Total Vehicle Alignment
Contents
1. characteristic The Ackermann Principle states that for any given corner the outside wheel should have less turn angle than the inside one because it is following a larger radius than the inside wheel Figure 6 7 This difference in wheel turning angles is determined by the length and angle of the steering arms that are attached to the hubs of the steer axle The theoretical Ackermann angle for a particular vehicle is determined by drawing a line through the pivoting axis ai Figure 6 7 Ackermann Principle which is the rear axle of a two axle vehicle to establish a pivot point for a turn then drawing lines to the pivot points of the two steer tires The Ackermann then is the angle the tires wheels needed to be turned to form a right angle with each of the lines extending from the turning pivot point to the tire wheel pivots This results in the steer tires toeing out when turning A vehicle s wheelbase is the most critical variable affecting the theoretical Ackermann for a vehicle Keep in mind that Ackermann is a purely geometric concept he argument that the Ackermann Principle was developed in the early 1900 s for very slow moving vehicles and does not k e Definition The Ackermann Principle States that for Any Given Corner the Outside Wheel Should Have Less Turn Angle Because It Is Running at a Larger dius than the Inside Wheel 47 consider the dynamic effect of many2. work is being directed through industry associations including The Maintenance Council of American Trucking Association The Society of Automotive Engineers and with individual OEM truck axle and Figure 6 2 Toe out suspension manufacturers Figure 6 3 Positive camber Figure 6 4 Negative camber 44 Total Vehicle Alignment In particular certain truck and axle manufacturers have responded to the requirements for more precise alignment settings These OEM s do not recommend delivery realignment of their vehicles at the dealer level Specific irregular wear patterns and their causes are discussed in detail in the Irregular Wear section of this service manual Years ago alignment meant simply a front end job But the steer axle is only the beginning of the total alignment story in the radial age We now know that proper attention to drive axles trailer axles and dolly axles completes the picture Not only does alignment affect tire wear but the amount of fuel used by a truck trailer combination as well See Section 9 for additional details Figure 6 5 Caster STEER AXLE ALIGNMENT The major front end alignment settings involve Toe Toe is defined as the difference in distance apart at the front and at the rear of the steering axle tires as seen in a top view of the truck Toe in exists when the tires are closer together in the front than in the rear Figure 6 1 and excessive toe in re
3. SECTION SIX Total Vehicle Alignment Vehicle alignment settings serve several purposes in vehicle operation They affect handling steerability stability and have a significant impact on tire performance Camber settings are not considered adjustable in the field NEVER ATTEMPT TO ADJUST THESE SETTINGS BY BENDING OR MODIFYING AXLE STEERING MECHANISM COMPONENTS 43 Total Vehicle Alignment Total Vehicle Alignment The long treadwear potential offered by modern radial linehaul truck tires can be reduced by the misalignment of tractor and or trailer wheels and axles Extensive research has demonstrated that total vehicle alignment programs can pay dividends in extended tire wear and improved fuel economy There has been increased attention to proper truck alignment procedures during the past few years and for good reason Current radial steer axle tires provide a much slower rate of wear than earlier generation radial or bias ply tires This also means that they may reflect the adverse effects of improper alignment that was unseen on faster wearing tires Opinions on proper alignment for radial tires often seem as varied as the number of authorities giving them For this reason Goodyear has been actively involved in working toward industry wide agreement to define the effects of improper alignment on tire wear durability and vehicle handling and to establish recommended alignment settings Much of this
4. endations for trailer alignment are as follows Preferred toe setting 1 32 inch toe in to 1 32 inch toe out or 2 7 minutes per spindle Acceptable toe setting 1 16 inch toe in to 1 16 inch toe out or 5 4 minutes per spindle Figure 6 44 Figure 6 12 The loaded axle camber can be up to negative 1 without affecting tire wear Axles should be parallel to each other within 1 8 inch measured between axles on both sides of the trailer at a 71 5 inch axle track This provides a scrub angle of 0 1 Axles should be perpendicular to the centerline of the trailer frame within 1 8 inch per side or 1 4 inch from side to side at a 71 5 inch axle track This provides a thrust angle of 0 2 IN SERVICE ALIGNMENT RECOMMENDATIONS Toe in is recognized throughout the industry as the most important contributor to optimizing steer tire treadwear In order of priority gains in tread life can be expected by focusing on the following vehicle alignment parameters e TOE e REAR TANDEM PARALLELISM e CAMBER NON ADJUSTABLE e REAR TANDEM PERPENDICULARITY e CASTER Most vehicle manufacturers in recent years have developed new factory equipment and procedures to control alignment to much narrower tolerances than was previously possible Today there is less need to adjust alignment on new vehicles than in the past Alignment accuracy and repeatability can best be achieved by proper training adherence to strict procedur
