If the drive chain jumps a gear from hitting an obstruction when you are riding your bike, replacing the chain usually solves the problem. However, sometimes a chain will come off a gear sprocket due to improper adjustment of either the front or rear derailleurs, the mechanisms that control where the drive chain sits on either the front chainring or the rear gear assembly. You can adjust the derailleurs with basic tools and a bike stand.
Preparation
Place the bicycle in the bike stand and secure it so the pedals and rear wheel turn without interference. Have an Allen wrench and Phillips screwdriver available to adjust the derailleurs.
Front Derailleur on Frame
Loosen the Allen screws on the clamp for the front derailleur and adjust its position on the frame of the bicycle, if necessary. Set the derailleur cage to just above the largest sprocket on the chainring. Set the angle of the derailleur cage parallel to the gears on the chainring. Tighten the screws in the clamp to secure the front derailleur to the bike frame.
Front Low-Gear Limit
Turn the pedals on the bike. Shift the rear derailleur to the biggest sprocket. Shift the front derailleur to the smallest chainring gear. Adjust the low-gear limit screw on the front derailleur with the screwdriver to align the derailleur cage with the smallest gear, so the cage prevents the chain from coming off the gear. Turn the pedals to check the setting.
Front High-Gear Limit
Turn the pedals again and shift the rear derailleur to the smallest sprocket and the front derailleur to the biggest gear on the chainring. Adjust the high-gear limit screw to align the derailleur cage to prevent the chain from coming off the gear. Turn the pedals and check the setting.
Final Front Adjustment
Turn the pedals again. Shift the rear derailleur to the biggest sprocket. Shift the front derailleur to the smallest chainring gear. Stop the pedals and wheel. Loosen the cable clamp bolt on the front derailleur using the Allen wrench. Tug the shift cable to remove slack in the cable. Tighten the cable clamp bolt.
Rear Derailleur Guide Pulley
Turn the pedals and shift the rear derailleur to the biggest sprocket. Check the space between the derailleur's guide pulley and the sprocket. Turn the derailleur body-adjustment screw clockwise to drop the pulley away from the sprocket or counterclockwise to bring it toward the sprocket until just enough space is left for the pulley to clear the sprocket.
Rear High-Gear Limit
Shift the rear derailleur to the smallest sprocket. Loosen the shift cable adjuster to release tension on the shift cable. Turn the pedals and watch the chain ride on the smallest sprocket. Turn the high-gear screw on the rear derailleur clockwise if the chain appears to come off the sprocket, or counterclockwise if the chain rides against the next sprocket.
Read Low-Gear Limit
Tighten the rear shifter cable adjuster to place tension back on the shift cable. Turn the pedals and shift the rear derailleur to the biggest sprocket. Watch how the chain rides on that sprocket. Turn the low-gear screw on the rear derailleur clockwise if the chain appears to come off the sprocket into the spokes, or counterclockwise if the chain rides against the next sprocket.
Final Rear Adjustment
Turn the pedals and shift the rear derailleur down, then up, then down, watching the chain as it turns on each rear sprocket. Tighten the rear shift cable adjuster to increase tension on the cable if the chain rides on the side of the next biggest sprocket, or loosen the tension if the chain wants to drop onto the next smaller sprocket.
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