INTRODUCTION

TO

AUTOMOTIVE

STEERING SYSTEM

TABLE OF CONTENTS1. INTRODUCTION

2. FUNCTIONS OF STEERING SYSTEM

3. STEERING SYSTEM COMPONENTS

4. RACK AND PINION STEERING SYSTEM

4.1. STEERING RATIO IN RACK AND PINION STEERING SYSTEM

4.2. VARIABLE RATIO STEERING

4.3. FACTORS AFEECTING HANDLING IN RACK AND PINION STEERING SYSTEM

5. ACKERMANN STEERING GEOMETRY

6. DESIGN CONSIDERATIONS IN STEERING GEOMETRY

6.1. CASTER ANGLE

6.2. Camber angle

6.3. Steering axis inclination

6.4. Toe

6.5. SUSPENSION HEIGHT

6.6 .THRUST ANGLE

6.7. TURNING CIRCLE RADIUS

7 VEHICLE DYNAMICS AND STEERING

7.1. UNDER STEER

7.2. OVER STEER

7.3. NEUTRAL STEER

8. FOUR WHEEL STEERING

9. POWER ASSISTED STEERING

9.1. HYDRAULIC POWER STEERING

9.2. ELECTRIC POWER STEERING

INTRODUCTIONThe steering system is the key interface between the driver and the vehicle. Steering is a set of mechanisms used to control the trajectory (path followed) by the vehicle. The basic aim of steering is to ensure that the wheels are pointing in the desired directions and to allow the driver to guide the vehicle. The most conventional steering arrangement is to turn the front wheels using a hand–operated steering wheel which is positioned in front of the driver.

The steering system maybe either manual or power. When the only energy source for steering system is the force the driver applies to the steering wheels, the vehicle has manual steering. Power steering uses a hydraulic pump or electric motor to assist the driver’s effort. Most vehicles have power steering to make parking easier .The basic operation is same for both manual and power steering.

As the driver turns the steering wheel, the movement is carried to the steering gear or; it changes rotary motion of the steering wheel into linear motion. The liner motion acts through steering linkage or tie rods attached to the steering arms. This moves the wheels and tires to the left or right for steering.

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FUNCTIONS OF STEERING SYSTEMA steering system performs the following functions-

Converts the rotary movement of the steering wheel into an angular turn of the front wheels and sometimes rear wheels and helps in swinging the wheels to the left or right and vice versa.

Multiplies the effort of the driver in order to make it fairly easy to turn the wheels (i.e. provide mechanical advantage to driver). The driver should be able to turn the vehicle with little effort, but not so easily that it is hard to control.

Absorbs a major part of the road shocks thereby preventing them to get transmitted to the hands of the driver

To provide directional stability of the vehicle while going straight ahead. In other words when a vehicle is being driven straight ahead, the steering system must keep it from wandering without requiring the driver to make constant corrections.

The steering mechanism should have self rightening effect so that when the driver release the steering wheel after negotiating the turn , the wheel should try to achieve straight ahead position.

Minimize tire wear and increase the life of the tires. For maximum tire life, the steering system should maintain the proper angle between the tires both during turns and straight-ahead driving. The steering system must help maintain proper tire-to-road contact

The steering system must also allow the driver to have some road feel (feedback through the steering wheel about road surface conditions).

Steering system also houses airbags (a safety device) and auxiliary systems as horn switch etc. These are easily accessible to driver.

STEERING SYSTEM COMPONENTSSteering system has following major components-

Steering wheel and steering shaft that transmits driver’s movement to the steering gear.

Steering gear that increases the mechanical advantage while changing the rotary motion of steering wheel to linear motion

Steering linkages that carries the linear motion to the steering armsThese components are common to all type of steering systems; other components may vary depending upon the type of mechanism used either recirculating ball or rack and pinion type.

RECIRCULATING BALL STEERING SYSTEM

In this report our discussion will be limited to rack and pinion steering mechanism and based on Ackermann steering geometry.

RACK AND PINION STEERING SYSTEM

Many modern cars use rack and pinion steering mechanisms, where the steering wheel turns the pinion gear; the pinion moves the rack, which is a linear gear that meshes with the pinion, converting circular motion into linear motion along the transverse axis of the car (side to side motion) This motion applies steering torque to the kingpins of the axle of the steered wheels via tie rods and a short lever arm called the steering arm. On most cars, it takes three to four complete revolutions of the steering wheel to make the wheels turn from lock to lock (from far left to far right). A rack-and-pinion gear set is

enclosed in a metal tube, with each end of the rack protruding from the tube. A rod, called a tie rod, connects to each end of the rack. The tie rod at each end of the rack connects to the steering arm on the spindle. The steering gear consists of the rack, pinion, and related housings and support bearings. The inner ends of the tie rods have balls which fit into ball sockets on the ends of the rack this allows the outer ends of the tie rods to move up and down with steering knuckles and wheels. Flexible rubber boots or bellows protect the steering gear from dust and water. Many steering systems have intermediate steering shaft between steering column and the steering gear. The intermediate shaft has a universal joint at the upper end and a flexible coupling at the lower end. These help prevent road shock and noise from passing up through the steering column to the driver.

