Issue link: http://read.dmtmag.com/i/34501
Parallelism: On a tractor with tandem drive axles, the two axles should be parallel to one another. Any deviation from this parallel position will create a tandem skew or scrub angle. This angle should be no larger than one tenth of a degree. An easy method of checking this angle is to measure the distance between the ends of the axle hubs on each side of the tractor. The difference between these two measurements should be no larger than 1⁄8 inch for a tandem tractor/truck and no larger than 1⁄16 inch on a tandem axle trailer. The easiest way of accomplishing this measurement is by using a trammel bar. The pointers on the trammel bar must fit in the axles’ centering holes on both sides of the vehicle. For example, if the ends of the drive axles on the left side of the vehicle are closer together than the axle ends on the right side, this will cause the vehicle to pull or drift to the left. AXLE PARALLELISM AND TRACKING In the straight-ahead position, the rear wheels of a vehicle should follow the front wheels in a parallel manner. Wheels that are out-of-track can cause excessive tire wear. Failure of the wheel to track is usually due to the following causes: –Master spring-leaf broken – Incorrect air spring (bag) height –Worn springs – Auxiliary leaves broken – Loose “U” bolts – Incorrect or reverse springs – Bent frame – Locating rods or torque rods improperly adjusted – Locating rod or torque rod bushings worn excessively Failure of the wheels to track is usually quite visible when one follows the vehicle on the highway. It is possible that, due to one of the above causes, no uneven wear manifests itself on the rear tires, but an uneven wear pattern may show itself on the front tires. This is because rear tires may push the vehicle off course and give some toe-out-on-turns in the straight-ahead position to the front tires. Hence, the driver makes a correction to offset the steering action caused by the rear wheels. If the rear axle of a vehicle is not at right angles to the chassis centerline, the front tires are affected, showing misaligned wear. In the diagram below, the position of the rear axle of the vehicle has been altered because of a weakened left side spring – so that the rear axle on the left side is further from the front axle than the rear axle on the right side. In this illustration of a 4x2 configuration, the angle of the Tandem Scrub Angle rear axle causes its wheels to point to the left side so that the rear end of the vehicle is, in fact, self-steered in that direction. The vehicle would then steer itself to the right – unless the driver takes corrective action. If the driver wishes to travel straight ahead, he will naturally compensate by turning his steering wheel. This action introduces a turning moment as if the vehicle were making a turn although it is moving in a straight line due to the toe-like posture of the front wheels. It is more difficult to identify this concept with additional drive axles and the placement of movable 5th wheels. For this reason, the onset of misalignment wear patterns on the front tires may be apparent, even though the lateral forces may be slight and the front wheel alignment settings may be correct. Top View Inside Wear Springs Springs Outside Wear Vehicle Pulls to Right Section Three: Extending Tire Life 51