Issue link: http://read.dmtmag.com/i/34501
STEER AXLE SETBACK (STEER AXLE SKEW) Any measured deviation left (negative) or right (positive) away from perpendicular to the centerline of the vehicle is called the setback. TMC RECOMMENDED ALIGNMENT TARGETS (Value representing industry-established midpoint.) For more information refer to TMC RP 642A, Total Vehicle Alignment: Recommendations for Maximizing Tire and Alignment-Related Component Life, Appendix 9. Alignment Specification (1) Steer Axle Total Toe Camber Caster Steer Axle Setback (Steer Axle Skew) Setback Scrub (Parallelism) Lateral Offset TOE-OUT-ON-TURNS (TURNING RADIUS) Toe-out-on-turns is the difference in the arcs described by the steering tires in a turn. The purpose is to prevent the inside tire from scrubbing around a turn since the outside tire (loaded tire) determines the turning radius of the steer axle. This is the Ackerman Principle. Improper geometry results in wheel scrub in turns, which generally appears as toe wear on the tire. More specifically, Ackerman wear shows itself as a rounded edge radial feather wear across the tread area of the tire. This angle is more important on a city vehicle with its many turns than on a line haul unit. Ackerman geometry is dependent upon the steering axle track-width and wheel base of a vehicle. When the turning angle or wheel base changes from the original specification, Ackerman is affected. Steering Arms Target Value (2) +1⁄16 inches (0.08 degrees, 0.06 inches, 1.5 mm/M) Less than 1⁄4 degree (3) Left: +3.5 degrees Right: +4.0 degrees 0 degrees / 0 inches Drive, Trailer, and Dolly Axles Thrust (Square) 0 degrees / 0 inches 0 degrees / 0 inches 0 inches (1) All specifications are measured with vehicle in static, unladen condition. (2) All specifications are stated in inches or degrees (where applicable). (3) Camber angle changes normally involve bending the axle beam, which may void the axle manufacturer’s warranty. If the measurement exceeds this value consult the vehicle, axle, and/or alignment equipment manufacturer. PERIODIC ALIGNMENT CHECKS An aggressive alignment preventative maintenance Basic Ackerman Steering Diagram Cross Bar (Tie Rod) Wheel Base program should include the following periodic checks: 1. Upon delivery of new vehicles. Even though OEMs make a concerted effort to properly align vehicles at the factory, shifting and settling can occur during delivery. Camber and caster may not change much, but toe and tandem axle parallelism may change sufficiently to set up undesirable tire wear patterns if not corrected upon receipt. 2. At the first maintenance check. Post break-in alignment checks should be done between 15,000- 30,000 miles, but no later than 90 days after the first in- service date. If shifting and settling did not occur during delivery, it may occur during the first few thousand miles of operation. Many OEMs recommend verification of torque on suspension/frame components after a few thousand miles of operation. A thorough alignment check should be made during this inspection (after torque verification). Consideration should be given to different torque requirements on metric and standard bolts. 3. When new steer tires are installed or front-end components are replaced. The steer tires coming out of service can tell a story of good or bad alignment. With this feedback, an alignment program can continue to improve. Without feedback, the best an alignment program can do is stay at its current level. 4. When tire wear indicates a concern. “Reading” tire Rear Axle Center of Rotation wear can help identify alignment issues. Unfortunately, correcting the alignment does not necessarily correct the tire wear pattern once an undesirable wear pattern has been established. Section Three: Extending Tire Life 49 Chassis