Booster

An extra axle assembly added behind the trailer to spread weight to the rear.

A booster is a trailing axle (or axles) added to the back of a heavy-haul trailer. Like a jeep up front, it spreads the load's weight over more axles to meet weight limits — boosters handle the rear, jeeps the front. The more an axle group can spread weight, the heavier (within reason) the load can legally be.