Many of the garbage trucks that service my area are large compactor trucks with 3 rolling axles plus one set of smaller wheels between the back 2 axles and the one up front, a la the truck in the pic on this web page.
The smaller wheel is clearly not for riding on, nor could it possibly be a spare. What is it for? Something to do with unloading?
"From my sources deep within the trucking industry comes the following: Apparently, it has to do with axle weight limits for trucks. The amount of weight that a truck can have per axle is decided by the department of transportation in each U.S. state.
When you load a dump truck with gravel, the weight can easily exceed the per-axle limit, so the extra wheels/axles are lowered to make the truck legal. They are raised at other times to improve gas mileage and eliminate wear on the tires."
Known as a “lift” axle, it can be lowered to support some of the trucks weight. There are laws determining the weight allowed per axle. In most cases the three “big ones” are enough to carry the load, but if the truck takes on a heavy load, or drives in an area with a lower allowed weight per axle exists, it can be lowered to the roadway and make the truck legal.
Here’s a manufacturer’s site.
Often concrete trucks have them on the back and they are raised and lowered hydraulically.
But their primary use is poppin’ and ridin’ wheelies, right?
Aha. Thank you!
So… is it purely a legal issue or does the extra axle actually provide any real practical benefit? The statement “the extra wheels/axles are lowered to make the truck legal” would seem to infer that it’s done strictly for legal reasons.
They do carry weight and I’m guessing that an experienced operator can adjust contact pressure to meet legal requirements. I’ve seen a concrete truck lower his axle and the truck frame did raise up somewhat.
tires have a maximum weight rating. if the truck’s load is heavy enough, dropping the lift axle will distribute the load weight across more tires.
So, as an interesting aside - since the question has been answered.
Have you ever noticed black marks/skid marks on the road right after a significant bump? You see it quite abit on 2 lane country roads where the speed limit is 55. Look close at bridge approaches and departures.
Those black marks are caused by the non spinning wheels on the helper axle hitting the road when the truck goes over those bumps.
Now I just need the “the more you know” music to play!
The benefit is the roads don’t break apart.
I have heard these called “Michigan Axles”. Michigan, apparently, has particularly low per-axle weight limits and is serious about enforcement.
have you driven our roads?
They’d be a lot worse without the limit.
They can carry a lot of weight - on things like large RVs and tour buses, the extra axle (often called a tag axle on these vehicles) can add 10,000-20,000 pounds of carrying capacity. They also add braking capacity.
AFAIK, the difference between a tag axle and a pusher or lift axle is just location. Tags are behind the driven rear axle, and lifts are in front of the driven rear axle.
Yes, for a couple weeks in Detroit area. It was unusual to see a heavy truck ther without the subject axles, while in many other places they are rare. I remarked on this to my customer, and the response was “oh yeah Michigan axles!”
Oh yeah I also drove up to near Kalamazoo to pick up my truck camper(eBay purchase) and that highway had me wishing for a sports bra. I’m male BTW
To elaborate a bit, it’s my understanding that the general rule of thumb for road designers is that that wear and tear a vehicle causes to the road is proportional to the weight per axle raised to the fourth power. So, exceeding the weight limits by not so much can have a huge impact. Lowering the extra axle (going from three axles to four) with the same weight reduces road wear to about 39% of what it was before lowering the axle, by my quick calculations.