Join Date: Sep 2003
Location: Memphis TN
Bikes: Raleigh, Benotto, Schwinn, Trek
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The following was copied from Sheldon Browns website:
The low-gear limit stop, stops the derailer from shifting past the smallest chainwheel and throwing the chain onto the bottom bracket shell. If it is too loose, the chain will fall off when you try to downshift to the small chainwheel. If it is too tight, you it will be difficult or impossible to shift down to the low chainweel.
On older front derailers, the low-gear stop is the one closer to the frame. Many newer designs reverse this position for reasons relating to the mechanism used.
The basic adjustment for the low-gear stop is to set it so that the chain just barely clears the inner plate of the cage when the lowest gear (small front, large rear) is selected. This will usually be the best position for double-chainwheel setups, and will permit the use of most or all of the rear sprockets with a minimum of trimming.
For triple chainweels, it will sometimes be necessary to adjust the low-gear stop a bit looser, so that the outer plate of the derailer can travel far enough to knock the chain off of the middle ring.