Camber, caster, toe, anti-roll bars, tires, springs......

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Alignment Angles Terminology

Caster Angle

Caster is the angle between an imaginary line drawn through the upper and lower steering pivots and a line perpendicular to the road surface (viewed from side of vehicle). If the top of the line tilts rearward, the vehicle is said to have "POSITIVE" caster. If the top of the line tilts forward, the vehicle is said to have "NEGATIVE" caster.




Positive caster can also be defined as when the spindle is tipped so that the pivot support centerline intersects the road surface at a point in front of the initial tire contact. Negative caster would then be the center line intersection to the road surface behind the initial tire contact.

Most vehicles produced today do not have adjustable caster angle. Many early model vehicles have adjustable caster in which road crown is compensated for (along with camber). By setting the caster angle on the driver's side 1/2 degree less than the passenger side for positive caster specifications or 1/2 degree more for negative caster specifications, the road crown should not cause vehicle pull in either direction.

Vehicles equipped with manual steering use very little positive or negative caster. This helps reduce the steering effort at the steering wheel. The advantage of caster adjusted toward negative is greater maneuverability; however, direction stability on open road driving is reduced. The advantage of positive caster is the strong directional stability and the ease of returning the steering to a straight-ahead position.

Caster will not cause tire wear unless extreme misadjustment or worn parts are involved. Always set caster (if adjustable) to specifications and within 1/2 degree from side to side. Keep road crown in mind and adjust as necessary if a pull is present after a proper alignment has been completed.

Front Camber Angle

The camber angle will affect the wear on the inner or outer edge of the tire. Camber is the inclination of the centerline of the wheel from the vertical as viewed from the front of the vehicle. Camber angle is measured in positive or negative degrees. Positive camber is the outward tilt of the top of the tire. Negative camber is the inward tilt of the tire at the top. If a tire was absolutely vertical, the degree of camber would be zero.



Unlike the caster angle, camber will change with vehicle load and ride height. With the weight of the driver in the vehicle, front left camber will increase and front right camber will decrease. As rough road conditions are encountered, the downward thrust of the vehicle body will cause front camber to go negative. As the vehicle body movement returns upward, front camber will go positive. As camber oscillates, toe adjustment will also change with each movement of the control arm.

A tire with positive camber can influence the vehicle with a directional pull. The vehicle will go towards the side that has the tire with the most positive camber. It is the normal tendency of the tire to roll around the center of a circle when the top of the tire is inclined towards the center of that circle.

Positive camber tends to place the tire-to-road contact area nearer the point of load. This assists in easier steering and forces the thicker inner portion of the spindle to carry most of the load. Modern suspension design has reduced the need for considerable positive camber. Many manufacturers specify a slight amount of negative camber.

Some manufacturers recommend an additional 1/4 to 1/2 degree positive camber on the left wheel to compensate for road crown. The car will then pull toward the side with greater positive camber. This will offset the pull effect of the road crown. Always set camber within specifications.

Rear Camber Angle - Front Wheel Drive

Rear wheel camber angle is being relied on for improved steering and general handling performance. In the past FWD vehicles and independent rear suspension vehicles were most likely to have adjustable rear camber. On vehicles currently being produced, rear camber adjustment capabilities are being found on all types of models.

Note: Always use full-floating tables under wheels whenever alignment is being done.

When alignment problems are reported on vehicles with fixed rear axles and no rear wheel camber adjustment capabilities, a thorough inspection of the rear suspension should be made. Damaged or worn components can cause alignment and/or steering problems. Replacing or repairing the defective components should bring the rear wheel assemblies into specification.

On vehicles where rear wheel camber is adjustable, all previous precautions apply. If camber adjustment requirements are excessive, a thorough inspection must be performed. Replacing any defective components could bring the camber into specification and adjustment may become unnecessary. As with the front suspension, DO NOT perform alignment on vehicles with damaged or worn components.

Whether the rear suspension is adjustable or not, if all components are in good condition and the proper specifications cannot be obtained, aftermarket correction kits may be needed.

