A variable pulley structure (10) arranged to be mounted to a shaft (17) with at least one of the sideplate elements (14) thereof movably mounted to the shaft for both axial and rotational movement. Disposition of the movable sideplate element (14) is controlled by a spring (26) extending between a spring retainer (22) fixed to the shaft (17) and a portion (24) of the sideplate element. The spring (26) is retained at its opposite ends to the retainer and sideplate element, respectively, suitably to be unwound as a result of increased torque developed in the movable sideplate element (14) by the drive system. The ends (29,30) of the spring (26) are releasably connected to the retainer and movable sideplate element as by reception in arcuate grooves (27,28) therein, by abutment with suitable stop shoulders (131,132) thereon, etc. Cross section of the spring, in one form, is round and in another form for use in transmitting high load torques, is rectangular.