What is the purpose of a tag axle?

Tag (also referred to as lift) axles allow more weight to be carried, by creating more of a contact surface with the road for weight to be distributed on. When it's behind the drive axle(s), it's a tag axle; when it's in front of the drive axles, it's a pusher axle.

Some companies set up tandem axle trucks where the rear drive axle is a tag axle, so that it can be lifted up to shorten the turn radius of the tractor. Schneider National used to have some Freightliner Argosy COEs which were set up this way.

In Europe, the use of a tag axle doesn't increase the amount of weight which can be totaled, but their use is encouraged because it puts less stress on the roadways than a 4x2 tractor.

Tag axles can be somewhat problematic - especially on a single axle truck - in that they effectively can turn the drive axle into a fulcrum, placing extra weight on the steer axle.

However, in many states, total allowable gross weight is determined by vehicle wheelbase, and tag axles may be used rather than pusher axles because they effectively extend the vehicle's wheelbase, thus allowing a higher gross weight.