Rear Camber Angle - Rear Wheel Drive

On RWD vehicles, where rear camber is usually not adjustable, camber will normally be fixed at zero. Even though this angle cannot be changed through adjustment, if rear suspension abnormalities exist, a thorough inspection must be made.

Not to be overlooked are the rear springs. Worn or weak rear springs will alter riding height and because of a reduction in tension, will bring the shock absorbers out of the optimum range of their dampening ability. The result will be excessive tire movement. This condition reduces operator control and contributes to abnormal tire wear. As in FWD vehicles, replacing worn or defective components may bring rear wheels within specification.

Toe-In and Toe-Out

Unlike caster and camber, which are measured in degrees, toe is most frequently measured in fractional inches, millimeters or decimal degrees. The system of measurement selected will depend on the type of equipment available. An incorrect toe setting is one of the main alignment factors that cause excessive tire wear. Front and rear toe are the same in definition, with the adjustment capabilities and procedures being the only actual difference.




Toe is the difference between the leading edge (or front) and trailing edge (or rear) of the tires. Toe-in is the measurement in fractions of an inch, millimeters or decimal of degrees that the tires are closer together in the front than they are in the back. Toe-out is the same measurement, except the tires are further apart in the front than in the rear.

Some manufacturers measure the angular change from straight-ahead in degrees. Slight toe-in is preferred to toe-out on most vehicles because steering is aligned while the vehicle is stationary. When the vehicle is moving, linkage components flex causing a change in alignment angles. This is classified as "Running Toe." Running toe should be zero to maximize tire life and achieve the least rolling resistance.

The usual tendency is for the tires to turn outward while the vehicle is in motion, so most vehicles are designed with a static toe-in setting. The static toe-in setting will become zero as the linkage flexes when the vehicle is in motion. Always set toe to the manufacturer's specifications.

On vehicles with toe adjustment capability on the rear, an alignment specialist can go beyond manufacturer's specifications according to vehicle usage and customer requirements. With the proper equipment, the rear axle can be adjusted to perform aggressively toward demanding load and road conditions. Vehicles with FWD and independent rear suspensions are more likely to have adjustable rear toe. As with rear camber, properly adjusted rear toe will contribute to improved steering and handling characteristics. Full floating tables must be used under the rear tires whenever toe is to be adjusted.

If rear toe is out of specification a thorough inspection must be done, whether or not rear toe is adjustable. Components found to be defective must be replaced. On vehicles that do not have rear toe adjustment capability and toe is not within specifications, replacing defective components may bring toe within specifications.

Toe-Out on Turns

When a vehicle enters a turn, the outer tire must travel a greater distance than the inner tire. The tire center is tangent to the turn circle. If the tires were to remain parallel in a turn, one tire would drag across the road surface. This would create tire squeal, excessive tire wear and reduce handling performance.



The outside front wheel must therefore be turned at less of an angle than that of the inside front wheel. This will keep both wheels tangent to their respective turning circles and prevent tire squeal and/or damage. As the vehicle enters a turn, the tie rod ends will travel an equal distance, but due to the angle of the steering arms the tires will progressively toe-out.

Although this angle is never adjustable, it is easily checked on the alignment rack by turning the tires 20 degrees on full floating tables. First turn the front right tire 20 degrees and read the indicator on the left wheel. This is the angle of toe-out for the left tire. Repeat the procedure for the remaining side. Compare reading with specifications. Readings not within specifications are an indication that the steering arms are bent and should be replaced. Never bend or heat components to repair them.

Steering Axis Inclination (SAI)

Steering Axis Inclination (SAI) can be a difficult angle to understand. SAI is also referred to as the ball joint angle or kingpin inclination (on I-Beam suspension). The easiest way to understand SAI is to first define steering axis. The steering axis is an imaginary line intersecting the spindle support. In a conventional steering system, the spindle supports are the upper and lower ball joints or the kingpins. With MacPherson strut systems, steering axis is the angle beginning at the ball joint and extended through the strut assembly.



