Let us also understand alignment angles explained well as below. There are the settings in a BMW that matter most - without it, it does not perform as how a BMW should.
Rear Camber
Rear Camber is the inward or outward tilt of the top of the tire/wheel assembly from true vertical. If the top of the tire/wheel assembly is Tilted Inward, it has a Negative Camber. If the top of
the tire/wheel assembly is Tilted Outward, it has a Positive Camber. If the tire/wheel assembly is straight up and down on a true vertical line, the Camber is measured at Zero (0°).
Rear camber is not adjustable on most rear wheel drive vehicles.
These vehicles are built with zero camber setting and are strong enough not to flex or bend under normal load. Most front wheel drive vehicles have a manufacturers specification calling for a slight
amount of rear camber, usually a small amount of negative camber for cornering stability. If the manufacturers specification allows, a setting of 0° to -.5° is preferred for tire wear and ride stability. If rear camber settings change, most vehicles can be adjusted by using an aftermarket type of adjustment, such as shims, cam bolts or bushings.
Rear Toe
Measuring rear toe using Geometric Centerline
Toe-Out (Negative Toe) is a condition where the front of the wheel is farther from the geometric centerline than the rear of the same wheel. Toe-In (Positive Toe) is a condition where the front of the wheel is closer to the geometric centerline than the rear of the same wheel.
Measuring rear toe using Total Toe
Toe-Out (Negative Toe) is a condition where the distance between the front of both wheels on a common axle are farther apart than the rear of the same wheels. Toe-In (Positive Toe) is a condition where the distance of the front of both wheels on a common axle are closer together than the rear of the same wheels.
Toe can be expressed in degrees, fractions or decimal inches. Do Not confuse degrees with inches when selecting your method of adjustment. Rear toe adjustment is the most critical factor regarding tire wear, mileage, and handling.
Geometric Center Line
The Geometric Center Line of the vehicle is established by connecting a line between the theoretical midpoint of the front spindles and the theoretical midpoint of the rear axle.
Rear Setback
Rear setback is a measurement referencing the rear wheels to an imaginary line perpendicular to the geometric centerline of the vehicle, and is measured as an angle. If a vehicle has rear setback, one rear tire/wheel assembly is further back from this imaginary line than the other.
Some causes of rear setback may be from frame, chassis, and rear cradle mis-alignment due to collision. If the vehicle has a setback condition, the vehicle may pull to the opposite side of the setback.
Thrust Line
Thrust line is determined by bisecting the rear total toe. To bisect the rear total toe, lines that are parallel to the tire/wheel assembly are drawn until they intersect. Another line that starts where the geometric centerline and rear axle intersect, is drawn to the intersection of the tire/wheel lines. This line is the Thrust Line. When you have a thrust line to the left it is considered Negative and when it is to the right it is considered Positive.
Thrust Angle
Thrust angle is the angle created between the geometric centerline and the newly created Thrust Line.
Front Camber
Camber is the inward or outward tilt of the tire/wheel assembly. This angle is measured from a true vertical line, i.e. perpendicular to the ground. A tire/wheel assembly that is tilted outward at the top is considered to have Positive camber. While a tire/wheel assembly tilted inward at the top, displays Negative camber. For a zero setting, the tire/wheel assembly is in the exact vertical position or perpendicular to the ground. To rephrase, if the top of the tire/wheel assembly is tilted inward towards the engine, it has a negative camber. If the top of the tire/wheel assembly is tilted outward from the engine, the camber is positive.
This is Negative Camber
This is Zero Camber
This is Positive Camber
Effects of Positive Camber
Slight positive camber results in a dynamic loading that allows the tire to run relatively flat against the road surface. Positive camber also directs the weight and shock load of the vehicle on the larger inner wheel bearing and inboard portion of the spindle rather than the outboard bearing. Positive camber in moderation results in longer bearing life, less likely sudden load failure, and as a side benefit, easier steering. Excessive positive camber wears the outside of the tire and can cause wear to suspension parts such as wheel bearings and spindles.
