section 1.2 Truck Characteristics Affecting Pavements; (c) Suspension Systems

Navigation: Page 1, Cover Page | Page 2.. Phase 1.1, Background. | 1.2 Truck Characteristics Affecting Pavements. (a) Axle Weights | Page 4, Section 1.2 [b] Tire Characteristics | Page 5, Section 1.2 (c) Suspension Systems | Page 6, Section 1.2; (d) Axle Spacing | Page 7, Section 1.2; (e) Liftable Axles | Page 8, Section 1.2; (f) Tridem Axles | Page 9, Section 2.1 Axle Weight Limits | Page 10, Section 2.2 Bridge Formula | Page 11, Section 2.3 - 80,000 Pound GVW Cap | Page 11, Section 2.4 Policies to Encourage Tridems | Page 11, section 2.5 Weight Limits Per Unit of Tire Width | Page 12, section 2.6 Turner Trucks | Page 13, section 2.7-New Approach; TRB Truck Weight Study | Page 14, section Section 3.0; Knowledge Gaps and Research Needs | Page 15, section 4.0 References for Pavements Working Paper

Comprehensive Truck Size and Weight (TS&W) Study

Phase 1—Synthesis

Working Paper 3—Pavements and TS&W Regulations

1.2 Truck Characteristics Affecting Pavements

(c) Suspension Systems

As a heavy truck travels along the highway, axle loads applied to the pavement surface fluctuate above and below their average values. The degree of fluctuation depends on factors such as pavement roughness, speed, radial stiffness of the tires, mechanical properties of the suspension system, and overall configuration of the vehicle. On the assumption that the pavement wear effects of dynamic loads are similar to those of static loads and follow a fourth-power relationship, increases in the degrees of fluctuation increase pavement wear. For example, a 22,000-pound load followed by an 18,000-pound load has 1.06 times the effect of two 20,000-pound loads. Rough estimates of the effects of suspensions assuming that the pavement wear effects of dynamic loads follow a fourth-power relationship support a finding by the Organization for Economic Cooperation and Development (OECD 1982) that reduction in dynamic effects due to improved suspension systems might reduce pavement wear effects by about 5 percent.

Rakheja and Woodrooffe investigated the role of suspension damping in enhancing the road friendliness of a heavy vehicle using a quarter-truck model to estimate the loads transmitted to the pavement by a tire. In this model, suspension effects are represented using a sprung mass, an unsprung mass, and restoring and dissipative effects due to suspension and tire. The tire is modeled assuming linear spring rate, viscous damping, and point contact with the road. They found that an increase in linear suspension damping tends to reduce the dynamic load coefficient and the dynamic tire forces, factors which are related to road wear. They conclude that linear and air spring suspensions with light linear damping offer significant potentials to enhance the road friendliness of the vehicle with a slight deterioration in ride quality.

Sousa, Lysmer and Monismith investigated the influence of dynamic effects on pavement life for different types of axle suspension systems. They calculated a Reduction of Pavement Life (RPL) index of 19 percent for torsion suspensions, 22 percent for four leaf suspensions, and 37 percent for walking beam suspensions (an ideal suspension would have RPL of 0). Similar results were found by Peterson in a study for Road and Transport Association of Canada: under rough roads at 80 kph (50 mph), air bag suspensions exhibited dynamic loading coefficients (DLC) of 16 percent, spring suspensions had a DLC of 24 percent, and rubber spring walking beam suspensions had a DLC of 39 percent. Problems with walking beam suspensions were also noted Gillespie et. al., who stated that on rough and moderately rough roads, walking-beam suspensions without shock absorbers are typically 50 percent more damaging than other suspension types.

[continued on next page, Page 6, Section 1.2; (d) Axle Spacing ]

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