Generally, this kind of speed-changing device is so constructed that the control lever is operated to move forward to thereby pull the wire so that the four member linkage mechanism can be transformed against the return spring provided at the four member linkage mechanism, whereby the chain guide can move forward. The control lever is also operable to return so as to allow the return spring to return so that the chain guide can also return.
However, in this construction, when the control lever is operated while applying thereto a rotational resistance which overcomes the return spring, it is necessary to keep the control lever in a predetermined functional position. Hence, especially when the control lever is operating against the return spring, it is necessary to apply to the lever a force which overcomes the force of the return spring and the rotational resistance, so that a significant amount of force is required to operate the control lever and a light touch speed-change operation cannot be performed.
With respect to the above problem, it has been proposed that one of the four members which constitute the linkage mechanism at the derailleur be provided with a positioning plate which has a number of recesses therein corresponding to the number of speed-changing stages. Another member moving relative to the member which is provided with the positioning plate is provided with a ball engageable with the recesses. When the above two members move relative to each other, the ball travels with respect to the recesses to thereby engage with a selected one of the recesses of a selected speed-changing position, whereby the speed-changing positions of the chain guide are determined and these positions can be maintained.
However, in the above construction, because the ball is resiliently engaged with the recesses for maintaining the positions of the chain guide the return spring can not be used to cause the chain guide to return through the returning function of the return spring. As a result, it is necessary to utilize two wires of a general pull-type wire or to utilize a push-pull type wire for forcibly reciprocating the chain guide.
According to the above construction, since the return spring is not used, operation of the lever can be performed only by a force with which the ball can disengage from the recesses, whereby operation of the lever can be performed lightly to that extent. However, this construction cannot utilize a pull-type wire as the control wire thereby causing a problem that the speed-change device is expensive to manufacture.