15355738 Steering Geometry Angles

8/3/2019 15355738 Steering Geometry Angles

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1.5.2. st e er i n g g e o m e t r y

The term "steer ing geometr y" (also known as "fr ont-end geometry") 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 steer ing geometry.

Traditionally, there are five steering geometry angles : Camber ( CAM ) , Caster

( CAS ) , Toe ( TOE ) , Steering axis inclination (SAI) , and Toe-out on turns ( R ).

There are two more steering geometry angles that are not specific to each wheel

but measure the spatial relationship among all four wheels. These angles are : Thrust

angle (TH ) and Setback ( SET ).

All steering geometry angles can be measured in degrees of a cycle. However,

TOE and SET can be measured in terms of distance and may be given in inches or

millimeters.

The 5 traditional alignment angles can be classified as tire wear angles or

directional control angles. A tire wear angle helps prevent tire wear when correct and

accelerates tire wear when incorrect. Of the 5 traditional alignment angles, the ones

affecting tire wear are : CAM , TOE , and R . A directional control angle affect steering

and handling. All 5 of the traditional alignment angles are directional control angles.

TH and SET are not usually discussed within these categories. Their importance is the

effect they have on other alignment angle.

A typical alignment involves checking and adjusting CAM and TOE at the rearwheels ; checking and adjusting CAM , CAS , and TOE at the front wheels ; and

checking SAI and R at the front wheels. CAM , CAS , and TOE of front wheels , and

CAM and TOE of rear wheels are usually adjustable, either through factory-installed

methods or through aftermarket devices. SAI and R are non-adjustable angles,

although camber adjustment can affect SAI. Usually a problem in these measurements

indicates damage within the suspension or steering system, calling for repair or

replacement of the damaged parts. TH and SET are reference angles, indicating the

relationship of the rear wheels to the rest of the auto and of same-axle wheels to eachother. These also are non-adjustable angles that can sometimes indicate damaged

components.

Adjustable suspension angles compensate for normal suspension wear. For

instance, as coil springs settle, CAM decreases. Adjusting the CAM returns the wheels

to their original angle without replacing the springs. Thus, alignment prevents the need

for more extensive suspension repair. This is true, of course, only as long as the wear is

slight. When spring sag becomes excessive, the springs must be replaced before CAM

can be adjusted correctly.

Assoc. Prof. Nguyen Van Nhan - Theory of Motor Vehicles


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We will look at the definitions, causes, and effects of all seven angles in this

section.

1) Camber

Camber is the angle between the centerline of the tire and a line perpendicular

to a level surface. More simply, camber is the tilt of a wheel and tire assembly, viewed

from the front of the vehicle. A wheel has zero camber when it is straight up and

down, so the centerline and the perpendicular line are the same. If the top leans

outward, away from the auto body, the wheel has positive camber. If the top leans

inward, the wheel has negative camber. CAM is measurable on both the front and rear

wheels.

Effects of Camber

Tire wear - Camber is a tire wear angle. Correct camber keeps the tire treadin good contact with the road. Zero camber while driving is the ideal position for thispurpose, but wheels and tires seldom maintain zero camber under actual driving

conditions. Too much positive camber makes the tire wear out faster on the outside,

and too much negative camber makes it wear out on the inside.

Steering stability - Camber is also a directional control angle. If camber isunequal side-to-side, the auto pulls toward the side with more camber.

The reason both for the tire wear and the steering pull is that a cambered tirerolls like a cone - as if one side of the ti re had a larger diameter than the other (Fig. 1-

20). The tread on the smaller side gets pushed under the tire and makes that side of the

Posit ive

camber( + )

Negative

camber( - )

Fig. 1-19. Camber



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tire wear faster, while the larger side tries to roll around the smaller side and pulls the

auto to one side.

Wheel bearing wear - Any excessive camber, whether positive or negative,causes extra wear on the wheel bearings because auto weight is not distributed in the

way the bearings were designed to support it. Positive camber places extra auto weight

on the outer wheel bearings, and negative camber puts extra weight on the inner wheel

bearings.

