Typical multi-speed bicycles have a crankset with two or three chainrings and a rear cassette with between five and eleven sprockets. Bicycle chains have a plurality of inner links and a plurality of outer links that are interconnected in an alternating manner by a plurality of pins. Typically, each of the inner links includes a pair of parallel inner link plates connected by a pair of parallel hollow cylinders or pins surrounded by cylindrical rollers. Each of the outer links includes a pair of parallel outer link plates connected to the adjacent inner links by the pins of the adjacent inner links.
Chainrings and sprockets may be referred to generally as gears, and their size may be specified in terms of the number of teeth on the gear: a relatively larger gear is larger in diameter and has more teeth; a relatively smaller gear is smaller in diameter and has fewer teeth. The rider selects a desired speed (i.e., gear ratio) by operating a front derailleur to move the bicycle chain laterally onto a desired chainring and a rear derailleur to move the chain laterally onto a desired rear sprocket. A relatively larger front chainring corresponds to a harder gear (upshifting). A relatively larger rear sprocket corresponds to an easier gear (downshifting).
Upshifting at the crankset and downshifting at the rear cassette (in both cases, moving onto a more-toothed, larger-diameter gear) conventionally require that the larger gear have specially-profiled shifting teeth, shifting ramps and/or pins to assist the chain in moving upward and laterally to engage with the larger gear. However, even when such features are appropriately deployed on a well-adjusted bicycle, selection of the larger gear can be noisy and subject to skipping, slipping, delayed chain engagement, chain suck, and other problems.
Prior art FIG. 1 is a side view that depicts an in-progress downshifting operation in which chain 10 is being disengaged with a smaller sprocket 11 and engaged with larger sprocket 12. During this operation, chain 10 is being moved laterally by a rear derailleur (not shown) in a direction into the page (i.e., away from the reader), with the smaller sprocket 11 being in front of the larger sprocket 12 (i.e., closer to the reader coming out of the page). Sprocket 12 is rotating clockwise, as indicated by the drive direction arrow DD in the figure.
FIG. 1 depicts three outer links: outer link 16; outer link 18; and outer link 20. Each outer link has two outer link plates, one of which is visible in the figure (the other is deeper into the page and thus obscured in this side view). Specifically, outer link plate 16a of outer link 16 is visible; outer link plate 18a of outer link 18 is visible; and outer link plate 20a of outer link 20 is visible.
Outer link 16 in the figure is fully engaged with sprocket 11. In other words, sprocket tooth 11a is fully received between the outer link plates 16a of outer link 16. In this depiction, links that engage later in time relative to other links may be referred to as “upstream links,” since they are upstream relative to the drive direction DD. For example, outer links 18 and 20 are upstream relative to outer link 16; outer link 20 is upstream relative to outer link 18; etc.
One problem that can occur while the chain 10 is being shifted from the smaller sprocket 11 to the larger sprocket 12 is that the outer link plate 18a on the opposite side (facing into the page and not visible in FIG. 1) of outer link 18 may come into contact with tooth 12a of sprocket 12 at point P, causing the outer link 18 to be lifted undesirably and ride up upon the tooth 12a as the sprocket 12 rotates. This can cause an unnatural bend in the chain upward as compared to the natural position 13 (shown in dotted lines) that the chain would assume if it was not undesirably lifted by tooth 12a. FIG. 6 is a cross-sectional view taken along line 6-6 in FIG. 1, and illustrates the contact between the sprocket tooth 12a and the outer link plate 18a at point P. This contact lifts outer link 18 an undesired lift distance D1. As a consequence of the chain being undesirably lifted in this manner, upstream links from outer link 18 are prevented from smoothly engaging with the sprocket 12 while the tooth 12a lifts the outer link 18. Further, outer link 18 may suddenly fall from the top of the tooth 12a, and although this may result in engagement of the tooth 12a and outer link 18, it may cause a shock to be transmitted from the sprocket 12 to the rider, resulting in an unpleasant shifting feel.