Wheel Alignment Fundamentals

Transcription

1 CHAPTER 67 Wheel Alignment Fundamentals OBJECTIVES Upon completion of this chapter, you should be able to: Describe each wheel alignment angle. Tell which alignment angles cause wear or pull. KEY TERMS Ackerman angle camber camber roll caster cradle crossmember dog tracking included angle oversteer radius SAI scuff setback slip angle toe alignment tracking turning radius understeer wheel base INTRODUCTION Correct alignment of the wheels allows a vehicle to run straight on a level highway with very little steering effort and minimal tire wear. This chapter deals with the principles of the different wheel alignment angles. WHEEL ALIGNMENT ANGLES There are five wheel alignment angles. They are: Toe Camber Caster Steering axis inclination (SAI) Turning radius TOE Toe is a comparison of the distances between the fronts and the rears of a pair of tires (Figure 67.1). It is the alignment angle most responsible for tire wear. Toe-in is when the tires are closer together at the front. Toe out is when the tires are further apart at the front. Every 1 16'' of toe-in results in 11 feet per mile of scuff. This means that the tires move sideways for 11 feet out of every mile traveled. If there is 1'' of toe, the tire is dragged 182 feet sideways every mile. This causes severe tire wear and decreased fuel economy Several things can cause incorrect toe: An incorrect adjustment Bent steering linkage due to a collision with a curb, a pothole, or an accident A change in either the caster or camber adjustment Looseness in the steering linkage due to wear Tie-rods and other steering linkage parts are built to be flexible, rather than brittle. If steering linkage parts were brittle, they would break when a tire hit a pothole or a curb, This could result in a dangerous loss of vehicle control. Because they are somewhat flexible, steering linkage parts will bend rather than break. A Figure 67.1 Toe is a comparison of the distances between the fronts and the rears of a pair of tires. B

2 Wheel Alignment Fundamentals 1249 Toe is generally not considered to be a directional control angle. A change in toe on one side of the vehicle will be split through the steering linkage with the wheel on the other side. NOTE: Bent steering linkage will cause the toe setting to change. This affects the position of the steering wheel, which will now be off-center when traveling straight. An off-center steering wheel is a good indicator of the need for a wheel alignment. Adjusting toe correctly will recenter the steering wheel. toe is adjustable on all vehicles and rear toe is adjustable on some. As a vehicle is driven, its toe alignment changes. The tires will deflect inward or outward when rolling, depending on whether the vehicle is front- or rearwheel drive. Rolling tires tend to toe out when driving on rear-wheel-drive (RWD) vehicles. The steering linkage deflects due to the rolling resistance of the tires, taking up any clearance that exists. During a wheel alignment, the front tire toe is adjusted to be as close as possible to zero while rolling. The specification usually calls for the tires to be slightly toed-in while on the alignment rack. -wheel-drive (FWD) vehicles react in an opposite fashion. FWD front wheels tend to toe inward as they are pushed by engine torque. Therefore, specifications usually call for a slight toe-out setting, or zero toe. Information on front and rear toe adjustment is found in Chapter 68. CAMBER Camber is the inward or outward tilt of a tire at the top. It is an adjustable angle on the front of most vehicles and on the rear of some others. Positive camber is when a tire tilts outward at the top (Figure 67.2). Negative camber is when a tire tilts inward at the top. Shorter Longer Figure 67.3 The tread will wear on the shorter side as the smaller diameter of the tire squirms against the road surface. Camber is a tire wearing angle. The radius measurements at the inside and outside edges of a cambered tire tread are different (Figure 67.3). Therefore, the inside and outside of the tread rotate at different speeds. When the tire leans to one side, the smaller diameter of the tread squirms against the road surface, causing the tread to wear on that side. The camber angle is controlled by the position of the control arms or struts. The control arms are strong and are not often bent, except during a collision. Running into a curb is not a normal cause of a change in the camber angle. On short-and-long arm (SLA) suspension systems, camber typically becomes negative as springs sag with age. When springs sag, the inside edges of the front tires often wear. Camber is a directional control angle. A cambered tire tends to roll in a circle, as if it were at the large end of a cone (Figure 67.4). Think about what happens if you put an ice cream cone on a table and attempt to roll it. This is called camber roll. Unequal camber between tires on opposite sides of the vehicle can cause a steering pull to the side with the most camber. True vertical 0 Center line of tire Federal-Mogul Corporation Tire buckles Figure 67.2 This tire has positive camber. Figure 67.4 A cambered tire tends to roll in a circle.

