Issue link: http://read.dmtmag.com/i/34501
Steer axle toe is adjustable to reduce wear to the leading edge of the tire and also to avoid road wander. Toe is adjusted in a static, unloaded condition so that the tires will run in a straight line under a dynamic, loaded condition. The toe measurement will probably change from unloaded to loaded condition. The amount of change will vary with axle manufacturer, axle rating, and steering arm geometry; but it is still fairly predictable. Front axles on most popular Class 8 long haul tractors will change in the direction of toe-out about 1⁄32" (0.8 mm or .05 degree) for each 1000 pounds of load increase on the steer axle. Cabover tractors with set-back-front-axles typically experience less steer axle change in load from bobtail to loaded than do other configurations. Wheelbase and fifth wheel location are also major factors affecting how much load change the steer axle will experience. Toe-in TANDEM AXLE PARALLELISM (SKEW - THRUST) Tandem axle parallelism is critical because it can have a detrimental effect on all ten tires on the tractor. Non- parallel drive axles tend to push the tractor into a turn in the direction that the axle ends are closest. In order for the vehicle to go straight, the driver must correct by steering in the opposite direction. The vehicle can then go straight, but all ten tires are at an angle to the direction of travel, causing scrubbing. Excessive tandem axle non-parallelism is usually detected in steer tire wear. If one steer tire is scrubbing from the outside inward and the other steer tire is scrubbing from the inside outward, then tandem axle alignment is suspect. A similar pattern can be generated by the driver’s compensation for a non- lubricated 5th wheel or from a dog tracking trailer. This should not be confused with a light level of toe-in on the right front and lighter toe-out wear on the left front that may be the result of secondary highway road crown. Tandem Scrub Angle Note: Additional consideration would be effects of air ride suspension systems, rack and pinion systems, and disc air brakes on steer tire wear. THRUST ANGLE (TRACKING) The relationship of the geometric centerline of the vehicle and the direction that the axle points generates a thrust angle. Ideally this relationship would result in a 0 degree value when the axle centerline is perpendicular to the geometric centerline. However, any deviation from this setting will increasingly cause the vehicle to travel away from the straight line, causing the tires to “dog track” and scrub. Tracking to the right generates a positive thrust angle; tracking to the left creates a negative thrust angle. A misaligned (dog-tracking) trailer may also be the cause of steer tire wear. See Section Nine, Appendix under Conversion Table on Page 117 for conversion of fractions in inches to millimeters and degrees. See Section Nine, Appendix under Alignment on Pages 118-119 for a field method for verification. Thrust Angle (Tracking) Section Three: Extending Tire Life 47