5. es and by properly maintaining and frequently calibrating alignment equipment THE VEHICLE MANUFACTURER S ALIGNMENT SPECIFICATIONS SHOULD BE ADHERED TO 49 The following guidelines have proved to be beneficial for improving overall tire treadwear STEER AXLES TOE IN unloaded Check Limits 1 16 1 16 Range 0 1 8 Reset Limits 1 16 1 32 When alignments are found within these limits adjustment is not necessary If outside of check lim its set to the reset limits DRIVE AXLES Tandem axles to be parallel within 1 8 measured at axle end Axles to be perpendicular to chassis centerline within 1 8 when measured from axle end to chassis centerline or within 1 4 when measured from left to right axle end TRAILER AXLES Tandem axles to be parallel within 1 8 measured at axle end Axles to be perpendicular to chassis centerline within 1 8 when measured from axle end to chassis centerline or within 1 4 when measured from left to right axle end Nominal toe setting O 1 32 Reference TMC Recommended Practice RP642 regarding total vehicle alignment for more detailed information
6. hich may result in asymmetrical steer tire wear A more severe case is shown in Figure 6 10 Here the drive axles are neither parallel to each other nor perpendicular to the chassis centerline The drive axle tires are trying to force the vehicle to turn left and the driver must compensate by turning to the right This will result in fast and irregular wear Figure 6 9 and as recent tests have shown in a much more severe way than the previous case These tests also indicated that the steer tire on the same side of the truck on which the drive tires are closest together will wear into an out of round condition as well Recommendations for drive axle alignment are as follows e Tandem axles should be parallel within 1 8 inch difference between the axles centers measured on the left and the right side of the vehicle Figure 6 11 e Axles should be perpendicular to the chassis centerline within 1 8 inch measured between axle end and vehicle centerline Figure 6 12 Figure 6 10 48 Total Vehicle Alignment TRAILER AXLE ALIGNMENT With more long wearing radial tires being applied to trailer axles their alignment has become an important issue Trailer axle tires have the potential for longer life more miles per thirty second inch of treadwear than any of the tires on the tractor They are therefore more susceptible to irregular wear due to misalignment than any other tires on the vehicle Goodyear s recomm
7. inimize or eliminate irregular wear Theoretically these new settings will result in steer tires running straight down the road in a 0 toe 0 camber mode Goodyear Proving Ground tests and independent field tests support this theory Tires with excessive camber will wear as shown in Figure 6 6 It can be seen that improper camber causes wear on one side of the tire this can be on the inside or outside of the tire depending on camber setting and tire position LF or RF 46 Camber Wear One side of thread worn excessively Figure 6 6 CASTER Generally caster is not considered to affect tire wear but is important in the handling and driveability of the vehicle Overall effects of caster can be summarized as follows Too little caster causes e Unstable steering e Constant corrections required e Wander and weave e Oversteer e Failure to return to straight ahead out of a turn e Roadwalk Too much caster causes e Hard steering e Shimmy e Road shock Vehicle manufacturers normally recommend caster settings for their vehicles Proper caster is that which gives best handling in combination with the camber and king pin inclination designed into the axle Total Vehicle Alignment ACKERMANN STEERING EFFECT ON TIRE WEAR There are many variables to check when determining the source of irregular tire wear patterns One potential cause for irregular wear on steer tires may be a truck s Ackermann
