BASIC WHEEL ALIGNMENT “You have got to know all the angles.”

Correct wheel alignment plays a huge part in a customer’s positive driving experience. Having it dialed in correctly is essential to proper vehicle handling, safety, maximum fuel economy and tire life. Webster’s dictionary defines the term “alignment” as “the proper positioning or state of adjustment of parts (as of a mechanical or electronic device) in relation to each other.” Correct alignment of a vehicle’s suspension means positioning the wheels to allow the vehicle to track smoothly. If a vehicle’s tires are rolling freely and evenly without pulling or scuffing down the road, it will help the vehicle to drive straight while minimizing steering effort. Definitely the way to go! With today’s fully computerized alignment machines, knowing all the angles of alignment is not necessary, right? Well, as it is with many mechanical things, it helps to know what is going on. So for the technician who is new to wheel alignments or for the one who wants to refresh CASTER his or her basic knowledge, this article will concentrate Caster is defined as the forward or rearward tilt of the projected on the eight different fundamental angles that fall under steering axis from true vertical as viewed from the side. Caster the heading of wheel alignment. Some of the angles we is sometimes called a directional control angle because it can list are a part of the chassis/suspension design known determine whether the vehicle travels straight or if it drifts (vehicle as fixed angles and would only be off-specification takes a lead to a direction other than straight) or pulls (steering if something were bent or broken. Other angles are wheel tries to turn) to the right or the left. However, as a rule, adjustable (depending on the vehicle’s design). So, let us caster will not cause tires to wear. Unless the chassis/suspension round up the usual (and not-so-usual) suspects when is radically out-of specification, it will not affect tire life. wheels are pointed in the wrong direction.

1 Caster is the measurement of the forward or backward tilt Negative caster tilts the top of the steering knuckle toward Since a chassis will always drift to the side that has the least of the steering knuckle (spindle support) arm when viewed the front of the vehicle. A great example of this is a shopping caster, you would think that caster, whatever the specified from the side of the vehicle. The caster setting controls cart’s front casters. The caster pivot point carries the load, but range, should always be set at exactly the same specification where the tires touch the road in relationship to an imaginary when the cart is pushed, the axle wants to swivel to behind on both sides of the chassis, right? Not necessarily so! Cross centerline drawn through the spindle support. Caster is the pivot point. Having the axle behind the load makes it easier caster can be used to offset the effect of road crown. Road measured in degrees (positive or negative), starting at the to steer the wheels, but the wheels will tend to swivel and crown is the normal slope of the road toward the outer edge true vertical (plumb line) of the steering knuckle (spindle follow the irregular surfaces and imperfections in the road (like of the road surface. Most road surfaces angle downward support) arm. on a vehicle chassis). from the center of the road to divert rainwater (sometimes to the right, depending on the area, and some areas more Positive, Negative & Cross Caster than others do). A chassis set at zero cross caster may drive straight down a flat road but tend to drift down a road crown. A positive caster reading means the top of the steering However, kick in a slight amount of extra negative caster on knuckle is tilted toward the rear of the vehicle. Great the opposite side of the drift and the drift is canceled out. This examples of this are bicycle (or motorcycle) forks, which is a pretty cool trick for some of you front-end experts out sometimes have radically positive caster. The bike frame’s there. Remember to always stay within the adjustment range load is carried by the fork steering knuckle, but the wheel specified for the chassis you are working on so that the vehicle axle is way out ahead. Vehicles usually have some positive behaves as designed for common road profiles. caster specified since this promotes directional stability. That angle shifts the actual load out ahead of the axle, increasing stability. It also helps with positive steering wheel return. With a high amount of positive caster, the camber changes that occur, especially at low speeds in tight turns, will cause the tires to show wear on their shoulders. In high speed cornering, the vehicle tends to continue straight ahead when the steering is initially turned. Due to this and the Important Note: amount of camber change that takes place when a spindle A vehicle will always lead or drift to the side that has travels through its arc of travel, the shoulders of the tires the least caster, side to side. This left-to-right caster on a vehicle may scrub and wear. This can result in vehicle relationship is called cross caster. wander or lack of directional stability. Not desirable traits!

