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Front-End Alignment Basics

Written by John Bellah

The cruiser comes into the shop with a front-end complaint: a shimmy, poor handling at high speed, or after jumping a curb or hitting a massive pothole, to check the front-end alignment. So exactly what is involved in an alignment? It is very different than the alignment on the police cars from the 1980s!

Alignment issues can involve vehicle safety, driver fatigue, fuel mileage, the service life of suspension components, vehicle ride, handling, and vibration, and tire mileage. Currently there are three types of alignment procedures:

First, the “old-fashioned front-end alignment,” a process considered old technology. It may still be acceptable for the most basic procedure on a rear-wheel drive, solid rear axle car. And a few older shops still exist that offer a front-wheel alignment. However, with the newer, front-wheel drive, independent rear-suspension vehicles on the road, and with much tighter tolerances with today’s vehicles, this type of alignment is virtually obsolete.

Second, the “thrust angle alignment.” This allows the technician to confirm that all four wheels are “square” with one-another, i.e., the front wheels are aligned in conjunction with the rear wheels and that the rears are not offset, or “dog-tracking.” The thrust angle alignment is acceptable with RWD vehicle with a solid rear axle, such as the Ford Crown Victoria Police Interceptor.

Third, the “four-wheel alignment.” This type of alignment is a must for vehicles with four-wheel independent suspension, such as the Dodge Charger and Chevy Impala with adjustable rear suspensions. It doesn’t matter if the car is FWD (Impala) or RWD (Charger). If it has an independent rear suspension, it needs a four-wheel alignment. The four-wheel alignment “squares” the front wheels with the rear wheels, and the rears are adjusted as needed.

Camber

Camber is how much the top of the tire tilts inward or outward with respect to the car. Looking at the vehicle from the front, the angle of the tire in relationship to the pavement is camber. If the top of the tire tilts inward toward the vehicle, this is negative camber. If the top of the tire tilts away from the vehicle, this is positive camber.

Camber adjustment affects both tire wear and handling. An aggressive driver may ask for more negative camber for better handling. The outside front tire with a lot of negative camber remains more upright on the road during hard cornering as the suspension changes due to body roll. However, too much negative camber will cause the inside edges of the tires to wear faster than the outside edges.

Positive camber will cause the vehicle to drift in the direction of the wheel with the most positive camber. This is because a tire with positive camber has less traction than one with negative camber. Some alignment specifications will increase positive camber by about a half-degree to compensate for the crown in the road, which decreases the tendency for the vehicle to drift toward the curb.

The camber angle will also change depending on the load in the vehicle. Improper tire pressures and rough road surfaces can also affect camber angles.

Caster

Caster is how much the top of the tire tilts forward or backward with respect to the steering pivot points. Looking at the vehicle from the side, draw an imaginary line from the top steering pivot point to the bottom pivot point. This is usually the ball joints or top strut-to-ball-joint on McPherson strut suspension. If this imaginary line tilts back toward the rear of the vehicle, that is positive caster. If the line tilts toward the front of the vehicle, that is negative caster.

With positive caster, the tire contact patch is behind the spot where the imaginary line hits the pavement. This situation makes the tire want to follow that imaginary spot on the pavement, i.e., the vehicle will track down the road rather than wander. A graphic example of this is the front forks of a chopper. The forks have an exaggerated positive caster angle, which allows the motor to remain stable while moving down the road surface.

Caster angle settings allow a balance between steering effort, cornering effort, straight line tracking, high-speed stability and self-centering of the steering wheel after a turn. An increase in positive caster will improve straight-line tracking, make the vehicle more stable at high speed, and improve cornering effectiveness.

Cornering is improved because as the suspension moves during body roll into a turn, lots of positive caster keeps results in negative camber, or at least less positive camber, and thus better tire adhesion. While out-of-specification caster settings won’t cause excessive tire wear, it will cause the vehicle to have problems in straight-line tracking and/or to pull to one side.

Cross-Camber, Cross-Caster

Cross-camber and cross-caster settings simply mean that camber and caster settings have been set slightly different from side-to-side. This allows compensation for the crown in the middle of the majority of our roads, which would cause the vehicle to drift toward the right curb. The right side would be set up with about ¼ degree additional negative camber and about ¼ degree additional positive caster. On the other hand with these settings, the vehicle may drift to the left if the pavement surface is perfectly flat.

Toe-in, Toe-out

Toe is the angle that the front tires are pointed with respect to one another. Toe-in is where the front of the front tires is pointed in toward the centerline of the vehicle. Toe-out is when the front of the tires point away from the centerline of the vehicle. The toe angles may be expressed in either inches or in degrees.

