Volvo Trucks/YouTube
If you spend a lot of time on the road, you might notice a few strange details about the semi trucks you see on a regular basis — things like Mansfield Bars, or, in this case, seemingly useless axles, floating above the pavement, with a truck's tires not touching the ground. From an outsider's perspective, it doesn't seem to do much of anything, but surely it's not just there to look pretty or to serve as a placeholder for a few spare tires. It must serve some function, right?
That odd-looking piece of tech is called a lift axle. I suppose you could say it does what it says on the tin — it lifts the axle. It's common on many heavy-duty haulers, semi-trucks, and flatbeds, and this extra set of wheels is only there to do the job part-time, according to Cush Suspensions, a heavy-duty suspension supplier based in Missouri (cool name, by the way). These axles — whether fitted to the trailer or the truck itself — are there to assist with weight distribution when carrying large loads.
The axles can be raised or lowered with the help of a pneumatic airbag controlled by the driver. Once the axle is dropped into place, it's locked there by air pressure in the trailer's air ride suspension system. For the more old-school operator, some trailer lift axles can actually be operated manually with a mechanical lift system, as well.
How much does a lift axle actually help?
STRONGA Innovations/YouTube
This might be a bit shocking to hear, but heavy-duty semi-trucks sometimes have to haul a lot of weight. The majority of that weight rides on the trailer's axles, which have to distribute the weight under load to the road. With lift axles, the amount of weight per axle can be adjusted in certain scenarios, like when the truck is going over a bridge that has a weight-per-axle restriction. So, if a truck is hauling 80,000 pounds on four axles, that breaks down to 20,000 pounds per axle. Drop that fifth lift axle, and suddenly it becomes 16,000 pounds per axle. That makes a big difference. Because of the weight distribution benefits, lift axles allow for heavier loads to be brought onboard.
Lift Axles provide other benefits as well. They can give the rig a smoother ride — especially on unpaved roads. It helps stabilize freight and keep it from shifting or falling off the trailer. However, because they can be lifted up when not in use, rolling resistance goes down, and fuel economy is improved. We all love that, don't we? I know truckers who pay for fuel out of their own pocket do.
They'll also love the fact that only using the tires part-time means they're going to last longer. What's more, some states allow lower tolls on vehicles using fewer axles on roads and bridges.