Steering Ratio in rack and pinion steering system

It is the relative number of turns of the steering wheel compared to the movement of the wheels. If the steering wheel must be turned one revolution to turn the front wheels one sixteenth of a turn, the steering ratio is 1 to 1/16. Reversing the numbers gives a ratio of 16 to 1, or 16:1.

The size of the pinion gear and the number of teeth on the gear determine the rack-and-pinion steering ratio. A large pinion gear with many teeth would turn quickly but would require a large steering effort. A small gear with a few teeth would turn easily, but require many revolutions of the steering wheel to make a turn.

The steering ratio must be carefully selected as a compromise between handling and steering effort. The average steering ratio on modern vehicles ranges from 12:1 to 24:1. A heavy vehicle will have a higher ratio than a lighter vehicle. If the vehicle has power steering, the ratio will be lower than that on the same vehicle with manual steering. Steering ratio affects the response of the front wheels to the movement of the steering wheel (handling) and the ease of turning the wheel. A small steering ratio means that slight steering wheel movement will turn the front wheels, but the effort required to turn the steering wheel will be relatively high. A large steering ratio means that more turns of the steering wheel are needed to turn the front wheels, but that the steering effort is less. A relatively high steering ratio also helps to absorb shocks from the road. If for instance the steering ratio is 16:1, road shocks are transmitted to the steering wheel at 1/16 of their original intensity.

Variable Ratio Steering

Some conventional steering gears are equipped with variable ratio steering. Variable ratio steering gears use specially designed gears. It uses a rack-and-pinion gear set that has a different tooth pitch (number of teeth per inch) in the center than it has on the outside. This makes the car respond quickly when starting a turn (the rack is near the center), and also reduces effort near the wheels turning limits. This makes hard turns easier, especially in low speed situations such as parking.

FACTORS AFFECTING HANDLING IN RACK AND PINION STEERING SYSTEM

Steering ratio, as well as overall handling and ease of steering, is determined by many factors. Some of these are as follows-

The size of the steering wheel. The relative size of the gears in the steering gear. The size and shape of the steering arms, and the angles formed by the

linkage action. The percentage of vehicle weight placed on the front wheels. Whether the vehicle has front- or rear-wheel drive.

ACKERMANN STEERING GEOMETRY

Ackermann steering geometry solves the problem of wheels on the inside and outside of a turn needing to trace out circles of different radius. The intention of Ackermann geometry is to avoid the need for tires to slip sideways when following the path around a curve. The geometrical solution to this is for all wheels to have their axles arranged as radii of a circle with a common instantaneous centre. As the rear wheels are fixed, this instantaneous centre must be on a line extended from the rear axle intersecting the axes of the front wheels (By Kennedy’s Theorem). Front wheel is turned through a greater angle than the outside wheel. When steering with perfect Ackermann, at any angle of steering, the centre point of all of the circles traced by all wheels will lie at a common point and hence the problem is solved.

As the steering moves, the wheels turned according to Ackermann, with the inner wheel turning further. The steering pivot points are joined by a rigid bar called the tie rod which can also be part of the steering mechanism, in the form of a rack and pinion. The length of the track rod (the moving link between the hubs) is kept shorter than that of the axle, so that the steering arms of the hubs appeared to “toe out.

DESIGN CONSIDERATIONS IN STEERING GEOMETRY

Front end steering geometry- It refers to the angular Relationship between suspension and steering parts, front wheels, and the road surface. Because alignment deals with angles and affects steering, the method of describing alignment measurements is called steering geometry. The basic factors involved are-

Suspension height, caster, camber, steering axis inclination, toe, turning radius and thrust angle

Caster angle

The angle between the vertical line and the kingpin centre line in the plane of the wheel (when viewed from the side) is called caster angle.

Function of caster is to achieve the self-centering action of steering, which affects the vehicle’s straight-line stability. Excessive caster angle will make the steering heavier, less responsive and increased steering effort. Caster angles over 7 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.

A rearward tilt provides positive caster and a forward tilt provides negative caster. Positive caster tends to keep the wheels pointed straight ahead and overcome any tendency for the vehicle to steer away from straight ahead. Negative caster makes steering easier. Positive caster tends to make the wheels toe in; negative caster tends to make the wheels toe out.

Camber angle

Camber is the inward or outward tilt of the wheel from the vertical when viewed from the front of the vehicle. If the top of the wheel is farther out than the bottom (that is, away from the axle), it is called positive camber; if the bottom of the wheel is farther out than the top, it is called negative camber.

