startbeforeyouareready.files.wordpress.com€¦ · Web view2016/02/01 · The thereby occurring steering forces are determined not only by the steering ratio, but also by the front-axle

AUTOMOTIVE MAINTENANCE TECHNOLOGY SUBJECT: CHASSIS, SUSPENSION TITLE: STEERING SYSTEM AND STEERING

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Duration: - 6 Page: - 2 /33

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1. INTRODUCTION.

The steering converts the steering- wheel rotary motion into a turn

motion of the steered wheels of the vehicle. The system allows the

driver to guide the car along the road and turn left or right as desired.

The system includes:-

a) the steering wheel, which the driver controls.

b) the steering gear, which changes the rotary motion of the wheel into straightline motion.

c) the steering linkages.

Most systems were manual until a few years ago.

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2. REQUIREMENTS ON THE STEERING SYSTEM.

The steering system must guarantee easy and safe steering of the

vehicle. It must be possible to turn the front wheels into the position

corresponding to a turning circle with a radius of 12 m within a

maximum of 6 seconds If the actuating force at the steering wheel

exceeds 250 N, power assistance is necessary If such power

assistance fails, 600 N must not be exceeded. The actuating force

must be harmonious from the center as far as the end stop and must



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not decrease. it must be possible to drive the vehicle accurately, ( i.e.

without any unusual steering corrections Play in the mechanical parts

is impermissible ) The entirety of the mechanical transmission

devices must be able to cope with all loads and stresses occurring in

operation. Unusual driving maneuvers, such as driving over

obstacles, accident Iike occurrences etc, must not lead to any cracks

or breakages.

3. STEERING BEHAVIOR

The requirements in terms of steering behavior can be summarized as

follows:

a) Jolts from irregularities in the road surface must be damped as much as possible in being transmitted to the steering wheel However, such damping must not cause the driver to lose contact with the road.

b) The basic design of the steering kinematics must satisfy the Ackermann conditions" the extensions of the wheel axes of the left and right front wheels, when at an angle, intersect on an extension of the rear axle.

c) By means of suitable stiffness of the steering system (particularly if rubber- elastic connections are used) the vehicle must react to minute steering corrections.

d) When the steering wheel is released, the wheels must return automatically to the straight-ahead position and must remain stable in this position.



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e) The steering should have as Iow a ratio as possible (number of steering- wheel turns from lock to lock) in order to obtain ease of handling. The thereby occurring steering forces are determined not only by the steering ratio, but also by the front-axle loading, the size of the turning circle, the wheel suspension (caster, steering-axis inclination, steering roll radius) and the tire tread.

4. TYPES OF STEERING BOX.

A steering box must have the following qualities.

no play in the straight-ahead position.

Low friction, resulting in high efficiency.

high rigidity.

readjustability.

For these reasons, only three types have become established to

date.

Rack and Pinion Steering

Worm and Roller Steering ( it is similar to the recirculating ball ).

Recirculating-ball Steering .



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RACK AND PINION STEERING GEAR

1. Most smaller and down-sized vehicles use a rack-and- pinion steering gear (Fig. 3).

2. It has a pinion gear on the end of the steering shaft that meshes with a flat rack of gear teeth.

3. Tie rods connect the ends of the rack to the steering arms.

4. As the steering wheel turns, the pinion gear moves the rack to the right or left.

5. This moves the tie rods and steering arms which turn the steering knuckles and wheels inward or outward.

6. The inner ends of the tie rods have balls which fit into ball sockets on the ends of the rack (Fig.3).

7. This allows the outer ends of the tie rods to move up and down with the steering knuckles and wheels.

8. Flexible rubber boots or bellows protect the steering gear from dust and water.

9. Many steering systems have an intermediate steering shaft between the steering column and the steering gear (Fig.3).

10.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.



