Tag axles are a way to increase the amount of concrete a truck can haul, while still complying with federal and state requirements. A tag axle is a retractable extra axle, mounted behind the drive axle on a ready-mix truck. It is lowered when transporting a heavy load, in order to distribute the truck's weight over more axles, and over a longer wheelbase.
The Federal Bridge Formula, Formula B, regulates the weight limits for trucks as applied to interstate highways and bridges. It uses the length of a truck's wheelbase to calculate the maximum weight that truck can carry. A tag axle's retractability makes it possible to lengthen the wheelbase of the truck and thereby increase the weight limit, while keeping the truck maneuverable in the tight confines of a construction area.
Having a tag axle on a ready mix truck means some adjustments have to be made to the way the truck is driven. The axle's hydraulic pressure has to be adjusted for each load it carries. The expanded wheelbase increases the truck's turning radius. Use of a tag axle requires special driver training to prevent incorrect use of the axle.