Effects of Negative Camber
Variations in negative camber can be used to improve the handling of a vehicle. A setting of 1/2° negative on both sides will improve cornering without affecting tire life greatly. This negative setting compensates for the slight positive camber change of the outside tire due to vehicle roll, thereby allowing a flatter tire contact patch during cornering. Excessive negative camber wears the inside of the tire and similar to positive camber, it can cause wear and stress on suspension parts.
Road Crown and Camber
A crowned road means that the outside/right hand side of the lane is lower than the left side of the lane. This improves the drainage of the road but adversely affects vehicle handling. Road crown must be compensated for in alignment settings because a vehicle driving on a crowned road leans to the right, causing some weight transfer to the right, and the camber changes slightly more positive. This combination creates a pull or drift to the right. Most alignment technicians adjust the vehicle with a slightly more positive camber, usually 1/4°, on the left to compensate for the road crown. This slightly more positive camber will not cause a noticeable pull when driving on a flat road. However, if camber is unequal from side to side with a difference greater than 1/2°, the vehicle will pull to the side with the most positive camber. If the specifications allow, 0° to ±.5° is usually best for tire life and vehicle handling.
Causes of Camber Changes
Always consult a ride height specification book prior to beginning alignment. If out of specification, attempt to correct. Changes in ride height from factory specifications affect camber. As a vehicle ages, the suspension has a tendency to sag. The weight of the vehicle can cause springs to weaken. Springs can also be damaged by excessive vehicle loading or abuse. Another factor to consider is sagging of center support or crossmember. Modifications to the vehicle such as raising or lowering the suspension or changing the total weight of the vehicle can also affect camber.
Front Cone Effect
When a tire/wheel assembly is tilted it creates a condition called the "cone effect".This angling of the tire/wheel assembly creates an imaginary cone that rotates in the direction the wheel is angled. The apex of the cone is created by the intersection of two lines: 1) the ground and 2) a line projected through the centerline of the spindle to the ground. The shape of the cone is then defined by the third line from the top of the tire to the intersection of 1 and 2. The wheel attempts to pivot around this intersection point. A positive cambered tire/wheel assembly will roll away from the center of the vehicle. Conversely, a negative cambered tire/wheel assembly will roll towards the center of the vehicle. If the vehicle is traveling on a flat, level road and the amount of camber offset is the same on both front wheels, the cone effects, although opposite, will offset each other and the vehicle will travel in a straight line. A maximum side to side variation of ± .5° is recommended.
Front Caster
Caster Definition
Caster can be defined as the forward or rearward tilt of the projected steering axis from true vertical, as viewed from the side. This line is formed by extending a line through the upper and lower steering knuckle pivot points. For vehicles with front control arms, visualize the line extending through the upper and lower ball joints. On strut equipped vehicles, the line extends through the lower ball joint to the center of the upper strut mount. Caster is always viewed from the side of the vehicle.When the upper pivot point is rearward of the lower pivot point, caster is positive. If the upper pivot is forward of the lower pivot point, caster is negative. When the two points are straight up and down from each other, the caster is zero. A maximum side to side variation of ±.5° is recommended on most vehicles. Caster is NOT a normal tire wearing angle and is used as a directional control for stability and steering returnability.
Front of Vehicle Caster Effect
Caster effect is necessary so that the load of the vehicle is "carried" through the steering axis line formed on the upper and lower pivot points. Positive caster gives a vehicle directional stability because the tire is being pulled along by the load which is projected in front of the center of the tire contact area. This causes a vehicle with positive caster (point of load ahead of the point of contact) to be harder to steer away from the straight ahead position. With Positive caster, road surface variations have a minimal effect on the tire, the tire will continue to go straight. When a tire has a Negative caster condition, where the projected steering axis point of load is behind the tire point of contact, a vehicle will have a tendency to be easier to steer but will lack directional stability. A vehicle with negative caster is affected by any road surface variation such as small road irregularities or bumps. With the point of load pushing the tire along (negative caster), any bumps or road irregularities which are encountered have a tendency to immediately affect directional stability and vehicle handling.
Front Caster Effects
Effects of Positive Caster
Vehicles usually have some positive caster specified since this promotes directional stability, however, excessive positive caster can cause two problems. The first is that excessive caster will cause a high level of road shock to be transmitted to the driver when the vehicle hits a bump, etc. The second problem is that a tire/wheel assembly with positive caster has a tendency to toe inward when the vehicle is being driven. If one side has more positive caster than the other, this causes it to toe inward with more force than the other side. This will cause a lead or pull to the side with least amount of positive caster.
