A solid axle is an axle structure that connects two wheels, with no relative motion between the two wheels on the same axle, forming a complete suspension system.
The history of the solid axle began almost simultaneously with the birth of the automobile, and its structure has not changed significantly to this day. A drive axle is mainly composed of a differential housing, axle tube, half-shafts, bearings, and other components, while a non-driven axle has a simpler structure and is now mostly used in trucks. There are two types of solid axle half-shaft structures: semi-floating and full-floating. A semi-floating half-shaft connects directly to the wheel hub, bearing both driving force and longitudinal torque, whereas a full-floating half-shaft connects to the wheel hub via a flange and bearings, only transmitting driving force, with the axle tube bearing the longitudinal torque, making it more widely applicable.
The differences in solid axle suspension mainly lie in the elastic elements paired with the suspension. Common types include leaf springs and coil springs, with some rarer air springs and torsion bar springs. Leaf springs offer strong load capacity and a simple structure, but tire travel is restricted, leading to average on-road performance and comfort; coil springs, on the other hand, provide greater tire free travel but require complex link connections, and maintenance is relatively more intricate.
Furthermore, to increase ground clearance, many off-road vehicles adopt a portal axle structure, where the half-shaft ends are higher than the wheel centers, driving the wheels through gear sets at both ends of the axle. This axle structure is more complex and is often used in models designed for extreme off-road conditions.