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RECIRCULATING-BALL STEERING GEAR

1. Trucks and large cars often have a recirculating-ball steering gear (Fig. 4).

2. It has a sector gear on the inner end of the output shaft. 3. A sector gear is a section of gear teeth from a gear wheel. 4. The output shaft is called the sector shaft or pitman-arm shaft. 5. The teeth on the sector gear mesh with the teeth of a ball nut. It

rides on the worm or worm gear that connects to the end of the steering shaft.

6. Balls roll in grooves inside the ball nut and in the worm. 7. As the steering shaft rotates, the worm forces the balls to roll in

the grooves. 8. The balls, as they roll, force the ball nut to move up or down the

worm. 9. Movement of the ball nut forces the pitman-arm shaft to turn. 10.This swings the pitman arm which forces the steering linkage to

pivot the wheels for steering. 11.The balls are the only contact between the worn and nut. 12.This reduces friction. The balls are recirculating balls because

they recirculate from one end of the ball nut to the other end during steering.

13. As the balls reach the end of the groove in the ball nut, they enter the return guides (Fig. 4). The balls then travel back to the other end of the ball nut.



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5. STEERING RATIO

With mechanical advantage, a small input force can produce a large output force. In the steering system, the driver applies a relatively small force to the steering wheel. This results in a much larger steering force at the wheels. For example, a 10-pound [44.5-N] force applied to the steering wheel may produce up to 270 pounds [1201 N] or higher at the wheels.

Steering ratio is the number of degrees that the steering wheel must turn to pivot the front wheels one degree. In Fig. 5, the steering wheel must tum 17.5 degrees to pivot the front wheels I degree. The steering ratio is 17.5:1.

Steering ratios vary, depending on the type of vehicle and operation. Typical automotive steering ratios range from about 24:1 with manual steering to 14:1 with power steering.

The higher the steering ratio, the easier it is to steer. However, the steering wheel must be turned more than with a lower steering ratio. If the steering ratio is 24: 1, the driver must turn the steering wheel 24 degrees to pivot the front wheels 1 degree. If the steering ratio is 14:1, the steering wheel must be turned only 14 degrees to pivot the front wheels I degree.

Steering ratio is determined by steering-linkage ratio and the gear ratio in the steering gear. Steering-linkage ratio for pitman-arm steering gears depends on the relative length of the pitman arm and the steering arm (Fig. 5). When the two are the same length, the steering-linkage ratio is 1:1. If the pitman arm is twice as long as the steering arm, the ratio is 1 :2. Every degree the pitman arm swings pivots the steering arm and wheels two degrees.

The gear ratio in the pitman-arm steering gear depends on the angle and pitch of the teeth on the worm and sector gears. In the rack-and-pinion steering gear, steiring ratio may be determined by the number of teeth on the pinion gear. The fewer the number of teeth, the higher the ratio.



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6. VARIABLE-RATIO STEERING

Many steering gears provide variable-ratio steering. The steering ratio varies as the steering wheel moves away from straight ahead. A typical change would be from 16:1 to 13:1 (Fig. 6). For the first 40 degrees of steering- wheel movement in either direction, the steering ratio is constant. This faster ratio provides good steering control for highway driving. The steering ratio decreases when the steering wheel turns more than 40 degrees (Fig. 6). The lower steering ratio helps in city driving during cornering or parking. It reduces the amount the steering wheel must turn for these maneuvers.

The variable ratio is produced by the shape of the gear teeth. In a recirculating-ball steering gear (Fig. 7), the center teeth on the sector gear are larger than the outer teeth. When the ball nut has moved far enough to bring the outer teeth into mesh, the effective leverage changes-

Further ball nut movement then produces a greater movement of the sector gear. In a rack-and-pinion steering gear, the shape of the rack teeth can provide variable-ratio steering-



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7. STEERING LINKAGE

a) The steering linkage connects the steering gear to the front- wheel steering arms (Figs. 3 And 5 ).

b) The linkage can be adjusted to produce the proper alignment of the front wheels.

c) Tie-rod ends connect to the steering arms with balls that fit into sockets (Figs. 3 and 8). These are ball sockets. They are smaller but similar in construction to front -suspension ball joints.

d) In parallelogram steering linkage, the idler arm (Figs. 2 and 5 ) and other parts connect through rubber bushings.

e) The bushings twist as the parts swing or move. f) They supply some force to help return the wheels to center after

completing a turn. Lubricating these bushings may cause them to slip and wear prematurely.



