AUTOMOBILE RUNNING SYSTEM AND POWER TRAIN 6. STEERING SYSTEM
The steering system gives control of the car in the hands of the driver, literally. It is used to steer a running vehicle to keep it on the desired track. The system should be so designed that it makes the car easy to handle and does not tire out the driver. At the same time it should keep the wear and tear of the tyres in check.
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Objectives After going through this lesson, you will be able to:
i. ii. iii. iv.
6.1
List the functions of the steering system List out the components of the steering system and give the function of each Describe the Ackermann steering mechanism Understand the importance of wheel alignment
Introduction
The basic function of the steering system is to convert the rotary motion of the steering wheel into angular displacement of the front wheels so that the car can be steered as desired. The steering system of a car performs the following functions: • It helps in turning the wheels to the left or the right as required. • It provides directional stability. • It helps control the wear and tear of tyres. • It helps in achieving a self-rightening effect, that is, the vehicle follows a straight path on its own after taking a turn. • It multiplies the turning effort applied by the driver, thus making steering less strenuous. Fig. 6.1: Steering system. • It absorbs road shocks and prevents them from being transmitted to the steering wheel.
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Components and Working
The various components of the steering system include the steering wheel, the steering shaft, the steering gearbox, the pitman arm, the drag link, the steering knuckle, the tie rod, the track arms, the stub axle, and the wheels. A steering wheel is used to help steer a four wheeler. It is made of hard plastic or polyurethane and consists of a circular rim with a hub at the centre. The steering wheel is attached to the upper end of a steering shaft with the help of serrations cut on the inside of the steering wheel hub and on the steering shaft. The steering shaft is enclosed within a tubular jacket and is held in bearings. The jacket is connected to the body of the vehicle by brackets and the jacket assembly together with the steering shaft is called the steering column. The steering column is always placed at an angle from vertical in view of ergonomic considerations and to make it convenient and comfortable for the driver. This angle is usually 20o for commercial vehicles and 50o for passenger cars. The horn, light switches, flasher unit, wiper control, and in some cases, the gear control lever are also fixed on the steering column. Sometimes, the steering columns are designed to be collapsible in order to reduce chances of injury to the driver in the case of an impact in an accident. The lower end of the steering shaft is attached to the steering gearbox. The various types of steering gearboxes used are: i. ii. iii. iv. v. vi.
Cam and peg type. Worm and sector type. Worm and roller type. Worm and nut type. Re-circulating ball and nut type. Rack and pinion type.
The steering gearbox provides the necessary leverage or the mechanical advantage, which is termed as the steering gear ratio. The steering gear ratio is the ratio of the angle by which the steering wheel is turned to the angle turned by the track arm. The value of this ratio typically lies between 10:1 and 15:1 for cars and from 15:1 to 25:1 for heavy vehicles. The greater is the steering gear ratio, the lesser is the effort required on the part of the driver to turn a vehicle. However, with increasing steering
Fig. 6.2: Steering system (a) Components. (b) Working.
gear ratio, the angle or the number of turns by which the steering wheel needs to be rotated also increases. The pitman arm, also called the drop arm, comes out of the steering gearbox. The rotary motion of the steering wheel is converted to oscillatory motion of the drop arm by the steering gearbox. The drop arm connects the steering gear box and the drag link. The oscillatory movement of the drop arm is further converted into linear motion of the drag link. The drag link connects the drop arm and the steering knuckle and has ball joints at both its ends. The motion of the drag link is transferred to the steering knuckle and from there to the stub axle and then to the wheel. Both the front wheels are attached to track arms and linked to each other through a tie rod. Hence, angular displacement of one wheel also causes the other wheel to turn, due to the displacement of the tie rod. The tie rod and track arms are connected with the help of ball joints.
Self-check Questions 1. The steering system converts the rotary motion of the ____________ into angular displacement of the ____________ 2. What is self-rightening effect? 3. Steering column is always placed at an angle from vertical which is ____ for commercial vehicles and ___ for passenger cars 4. Define steering gear ratio. 5. Value of steering gear ratio typically lies between __________ for cars and from _________ for heavy vehicles 6. If steering gear ratio increases, effort required on the part of the driver to turn a vehicle _____________ but the angle or the number of turns by which the steering wheel needs to be rotated ______________
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Classification
Fig. 6.3: Types of steering system (a) Manual steering. (b) Power steering.
The steering systems can be classified as: i.
Manual steering in which effort is applied by the driver and it is multiplied by the steering gearbox. ii. Power assisted steering system or power steering in which driver effort is assisted by engine power or electric power in order to reduce the driver’s effort at the steering wheel. Power steering is standard equipment on most cars nowadays. 6.4
Steering Mechanism
All components of the steering system are connected using some mechanism which is called the steering mechanism. The steering mechanism helps obtain pure rolling motion of the front wheels on the road without any sliding or lateral slip, thereby increasing the life of the tyre.
Fig. 6.4: Ackermann’s steering mechanism (a) Principle. (b) Assembly.
The most commonly used mechanism in vehicles is the Ackermann steering mechanism, which is based on the principle of geometrically correct steering. This is also called the Ackerman’s principle and it states that when a vehicle travels on a curved path, its wheels should describe circles around the same centre. A wheel which follows a different path will slide, causing wear of the tyre. Imaginary lines drawn through the axes of all the wheels should pass through the same point, that is, the centre of the curve on which the car is traveling, or they should be very near to this point. This is achieved through a proper assembly of the steering. In Fig. 6.4(b), the longer bar represents the front axle assembly and the shorter bar represents the steering linkage operated by the steering wheel. Because the steering linkage is shorter than the axle assembly, it moves the right wheel through a
larger angle than the left when the car turns right and vice versa when the car takes a left turn.
