Axles and Suspensions

Axles were originally defined as rigid lateral connections between two wheels which could be steered together. This type of axle helps ensure rolling stability and simplifies assembly by providing a connection between the wheels and body. This axle definition is valid for all rigid and semi-rigid axles. Such designs feature wheels that are attached directly to the axle. This type of axle follows the motion of the wheels at all times. The direct connection between the wheels was eliminated with the introduction of independent suspension systems, which feature wheels that are mounted to a steerable wheel carrier. This wheel carrier is part of a kinematic linkage which connects the wheel to the axle. The term “axle” is less easily defined for these systems. In some cases, the axle is defined as the entire system, including the independent suspension and wheel carrier. In other instances, the new components (suspension linkage and wheel carrier) are considered a separate assembly (known in North America as a “corner module”) in addition to the axle and subframe. This book will use the latter definition, as the former is based on the concept of rigid axles, which are increasingly irrelevant for passenger vehicle design.