Caster Camber Tire-Wear Angles

Home , Camber angle, Caster angle

BASIC WHEEL ALIGNMENT odern steering and ples. Therefore, let’s review these basic the effort needed to turn the wheel. suspension systems alignment angles with an eye toward Power steering allows the use of more are great examples of typical complaints and troubleshooting. positive caster than would be accept- solid geometry at able with manual steering. work. Wheel align- Caster Too little caster can make steering ment integrates all the factors of steer- Caster is the tilt of the steering axis of unstable and cause wheel shimmy. Ex- Ming and suspension geometry to pro- each front wheel as viewed from the tremely negative caster and the related vide safe handling, good ride quality side of the vehicle. Caster is measured shimmy can contribute to cupped wear and maximum tire life. in degrees of an angle. If the steering of the front tires. If caster is unequal Front wheel alignment is described axis tilts backward—that is, the upper from side to side, the vehicle will pull in terms of angles formed by steering ball joint or strut mounting point is be- toward the side with less positive (or and suspension components. Tradi- hind the lower ball joint—the caster more negative) caster. Remember this tionally, five alignment angles are angle is positive. If the steering axis tilts when troubleshooting a complaint of checked at the front wheels—caster, forward, the caster angle is negative. vehicle pull or wander. camber, toe, steering axis inclination Caster is not measured for rear wheels. (SAI) and toe-out on turns. When we Caster affects straightline stability Camber move from two-wheel to four-wheel and steering wheel return. High posi- Camber is the tilt of the wheel from true alignment, we add setback and the ve- tive caster makes the front wheels vertical as viewed from the front of the hicle thrust angle into the equation. want to go straight ahead. A normal vehicle. Like caster, camber is measured Rear wheel camber and toe also must amount of positive caster provides sta- in degrees of an angle. If the tire ap- be checked for a four-wheel alignment. bility and makes the steering wheel pears to tilt outward at the top, the cam- straighten out after turning. On the ber angle is positive. If the top of the tire Tire Wear & other hand, positive caster increases tilts inward, the camber angle is negative. Directional Control Camber, toe and toe-out on turns are Caster Camber tire-wear angles. If they’re incorrect, Camber Angle the tires will wear unevenly and faster Caster Angle than normal. Because camber is relat- True Wheel ed to steering axis inclination, SAI also Vertical Centerline can be considered a tire-wearing angle. True Vertical (0°) Ball Joint (0°) Caster and setback are not tire-wear Centerline angles unless extremely out of specification. All alignment angles are directional control angles, which means they affect steering and vehicle control. Troubleshooting a complaint about vehicle handling, ride or vibration starts with knowing what a particular alignment angle does and how all the angles Front work together. Before you plunge into of Car system diagnosis, take a moment to think about the system operating princi-

BY KEN LAYNE

June 2002 43 To e Toe Change

Zero Toe Tie Rod Too Short Suspension Travel Front of Car Tie Rod End Travel

Toe-In

Outer Tie Rod Suspension End Too High Travel

Toe-Out Tie Rod End Travel

Zero camber—a perfectly vertical affects straight-ahead stability and race cars and some high-performance wheel and tire—causes the least tire steering wheel return. As the vehicle street vehicles have negative camber. wear. Positive camber causes the outer turns, the outside suspension tends to Rear wheels usually have zero cam- tread of the tire to wear more than the rise on the wheel because of positive ber, but some independent rear sus- inner tread; negative camber has the camber. When the wheel returns to pensions are designed with some opposite effect. Engineers design small straight ahead, the vehicle’s weight amount of (usually negative) camber amounts of positive or negative camber presses down on the steering axis and angle. If front camber angles are un- into vehicle suspensions to aid handling helps straighten the wheel. equal side to side, the vehicle pulls to- and steering. Normal camber angles Negative camber resists the tendency ward the side with the greater positive have little visible effect on tire wear, but of the tire to slip sideways during cor- camber. Unequal rear camber also can extreme camber causes noticeably ab- nering. It also can increase steering ef- affect vehicle handling. normal tire wear and shortens tire life. fort. Most cars and light trucks are de- Positive camber, like positive caster, signed with positive camber, but many Toe Toe is how the wheels are aimed, as Toe-Out On Turns viewed from above. A pair of front or rear wheels aimed inward at the forward edges have toe-in; wheels aimed outward have toe-out. The toe angle Centerline of Right Front Wheel for front or rear wheels is measured in fractions of an inch, millimeters or fractions of a degree. Zero toe—wheels aimed straight ahead—causes the least tire wear. Ex- Turning Center treme toe-in or toe-out causes feather- edged wear across the tire tread. Too much toe-in wears the outside tread edges, with feathered edges on the in- Centerline side of each tread row. Too much toe- 20° out has the opposite effect. of Left 18° Front Wheel Front wheels are usually toed in on rear-drive vehicles and toed out on front-drives to compensate for changes Centerline of in the steering linkage and tires when Rear Wheels the vehicle is moving. When the vehi-