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8. WHEEL ALIGNMENT

Why Alignment is important ?

It is not enough to merely place the front wheels on hubs, stand them up straight and devise a method of swiveling them from left to right. The car could be driven, but it would steer poorly. At high speed, it would become dangerous to handle; tire life would be short. That's why to secure easy steering, sure handling, smooth operation and good tire wear wheel alignment need to be done.

9. FRONT-END GEOMETRY

Front-end geometry (also called wheel-alignment angularity) is the relationship of the angles among the front wheels, the front-wheel attaching parts, and the ground.

The various factors that enter into front-end geometry are :-

I. Front-suspension heightII. Camber III. Steering-axis inclination IV. CasterV. Toe VI. Tuming radius.

A. SUSPENSION HEIGHT

a) This is the distance measured from some specific point on the body, frame, or suspension to the ground (Fig. 9).

b) If suspension height is not correct, it can affect the angles in the suspension system (the wheel alignment). Incorrect height could result from sagging coil or leaf springs, or incorrect torsion-bar adjustment.



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B. CAMBER

a) Camber is the tilting in or out of the front wheels from the vertical when viewed from the front of the vehicle (Fig. 10).



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b) If the top of the wheel tilts out, it has positive camber.

c) If the top of the wheel tilts in, it has negative camber.

d) The amount of tilt, measured in degrees from the vertical, is called the camber angle.

e) On many cars, the wheels are given a slight outward tilt when camber is correctly set. Then, when the car is loaded and rolling along the road, the wheels should run straight up and down in a true vertical position.

f) This "zero camber" position puts the full width of the tire tread on the road surface. The load and wear are evenly distributed across the tread.

g) However, an average .'running" camber of zero does not occur at all times during driving.

h) This is because camber changes as the body and wheels move up and down.

i) Figure 11 shows how the camber goes negative when the tire hits a bump.



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j) hen the tire drops into a hole in the road, the camber changes from zero to slightly positive (Fig. 12).

k) In wheel-alignment and suspension work, the upward movement of the wheel is called jounce.

l) The downward movement of the wheel is called rebound.

m) Any amount of camber, either positive ( + ) or negative ( -), tends to cause uneven and more rapid tire wear.

n) Tilting the wheel puts more load on one side of the tire tread than on the other side. This is why camber is called a tire-wear angle.

o) Excessive positive camber causes the outside of the tire tread to wear.



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p) Excessive negative camber causes the inside of the tire tread to wear.

q) The camber angle specified by the manufacturer is the setting that will normally provide maximum tire life.

r) If the vehicle is rolling on a perfectly level road, the ideal camber would be the same for both front wheels. This ideal camber would be just sufficient to bring the front wheels to the vertical position when the vehicle is normally loaded and moving.

s) However, roads are seldom perfectly level. Most roads are crowned, slightly higher at the center than on the two sides. When a car is moving along one side of a crowned road, the car tends to lean out slightly and drift to the right. This could cause the outside of the tread on the right front tire to wear excessively because it carries more than its share of car weight.

t) To overcome this, some vehicles have camber settings that give the left front wheel 1/4 degree more positive camber than the right.

u) Since the car tends to pull toward the wheel with the most positive camber, the effects of the average crowned road are overcome.

v) The car travels forward in a straight line. Another reason for the increased positive camber on the left front wheel is that most cars are operated with only the driver in them. When the driver gets in the car, the driver's weight tends to slightly lower the left side of the car. This, in turn, reduces the positive camber of the left front wheel.

w) Camber changes have little effect on directional stability when the car is traveling straight ahead. However, in cornering, centrifugal force causes the body to lean toward the out side of the turn (Fig. 13).

x) Tire traction resists the resulting tendency to skid.



