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Feb,08 05 09:24a Cit~ Or Falcon Heights 6516448675 . . Tandem suspensions-Theoretically, tandem axles have the potential to be no more damaging to roads than single axles with equivalent load per axle (i.e., a 36-kip tandem can be no more damaging than two IS-kip singles). In practice, certain deficiencies in the performance of tandem suspensions preclude these benefits: _ Inequalities in static load sharing cause disproportionate fatigue from the heavily loaded axle. Load sharing coefficients Ooad on the heaviest axle normalized by the average of both axles) have been observed to vary from 1.02 to 1.21. A 34- kip tandem with a load sharing coefficient of L 15 produces damage equivalent to two 18-kip axles (ESALs). _ Most tandem suspensions produce dynamic loads comparable to their single axle equivalents. The walking-beam tandem suspension is an exception in that it produces unusually high dynamic loads. On rough and moderately-rough roads. walking-beam suspensions (without shock absorbers) are typically 50% more damaging than other suspension types. . Axle spacing-Aside from the suspension effects discussed above, locating axles at a close spacing does not contribute to pavement damage. Damage on flexible pavements is largely insensitive to axle spacing down to the limits dictated by cunent tire diameters. Rigid pavements actually benefit from stress interaetions between axles and produce less fatigue with closely-spaced axles. Thus, axle spacing is not an important truck characteristic affecting pavement damage. . Tire inflation pressure-Elevated tire inflation pressure greatly increases the fatigue damage of flexible pavements. Over-inflation of conventional tires (e.g., llR22.5) by 25 psi nearly doubles flexible pavement fatigue. Similarly, over-inflation of wide- base single tires is especially critical, increasing fatigue by a factor of four. Tire pressure has a moderate influence on rigid pavement fatigue. . Tire configuration-Of the various tire configurations used on trucks, the most significant to damage is the heavily loaded conventional tire on steer axles. Single tires, typically loaded to 12 kips, cause the steer axle to be more damaging in fatigue and rutting to flexible pavement than a 2O-kip axle (the current legal limit) with dual tires. Steer axle loads should be reduced to 11 kips or less to eliminate this disparity. Wide-base singles at their rated load capacity cause more fatigue and rutting damage than conventional dual tires on a 2O-kip axle. Load on wide-base singles would have to be limited to approximately 90% of their rated capacity to eliminate this disparity. Of the truck properties discussed above, axle loads have the greatest influence on fatigue damage of flexible pavements. However, flexible pavements may also be damaged by rutring. The permanent deformation of the asphalt concrete layer caused by a vehicle is directly dependent on its gross vehicle weight. To the extent that frieghl must be carried by trucks, rutting cannot be alleviated by regulating truck gross weight, because lower weight limits will ouly put more trucks on the road to meet commercial hauling needs. Among the vehicle factors, tire type and inflation pressure have small influences on rutting. . . x p.4