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Chain Deflector
In some triple-chainring installations, typically when the "granny" gear is unusually small, it may be impossible to get good shifting to the "granny" chainring with the normal derailer adjustments.
A loose adjustment of the low-gear stop causes the chain to derail past the small chainring, but a tighter setting results in slow downshifting to the small ring.
In such cases, a good, if inelegant, solution is sometimes to install a chain deflector, an anti-derailment device that clamps to the seat tube. These products, such as the 3rd Eye Chain Watcher ® and the N-Gear Jump Stop ® set up a barrier preventing the chain from overshooting the small ring, no matter how loose the low-gear stop is set. This allows the low-gear stop to be set to allow the derailer to move farther inboard for faster, more precise shifting, even under some load. These devices can often save the day when extra-wide range gearing is used on a mountain bike or tandem.
