1. Field of the Invention
The present invention relates to a multistep sprocket wheel device for a bicycle. More particularly, the invention relates a sprocket wheel device including at least one large sprocket wheel and one small sprocket wheel disposed adjacent to each other, each sprocket wheel defining a plurality of teeth peripherally thereof for engaging a chain. The two sprocket wheels are assembled such that a distance between a first pitch center of one of the teeth on the large sprocket wheel and a second pitch center on the small sprocket wheel adjacent a tangential line of the small sprocket wheel extending rearwardly from the first pitch center with respect to a direction of rotation of the sprocket wheels is substantially a product of a chain pitch multiplied by an integer.
2. Description of the Prior Art
A term used in this specification will be defined here. "The chain disengages or is disengaged from a certain tooth of the large sprocket wheel." means "The chain remains engaged with teeth located forwardly of the above certain tooth with respect to a direction of rotation of the sprocket wheels, and disengages or become disengaged from the certain tooth and teeth located rearwardly with respect to the direction of rotation to move toward the small sprocket wheel."
A known multistep sprocket wheel device for a bicycle as noted above is disclosed in U.S. Pat. No. 4,268,259, for example.
This known device includes a large sprocket wheel defining a ready engagement type tooth among its peripheral teeth. In order for the chain to correctly engage a tooth adjacent the ready engagement type tooth when changing from small sprocket wheel to large sprocket wheel, a distance between one pitch center adjacent the ready engagement type tooth and one pitch center adjacent of one of the teeth of the small sprocket wheel from which the chain disengages is substantially a product of a chain pitch multiplied by an integer as noted above.
The above known device may be modified to assemble the two sprocket wheels such that, in order to allow a smooth change from large sprocket wheel to small sprocket wheel, the above distance between the first and second pitch centers has an appropriate value.
Such a modification, however, does not provide for a smooth change from large sprocket wheel to small sprocket wheel when the chain disengages from a tooth other than the (first) tooth serving as a reference for adjusting the distance between the first and second pitch centers. In this case, the chain will mount tooth crests instead of correctly engaging the teeth of the small sprocket wheel since the chain is displaced to a great extent from the first pitch center toward the center of the sprocket wheels.
In the above modification, the large sprocket wheel may define a ready disengagement tooth or teeth for allowing the chain to readily disengage toward the small sprocket wheel from at least the first tooth and teeth adjacent and forwardly thereof in the direction of rotation, and may be formed of a metal such as aluminum which has a low abrasion resistance. In this case, the large sprocket wheel will become worn locally at positions adjacent the first tooth and on the side opposed to the small sprocket wheel through sliding contact with the chain. At an advanced stage of wear of the large sprocket wheel, as shown in FIG. 6, the chain 3' having disengaged from the first tooth or adjacent teeth and moving to the small sprocket wheel 2' falls toward the center of the sprocket wheels and becomes inclined at a large angle 40' to the large sprocket wheel compared with an originally designed angle as shown in a solid line. As a result, the curve of the chain 3' being shifted reaches its limit, whereby the chain tends to fall over to the side of the small sprocket wheel 2' away from the large sprocket wheel 1'.