Fig. 1-20. A cambered tire rolls like a cone, pulling the vehicle toward the side with

more positive camber and wearing out the shoulder of the tire.


(+) (-)

(-)

(+)

a) b)

(-) (-) (+)

c) d)


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Fig. 1-21. Influences on camber

a) Uneven loading of the vehicle ; b) Road crown

c) Worn-out suspension ; d) Str iking a curb.

Influences on Camber

Once a vehicle is out on the road, a number of factors begin to affect camber :

Uneven loading of the vehicle Body roll during turns Road crown Rough road surface Suspension wear Suspension damage Tire size CasterSome of these factors cause long-term changes in CAM . Others produce temporary

effects.Uneven cargo or passenger loading can cause body tilt (Fig. 1-xa). Another type of

body tilt is roll during turns. Centrifugal force shifts the weight of the vehicle toward

the outside of the turn, and the body tilts, placing more weight on the wheel and tire at

the outside of the turn while taking weight off the inside wheel and tire. CAM decreases

at the outside wheel and increases at the inside one. The faster the vehicle corners, the

greater the effects on CAM, which is why high-speed cornering wears out the edges of

tires.

Most roads are built higher at the middle than the sides to keep rainwater fromgathering in the road. This slope is called road crown. Road crown tends to cause

positive camber at the right wheel and negative camber at the left. This could

ultimately cause the outside of the tread on the right front tire to wear excessively

because this part of the tread would be taking most of the action. In passenger car,

this is not particularly important because the vehicle weight is relatively light. On the

other hand, the ti res on heavy-duty vehicles could show this sort of wear at relatively

low mileages. For this reason, some heavy-duty vehicles have a front adjustment that

gives the right front wheel less positive camber than the left front wheel.



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Suspension wear is another factor. Over time , as springs settle and sag, and other

suspension parts wear down, wheels tend to develop negative camber (Fig. 1-xc) .

Striking a curb, or driving into a hole in the road can bend suspension parts in ways

that cause permanent positive camber. Installing new tires of a different size after an

alignment changes the camber.

2) Caster

Caster is the angle between the steering axis and a vertical line running through the

center of the wheel and tire, viewed from the side. More simply, caster is the forward

or backward tilt of the steering axis. If the steering axis leans toward the back of the

vehicle, the wheel has positive caster. If the steering axis tilts toward the front of the

vehicle, the wheel has negative caster. If the caster line is vertical, the wheel has zero

caster.

Steering axis - On an SLA or strut/SLA suspension, the steering axis is an imaginary

line running through the center of the upper and lower ball joints. On a strut

suspension, the line runs through the top of the strut, at the pivot, and the lower ball

joint. On a kingpin suspension, the line runs through the kingpin axis .

Effects of caster- Caster angle has effects on :

Straight-ahead stability Steering wheel returnability Steering sti ffnessWheels with positive caster "want" to go straight, resist turning, and return to their

straight-ahead position as soon as possible. Generally, the straight-ahead stability and

steering wheel returnability associated with high caster are considered good qualities,

while the increased steering stiffness is considered a problem. Also, increased caster

often increases the amount of road shock that the driver and passengers feel because it

in effect "aims" the bumps at the passenger compartment. Decreasing CAS decreases

steering stability when the vehicle is travelling straight ahead and decreases the

tendency of the steering wheel to re-center after a turn. There is little resistance tochanging direction, however, so steering is easy. Negative caster specifications are rare

in late-model cars, but in the 1960s and 1970s some large, heavy cars had negative

caster to make steering less difficult.

Caster trail - is the distance between where the caster line intersects the ground and the

center of the tire contact patch.

Part of the reason that increased caster tends to increase steering stiffness is because it

also tends to increase caster trail. With positive caster, the caster trail increases a

wheel's resistance to turning. To illustrate, imagine a caster under a tool chest (Fig. 1-

23a). The steering axis is vertical, although not through the hub of the wheel. When we



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roll the tool chest so that the wheel is ahead of the steering axis, the resistance of the

floor opposes the wheel, and it is easily deflected by anything in its path. It wobbles,

and the cabinet has little travelling stability. The natural tendency is for the caster to

spin around so that the wheel trails behind the steering axis, giving it positive caster.

