due to road shocks • These are used with independent front and rear suspensions DepartmentofMechanical Engineering,MITSPuthencruz 67
- 68. Rubber springs • Absorbs oscillations through generation of internal friction when they are deformed by an external force. • More compact system • Excellent vibration damping properties • Absence of squeaking • Greater reliability • Good fatigue strength in compression DepartmentofMechanical Engineering,MITSPuthencruz 68
- 69. Air Springs • It is based upon the elasticity of compressed air • System employs rubber bellows which has compressed air. • The air filled bellows acts as suspension • Vibration and shocks are absorbed • Air pressure can be controlled for optimum riding comfort • Complexity of compressed air and accessory system DepartmentofMechanical Engineering,MITSPuthencruz 69
- 70. Stabilizer (Sway bar/Anti roll bar) • This prevents vehicle from rolling while taking a turn • Fastened to lower control arms • During cornering, centrifugal force makes the outside of body drop and inside raise. • The bar’s resistance to twisting motion limits body lean in corners. DepartmentofMechanical Engineering,MITSPuthencruz 70
- 71. Sprung and un-sprung weight • Refers to the weight of those parts of the vehicle that are supported on front and rear springs eg: vehicle frame, body, engine, power train Jounce and Rebound The suspension springs gets compressed when the vehicle goes over a bump. This state is known as jounce, which is the energy stored in the spring that forces the spring to original state. While returning the spring over runs the neutral position and this action is termed as rebound DepartmentofMechanical Engineering,MITSPuthencruz 71
- 72. Dampers/ Shock Absorbers • These are used in suspension system to check any continuous vibration which may follow the initial force on the system • Shock absorbers are necessary because springs do not settle down fast enough. • A dampening device is needed to control the spring oscillations while bouncing, such device is called a shock absorber • The shock absorber absorbs the energy of shock converted into vertical movement of axle by producing damping and dissipating the same into heat. • Damper is a better term technically to describe a shock absorber DepartmentofMechanical Engineering,MITSPuthencruz 72
- 73. Principle of operation Hydraulic Dampers In these dampers hydraulic fluid is used as the damping agent. “When a piston forces a fluid in a cylinder to pass through some hole, a high resistance to the movement of the piston is developed which provides the damping effect” • An advantage of hydraulic damper is that damping is proportional to the square of the speed. DepartmentofMechanical Engineering,MITSPuthencruz 73
- 75. When the shock absorber is under compression • Fluid in the cylinder passes upwards through the restricted valves of the piston • At the same time the fluid passes down through a small valve of the cylinder tube • Due to this arrangement the piston is able to move against the resistance of the fluid, thus the shock is absorbed. When the shock absorber is under tension: • Liquid from the top portion of piston is forced downward through the piston valves • At the same time the fluid enters through the valve at the bottom of the cylinder tube. • The lengthening of shock absorber is thus made very slow, since it has to overcome the resistance of the fluid DepartmentofMechanical Engineering,MITSPuthencruz 75
- 76. Gas filled shock absorber • Here instead of oil, a mixture of oil and gas is used for the dampening effect Advantages • Provides longer life to tyres and other related suspension components • Can tolerate heat to a greater degree • This can be mounted in any position • Increases ground clearance • Prevents the formation of bubbles DepartmentofMechanical Engineering,MITSPuthencruz 76
- 77. Independent Suspension • It is an arrangement by which wheels are connected to the carriage unit in a manner such that the rise and fall of one wheel has no effect on the others. eg: passenger cars Advantages • Engine position can be moved forward, hence passenger space is greatly increased • Gyroscopic effects are reduced to minimum • Diminished wheel wobble and steering tramp • Reduced un-sprung weight • Provides a greater springing movement DepartmentofMechanical Engineering,MITSPuthencruz 77
- 78. Disadvantages • High initial cost • Due to large no. of bearings greater maintenance is required • More rigid sub frames are required • Force due to unbalanced wheels are transmitted easily to steering • Misalignment of steering geometry due to wear of component, hence requires more attention DepartmentofMechanical Engineering,MITSPuthencruz 78
- 79. Front wheel Independent Suspension • Wishbone arm system • Trailing link system • Sliding pillar system DepartmentofMechanical Engineering,MITSPuthencruz 79
- 85. Rear wheel Suspension system • Leaf spring rear suspension system (rigid suspension) • Independent rear suspension • Swinging half axle • Macpherson strut • Trailing Arm Design DepartmentofMechanical Engineering,MITSPuthencruz 85