Camber angle alters the handling qualities of a particular suspension design; positive camber angle helps to achieve a lower steering effort, negative camber improves grip when cornering. This is because it places the tire at a better angle to the road, transmitting the forces through the vertical plane of the tire rather than through a shear force across it. Another reason for negative camber is that a rubber tire tends to roll on itself while cornering. The inside edge of the contact patch would begin to lift off of the ground if the tire had zero camber, reducing the area of the contact patch. This effect is compensated for by applying negative camber, maximizing the contact patch area. This is only true for the outside tire during the turn; the inside tire would benefit most from positive camber.

On the other hand, for maximum straight-line acceleration, the greatest traction will be attained when the camber angle is zero and the tread is flat on the road. Excessive camber angle can lead to uneven and increased tire wear. Tilting the wheels puts more load and wear on one side of the tire tread. Incorrect camber at both wheels can cause hard and unstable steering and wander.

Steering axis inclination

This is also known as king pin inclination.The angle between the vertical line and centre of the king pin or steering axle, when viewed from the front of the vehicle is known as king pin inclination or steering axle inclination. SAI is not adjustable. It is designed into steering knuckle.

This inward tilt is desirable for following reasons- It helps steering stability by returning the wheels to straight ahead after

a turn is completed. This is called steering wheel returnability. It reduces steering effort, especially if the vehicle is not moving. It reduces tire wear.

Toe

Toe, also known as tracking, is the symmetric angle that each wheel makes with the longitudinal axis of the vehicle. In a rear wheel drive car, increased front toe in provides greater straight-line stability at the cost of some sluggishness of turning response. The wear on the tires is marginally increased as the tires are under slight side slip conditions. On front wheel drive cars, the situation is more complex. The greater the toe, the faster the tires wear.

TOP VIEW

Positive toe, or toe in, is the front of the wheel pointing in towards the centerline of the vehicle.

Negative toe, or toe out, is the front of the wheel pointing away from the centerline of the vehicle.

FRONT VIEW

SUSPENSION HEIGHT

It is the distance measured from some specific point on the body, frame, or suspension to the ground. If suspension height is not correct it can affect the angles in the steering and suspension systems.

THRUST ANGLE

When all four wheels are properly aligned and the steering wheel I is centred the vehicle should travel forward in straight line. However if a rear wheel has improper alignment, when the vehicle moves forward it may not move straight ahead. The direction of travel is determined by geometric centreline and thrust line. Thrust angle affects handling by causing a pull in the direction away from

the thrust line. TURNING CIRCLE RADIUS

The turning circle of a car is the diameter of the circle described by the outside wheels when turning on full lock. There is no hard and fast formula to calculate the turning circle but you can get close by using this:

Turning circle radius = (track/2) + (wheelbase/sin(average steer angle))

VEHICLE DYNAMICS AND STEERING

Under steer

Over steer

Neutral steer

FOUR WHEEL STEERING

A variety of vehicles have four wheel steering (4WS) like military and off road vehicles. Four-wheel steering found its most widespread use in monster trucks, where maneuverability in small arenas is critical, and it is also popular in large farm vehicles and trucks. Some of the modern European Intercity buses also utilize four-wheel steering to assist maneuverability in bus terminals, and also to improve road stability. The system automatically steers the rear wheels according to the speed of the vehicle and steering angle of the front wheels.For increased manoeuvrability at lower speeds, the rear wheels may steer in opposite direction of the front wheels. At higher speeds, the rear wheels steer in the same direction as the front wheels. This improves handling and allows tighter turns.

In an active four-wheel steering system, all four wheels turn at the same time when the driver steers. In most active four-wheel steering systems, the rear wheels are steered by a computer and actuators. The rear wheels generally cannot turn as far as the front wheels. There can be controls to switch off the rear steer and options to steer only the rear wheel independent of the front wheels.

POWER ASSISTED STEERING

Power steering helps the driver of a vehicle to steer by directing some of the power to assist in swiveling the steered road wheels about their steering axes. As vehicles have become heavier and switched to front wheel drive, particularly using negative offset geometry, along with increases in tire width and diameter, the effort needed to turn the wheels about their steering axis has increased, often to the point where major physical exertion would be needed were it not for power assistance. To alleviate this auto makers have developed power steering systems or more correctly power-assisted steering.

There are two types of power steering systems; hydraulic and electric/electronic. A hydraulic-electric hybrid system is also possible.A hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel. Electric power steering (EPS) is more efficient than the hydraulic power steering, since the electric power steering motor only needs to provide assistance when the steering wheel is turned, whereas the hydraulic pump must run constantly. In EPS, the amount of assistance is easily tunable to the vehicle type, road speed, and even driver preference. An added benefit is the elimination of environmental hazard posed by leakage and disposal of hydraulic power steering fluid. In addition, electrical assistance is not lost when the engine fails or stalls, whereas hydraulic assistance stops working if the engine stops, making the steering doubly heavy as the driver must now turn not only the very heavy steering— without any help—but also the power-assistance system itself.

HYDRAULIC POWER STEERING

ELECTRIC POWER STEERING