3 1250 CHAPTER 67 Weight of vehicle Positive camber Negative 0 Positive Intersects near inner wheel bearing Figure 67.5 Positive camber loads the inner wheel bearing. The inner wheel bearing is larger because it is designed to support the vehicle s load. Camber is usually positive, which loads the inner wheel bearing (Figure 67.5). CASTER Caster is the forward or rearward tilt of the spindle support arm (Figure 67.6). Positive caster is when the top tilts to the rear as shown in Figure This applies the lead point, or point of load, in front of true Figure 67.6 Caster. True vertical Centerline of wheel Point of load Point of contact (+) caster Lead point Pivot axis centerline Pivot or caster center line Figure 67.7 Bicycles and motorcycles have positive caster. Figure 67.8 Comparison between positive and negative caster. vertical. Negative caster is when the steering axis tilts forward, moving the point of load behind the wheel. Figure 67.8 compares positive and negative caster. Caster is sometimes adjustable on front wheels, although very few cars have rear caster adjustment. It is a directional control angle but does not cause tire wear when the vehicle is going straight, however. A bicycle has positive caster, which makes it want to move in a straight-ahead direction when you let go of the handle bars. If you turn the handle bars around in the opposite direction, negative caster results and the bicycle loses its stability. When the front wheels have different caster settings, a vehicle will pull toward the side with the most negative caster. That wheel will have its point of load behind the wheel on the other side of the vehicle. During a turn, caster can cause the spindle to move either toward or away from the ground, depending on whether the caster is negative or positive. Positive caster causes the spindle to move toward the ground when the wheel is steered to the left. The tire cannot move closer to the ground, so the vehicle must lift (Figure 67.9). When the turn is finished, turning effort is removed from the steering wheel and the tire attempts to move back to center. Steering a positive-castered wheel to the right causes the spindle to move up, with the opposite results. Wheels with equal caster will cancel each other out and the vehicle will steer straight. Figure 67.9 During a turn, the tire cannot move closer to the ground, so the vehicle must lift. During a left turn the left side spindle moves downward

4 Wheel Alignment Fundamentals 1251 of vehicle Pivot axis C/L (+) Caster Road shock Figure Positive caster results in road shock being transmitted through the steering column. NOTE: Steering axis inclination (SAI) causes a similar return force to straight ahead, but in a different and consistent direction. SAI is covered later in the chapter. Positive caster aligns the steering axis with bumps encountered in the road, so road shock is more likely to be felt through the steering wheel (Figure 67.10). Too much positive caster can cause front-wheel shimmy so some vehicles with a large amount of positive caster use a steering damper to prevent this. When the steering damper wears out, the front wheels will shimmy uncontrollably when a front tire hits a small bump. Too much positive caster can cause cupped tire wear if the resulting shimmy is allowed to continue. Negative caster makes a vehicle easier to steer, but it can also cause wander and weave on the highway. Old cars had narrow tires; their alignment specification called for positive caster to keep the car going straight without holding the steering wheel. Newer cars usually have wider tires, which tend to keep rolling straight (Figure 67.11). NOTE: Light trucks have different caster settings depending on their rear axle ride height. A measurement of frame angle is done during a wheel alignment. Changing the caster setting can have a drastic effect on the driveability of the vehicle. a directional control angle, although it can cause tire wear, too. Alignment equipment manufacturers teach this differently. Some say that tire wear is not a result of caster. A high amount of caster can cause tire wear if the vehicle is driven excessively on winding roads or in the city. This is because a high amount of caster influences camber roll. STEERING AXIS INCLINATION Steering axis inclination (SAI) is the amount that the spindle support arm leans in at the top (Figure 67.12). SAI is also known as ball joint inclination (BJI) or king pin inclination (KPI). It is not a tire wearing angle; it helps the vehicle steer straight ahead. How SAI Works When camber is 0 degrees, the spindle is horizontal to the ground when the wheels are pointed straight ahead. During a turn, the spindle attempts to move closer to the ground (see Figure 67.9). However, the spindle cannot move any closer to the ground because of the tire, so the vehicle must lift. During a turn, the weight of the vehicle puts pressure on the wheel, causing it to want to return it to the straight-ahead position. SAI has three functions: After a turn, SAI helps the vehicle return to straight ahead (Figure 67.13). SAI keeps the vehicle going straight down the road. Although positive caster does this as well, more positive caster makes it harder to steer. Steering axis inclination Included angle Camber angle Vertical Caster and Tire Wear Caster is a measurement of the camber angle as it changes during a turn. Caster is known as primarily Log Wide contact area tends to roll straight Hoop Figure Wider tires tend to roll straight. Narrow contact area tends to wander Point of load Scrub radius Figure Steering axis inclination puts the pivot point under the tire.

