Extra axle/wheels on trucks

Every now and then I’ll see a cement mixer or dump truck that looks like it has an extra axle and wheels between the front axle and the two rear axles. These wheels are usually smaller than the rest and don’t hang down far enough to touch the ground.

What do these wheels do? Can they be lowered to provide more power? Or do they help spread out the weight of the load?


IANATD (TD=Truck Driver), but I think they’re called “Tag Axles*”. Their only purpose is to spread-out the load on the highway. I’m pretty sure they don’t supply any power (not drive wheels) and they don’t even have brakes.
Awaiting more knowledgeable dopers who (I’m sure) will be along soon…
*from a distant memory of a long-ago ride with a truck driver relative

There are legal limits as to houw much weight a 2 axle or a 3 axle truck may carry. By having an extra axle that is deployed when the truck is full, more weight can legally be carried on a smaller truck.
In my experience you see these extra axles on dumptrucks, and cement trucks.

All correct except that they can have brakes.

I work with lots of ready mix companies, and occasionally when I’ve been in a dispatch office somewhere I’ve heard one of the drivers call in on the radio that he’s been stopped by the Highway Patrol, local LEO, or the DOT for inspection or some violation.

On a couple of these occasions I know the drivers have had to demonstrate to the officer that the brakes on the tag axel were operational.

So in some jurisdictions at least, they do have working brakes.