Effects of Caster on Tire Wear
When set with a substantial amount of caster, the spindle travels in a vertical arc, causing it to move up and down and raise and lower the wheels as the steering wheel is turned. Because of this, camber changes occur. With a high amount of positive caster, the camber changes that occur, especially at low speeds in tight turns, 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 amount of camber change that takes place when a spindle travels through its arc of travel, the shoulders of the tires on a vehicle may scrub and wear. When a left turn is made at a fairly high rate of speed with a vehicle which has positive caster, the caster of the left front wheel changes toward positive but the momentum of the vehicle is in a straight ahead direction. This causes the inside of the left front tire to scrub as it is turned. Just the opposite effect takes place on the right wheel as the vehicle is turned left at high speed. The right front wheel's camber will go negative but the outside edge of the tire is scrubbed because of the vehicle's momentum to go straight. On some vehicles setting caster more than +2.5° will cause scrub problems.
Front Toe
Toe Definition
Toe relates to the difference in the distance between the front of the tires and the rear of the tires on the same axle, or to the vehicle centerline. Toe-in, or positive toe, is defined as the front of the tires being closer together than the rear of the tires. Toe-out, or negative toe, is when the rear of the tires are closer together than the front of the tires. Zero toe is when the tires are parallel to each other.
Measuring Toe
Toe on an individual tire/wheel assembly is understood to be the difference between the distance of the front and rear of one tire in reference to the vehicle centerline. Since most alignment specifications show toe as total toe, i.e. both wheels, it is important to understand two points: (1) 1/2 of the specified total toe should be applied to each front wheel. (2) a minus(-) sign would actually indicate a toe-out setting as being specified. It is important to note that although toe has historically been measured as a distance in fractions of an inch, and then decimal inches, it is becoming more common for vehicle manufacturers to express toe in degrees. The idea is that the angle, rather than an arbitrary distance, determines the side slip of the tire and the resulting scrub of the tread. This should not be affected by the tire size, but rather should be constant for a given measurement. Most alignment equipment displays toe-out as a minus (-) and toe-in as a positive (+).
Effects of Toe
Excessive toe increases tire scuffing and results in tire wear and drag on the vehicle. Excessive toe-in, or positive toe, increases scuffing on the outside of the tire.Excessive toe-out, or negative toe, increases scuffing on the inside of the tire, and in some cases can cause a darting or wandering problem. Bias or bias-belted tires will commonly show a featheredge or saw-tooth toe wear pattern across the entire tire tread area. Any tire wear pattern caused by a toe condition can be further affected by an excess camber condition and may result in irregular wear patterns.
Toe Out On Turns
When a vehicle is turned, the inner front wheel must toe-out more than the outer wheel. The inner wheel must turn this tighter radius to avoid scrubbing. This is also known as the Ackerman effect. Viewing the vehicle from the top as it is turning, the front wheels should turn on two different radii.
Toe Out On Turns, also known as TOOT, is built into the front steering arms and is not adjustable. Before checking toe out on turns, make sure that all alignment settings are within manufacturers specifications. If using degree marks on the turn tables, make sure that the tire/wheel assembly is centered on the tables, this will reduce erroneous readings. To check toe out on turns, steer the wheels to the left so that the inner wheel is at 20°, the out wheel should be less than 20°, optimal reading is 18°. Repeat the test in the other direction, and determine if there are any problems be comparing the manufacturers specifications. A variation from specifications indicates damaged steering arms and one or both arms should be replaced.
Front Setback
Front setback is a measurement referencing the front wheels to a line placed perpendicular to the vehicle centerline. This line would be parallel to a line drawn through the centers of the spindle. If a vehicle has setback, one front tire/wheel assembly is farther back from this imaginary reference line across the front of the vehicle than the other.