6.5
Wheel Alignment
Wheel alignment is the positioning of the front wheels and the steering mechanism in such a way that the geometry imparts: • good directional stability, • ease of steering, • minimum tyre wear, • reversibility of steering gear, and, • good riding qualities of the vehicle.
• • • •
The front wheel alignment depends on some geometrical characteristics, namely: Camber, Caster, Kingpin inclination, and, Toe
The front wheels of most cars tilt slightly, when seen from the top. This tilt is called the camber and the amount of tilt is known as the camber angle. The camber is usually positive when the wheels lean outwards such that they are farther away at the top than at the bottom. Wheels closer at the top are said to have a negative camber. If the camber is kept equal on both the wheels, it reduces the steering effort and imparts directional stability. Fig. 6.5: Camber angle.
The inclination of the kingpin axis forwards or backwards from the vertical is called the caster angle. If the king-pin is tilted backwards at the top, the caster is positive. Usually, the caster angle is kept between 2O and 7O and it provides directional stability while also promoting self-rightening effect, that is, helping the car return to the straight ahead position after a turn.
Fig. 6.6: Caster angle.
The kingpin inclination is the angle between the vertical and the kingpin. It is kept in the range between 3.5O and 8.5O. This angle, like caster, provides directional stability and promotes selfrightening effect. Setting the front wheels to point inwards is known as toe-in. It is used in most rear-wheel driven cars as it offsets the tendency of the wheels to move outwards. Toe-out is when the front wheels are set to point outwards and this is used in some front-wheel driven cars. Fig. 6.7: Kingpin inclination.
Fig. 6.8: (a) Toe-in. (b) Toe-out.
The wheel alignment angles and settings can be checked and adjusted in most vehicles, even though for cars that have a monocoque construction, the toe alone can be adjusted.
Self-check Questions 7. ______________ steering mechanism is used nowadays 8. State Ackerman’s principle 9. The steering mechanism moves the right wheel through a _______ angle than the left when the car turns right 10. Name the parameters of wheel alignment 11. Camber is positive when the wheels lean __________ at the top 12. What does camber do? 13. Inclination of the kingpin axis forwards or backwards from the vertical is called the __________ angle 14. If the king-pin is tilted backwards at the top, the caster is ___________
15. caster angle is kept between ___ and ____ 16. kingpin inclination is kept in the range between ____ and ____ 17. ____________ and _____________ provide directional stability and promote self-rightening effect 18. Setting the front wheels to point inwards is known as ________
6.6
Activity
1. Check out different vehicles and note how many turns of steering wheel are
required to turn the vehicle at a given turn. 2. Try to turn the steering wheel of a car from one to the other and note when it
cannot be turned and further. Measure the lock to lock angle.
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Summing Up
The steering system is used to steer a running vehicle to keep it on the desired track. It is so designed as to improve driveability and minimize wear and tear of tyres. It starts at the steering wheel and ends at the road wheels. Various types of steering gearboxes are used to reduce the effort on the part of the driver to steer the vehicle. The steering gear ratio is between 10:1 and 15:1 for cars and from 15:1 to 25:1 for heavy vehicles. The greater the steering gear ratio, the lesser is the effort required on the part of the driver to steer a vehicle though the number of turns by which the steering wheel needs to be rotated increases Steering system could be manual or power assisted All components of the steering system are connected using some mechanism. Now Ackermann steering mechanism is used. Wheel alignment is the positioning of the front wheels and the steering mechanism in such a way that we get good driveability and minimum tyre wear. The front wheel alignment depends on some geometrical characteristics, namely camber, caster, kingpin inclination, and toe.
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Possible Answers to Self-check Questions
1. steering wheel, front wheels 2. the vehicle returns to a straight path on its own after taking a turn without any effort by the driver to turn the steering wheel 3. 20 O, 50O 4. Steering gear ratio is the ratio of the angle by which the steering wheel is turned to the angle turned by the track arm. 5. 10:1 and 15:1, 15:1 to 25:1 6. decreases, increases
7. Ackermann 8. Ackerman’s principle states that when a vehicle travels on a curved path, its wheels should describe circles around the same centre 9. larger 10. camber, caster, KPI, toe-in 11. outwards 12. reduces the steering effort and imparts directional stability 13. caster 14. positive 15. 2O, 7O 16. 3.5O, 8.5O 17. caster angle, kingpin inclination 18. toe-in
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Terminal Questions
1. What are the various functions that the steering system has to perform? 2. List the components of the steering system and give the function of each. 3. List the various steering gear boxes and state which member of each is connected to the steering wheel and which to the wheel through links. 4. State Ackermann’s principle and describe Ackermann Steering mechanism. 5. What is meant by Wheel Alignment? What are its parameters? Give their representative values.
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References and Suggested Further Reading
1. AA BOOK OF THE CAR. Drive Publications Ltd, London. 2. Maruti 800 Service Manual 3. http://www.howstuffworks.com
6.11
Glossary
Manual steering
Car is steered by manual effort.
Mechanical advantage
Ratio of load to effort.
Power assisted steering
Car is steered with assistance of hydraulic or electric power.
Self-rightening effect
Vehicle returns to a straight path on its own after taking a turn without the driver rotating the steering wheel.
Steering gear ratio
Ratio of the angle by which the steering wheel is turned to the angle turned by the track arm
greater is the steering gear ratio, the lesser is the effort required on the part of the driver to turn a vehicle angle or the number of turns by which the steering wheel needs to be rotated also increases. Steering mechanism
System of linkages which transmits motion from steering wheel to the road wheels.
Wheel alignment
Proper positioning of the front wheels and the steering mechanism to obtain good driveability and tyre life.