Toe used to be measured in terms of inches. The distance between the front of the front tires was simplistically compared to the distance between the rear of the front tires. Toe-in was where the distance between the fronts of the tire was less than the rears of the tire. Today, the actual angle of each tire is measured. A positive (+) reading is toe-in, while a negative reading (-) is toe-out. Toe is adjusted individually for each front wheel by lengthening or shortening the tie-rod ends on each side. While proper toe sittings are critical, the recommended settings allow tolerances to compensate for normal suspension component wear and allow for the compression of the suspension arm bushings when the vehicle is at speed.

Correct toe settings are critical. Out-of-specification toe angles will waste fuel, prematurely wear tires, and can cause handling problems, both low-speed and high-speed, especially under wet conditions. With the front tires pointing in slightly different directions, when one tire loses its grip during hydroplane, the other tire may push the vehicle in a different direction. This will make the vehicle feel very unstable to the driver.

At high speeds, and due to aerodynamic lift, the front may actually raise. This will change the alignment. Thus, the toe settings will vary from when the vehicle is stationary, and at speed toe settings can affect the way a vehicle handles. An increase in toe-in angle will usually reduce oversteer, and make the vehicle more stable at high speeds.

The technician can also “finesse” the vehicle’s handling characteristics by varying toe settings, as long as they are within the specified limits. An increase in toe-in (decrease in front tire traction while cornering) will result in reduced oversteer, while an increase in toe-out (increase in front tire traction while cornering) will reduce understeer.

Adjustment of the tie-rods also centers the steering wheel. One of the indicators that toe might be out of specification is when the steering wheel is canted off-center while the vehicle is driving down a flat and level roadway surface. After the steering wheel is centered and left and right toe settings have been made, the adjustments must be within the combined or total toe range—which is the settings of both sides added together.

For example, the toe settings on a late-model Ford CVPI call for each side to be set at the nominal (preferred) setting of -0.19 degrees (toe-out) with an allowable tolerance of +/- 0.07 degrees. The combined toe, the total of both sides added together the adjusted, is to be -0.37 degrees with a tolerance +/- 0.13 degrees.

Thrust Angle

Thrust angle is the angle the rear tires are facing compared with the centerline of the vehicle. This adjustment confirms that the rear tires are running parallel to the front wheels, or “square.” Variations in wheelbase from manufacturer’s tolerances, such as collision damage, will cause the vehicle to “dog-track,” and the steering wheel will not be centered. These variations will also cause the vehicle to turn differently during left and right turns.

If the thrust angle is not correct on a rear-wheel-drive vehicle with a solid rear axle, such as a Ford Crown Victoria, it may need to go to a facility with frame-straightening equipment to properly reposition the rear axle. If the rear suspension is not adjustable, then the front toe must be set to compensate for the difference.

Front-wheel-drive vehicles, such as the Chevrolet Impala, and vehicles with independent rear suspension, such as the Dodge Charger, will require a 4-wheel alignment, to ensure the rear-wheel alignment adjusted within specifications.

The Alignment

Police suspension components take a severe pounding from high-speed driving, hitting curbs, going over pot-holes in the road, being driven over median strips, sidewalks, and at times, off the pavement completely. Therefore, it is imperative that the vehicle be road-tested and the suspension completely inspected for worn and/or bent components before any alignment services are performed. Otherwise trying to align a vehicle with a sagging spring, worn bushings, tie-rod ends, idler arms, ball-joints, loose steering box or rack, if it is a rack and pinion unit, or other bent components is a useless waste of money.

Temperatures can also make a difference in spring rates and ride height. The alignment service should be performed at close to the operating temperature of the vehicle, and if it has been sitting outside in freezing temperatures all night and then driven on the alignment, settings may change when the car warms up to operating temperature.

Tire pressures should be checked and set to the manufacturer’s recommended pressures. Ride height is also important as a sagging spring can cause variances in front-end geometry. If there is doubt, the ride height should be measured in accordance to manufacturer’s recommendations.

Heads up! The alignment should be performed with the vehicle fully loaded. The fuel tank should be full, and all the police equipment should be in place when the alignment is performed. Whatever is carried in the trunk should be in the trunk. If special weapons or gear is removed from the trunk before sending the car out for alignment, then a ballast of the same weight must be placed in the trunk. Weight in the trunk affects the front-end alignment!

Earl Gautsche, GM Fleet’s national service manager, has a tech tip for the alignment on the Impala. The tire pressures are different between the police and civilian models. As a matter of practice, technicians adjust the tire pressure on all four tires before starting the alignment. If alignment is subcontracted to an outside shop, Gautsche emphasized that the technician be told to set the tire pressures for the police version when working on police cars. Out of habit, they may set them to the lower retail pressures.

On this same topic, police vehicle specifications may differ from the civilian vehicle because of the difference in tires, suspension components, spring rates, ride heights and desired handling characteristics. Be sure your alignment shop uses “police” settings!

John Bellah is the technical editor of Police Fleet Manager and a corporal with the California State University, Long Beach Police. He can be reached at jbellah@csulb.edu.


Published in Police Fleet Manager, Nov/Dec 2006

Rating : 7.4


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