Positive setback indicates that the right front wheel is setback further than the left. Negative setback refers to the left front wheel being further back than the right. Front setback can be checked during a normal alignment, and is used to diagnose collision damage or cradle mis-adjustment. If the cradle is adjusted incorrectly, or damage is present, it is not unusual to also see a reduced positive caster reading on the side with the setback condition. Excessive setback can cause an alignment pull to the side with the setback. If the rear axle is positioned correctly and all other parts and systems of the vehicle are in good working order, a setback condition will also create different wheelbase measurement side to side.
Steering Axis Inclination (SAI)
SAI Definition
The angle between the centerline of the steering axis and vertical line from center contact area of the tire (as viewed from the front). SAI is typically not adjustable, but deviations from specification can indicate vehicle damage. A maximum variation side to side of ± 1.0° may also indicate vehicle damage. This topic is covered in detailed charts later.
Effects of SAI
SAI urges the wheels to a straight ahead position after a turn. By inclining the steering axis inward, it causes the spindle to rise and fall as the wheels are turned in one direction or the other. Because the tire cannot be forced into the ground as the spindle travels in an arc, the tire/wheel assembly raises the suspension and thus causes the tire/wheel assembly to seek the low (center) return point when it is allowed to return. Thus, since it has a tendency to maintain or seek a straight ahead position, less positive caster is needed to maintain directional stability. A vehicle provides stable handling without any of the drawbacks of high positive caster because of SAI.
Included Angle (IA)
I/A Definition
Included Angle is the combination of SAI and camber. Viewed from the front, the included angle is SAI plus camber if the camber is positive. If the camber is negative the included angle is SAI minus camber. If a side to side variation greater than ± 1.5° exists, check for vehicle damage.
Angle + Camber = Included Angle (I/A)
Measuring Procedures
SAI should always be measured after you have adjusted the camber and caster to the proper specifications or as close to the specifications as possible. Check for worn suspension parts. SAI is best measured with the front wheels off the ground, brakes applied and alignment equipment leveled and locked. Raise the vehicle underneath the lower control arms but, do not relax the suspension. Not raising the vehicle from the turntables can cause the control arm bushings to move when wheels are turned, resulting in an inaccurate reading. Always refer to the manufacturers SAI specifications and measuring procedures. If the vehicle has a solid front axle, the measurement can be taken with the wheels on the turntables.
Scrub Radius
Scrub Radius Definition
Scrub radius is the distance at the road surface between the tire line and the SAI line extended downward through the steering axis. The line through the steering axis creates a pivot point around which the tire turns. If these lines intersect at the road surface, a zero scrub radius would be present. When the intersection is below the surface of the road, this is positive scrub radius. Conversely, when the lines intersect above the road, negative scrub radius is present. The point where the steering axis (sai) line contacts the road is the fulcrum pivot point on which the tire is turned.
Squirm
Squirm occurs when the scrub radius is at zero. When the pivot point is in the exact center of the tire footprint, this causes scrubbing action in opposite directions when the wheels are turned. Tire wear and some instability in corners is the result.
Applications in Suspensions
MacPherson strut equipped vehicles usually have a negative scrub radius. Even though scrub radius in itself is not directly adjustable, it will be changed if the upper steering axis point or spindle angle is changed when adjusting camber. This is the case on a MacPherson strut which has the camber adjustment at the steering knuckle. Because camber is usually kept within 1/4° side to side, the resulting scrub radius difference is negligible. Negative scrub radius decreases torque steer and improves stability in the event of brake failure. SLA suspensions usually have a positive scrub radius. With this suspension, the scrub radius is not adjustable. The greater the scrub radius (positive or negative), the greater the steering effort and the more road shock and pivot binding that takes place. When the vehicle has been modified with offset wheels, larger tires, height adjustments and side to side camber differences, the scrub radius will be changed and the handling and stability of the vehicle will be affected.
Ball Joint Inspection
Test ball joints for radial (horizontal) play by grasping the tire at the top and bottom and attempting to move the ball joint in and out. Also, test load carrying ball joint for axial play (vertical) by lifting with an appropriate tool (pry bar). If any loose or bent ball joints are detected, they should be replaced. Do Not mistake wheel bearing play for ball joint wear.
For suspensions with coil springs resting on the lower control arm lift as shown below.
This method will properly unload ball joint for inspection.
For suspensions with coil springs resting on the upper control arm, lift as illustrated at right. This will properly unload the ball joint for inspection.
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