5 1252 CHAPTER 67 View from side Steering axis Centerline Spindle arc during a turn Figure SAI aids in returning to a straight-ahead position after a turn when vehicle weight causes the spindle to aim straight ahead. Tire toes out Figure With positive scrub radius, the tire toes out when rolling. Steering axis Centerline SAI allows the vehicle to have less positive caster (for easier steering) while still having good directional stability. The combination of SAI and camber is called the included angle. Some vehicles with a large amount of SAI will wear the outsides of the tires because of the excessive amount that the wheel cambers during turns (see Figure 67.9). These vehicles do not require as much positive caster to keep the vehicle from wandering. In city driving, right-hand turns are made about 80% of the time. Delivery trucks are especially hard on right-hand springs, bearings, and tires (which must turn sharper). NOTE: SAI can be used as a diagnostic angle in determining whether parts are bent. SCRUB RADIUS Scrub radius is a factor of steering axis inclination. It is the pivot point for the front tire s footprint. Scrub radius is the distance at the road surface between the centerline of true vertical at the center of the tire tread and the steering axis pivot centerline. The junction of the steering axis and centerline pivot point is normally below the surface of the road (see Figure 67.12). This is positive scrub radius. How far it is below the surface of the road determines the amount of scrub radius. More scrub radius makes it harder to steer. Positive camber reduces scrub radius, but tire life is best when the running camber angle is zero. Outside Negative scrub radius Tire toes in Figure With negative scrub radius, the tire toes in when rolling. RWD vehicles generally have positive scrub radius. The front wheels toe out when rolling (Figure 67.14). FWD vehicles usually have negative scrub radius. The tires toe in when rolling (Figure 67.15). NOTE: If there is a tire blowout on the left side of an FWD vehicle with positive scrub radius, the tire will pull hard to the left. The still-inflated right tire will pull inward. To prevent this dangerous situation, SAI has been increased on FWD vehicles. This results in negative scrub radius, which causes the blown tire to pull inward. The vehicle will continue to go straight because this direction of motion counteracts the motion of the right front tire. Incorrect Scrub Radius Changing ride height will affect scrub radius. Installing lower profile tires and offset wheel rims will greatly

6 Wheel Alignment Fundamentals O.E.M. wheel negative scrub radius Offset wheel positive scrub radius Figure Installing lower profile tires and offset wheel rims on a vehicle will change scrub radius. SPX Service Solutions Figure tires must toe out during a turn. of vehicle Hunter Engineering Company increase scrub radius (Figure 67.16). The pivot point can actually move outside of the tires footprint area. The result is much harder steering, wheel shimmy, and a tendency to wander. When tires and wheels that are too tall are installed on an RWD vehicle, negative scrub radius can result. This can cause instability. When a tire on an SLA suspension system goes over a bump, the scrub radius can change from positive to negative as the camber changes. Vehicle handling can become dangerous. NOTE: When changing to tires and wheels of a different size, modifications to the suspension must be made. Wheel alignment cannot compensate for this. Other causes of incorrect scrub radius are a bent front suspension part or damage to the frame at the crossmember or strut tower. The crossmember, or cradle, is the large steel part of the frame beneath the engine and between the front wheels. The strut tower is the area inside of the front fenders that supports a Macpherson strut. TURNING RADIUS When a vehicle makes a turn, the outside wheel must travel in a wider arc than the inside wheel (Figure 67.17). The alignment angle that controls this is called turning radius, which is also known as toe-out-onturns, or the Ackerman angle. The tires toe out during a turn because the steering arms are angled inward or outward (Figure 67.18a). When the wheels are turned, they move at different amounts as they move through their arc of travel (Figure 67.18b). (a) Tie-rod Inside of turn 20 (b) Center link of vehicle Linkage Steering arms Steering arm Outside of turn 18 Figure (a) The steering arms are angled. (b) The tires turn different amounts as the steering arms move through their arcs of travel. HISTORY NOTE Horse-drawn wagons used a steering system that pivoted at the center. In 1884, Rudolf Ackerman patented a unique steering system with both of the wheels pivoting at the outside of the axle. The angle he designed into the steering arms caused the front wheels to toe out when turning.