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y) As a result, the side forces against the bottom of the tires cause their tops to tilt toward the inside of the turn.

z) In relation to the car frame, the outer wheel tilts in at the top, producing negative camber. The inner wheel tilts out at the top, producing positive camber.

aa) Improper camber on both wheels can cause hard steering, unstable steering, and wander. Excessive, unequal camber between front wheels contributes to low-speed shimmy.

bb)Incorrect chassis height, caused by sagging front or rear springs, can also affect camber.

cc) When a rear spring is weak, it can cause a camber change at the diagonally opposite front wheel.

dd)In some cars, this camber change may be as much as 3/4 degree for each inch [25 mm] of rear-spring sag.

ee)This is the reason that front-suspension height is checked and corrected before the front wheels are aligned.



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ff) Camber is an adjustable angle (except on some cars with Macpherson-strut front suspension).

C. STEERING-AXIS / KING PIN INCLINATION.

a) In older cars, all steering systems had a kingpin that attached the steering knuckle to a support (Fig. 14).

b) Then ball joints were adopted, allowing the steering knuckle and steering-knuckle support to be combined into a single part.



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c) With ball joints, no kingpin is used. Instead, the steering knuckle is supported through ball joints by upper and lower control arms.

d) A line drawn through the centers of the ball joints is called the steering axis (Fig. 15).

e) This is the line around which the steering knuckle pivots as the wheel swings to the right or left. Therefore, steering axis is defined as the center line around which the front wheel swings for steering.

f) Steering-axis inclination (SAI) is also called ball-joint inclination (or kingpin inclination (KPI) on vehicles that have kingpins



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g) It is the angle, measured in degrees, between the vertical and a line drawn through the centers of the ball joints, when viewed from the front of the vehicle (Fig. 16).

h) Another definition is the inward tilt of the ball-joint center line from the vertical (Fig. 15).



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i) Inward tilt of the ball-joint center line is desirable for several reasons.

First, it helps provide steering stability by re- turning the wheels to a straight-ahead position after the car has turned. This is called returnability.

Second, SA! reduces steering effort, particularly when the car is stationary.

Third, it tends to keep the front wheels rolling straight ahead. This is because the inward tilt of the steering axis causes the front end to raise slightly as the front wheels are swung away from the straight-ahead position.

j) When the front wheel is in the straight-ahead position, the wheel spindle is at its highest point. As the spindle pivots forward or backward, it begins to drop.

k) This is because the spindle pivots around the steering axis, which is tilted inward. However, the tire is already in contact with the ground and cannot move down.

l) So, as the wheel is swung away from the straight-ahead position, the steering knuckle, ball joints, and car body and frame are lifted upward.

m) The lift is slight- only about 1 inch [25 mm] or less. But it is enough for the weight of the car to help return the wheels to straight ahead after the turn is completed.

n) This same action tends to make rolling wheels resist any small force trying to move them away from the straight- ahead position. The resistance is not enough to cause hard steering, only good steering stability .



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o) SAI is not adjustable. The amount of the inclination, or tilt, is designed into the steering knuckle. Generally, if camber can be adjusted to specifications, the SAI is correct.

p) However , a change in camber will cause a similar change in SAI.

q) When the SAI is not within specifications, the spindle, Ball joints, or other parts are bent or worn and should be replaced.

D. INCLUDED ANGLE

a) In front-end geometry, the included angle is the camber angle plus the SAI angle (Figs. 16 and 17).



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b) This can be written as

c) The included angle determines the point of intersection of the tire center line with the steering axis or ball-joint center line (Fig. 17).

d) This point determines whether the rolling wheel tends to toe in or toe out.


