Some trucks raise an axle off the road when it isn't needed
Heavy dump trucks and trailers often carry a lift axle that can be lowered to spread a load across more tyres, say to cross a weight-limited bridge, then hoisted clear of the road when empty. Some systems drop it automatically once the main axle hits its weight limit.
Strictly, an axle is the central shaft for a wheel or gear. It can be fixed to the wheels and turn with them, supported by bearings where it meets the vehicle, or fixed to the vehicle while the wheels spin around it on bearings in their hubs; on bicycles that second kind is often called a spindle. In looser speech, axle can mean the whole housing around the shaft, or simply any pair of wheels across a vehicle. That broad sense is what toll roads and vehicle taxes count, as a rough stand-in for weight and road wear.
A straight axle is one rigid shaft joining the left and right wheels, keeping them steady under heavy stress, which is why trains, truck rear ends and heavy off-road vehicles use it. Split axles give each wheel its own shaft, and modern cars have split drive axles. That allows a differential, letting the outer wheel turn faster than the inner one in a corner, which improves traction and extends tyre life, and in some designs it permits independent suspension for a smoother ride. Semi-trailers typically use a tandem pair of axles set close together for extra capacity.
In a front-wheel-drive car, gearbox, differential and front axle merge into one unit called a transaxle, and each half-shaft reaches its wheel through a constant-velocity joint so the wheel can bounce and steer. Rear-wheel-drive vehicles send power down a driveshaft to a rear axle, which in modern cars is usually a pair of half-shafts on either side of a differential. In an independent front-drive layout, these shafts carry none of the vehicle's weight.
Most axles are made from one of two steel families: carbon steel, SAE grade 10xx, or chrome-molybdenum steel, SAE grade 41xx. Chrome-moly resists bending and breaking far better but is hard to weld without a professional shop.
Source: Axle