8. ments On steer axles toe and camber settings are related and should be considered together for optimum tire life especially in line haul service where treadwear rates are slow Positive camber refer to illustration creates a slightly shorter rolling radius on the outside shoulder of a radial tire than on the inside shoulder This creates a tendency for the tire to roll toward the outside a toe out condition Since all the working tolerances in the tie rod ends and kingpins must be taken up before the tendency to toe out is restrained an initial static toe in setting is essential NOTE Alignment recommendations may need to be customized for certain vehicle tire service conditions Total Vehicle Alignment TOE Toe settings generally have the greatest effect on truck tire treadwear Toe is also the easiest front end alignment variable to adjust in the shop Road tests were made using three trucks with different amounts of loaded truck toe in 1 32 inch 1 8 inch and 1 4 inch with radial tires on the steering axles The test results showed e Tire tread mileage decreases with increased toe in The 1 32 inch toe in showed the best treadwear rate miles per 32nd of tread depth e Assigning a value of 100 to the treadwear rate with 1 32 inch toe in the treadwear rate values compared as follows Loaded Toe in Comparative Value Treadwear Inches Rates 1 32 100 1 8 82 1 4 76 In addition to wea
9. outside influences on the path a vehicle takes through a turn is somewhat correct To further complicate the Ackermann Principle as it applies to trucks remember that the turning axis must be drawn to determine a pivoting point about which the vehicle turns Its more difficult to define this axis for vehicles with more than one drive axle Fifth wheels depending on their location can also alter where this line would fall The trend is for vehicle manufacturers to provide different Ackermann arms for different wheel bases and different fleet vocations Total Vehicle Alignment Drive axle alignment is very important Tandem drive axles that are not parallel to each other have a definite effect on steer tire wear Figure 6 8 shows a model of a tandem drive axle tractor with both drive axles in proper alignment In this case the driver simply steers the truck straight ahead and neither fast wear nor irregular wear would be expected as a result of the driving axles However Figure 6 9 is an exaggerated view of a truck with drive axles parallel but not perpendicular to the chassis Figure 6 8 centerline The eight driving tires create a thrust angle to the left at the rear of the truck Turning the steering wheel slightly to the left aligns the steer and drive tires to run parallel but the vehicle however will dog track Even though lateral forces on the steer tires are minimal the steering geometry is affected w
10. r drivers reactions to the toe in settings without power steering indicated that the 1 4 inch toe in caused the truck to roadwalk badly The 1 32 inch value was considered to have the best handling Tire wear due to excessive toe in on radial tires shows up initially as irregular wear more so on the outside than the inside grooves of the tires and more so on the right front than on the left front tire Excessive toe out will show a reversed effect more wear on the inside than the outside grooves and more so on the left front than the right front tire At high values of toe in or toe out and at relatively early mileage the tread of the outside or inside ribs can be completely worn away For 5 16 inch toe in condition this can occur after only 19 000 miles of highway travel Gauges for measuring toe in setting are relatively simple and inexpensive Every maintenance shop should use them frequently It is not necessary to send a truck to an alignment shop to check toe in settings Setting toe alone is usually not sufficient A total vehicle alignment toe and axle is recommended per TMC RP642 CAMBER After years of recommending camber settings of 1 4 degree for left front and 0 degrees for right front major axle manufacturers have changed to 0 degree settings for both left and right steer axle positions on axles designed for line haul service The objective of this change is to optimize steer tire wear and m
11. sults in feather wear in the direction shown by the arrows Toe out exists when the tires are closer together in the rear than in the front Figure 6 2 and excessive toe out results in the feather wear in the direction shown by the arrows Camber Camber is the tilt of the tires as seen in a front view of the truck Positive camber exists when the tires are closer together at the bottom point of road contact Figure 6 3 Negative camber exists when the tires are closer together at the top Figure 6 4 Caster Positive caster is provided by a backward rotational tilt of the top of the axle or backward inclination of the kingpin at the top as seen in a side view of the truck Figure 6 5 Negative caster would be a corresponding tilt forward at the top Before any alignment adjustment is performed always check the vehicle for loose kingpins worn wheel bearings tie rod ends or any looseness in the steering system Adjust wheel bearing end play in accordance with the recommendations of the OE manufacturer Attempts to correct alignment on a vehicle with worn or loose components are pointless 45 LOADED VS UNLOADED ALIGNMENT SETTINGS Alignment changes as load changes especially steer axle camber caster and toe Since springs axles and suspension mountings vary by truck and components with different weight ratings are often chosen different unloaded truck alignment settings may be required to obtain optimum loaded truck align
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