Included angle = camber + SAI


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e) Toe-in is the amount that the front wheels point inward (Fig. 18). It is measured in inches, millimeters, or degrees. Toe-out is just the opposite.

f) A wheel that has toe-out points outward as it rolls. The tire on a wheel that rolls with toe-in or toe-out will wear rapidly.

g) The tire has to travel in the direction that the car is moving. But if the tire has toe-in or toe-out, it is dragged sideways as it rolls forward.

h) The more toe-in or toe-out the tire has, the more it is dragged sideways and the faster it wears.



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i) Ideally, the wheel rolls straight ahead, with neither toe-in nor toe-out. Figure 17 shows two opposing forces acting on the wheel. One is the forward push through the ball joints. The other is the road resistance to the tire.

j) If these two forces are exactly in line, the wheel will have no tendency toward toe-in or toe-out. However, the two forces are in line with each other only when the point of intersection of the tire center line with the steering axis is at the road surface.

k) When the point of intersection is below the road surface (Fig. 17 A), the wheel tends to toe out.

l) Since the forward push through the center line of the ball joints is inside the tire center line at the road surface, the wheel attempts to swing outward.

m) When the point of intersection is above road level {Fig. 17 B), the wheel attempts to swing inward, or toe in.

E. CASTER

a) Caster is the angle {measured in degrees) formed by the forward or rearward tilt of the steering axis from vertical, when viewed from the side of the wheel {Fig. 19). the angle is positive { + ) when the steering axis tilts backward {Fig. 20, left). With positive caster, the upper ball joint is behind the lower ball joint. Caster is negative { -) when the steering axis tilts forward {Fig. 20, right).



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b) Then the upper ball joint is ahead of the lower ball joint. When the upper ball joint is directly above the lower one, there is zero caster .

c) Although the caster angle is adjustable (except on some cars with MacPherson struts), caster has little effect on tire wear.

d) There are three reasons why caster is used. (Note that tire wear is not one of them.) These are:

1. To maintain directional stability and control

2. To increase steering returnability

3. To reduce steering effort

e) Positive caster aids directional stability. The center line of the ball joints passes through the road surface ahead of the center line of the wheel (Fig. 19).

f) Therefore, the push on the ball joints is ahead of the road resistance to the tire (Fig. 17).

g) The tire is trailing behind. A car wheel that is pulled has greater directional and steering stability than a wheel that is pushed.

h) In addition, positive caster tends to keep the wheels pointed straight ahead. It helps overcome any tendency for the car to wander or steer away from the straight-ahead position.

i) Negative caster does not aid directional stability. SAI contributes more than caster to directional stability of the tire. This is why cars with manual steering, which often have a slight negative caster, still retain good directional stability.

j) Caster also affects the tendency of the steering wheel to return to its center position after making a turn. This is called returnability.

k) Positive caster increases steering-wheel returnability. Therefore, cars with power steering often have slightly more positive caster than manual-steering cars.



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l) The additional positive caster helps overcome the tendency of the power steering to hold the front wheels in a turn.

m) Although the additional positive caster requires greater turning effort, the driver does not notice it because of the power steering.

n) Positive and negative caster have different effects on the actions of the wheels and car body. When both front wheels have positive caster, the body leans toward the outside of the turn (Fig. 13).

o) But if the front wheels have negative caster, the car tends to lean into the turn (Fig. 21).



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p) When the left front wheel has positive caster, during a left turn the left front wheel tries to move down against the road surface. This causes the ball joints to be lifted, which lifts the left side of the car body. At the same time, positive caster causes the right side of the car to be lowered slightly during a left turn.

q) When the right side of the car is lowered and the left side is lifted as a left turn is made, the car leans out on the turn (Fig. 13). This is just the opposite of what is desirable, since it adds to the effect of centrifugal force on the turn.

r) By using negative caster, which tilts the center line of the ball joints forward as in Fig. 20 (right), the car can be made to lean in on a turn.

s) For example, with negative caster, the left side of the car drops during a left turn and the right side lifts. This decreases the roll-out effect of centrifugal force.

t) Caster has another effect. Positive caster tends to make the front wheels toe in. With positive caster, the car is lowered as the front of the wheel pivots inward. Therefore, the weight of the car is always exerting force to make the wheels toe in.