Viewed from the front of the vehicle, SAI is the angle between the steering axis and a true vertical line established through the tire. The SAI is a stability angle and is measured in degrees. If these imaginary lines were extended to the road surface, the area covered between them would be identified as the point of load or scrub radius.

The vehicle body will be closest to the road surface when the wheels are pointed straight-ahead as a result of SAI. A spindle with SAI will have the outer end of that spindle at the highest point when the wheels are pointed straight-ahead. Therefore, as the weight of the vehicle pushes downward, the spindle will always attempt to move upward to return the wheels to a straight-ahead position.

After a turn, the SAI helps to return the tires to straight-ahead position. SAI also aids in vehicle directional stability by resisting road irregularities that attempt to turn the wheels away from the straight-ahead position. SAI produces many of the same benefits that improve steering stability as positive caster. Correct engineering of SAI can reduce the need for high positive camber.

The effect of SAI on directional stability is usually greater then that of caster. Some vehicles with power steering require a greater amount of steering wheel returning force than those with manual steering. SAI is often used with positive caster on power steering equipped vehicles to assist in steering wheel returnability.

Scrub Radius

Scrub radius is the term used to describe the distance between the projected steering axis and the tread centerline at the road surface. Scrub radius is positive when the centerline of the tire lies outside the projected steering axis. It is negative when the centerline of the tire is inside the projected steering axis. The scrub radius is a distance measurement and it is therefore measured in inches or millimeters.

The size of the scrub radius depends on the steering axis inclination, wheel offset and the distance the spindle centerline is above the road surface. By carefully considering the correct SAI and the proper wheel offset for the designed spindle height, the required amount of scrub radius is designed into the suspension.

Although the spindle height has an effect on the scrub radius, little can be done to change this height because tire height is limited by the clearance space under the fender and body. Since all handling sensations pass between the tire and the road, the scrub radius provides the necessary feedback to give the driver road feel.

Setback

Setback or front end squareness is a condition in which one wheel is rearward of the other. If setback is present the turning radius will not be correct when the vehicle turns. With this condition, the tires will wear very much in the same manner as if they were under inflated. Generally, setback is the result of collision damage. It is preferable to have the front tires square with each other before alignment is done.



Considering the many different types of alignment equipment available, it is not possible to cover each checking procedure. Use the alignment machine manufacturer's instructions for checking setback. The most accurate way of checking is with four wheel alignment equipment.

Depending on the severity of setback and the type of alignment equipment being used, false readings can mislead a technician into thinking that an incorrect adjustment is within specification. These false readings are experienced more frequently with two wheel alignment methods.

Thrust Angle

Thrust angle is the line that divides the total angle of the rear wheels. The rear tires are not just following the front tires, they are actually establishing direction of the vehicle. In doing so, a direction of thrust is developed. The thrust angle created by the rear wheels is used as a reference for aligning the front wheels. Ideally, the thrust angle should be identical to the geometric centerline of the vehicle.




If thrust angle and geometric centerline are identical, the position of the tires would then form an absolute rectangle and the front tires could be aligned to the rear tires, resulting in a perfectly centered steering wheel.

Because of unitized construction, factory tolerances and a varying degree of damage and/or wear, it is increasingly unlikely that the axles will be parallel. When the rear axle projects a different angle than the front axle, the driver will need to turn the steering wheel to compensate in order to drive in a straight line.

On situations where the thrust line and geometric centerline are not identical, a thorough inspection of the rear axle and suspension system must be done. Replacing defective components should aid in positioning thrust angle close to the geometric centerline.

If the thrust angle is not identical to the geometric centerline and there are no defective components, align the vehicle using the thrust angle instead of the geometric centerline. Aligning the front wheels to the thrust angle is preferred to aligning to the geometric centerline. The ability to do this is a significant advantage of four wheel alignment. The result should be a straight steering wheel as the vehicle moves straight-ahead.

Adapted from http://www.specprod.com/
Copyrighted by Specialty Products Company, Longmont , Colorado, USA

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(Update: October 07, 2004)

Pointed the Right Way
By: John Hagerman

Camber, Caster and Toe: What Do They Mean?