2 CAMBER TOE Toe is one angle that tends to be generalized in a one- Camber is the inward or outward tilt of a wheel when Toe is definitely the easiest of all to visualize. If a pair of size-fits-all mentality. But, some chassis, like our 370Z, viewed from the front of the car. A wheel tilting outward wheels on the same axle are closer together at the front are very sensitive to having exactly the right settings. Do at the top has positive camber, and a wheel tilting inward than at the rear, they are toed-in. If they are closer at the not assume anything and always check the applicable at the top has negative camber. Camber is a tire-wearing back than at the front, they are toed-out. If the wheels Nissan or Infiniti Electronic Service Manual (ESM) for the angle, appearing as smooth (not feathered) wear. If there are exactly parallel, they are at zero toe. It is the position of exact specifications. is too much positive camber, the wear will be on the tire’s the wheel in relation to the centerline of the chassis when outer edge. If there is too much negative camber, it will viewed from the top. The amount of toe can be expressed be on the inside edge. Negative and positive camber in linear measurements (fractions of an inch or millimeters) or are measured from the true vertical (plumb line) and are degrees as the angle to which the wheels are not parallel. measured in degrees. If the wheels are aligned at true Toe is an angle that can really tear up tires. Excessive toe vertical, camber is zero (neutral). Camber can also be a results in tire wear and drag on the vehicle. If the toe setting directional control angle if there is too much of it. A chassis is incorrect, the tires scuff or skid sideways as the vehicle is will lead or drift to the side that has the most positive moving ahead. Excessive toe wear is sometimes described camber. A crowned road means that the outside/right as feathering since the wear pattern is a serrated (not hand side of the lane is lower than the left side of the lane. smooth) wear across the face of the tire. Slight feathering However, camber should never be adjusted to compensate can be felt by hand when sliding the palm across the tread for road crown since uneven and excessive tire wear will side to side. It feels rough one way and smooth the other. result if too much camber (positive or negative) is set.

Caster, camber and toe are definitely the top three alignable adjustment angles you need to understand in order to get a chassis pointed straight. But not to be left out, there are other factors in the alignment equation. The following measurements are almost never adjustable.

Note: Only 3 millimeters of improper toe cause Note: Vehicles with wide tires are very sensitive to tire wear equal to dragging a tire sideways the camber being off. 28 feet for every mile traveled.

3 STEERING AXIS INCLINATION The spindle (or axle) is horizontal with the wheels pointed INCLUDED ANGLE (and scrub radius) straight ahead. With this inboard tilt designed into the Included Angle is simply the combination of SAI plus Steering axis inclination (SAI) is the angle measured steering knuckle or strut, as the steering turns, the axle/ camber. For example, on a chassis with 10º of SAI and 1º between true vertical and the inward tilt of the steering spindle will try to move closer to the ground. Because the of positive camber, the included angle is 11º. Alone, this knuckle, king pin or McPherson strut tube. SAI is typically spindle cannot move any closer due to the tire, the car angle does not mean much. But back in the day, some not adjustable, but any deviation from specification can must lift. The effect of SAI, coupled with the vehicle’s own alignment machines measured included angle, not SAI. To indicate vehicle damage. It is designed into the suspension weight, helps re-center the steering from turns. SAI also get SAI, you had to do the math, deducting camber from system, so if the angle is not correct, it is necessary to helps reduce steering effort by reducing the scrub radius. the included angle. diagnose and possibly replace bent parts, like spindles or Scrub radius is a factor of steering axis inclination. It struts, to correct the situation. SAI is always an inward tilt is the distance (at the road surface) between the true regardless if the wheel has positive or negative camber. vertical at the center of the tire tread and the steering axis pivot centerline. The intersection of the steering axis and centerline pivot point is usually slightly below the road. When the intersection is below the surface of the road, this is positive scrub radius. The distance below the surface of the road determines the amount of scrub radius. Conversely, when the lines intersect above the road, negative scrub radius is present. When a vehicle has been changed into lower height tires and wheels with increased positive offset, it causes a big increase in scrub radius. This may result in harder steering and a tendency for the wheels to wander or shimmy.

