Presented are crankshaft assemblies with internal stiffening structures, methods for making/using such crankshaft assemblies, and internal combustion engines equipped with such crankshaft assemblies. A crankshaft body, which is formed with a first material, includes multiple bearing journals that are mutually coaxial to rotate on a crankshaft axis and spaced from each other along the length of the crankshaft. Each bearing journal has an internal journal cavity. Multiple crankpins are longitudinally spaced from each and axially offset from the crankshaft's rotational axis. Each crankpin has an internal crankpin cavity. Multiple crank webs project radially from the crankshaft axis and interconnect the bearing journals with the crankpins. Each crank web has an internal web cavity. Disposed within the journal cavities, crankpin cavities, and/or web cavities is a stiffening bar formed with a second material having a modulus of elasticity that is greater than the modulus of elasticity of the first material.