Why trailers have floating wheels that never touch the ground @ma3route

Ever notice that some big trucks sometimes have random-looking extra tyres attached to them that seem to be pointlessly floating? Those aren’t spares and they’re not for decoration, even when they’re mounted on the top of the truck.

The simplest explanation is that the more axles a truck has, the more weight it can carry. But the more axles a truck uses, the less manoeuvrable it is. More axles also make a vehicle less efficient by adding rolling resistance and even cost the operator extra money through tolls. Plus, of course, more tires in use means more tyres to replace, and big truck tyres aren’t cheap.

Hence why some heavy load-hauling trucks have what are known as “drop axles,” which are exactly what they sound like. Drop axles spend most of their time tucked up against the truck’s belly until extra weight capacity is required, then can be lowered when needed.

Drop axles are also significant from a legal standpoint. Legal weight limits for commercial vehicles, which vary by country, are posted per axle. So not only do the extra wheels help bear physical weight, they’re required for compliance with trucking laws.