from the Park tools website: www.parktool.comRepair Help


Shifting issues related to chainline
If either the front or rear sprockets are either inward or outward relative to the other, there may be certain shifting problems. However, if a bike chainline measures off, yet the bike has no problems shifting, the bike should be considered acceptable. No "fixing" of the chainline is then required. The following are typical problems that may be caused by chainline issues.
Chain jumping off large chainring when front derailleur is correctly adjusted.
Chain riding off lower derailleur pulley when derailleur or hanger is not bent
Chain rattling on inner faces of front chainrings.
Chain derailling off inner chainring when front derailleur correctly adjusted.
Front derailleur cannot be adjusted to stop over shifts while still allowing good shifting.
On certain bikes, the chain may tend rub and rattle against the front rings while riding in certain gear combinations. This is common on many bike when riding in the so-called "cross-chaining" combination of the smallest front ring, and the smallest rear cog. An example of a gear combination that is likely to rub is shown below.



Generally, the shorter distance from the bottom bracket to the rear hub, the more likely a rattle from gear combinations will occur. There may in fact be several gears that are unuseable on any given bike. It is possible to minimize or eliminate this problem by moving either the front or rear cogs. Even if the bike is has so-called perfect chainline, it may help the riding and shifting to create an "error" to solve shifting existing issues.