The three major alignment parameters on a car are toe, camber, and caster. Most enthusiasts have a good understanding of what these settings are and what they involve, but many may not know why a particular setting is called for, or how it affects performance. Let's take a quick look at this basic aspect of suspension tuning.

UNDERSTANDING TOE

When a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tires or wheels. Toe settings affect three major areas of performance: tire wear, straight-line stability and corner entry handling characteristics.

For minimum tire wear and power loss, the wheels on a given axle of a car should point directly ahead when the car is running in a straight line. Excessive toe-in or toe-out causes the tires to scrub, since they are always turned relative to the direction of travel. Too much toe-in causes accelerated wear at the outboard edges of the tires, while too much toe-out causes wear at the inboard edges.

So if minimum tire wear and power loss are achieved with zero toe, why have any toe angles at all? The answer is that toe settings have a major impact on directional stability. The illustrations at right show the mechanisms involved. With the steering wheel centered, toe-in causes the wheels to tend to roll along paths that intersect each other. Under this condition, the wheels are at odds with each other, and no turn results.

When the wheel on one side of the car encounters a disturbance, that wheel is pulled rearward about its steering axis. This action also pulls the other wheel in the same steering direction. If it's a minor disturbance, the disturbed wheel will steer only a small amount, perhaps so that it's rolling straight ahead instead of toed-in slightly. But note that with this slight steering input, the rolling paths of the wheels still don't describe a turn. The wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability.

If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So it's clear that toe-out encourages the initiation of a turn, while toe-in discourages it.

The toe setting on a particular car becomes a tradeoff between the straight-line stability afforded by toe-in and the quick steering response promoted by toe-out. Nobody wants their street car to constantly wander over tar strips-then ever-ending steering corrections required would drive anyone batty. But racers are willing to sacrifice a bit of stability on the straightaway for a sharper turn-in to the corners. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.

With four-wheel independent suspension, the toe must also be set at there are of the car. Toe settings at the rear have essentially the same effect on wear, directional stability and turn-in as they do on the front. However, it is rare to set up a rear-drive race car toed out in the rear, since doing so causes excessive oversteer, particularly when power is applied. Front-wheel-drive race cars, on the other hand, are often set up with a bit of toe-out, as this induces a bit of oversteer to counteract the greater tendency of front-wheel-drive cars to understeer.

Remember also that toe will change slightly from a static situation to a dynamic one. This is most noticeable on a front-wheel-drive car or independently-suspended rear-drive car. When driving torque is applied to the wheels, they pull themselves forward and try to create toe-in. This is another reason why many front-drivers are set up with toe-out in the front. Likewise, when pushed down the road, a non-driven wheel will tend to toe itself out. This is most noticeable in rear-drive cars.

The amount of toe-in or toe-out dialed into a given car is dependent on the compliance of the suspension and the desired handling characteristics. To improve ride quality, street cars are equipped with relatively soft rubber bushings at their suspension links, and thus the links move a fair amount when they are loaded. Race cars, in contrast, are fitted with steel spherical bearings or very hard urethane, metal or plastic bushings to provide optimum rigidity and control of suspension links. Thus, a street car requires a greater static toe-in than does a race car, so as to avoid the condition wherein bushing compliance allows the wheels to assume a toe-out condition.

It should be noted that in recent years, designers have been using bushing compliance in street cars to their advantage. To maximize transient response ,it is desirable to use a little toe-in at the rear to hasten the generation of slip angles and thus cornering forces in the rear tires. By allowing a bit of compliance in the front lateral links of an A-arm type suspension, the rear axle will toe-in when the car enters a hard corner; on a straight away where no cornering loads are present, the bushings remain undistorted and allow the toe to be set to an angle that enhances tire wear and stability characteristics. Such a design is a type of passive four-wheel steering system.

THE EFFECTS OF CASTER

Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, hen the caster is negative.

Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. The mechanism that causes this tendency is clearly illustrated by the castering front wheels of a shopping cart (above). The steering axis of a shopping cartwheel is set forward of where the wheel contacts the ground. As the cart is pushed forward, the steering axis pulls the wheel along, and since the wheel drags along the ground, it falls directly in line behind the steering axis. The force that causes the wheel to follow the steering axis is proportional to the distance between the steering axis and the wheel-to-ground contact patch-the greater the distance, the greater the force. This distance is referred to as "trail.”

Due to many design considerations, it is desirable to have the steering axis of a car's wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tire contact patch. The trail would be zero, and no castering would be generated. The wheel would be essentially free to spin about the patch (actually, the tire itself generates a bit of a castering effect due to a phenomenon known as "pneumatic trail," but this effect is much smaller than that created by mechanical castering, so we'll ignore it here). Fortunately, it is possible to create castering by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tire contact patch, and thus the same effect as seen in the shopping cart casters is achieved.

The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by imagining the unrealistically extreme case where the steering axis would be horizontal-as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it.

Most cars are not particularly sensitive to caster settings. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings, with lower angles being used on heavier vehicles to keep the steering effort reasonable.

WHAT IS CAMBER?

Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the car, it has positive camber. The cornering force that a tire can develop is highly dependent on its angle relative to the road surface, and so wheel camber has a major effect on the road holding of a car. It's interesting to note that a tire develops its maximum cornering force at a small negative camber angle, typically around neg. 1/2 degree. This fact is due to the contribution of camber thrust ,which is an additional lateral force generated by elastic deformation as the tread rubber pulls through the tire/road interface (the contact patch).

To optimize a tire's performance in a corner, it's the job of the suspension designer to assume that the tire is always operating at a slightly negative camber angle. This can be a very difficult task, since, as the chassis rolls in a corner, the suspension must deflect vertically some distance. Since the wheel is connected to the chassis by several links which must rotate to allow for the wheel deflection, the wheel can be subject to large camber changes as the suspension moves up and down. For this reason, the more the wheel must deflect from its static position, the more difficult it is to maintain an ideal camber angle. Thus, the relatively large wheel travel and soft roll stiffness needed to provide a smooth ride in passenger cars presents a difficult design challenge, while the small wheel travel and high roll stiffness inherent in racing cars reduces the engineer's headaches.

It's important to draw the distinction between camber relative to the road, and camber relative to the chassis. To maintain the ideal camber relative to the road, the suspension must be designed so that wheel camber relative to the chassis becomes increasingly negative as the suspension deflects upward. If the suspension were designed so as to maintain no camber change relative to the chassis, then body roll would induce positive camber of the wheel relative to the road. Thus, to negate the effect of body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e., gains negative camber) as it is deflected upwards.

While maintaining the ideal camber angle throughout the suspension travel assures that the tire is operating at peak efficiency, designers often configure the front suspensions of passenger cars so that the wheels gain positive camber as they are deflected upward. The purpose of such a design is to reduce the cornering power of the front end relative to the rear end, so that the car will understeer in steadily greater amounts up to the limit of adhesion. Understeer is inherently a much safer and more stable condition than oversteer, and thus is preferable for cars intended for the public.

Since most independent suspensions are designed so that the camber varies as the wheel moves up and down relative to the chassis, the camber angle that we set when we align the car is not typically what is seen when the car is in a corner. Nevertheless, it's really the only reference we have to make camber adjustments. For competition, it's necessary to set the camber under the static condition, test the car, then alter the static setting in the direction that is indicated by the test results.

The best way to determine the proper camber for competition is to measure the temperature profile across the tire tread immediately after completing some hot laps. In general, it's desirable to have the inboard edge of the tire slightly hotter than the outboard edge. However, it's far more important to ensure that the tire is up to its proper operating temperature than it is to have an "ideal" temperature profile. Thus, it may be advantageous to run extra negative camber to work the tires up to temperature.

TESTING IS IMPORTANT

Car manufacturers will always have recommended toe, caster, and camber settings. They arrived at these numbers through exhaustive testing. Yet the goals of the manufacturer were probably different from yours, the competitor. And what works best at one race track may be off the mark at another. So the "proper" alignment settings are best determined by you-it all boils down to testing and experimentation.