Issue link: http://read.dmtmag.com/i/34501
MAINTAINING THE VEHICLE Many tire problems can be traced to mechanical conditions in the vehicle. Therefore, to obtain maximized tire performance, vehicles must be properly maintained. MAJOR VEHICLE FACTORS WHICH AFFECT TIRE LIFE: ALIGNMENT Alignment refers not only to the various angles of the steer axle geometry, but also to the tracking of all axles on a vehicle, including the trailer. The dual purpose of proper alignment is to minimize tire wear and to maximize predictable vehicle handling and driver control. Toe misalignment is the number one cause of steer tire irregular wear, followed by rear axle skew (parallelism or thrust). One of the challenges of meeting this goal is that alignments are typically performed on a static, unloaded vehicle sitting on a level floor. The vehicle then operates over varying contoured surfaces, under loaded conditions, with dynamic forces acting upon it. Predicting the amount of change between static/unloaded/level - versus – dynamic/loaded/contoured is difficult because many variables affect the amount of change. Variables such as Steering System Compliance (i.e. “play”) must be considered in making alignment setting recommendations. All of these misalignment conditions may exist alone or (more likely) in combination with other misalignment conditions. Sometimes it is these interactions that produce the outcomes that are especially undesirable. As an example, a tire running at slightly negative camber may perform especially badly if it is also subjected to tandem thrust misalignment. The conceptual understanding for this phenomenon is that because of the camber issue, the wear burden imposed by the thrust misalignment is not being shared equally by the entire tread surface. Further, a tire that is being operated in a misaligned condition may well transmit forces into the suspension from its interaction with the road. Some suspension systems manage those forces favorably. Others react in a way that imposes motions in the tire that are very unfavorable to the tire's ability to yield a favorable wear outcome. • Tires that are not operated at a normal (perpendicular) angle to the road surface typically produce uneven tire wear. Tires that are fighting each other (because of conflicting alignment operating angles) produce unfavorable and sometimes irregular tire wear. Tires that are fighting 46 Section Three: Extending Tire Life each other due to highly compliant suspension components (compression/extension in the bushings or joints, or deflection of solid parts) will likely produce irregular wear forms. • Alignments should be performed carefully using best alignment practices. (For example, ensuring that the suspension is at the correct ride height and that the suspension has been settled out by being moved forwards/backwards, etc.) • Alignments should be conducted in the most representative loading condition and ride height for the expected usage. We therefore recommend referring to TMC RP 642A, Total Vehicle Alignment Recommendations for Maximizing Tire and Alignment Related Component Life, which has established industry recommended target values for the alignment of vehicles. STEER AXLE GEOMETRY Since very few vehicles continue to use Center Point Steering, the following recommendations are based on the more common Inclined Kingpin Steer Axle Geometry. Inclined Kingpin TOE Toe is typically the most critical alignment condition affecting steer axle tire wear. The purpose of setting toe at a given specification is to allow the tire to run straight during normal operating conditions. Too much toe-in results in scrubbing from the outside inward on both tires, and too much toe-out results in scrubbing from the inside outward on both tires. Total toe is the angle formed by two horizontal lines through the planes of two wheels. Toe-in is when the horizontal lines intersect in front of the wheels or the wheels are closer together in front than in back. Toe-out is when the horizontal lines intersect behind the wheels or the wheels are closer together in back than in front. Toe-in is commonly designated as positive and toe-out as negative.