Abstract: An automatic derailleur device comprises an arm, a sprocket roller meshed with one of a set of different-sized sprocket wheels on, a control member, a tension wheel meshed with a sprocket chain and secured to the arm, a cylinder including a shaft secured to the arm, an intermediate collar on the shaft, an elastic member on the shaft and biased between the collar and the cylinder, and a hollow cylindrical block with the shaft passed through, and a pump in the cylinder and including a piston rod rotatably coupled to the sprocket roller, a plurality of blades extended from the piston rod, and a cylindrical passage formed between the cylinder and the block. The sprocket chain is shiftable from a meshing with a largest sprocket wheel to a smallest sprocket wheel automatically as the bicycle is accelerating from a stop or low speed to a high speed and vice versa.

Abstract: An automatic derailleur device comprises an arm, a sprocket roller meshed with one of a set of different-sized sprocket wheels on, a control member, a tension wheel meshed with a sprocket chain and secured to the arm, a cylinder including a shaft secured to the arm, an intermediate collar on the shaft, an elastic member on the shaft and biased between the collar and the cylinder, and a hollow cylindrical block with the shaft passed through, and a pump in the cylinder and including a piston rod rotatably coupled to the sprocket roller, a plurality of blades extended from the piston rod, and a cylindrical passage formed between the cylinder and the block. The sprocket chain is shiftable from a meshing with a largest sprocket wheel to a smallest sprocket wheel automatically as the bicycle is accelerating from a stop or low speed to a high speed and vice versa.