TURNING RADIUS/TOE-OUT ON TURNS Toe-out on turns, also called turning radius, is the SAI has three functions: amount the front wheels toe-out when turning corners. As the vehicle goes around a turn, the inside tire must • Helps the vehicle return to the straight position turn more sharply and in a smaller radius (circle) than the after a turn. outside tire to maintain good traction and low tire wear. Steering systems are designed to allow the vehicle’s wheels • Helps the vehicle track straight ahead. to move through different arcs of travel and turn different • Gives the vehicle good directional stability with less amounts. Turning radius/toe-out is not an adjustable angle; positive caster designed in. it is controlled by the built-in angle of the steering arms. If the turning radius is incorrect, it indicates damaged or bent steering parts.

4 TRACKING SETBACK PRELIMINARY CHECKS (thrust line or thrust angle) Setback is just as it sounds; the amount by which one Proper pre-flight checks are critical for airplanes. And to Incorrect tracking (or excessive thrust angle) front wheel is further back from the front of the vehicle than keep your customers “flying straight,” it is essential, before happens when the centerline of the rear tires is not parallel to the other. Setback should usually be around zero, but beginning a wheel alignment, to check the following: some vehicles have asymmetrical suspensions by design. the chassis centerline. This can also be caused when the rear 1. Tires for wear or improper air pressure. wheel centerlines are not equal side to side. Have you ever Positive setback indicates that the right front wheel is seen a car going down the road and the rear tires are not setback further than the left. Negative setback refers to the 2. Road wheels for runout. tracking in line with the fronts? That is too much thrust angle. left front wheel being further back than the right. 3. Wheel bearing axial end play. As you can imagine, zero thrust angle is ideal for the best fuel Setback is measured with both wheels straight ahead economy and tire wear. A common culprit here is incorrect rear and is used as a diagnostic angle along with caster 4. Ball joint axial end play of compression rod, upper link toe settings, but bent parts can also be the cause. to identify chassis misalignment or a larger problem and steering knuckle. that was missed when measuring a collision-damaged 5. Shock absorber operation. vehicle. The presence of setback can also cause differences in toe-out on turn angle readings side to side. 6. Each mounting part of the axle and suspension for A setback condition can possibly occur if the vehicle hit a looseness and deformation. pothole or curb or was involved in a collision that affected the vehicle’s undercarriage. 7. Each link, rod and member for cracks, deformation and other damage. 8. Vehicle posture (height, rake and side to side).

Note: Always measure wheel alignment under unladen conditions. This means everything in place but the driver. The vehicle’s fuel, engine coolant and all lubricants are full. If the thrust angle is not zero, it can cause the vehicle to In addition, the spare tire, jack tools and handle differently when turning as the driver compensates floor mats should be installed in their for the thrust angle or tries to drive straight down the road. designated locations. CLEAR-VISION Clear-vision has become a generic term describing if Remember, our customer’s appreciation of his or her the steering wheel is properly centered or leveled during vehicle’s drivability is greatly affected by its wheel alignment! alignment adjustment. Clear-vision can be controlled from So, if a customer happens to complain about the extra time the front toe settings. A good practice is to set the front it takes to “get it right,” help them to be patient by reassuring toe with the vehicle running so that the power steering them and communicating that correct wheel alignment will system is pressurized. Make sure the steering wheel is not improve his or her driving satisfaction. Keeping his or her crooked and is adjusted to the straight ahead position to be wheels aligned can help prevent tire wear, increase his or certain there will not be a spoke blocking the driver’s view. her fuel mileage by reducing road friction and improve the Then adjust the left and right tie rods so that the total toe is vehicle’s handling. equally divided between both sides.

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