Chain-link Wear and Loading on alternate teeth
In addition to the high loading placed on the teeth of small chain-rings, and the general overloading of upper teeth when the chain "stretches" due to wear, there is an additional overloading effect from the stretching.
The pitch of inner and outer links starts out essentially identical when new but, because of the nature of bicycle chain design, subsequent wear changes this. Assuming wear in chain joints is essentially even along the chain length, then :
The contact pitch of rollers bridged by inner-links remains basically constant - the rollers are carried directly by the bushings (swaged on the inner link plates), and thus have a fixed separation even when their interface wears
The contact pitch of rollers bridged by outer-links increases - the inner faces of their link-pins wear against the outer half of the bores of the bushings (swaged on the inner link plates) ; this causes each set of inner link plates (as a whole) to move farther apart from the next set of inner links ; since each set of link plates carries a roller-pair, each set of roller-pairs will move apart from the next set of roller pairs
This results in play caused by wear within the chain joints to be manifested only across the outer-links - and chain stretch presents only over these alternate links.
Because of this, every second roller now applies all (or more) of the chain load from a link-pair to the teeth. Thus only every second tooth is loaded (or more highly loaded). This re-distribution of tooth loading contributes to over-loading of individual teeth; it starts even when the amount of chain wear is quite small.
Copyright © Jonathan Levy, 2000. All rights reserved.