7 1254 CHAPTER CL FRONT TRACK REAR TRACK Figure When tracking is off, the front wheels do not follow the rear wheels. TRACKING The distance between the front and rear tires is called the wheel base. The side-to-side distance between an axle s tires is called the track. For minimum tire wear and good fuel economy, the wheels must run on track. The rear tires are supposed to follow in the tracks of the front tires. All four wheels should form an exact rectangle. Tracking is a term that refers to the relationship between the average direction that the rear tires point when compared to the front tires. When tracking is off, the front wheels do not follow the rear wheels (Figure 67.19). This causes the steering wheel to turn to the side as the front wheels try to follow the direction of the rear wheels. If the rear axle is out of line to the right, it will cause the steering wheel to be aimed to the right. This is often referred to as dog tracking due to its resemblance to the way that some dogs walk, with their rear feet not in alignment with their front feet. compares the wheel base between the two sides of the vehicle and is measured in degrees. A negative angle indicates that the wheel on the left side is set back. This will cause the vehicle to steer to the left and can also cause brake pull. A vehicle will generally pull to the side with the least wheel base. SPECIAL HANDLING CHARACTERISTICS Slip angle, understeer, and oversteer are terms that describe vehicle handling characteristics Slip Angle Tire manufacturers sometimes recommend different inflation pressures or alignment angles to change a vehicle s handling characteristics. A tire with lower inflation has a greater slip angle. Slip angle is the tendency during a turn for a tire to continue turning in the same direction, even though the steering wheel has been turned in the opposite direction (Figure 67.21). SETBACK A collision can cause incorrect setback. Setback is the amount that one front wheel is behind the one on the opposite side of the vehicle (Figure 67.20). It Setback Tire slip Figure Setback is the amount that one front wheel is behind the one on the other side of the car. Turning center ahead of rear axle centerline Figure Slip angle.

8 Wheel Alignment Fundamentals 1255 The amount of slip angle depends on the weight exerted vertically on the tire, the tire pressure, structure and type, and the wheel alignment setting. Positive camber causes a tire to have a greater slip angle. Desired path Understeer and Oversteer Understeer is when a vehicle does not seem to respond to movement of the steering wheel during a hard turn. Oversteer is when a vehicle turns too far in response to steering wheel movement (Figure 67.22). Desirable steering is said to be neutral. On RWD vehicles, lower inflation pressure is sometimes specified for the rear tires to prevent understeer. When a vehicle begins to slide and the driver lets up on the accelerator, this will allow the driver to regain control. If the vehicle had oversteer, easing up on the throttle would cause a spinout. When a vehicle has four-wheel alignment capability, the tendency to oversteer can be minimized by adjusting the camber on the rear wheels to a more negative setting than the front wheels. When tires of different profiles are installed on the same vehicle, the tires of the lower height are installed on the rear to prevent oversteer. Desired path Understeer Figure Understeer and oversteer. Oversteer REVIEW QUESTIONS 1. List the five wheel alignment angles. 2. Which alignment angle is most responsible for excessive tire wear? 3. If a tire has 1'' of toe-in, how far is it dragged sideways during 1 mile of driving? 4. Which kind of vehicle has its toe set inward when not rolling, front-wheel drive or rear-wheel drive? 5. When the top of a tire tilts inward (toward the center of the vehicle), what positive or negative angle is this? 6. The angle that keeps a bicycle going straight when your hands are off the handle bars is called. 7. What is the name of the alignment angle that keeps the vehicle going straight while allowing easy steering? 8. When tires of a different height or wheels with a different offset are installed on a vehicle, what alignment factor changes? 9. During a turn, which front wheel turns sharper, the outside or the inside? 10. When a vehicle does not seem to respond to movement of the steering wheel during a hard turn, what is this called? ASE-STYLE REVIEW QUESTIONS 1. Technician A says that camber can cause a vehicle to pull to one side. Technician B says that camber can cause a tire to wear on one side of its tread. Who is right? 2. Which of the following is/are true about caster? a. When aging springs sag, caster changes. b. Caster is a measurement of camber change as the wheel is turned. c. Caster is a directional control angle. d. All of the above 3. Technician A says that scrub radius is what causes the tires to toe in or out when rolling. Technician B says that front-wheel-drive vehicles usually have positive scrub radius. Who is right?

9 1256 CHAPTER Technician A says that front tires toe out during a turn. Technician B says that turning radius is caused by the toe setting. Who is right? 5. All of the following will cause a vehicle to pull to one side except: a. Setback c. A low tire b. Tracking d. Toe-in 6. Two technicians are discussing setback. Technician A says that setback can cause a brake pull. Technician B says that incorrect setback can be due to a collision. Who is right? 7. Which of the following is/are true about frontwheel toe? a. Wheels on front-wheel-drive vehicles tend to toe inward when driving. b. Wheels on rear-wheel-drive vehicles tend to toe outward when driving. c. Both A and B d. Neither A nor B 8. Technician A says that uneven caster can cause a vehicle to pull to the side. Technician B says that excessive caster will cause a tire to wear on one side of its tread. Who is right? 9. Which of the following is/are true about camber? a. Camber is affected by vehicle ride height. b. Negative camber loads the inner wheel bearing. c. Both A and B d. Neither A nor B 10. When a vehicle makes a turn, the outside wheel must travel in a wider arc than the inside wheel. The alignment angle that controls this is called: a. Turning radius b. Toe-out-on-turns c. The Ackerman angle d. All of the above