Why some truck wheels never touch the road

Some truck wheels do their best work by not touching the road. They're attached to lift axles, which can be raised when the vehicle doesn't need their support. Leaving every wheel down sounds like the sensible choice, but extra axles can make a lightly loaded vehicle harder to maneuver and give it more tires to wear out. The useful arrangement changes with the load. A truck carrying cargo and the same truck returning empty have different needs, even though they're hauling around exactly the same hardware. Lift axles let that hardware get out of the way.

The reason to lower an extra axle starts with where the weight goes. A truck's total weight isn't the whole story: the load carried by individual axles and groups of axles matters too. So does the distance between them. Two vehicles can have the same total weight and the same individual axle weights, yet place different stresses on a bridge because one spreads its load over a longer distance. That's why the United States' federal bridge formula considers both axle count and spacing. More wheels aren't a magic permission slip to carry anything. Their position matters, and each axle group still has to meet the applicable limits.

Getting those wheels into the air takes a lifting mechanism built into the suspension. Air-operated designs use an air spring as part of that mechanism, moving the axle assembly rather than folding each wheel away separately. The tires stay attached to the axle; the assembly rises far enough to clear the surface. Lift systems need controls to raise and lower them, and some designs handle the decision automatically. On Volvo's Tandem Axle Lift, a driven axle can disengage and rise when the truck is unloaded, then automatically lower and engage again when the truck is loaded. The vehicle changes which wheels are working without anyone removing a wheel.

Corners explain another reason those suspended tires are useful. A fixed, non-steering auxiliary axle can scrub its tires across the surface during a tight turn. That sideways resistance doesn't just eat rubber. It also puts lateral stress into the wheels and chassis components. Raising the axle removes that particular argument between the tires and the road. A steerable auxiliary axle offers another way to deal with the problem while keeping its wheels down. Which arrangement makes sense depends on the vehicle's work and the rules where it operates, since some authorities require auxiliary axles to remain lowered during turns.

The payoff can reach beyond tire life. Volvo's liftable driven-axle system can reduce fuel consumption when the truck is unloaded and allow a tighter turning radius. That's the larger idea behind the apparently useless wheels dangling above the pavement: carrying capacity is valuable when there's weight to support, but keeping every axle working isn't always the best choice after unloading. The truck still carries the extra equipment. It simply stops asking that equipment to touch the road when doing so brings more cost than benefit. Those wheels haven't been forgotten. Their job includes knowing when to sit a trip out.