With positive caster, the resistance of the floor works to keep the wheel in position. In

other works, the caster trail resists changes in direction.

Some manufacturers have gotten around the problem of increased caster trail causing

increased steering stiffness by increasing the caster angle without increasing caster

trail. They do this by placing the caster angle behind the center of the wheel. In this

way, the caster angle can lean far back but the distance from where the caster line

intersects the ground to the center of the wheel - SCAS - remains small (Fig. 1-23b).

Camber roll - The amount that CAM changes when wheels are steered to either side.

CASdetermines camber roll.

When both front wheels have positive caster, the vehicle tends to roll out or lean out on

turns. But if the front wheels have negative caster, then the vehicle tends to bank (lean

in) on turns. Let us demonstrate why this is so. Fasten the cardboard disk and the

pencil together as shown in Fig. 1-xa. The disk represents the left front wheel. Note

that we do not include any steering-axis inclination here. We want to show only the

effect of positive caster. Hold the disk vertical with the pencil at an angle so that both

the pencil point and the edge of the disk rest on the tabletop. Now, rotate the pencil as

shown. Note that the disk is lifted from the tabletop. Actually, in the vehicle, the wheel(the disk) would not be lifted. Instead, the ball joints (the pencil) would move down. In

other words, on a right turn, the left side of the vehicle would drop.


0 0

0 0

0 0

()CAS

(+) CAS

(+) CAS0 0

SCAS


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Fig. 1-22. Caster

Fi g. 1-23. Caster trail

Right wheel in right turn

Lift ofpencil

Straight ahead

Lift ofdisk

Left wheel in right turn

(+)CAS Front

a)

b)

SCAS

Movement

SCAS

b)a)



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Fi g. 1-24. Effect of (+ ) CASon the front wheel during a right turn

Now, let us see what happens at the right front wheel. As the right turn is made, the

wheel pivots on the road surface, causing the ball joints (the pencil) to be lifted. The

right side of the vehicle is lifted.

When the left side of the vehicle is lowered and the right side is lifted as a right turn is

made (as described above), then the vehicle leans out on the turn. This decreases

camber on the outside wheel and increase it at the inside wheel. The higher the CAS ,

the more it affects camber during turns. Negative caster affects CAM in the opposite

direction.

Because of camber roll, a wheel has CAM during turns even if the camber reading is

zero when the wheel is straight ahead. Camber roll wears both edges of the tires.

3) Toe

Wheel and tire technicians often talk about a tire as if it were a foot. They call the front

edge of the tire the " toe", the back edge the "heel", and the tire contact patch the " foot

print".

Toe is the angle between the direction a wheel is aimed and a line parallel to the

centerline of the auto. When measured linearly, toe is the distance between the leading

edges of the tires subtracted from the distance between the trailing edges. If the toes

point straight ahead, the wheels have zero toe. If the toes point toward each other, thewheels have toe-in, or positive toe. If the toes point away from each other, the wheels

have toe-out, or negative toe.

Changes in camber always cause changes in toe. This means that all the factors that

affect camber, such as vehicle load and suspension wear, can also affect toe. Caster

changes affect toe as well. This is why alignment technicians correct camber and caster

before making toe adjustments.

Effects of Toe

Toe is the most important tire wear angle, and zero toe is the ideal for preventing wearduring driving. When a wheel is not pointed straight ahead, the tire scuffs sideways

along the road surface as it rolls forward. Toe is also a directional control angle.

Incorrect toe, whether toe-in or toe-out, makes the auto wander and the wheels

shimmy.

Older, bias-ply tires are stiffer than radials and tend to wear in a feathered pattern

when toe is excessive, but toe wear on radial tires is difficult to tell from camber wear.

If a wheel is toed-in, the tire will wear on the outside edge, as it would with positive

camber, while toe-out causes wear on the inner edge of tire, like negative camber.

However, toe wear tends to be the same on both same-axle tires because the drag of the



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road surface tends to equalize toe side-to-side even if one wheel has more toe than the

other.

