According to the Specialized Carriers and Rigging Assn., lift axles are allowed for extralegal weight if they meet two tests:
1. All axles within an axle loading group must have the same suspension system.
2. Each axle in an axle loading group must carry an equal share of the axle loading group weight, within 10% of the average weight per axle in the loading group.
A lifting device that raises an axle for backing is allowed if its function does not interfere with the equal weight distribution requirement.
Highway-legal trucks have used life axles in California for more than 30 years.