How to Use the Trimmer on the Shimano 105
The two gear shifters of the Shimano 105 each control a derailleur, which changes gears by moving the chain from one sprocket to the next. There is one derailleur at the front and one at the rear. As the rear derailleur is shifted, the angle of the chain can change and it can rub against the side of the front derailleur cage, causing wear. A trimmer allows fine adjustments to be made to the position of the front derailleur to prevent rubbing.
Inspect the Shimano 105 derailleur cage and determine which side the chain is rubbing against. If the chain is on the biggest sprocket in the front, the chain is rubbing the inside. If the chain is on the smallest sprocket in the front, it is rubbing the outside.
Push the smaller of the two gear shift levers downwards slightly if the chain is rubbing the inside. Check that the chain is no longer rubbing.
Push the larger of the two gear shift levers downwards slightly if the chain is rubbing the outside. Check that the chain is no longer rubbing.
Use the trimmer function whenever the chain rubs. If you ride with the chain rubbing, it will wear a groove in the side of the front derailleur cage and make it hard to shift gears.
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