If the rear wheels of a FWD auto have excessive toe, they tend to develop a unique

wear pattern called diagonal wipe. Because there is little weight on these tires, when

toe makes the tires scuff along the road, they drag for a little while then hop. After the

tires alternately drag and hop for some distance, they begin to wear in stripes running

diagonally across the tread.

Fig. 1-25. Toe

SCF - Distance between toes, SCR - D istance between heels

SCF

SCR

FRONT OFVEHICLE

Zero toe

Toe - out

Toe - in



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4) Steering Axis Inclination

Steering axis inclination (SAI) is the angle between the steering axis and a

vertical line, viewed from the front of the vehicle. Sometimes service literature refers to

SAI as "kingpin i ncli nation- KPI" for kingpin suspension, or as "ball joint i ncli nation-

BJI".

Caster and SAI, both measuring steering axis tilt, but caster is seen from the side of the

vehicle and SAI from the front. These angles are generally measured only on the front

wheels because, except in vehicle with 4WS, rear wheels do not have a steering axis.

Of the three major independent front suspension design, strut suspensions have

high SAI, and SLA and strut/SLA suspensions with a low knuckle have less SAI.

SAI measurements

Generally, alignment technicians measure SAI when they suspect suspension

damage. The SAI measurement indicates the position of the ball joints, or ball joint and

strut, and the steering knuckle and spindle. On a kingpin suspension, SAI indicates the

position of the kingpin, steering knuckle, and spindle. If any of these parts are bent, it

will throw off SAI. The only way to correct it is to replace the bent part.

To trace bent suspension parts, alignment technicians usually measure SAI,

CAM , and the combined angle( COM ). The included angle is the SAI angle plus or

minus the camber angle. If camber is positive, add it to the SAI measurement for theincluded angle. If camber is negative, subtract it from the SAI measurement. By

comparing these measurements and seeing which one is incorrect, a technician can tell

which suspension or steering parts are causing the problem [A-51/357].

Fig. 1-26. SAI andCOMEffects of SAI

SAI is desirable for several reasons :

SAI

0 0 00

SAI CAM

COM



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Improving steering stability - SAI tends to keep the wheels straight-ahead . Ithelps recovery, or the return of the wheels to the straight-ahead position after a turn has

been made. We can make a tabletop demonstration of why this is so with a pencil, a

rubber bend, a cardboard disk, and a piece of cardboard. Put them together as shown in

Fig. 1-27. The cardboard disk represents the wheel, the pencil - the steering axis. The

cardboard brace at the top holds the two apart there so as to give "steering-axis

inclination". Needless to say, the angle is greatly exaggerated in the illustration. No,

hold the pencil at an angle with the tabletop so the wheel is vertical. Then rotate the

pencil, but do not change its angle with the tabletop.

SAI

hh1

A B C

Fig. 1-27. The effect of SAI. Thecardboard disk represents the leftfront wheel as viewed from thedr iver 's seat.

A) Straight-ahead position.B) Right turnC) Left turn.

Notice that as we turn the pencil, the wheel is carried around and down toward

the tabletop. If the wheel could not move down, what would happen ? As we turned the



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pencil, the pencil would have to be moved up, always maintaining the same angle with

the tabletop. This last movement is what actually takes place in the auto. The wheel is

in contact with the ground. It cannot move down. Therefore, as it is swung away from

straight-ahead , the ball joints and supporting parts are moved upward. This means that

the auto body is actually lifted. In other words, SAI causes the auto to be raised every

time the front wheels are swung away from straight ahead. Then the weight of the auto

brings the wheel back to the straight ahead after the turn is completed and the steering

wheel is released.

SAI reduces steering effort, particularly when the auto is stationary.SAI is an especially important angle for strut suspensions. Because the strut serves a

dual purpose - suspension arm and shock absorber - and because the upper strut mount

attaches to the a fender well, the strut cannot be tilted back very far, and this limits the

amount of caster provided in these suspensions. However, the strut can be tilted inward

to some degree, and designers do this so that high SAI will makes up for small caster.

In the FWD auto that typically use strut suspensions, SAI is much more important than

caster in ensuring steering stability.


15355738 Steering Geometry Angles

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