44 June 2002 cle is moving, toe decreases (or disap- pears) because the wheels straighten Steering Axis Included Angle out under acceleration and the steer- Inclination (SAI) 0° ing linkage flexes slightly. Included Steering Axis 0° Angle Toe change, or bump steer, occurs Inclination Vertical Camber when a steering tie rod is the wrong Steering length or is installed at the wrong an- Steering Axis Axis Inclination gle. The outer tie-rod end moves up Centerline and down as the suspension compress- es and extends. If tie-rod length or angle is incorrect, it pulls or pushes the steering arm and aims the wheel in a new direction. The driver feels this when the steering wheel jerks to one side as the car goes over a bump or dip. Toe-Out on Turns Toe-out on turns also is called turning radius, or the Ackerman angle. As a vehicle turns, the outside front wheel turns at a lesser angle than the inside wheel. This causes the front tires to toe out during cornering. Thrust Angle the thrust angle, but the rear suspen- Some amount of toe-out on turns is The thrust angle is the angle between sion design may not allow toe correc- necessary because the outside wheel the vehicle’s geometric centerline and tion. If a thrust angle can’t be elimi- must turn on a larger radius than the in- the direction in which the rear nated, align the front wheels to the side wheel. If wheel turning angles wheels are aimed. If the rear wheels thrust line rather than to the vehicle’s were equal, the outside tire would scuff point straight ahead, the thrust line geometric centerline. Aligning the as it tried to turn on a shorter radius. and the geometric centerline are the front wheels to the centerline with Toe-out on turns is designed into a same, and there is no thrust angle. the rear wheels driving along a differ- vehicle’s steering geometry and must When a vehicle is being driven ent thrust line can result in any or all be equal in both right and left direc- straight ahead, the rear wheels steer of the following: a crooked steering tions. Toe-out on turns is not ad- it along the thrust line, so a zero wheel, incorrect front camber and justable. Angles that are unequal from thrust angle is ideal. toe while moving, accelerated tire side to side or out of specification are Adjusting rear toe should correct wear or pull. caused by vehicle damage. Thrust Angle Setback Steering Axis Inclination Right Geometric Thrust Wheel Steering axis inclination (SAI) is the tilt Centerline Line Set of the steering axis from vertical as Back viewed from the front. It’s an angle formed by a line through the centers of the lower and upper ball joints of the strut mount. Like caster, SAI affects steering feel and stability. In a suspen- Measured sion that doesn’t have much caster, in Inches high SAI can provide solid steering Behind feel and stability. Left Wheel SAI plus the camber angle form what’s called the included angle. If cam- Thrust ber is positive, the included angle is Angle greater than SAI; if camber is negative, the included angle is less than SAI. Knowing the SAI, camber and included angle can help you diagnose problems in the steering knuckles and suspension.

June 2002 45 Setback at the bottom of a front or rear rocker ings. Their complaints usually identify Setback is a condition in which one panel or the top of a wheel well. Be- the effects of steering and suspension wheel on an axle is in front of or because vehicle body panels are not the problems rather than the causes. Own- hind the other wheel in relation to the most accurate data points (with the pos- er complaints that relate to wheel chassis. Setback is actually designed in- sibility of collision damage), ride height alignment generally relate to poor han- to some vehicles, such as old Ford is more accurately measured from dling, hard steering, vibration or tire trucks with twin I-beam front axles, points on the suspension or frame. wear. The first step in diagnosing a but incorrect setback most often re- Many trucks are fitted with a variety problem is to determine the category sults from collision damage. Extremely of wheel and tire sizes, suspension op- into which it fits. A careful road test uneven caster also can cause setback at tions and lift kits. Therefore, truck followed by a thorough inspection will the front wheels. manufacturers often specify different get you off to a proper start. caster angles for different ride heights Ride Height on the same basic truck model. A Final Word: Strictly speaking, ride height is not an Check the Tires alignment angle. It can, however, affect Start With the Before you begin your road test, take a other angles, especially caster. Many Customer Complaint close look at the tires. Most cars and manufacturers specify ride height mea- Customers rarely complain about “an light trucks should have four tires of surement positions for alignment ad- incorrect Ackerman angle,” “too much the same size and tread pattern. At justment. Ride height often is measured caster” or even worn control arm bush- least the pair on each axle should be the same size and have approximately Ride Height equal wear patterns. Next, check the tire pressures. Oc- casionally, you’ll find a tire that’s over- inflated, but probably two-thirds of the tires rolling down the highways today are underinflated. Bring all four tires Front Rear to their recommended inflation pressures. Most car owners will be sur- prised by how a simple tire pressure check can solve problems like vehicle wander, brake pull, hard steering and a spongy ride. During your road test evaluation and alignment measurements, remember Measure the basic purposes of alignment angles Distance and how they all work together to main- tain vehicle motion balance.

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Answers to Previous Assessment Quiz (Battery Operation & Testing, Apr. 2002) 1-C 2-C 3-D 4-C 5-B 6-C 7-D 8-B 9-A 10-C

46 June 2002