section 1.2 Truck Characteristics Affecting Pavements; (e) Liftable Axles

Billing et. al. investigated the use of liftable axles. They found widespread use of these axles in Canada. For example, a 1988 and 1989 surveys in Ontario and Quebec found 17 and 21 percent (respectively) of trucks on the highways had liftable axles. Truckers frequently adopt liftable axles in response to weight limits under which maximum gross weights are higher for trucks with more axles. Also, trucks with multiple, widely spaced axles have difficulty turning on dry roads. Industry has in some cases resolved this difficulty through the use of liftable axles, which can be raised or lowered by the driver, usually with air pressure. The driver raises a liftable axle when a turn is being made and lowers it when the turn is completed. The axles can also be raised when cruising along the highway to improve fuel consumption and reduce tire wear.

On the negative side, liftable axles make compliance with and enforcement of axle weight limits difficult. There are many concerns about the use of liftable axles and damage to roads and bridges. Improperly adjusted liftable axles can be extremely damaging to pavements. The liftable axle can be adjusted to any level by the driver. If the liftable axle load is too high, the liftable axle is overloaded. If it is too low, other axles may be overloaded (Billing et al). For example, under current Federal limits, a four-axle single-unit truck with a wheelbase of 30 feet can carry 62,000 pounds: 20,000 pounds on the steering axle and 42,000 pounds on the rear tridem. This vehicle would produce approximately 2.1 ESALs on flexible pavements. However, if the first axle of the tridem is a lift axle that is carrying no weight, this vehicle would produce approximately 4.0 ESALs.

[continued on next page, Page 8, Section 1.2; (f) Tridem Axles ]

« Previous 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 | Next Page »

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