Chapter 25 Wheel Alignment Objectives Define the term

Chapter 25 Wheel Alignment

Objectives • Define the term “wheel alignment” • Inspect tires, steering, and suspension systems before alignment • Check and adjust caster, camber, and toe • Summarize alignment equipment variations

Introduction • Wheel alignment involves positioning the vehicle's tires so they roll properly • Essential to safety, fuel economy and tire life • A vehicle may require an alignment if: – Damage to any steering or suspension parts or mounting locations – Engine cradle damage or position change or suspension or steering parts removed – Damage to major structural components

Wheel Alignment Basics • Caster is the angle of the steering axis of a wheel from true vertical, and has little effect on tire wear • Positive caster tilts the tops of the steering knuckles toward the rear of the vehicle • Negative caster tilts the tops of the steering knuckles toward the front of the vehicle • Unequal caster will cause the vehicle to steer toward the side with less caster • Most common problem affecting caster is worn strut rod and control arm bushings

Figure 25 -2. Note the difference between positive and negative caster. Caster affects whether the car’s steering wheel pulls to the left or right while driving.

Camber • Camber is the angle represented by the tilt of the wheels inward or outward from the centerline • Positive camber has the top of the wheel tilted out, when viewed from the front • Negative camber has the top of the wheel tilted inward when viewed from the front • Camber is controlled by the control arms and their pivots • Camber is affected by worn or loose ball joints, control arm bushings, and wheel bearings

Figure 25 -3. Camber is the tilt of the wheel in or out from the centerline when viewed from the front of the vehicle. Check with vehicle manufacture specs for camber settings. Camber affects whether the whole tire tread or just the edges of the tread wears.

Toe • Toe is the difference in the distance between the front and rear of the left- and right-hand wheels • Toe is critical to tire wear • Toe-in: front of the wheels are set closer than the rear • Toe-out: front of the wheels farther apart than the rear • Rear-wheel-drive vehicles are often adjusted to have toe-in, front-wheel-drive vehicles are adjusted to have a slight toe-out

Steering Axis Inclination • Steering Axis Inclination (SAI) is the angle between true vertical and the line through the upper and lower pivot points • SAI can be used in structural measurements to diagnose: – Strut lower misalignment – Shifted engine cradle or crossmember – Control arm mounting location damage – Misalignment frame or body structure

Turning Radius • Turning radius is the different amount each wheel moves during a turn • Turning radius allows the inside wheel to turn a few degrees more than the outside wheel • Tracking is the parallel alignment of the rear wheels • Rear-wheel tracks should be an equal distance from the front-wheel tracks • Geometric centerline of the vehicle should parallel the road direction

Thrust Angle • Thrust angle is the angle between the thrust line and the vehicle centerline • Positive thrust angle results if the thrust line projects to the right of the centerline • Negative thrust angle results if the thrust line projects to the left of the centerline • Included angle is the sum of both camber and SAI angles

Wheelbase • Wheelbase is the measurement between the center of the front and rear wheel hubs • Wheelbase is measured to determine the forward and rearward position of each wheel

Set-Back • Set-back is when one wheel is moved back – May be caused by an impact to a front wheel assembly that moves a lower control arm, engine cradle, or radius rod backwards – May be designed into some suspension systems

Prealignment Checks • Visually inspect everything while the vehicle sits on the shop floor – Reading tires involves inspecting the tire tread wear and diagnosing the cause • Measure the ride height from the shop floor to specific points on the vehicle • Vehicle load affects the weight on the front suspension and affects alignment angles • Load in the trunk takes weight off the front and changes caster and camber angles

Prealignment Checks (continued) • Inspect the underside of the vehicle with the vehicle raised • Damaged parts must be repaired before adjusting alignment angles • Road test the vehicle – check that the steering wheel is straight • Feel for vibration in the steering wheel and floor of seats • Notice any pulling or handling problems, and make sure the steering wheel is centered

Diagnostic Checks • First diagnostic check should be a visual inspection of the entire vehicle • Jounce-rebound check – Jounce is the motion caused by a wheel going over a bump and compressing the spring – Rebound is the motion caused by a wheel going into a dip and extending the spring • Jounce-rebound check determines if there is misalignment in the rack-and-pinion gear

Figure 25 -9. Bent wheels are a common problem in collision repair. A dial indicator will let you accurately check axial and radial runout of the wheels.

Wheel Alignment Procedures • The order of adjustment – caster, camber, and then toe – is recommended for all vehicles • Refer to the manufacturer’s service manual for details • Mount the alignment equipment, following the equipment manufacturer's instructions • Check steering and axis inclination and camber, caster, turning radius and toe • Document your readings and adjustment values

Alignment Equipment Variations • Most modern wheel alignment equipment is computer controlled – Greatly simplifies the wheel alignment process • Approximately 80 percent of today's vehicles require front- and rear-wheel alignment • Collision repair usually requires at least a fourwheel alignment check

Road Test After Alignment • After making wheel alignment adjustments, road test the vehicle • Make sure the vehicle does not pull to one side of the road, vibrate, or exhibit other troubles

Summary • Wheel alignment involves positioning the vehicle's tires so they roll properly over the road – Essential to safety, handling, fuel economy and tire life • Caster is the angle of the steering axis of a wheel from true vertical as viewed from the side • Camber is the angle presented by the tilt of the wheels when viewed from the front of the vehicle

Summary (continued) • Camber assures that all of the tire tread contacts the road surface • Camber is usually the second angle adjusted during wheel alignment • Toe is the difference in the distance between the front and rear of the left- and right-hand wheels • Toe is critical to tire wear