A bicycle is disclosed as having a seat and a seat post assembly supported supporting the seat. The seat post assembly includes a lower seat support, an upper seat support secured to the seat and movable (e.g., telescopically) relative to the lower seat support, a locking mechanism for selectively locking and unlocking the upper seat support relative to the lower seat support, and an actuating mechanism to facilitate moving the upper seat support relative to the lower seat support. The actuating mechanism comprises a spring having first and second nodes, and a drive system adapted drive the second node while the first node is engaged with the upper seat support to energize the spring. In one embodiment, the drive system comprises a reciprocating driver constructed to drive either one of the first node and the second node. Preferably, the reciprocating driver is driven by a unidirectional rotor.