u) With negative caster, the wheels tend to toe out, although the car steers easier. Then the force to return the front wheels to straight ahead after a turn is provided by SAI.

v) Positive caster increases the force required to steer because it tries to keep the wheels running straight ahead. SAI also tried to keep the wheels straight ahead.

w) Therefore, to make a turn, the steering effort must overcome the effects of both positive caster and SAI.

x) Excessive positive caster may cause too much steering effort, snap-back of the steering wheel after a turn, wander, low-speed shimmy, and an increased amount of road shock.

y) Improper caster may occur as a result of spring sag. For example, front-spring sag decreases positive caster (Fig. 22). For this reason, front-suspension height is checked and corrected before the front end is aligned.



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F. TOE.

a) Toe is the amount in inches, millimeters, or degrees by which the front wheels point inward or outward.

b) Its purpose is to ensure parallel rolling of the front wheels, to stabilize steering, and to prevent side-slipping and excessive wear of the tires.



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c) With toe-in, the front wheels attempt to roll inward instead of straight ahead. This is because the tires are slightly closer together at the front than at the rear {Fig. 18).

d) Front-wheel toe is set with the car standing still.

e) The toe offsets the play in the steering linkage, which is eliminated when the car is moving forward {Fig. 23).

f) This change in toe is due to the rolling resistance of the tires against the road. The amount of toe should be sufficient to prevent any toe-out or toe-in while the car is moving forward.

g) This is called running toe and may be measured by some types of dynamic wheel analyzers. Ideally, the toe setting will bring the actual running toe to zero when the car is moving.

h) As a car with toe-in begins to move forward, the backward push of the ground against the tires {shown in Fig. 23) tries to force the wheels to toe out.



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i) Since the tie rods are behind the wheels, this force compresses the steering linkage and takes up any play. As a result, the tires become parallel and roll straight ahead.

j) Toe greatly affects tire wear. Therefore toe can be adjusted.

k) Excessive toe-in or toe-out causes the tire tread to scrub the road. This happens when the car is moving straight, with the tire turned slightly in or out. Whatever the toe setting, the wheels must not run with toe-out. This can cause wander.

G. TURNING RADIUS

a) Turning radius is sometimes called toe-out during turns and turning angle.

b) It is the difference between the two angles formed by the two front wheels and the car frame during turns. Then the inner wheel is following the radius of a smaller circle than the outer wheel {Fig. 24).



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c) Therefore the inner wheel must toe out more to prevent tire sideslip and excessive wear .

d) When the front wheels are steered to make the turn shown in Fig. 24, the inner wheel turns at an angle of 23 degrees, while the outer wheel turns only 20 degrees. This permits the inner wheel to follow a shorter radius than the outer wheel. The two front wheels turn on concentric circles, which have a common center (Fig. 24).

e) Toe-out during turns is achieved by the proper relationship between the steering arms, tie rods, and steering gear {Fig. 25).

f) For this reason, the steering arms are angled inward {Fig. 18). If the steering arms were parallel to the wheels, then the wheels would remain parallel during a turn. This would scuff the tires.

g) The relationship of the parts ensures that the inner wheel on a curve always has more toe-out than the outer wheel.



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h) As the dotted line in Fig. 25 shows, when the tie rod is moved to the left during a right turn, it pushes at almost a right angle against the left steering arm. At the same time, the right end of the tie rod moves to the left and swings forward.

i) This turns the right wheel an additional amount. When a left turn is made, the left wheel is turned more than the right wheel.

j) Figure 25 shows a parallelogram type of steering linkage.

k) Other types of steering linkage provide a similar toe-out of the inner wheel during turns. Turning radius cannot be adjusted. If it is not correct, one or both of the steering arms are bent and should be replaced.


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startbeforeyouareready.files.wordpress.com€¦ · Web view2016/02/01 · The thereby occurring steering forces are determined not only by the steering ratio, but also by the front-axle