How to Check for a Bent Derailleur

Your bicycle's derailleur shifts the chain from one gear to another. If the hanger upon which the derailleur is attached is bent, however, shifting will no longer be accurate. A bent derailleur hanger will often happen as a result of an accident, particularly if the bike falls on its right side, where the derailleur and its hanger are positioned. A tool called a derailleur alignment gauge (DAG) exists for checking the horizontal and vertical alignment of the derailleur hanger. Learn to use this tool to not only determine alignment, but also to correct any misalignment.

Things You'll Need

  • Bicycle work stand
  • Derailleur alignment gauge
  • 5 mm Allen wrench
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Instructions

    • 1

      Elevate your bicycle's rear wheel off the ground, either by having an assistant raise the rear of the bike or by placing the bike in a bicycle work stand.

    • 2

      Use the rear gear shifter to shift the chain down to the smallest rear cog.

    • 3

      Use a 5 mm Allen wrench and loosen the bolt that attaches the derailleur to the derailleur hanger. Pull the derailleur away from the bicycle. The derailleur will still be attached to the bicycle by its cable as well as by the chain.

    • 4

      Thread the derailleur alignment gauge (DAG) shaft into the derailleur mount on the derailleur hanger. Use the knob on the gauge to tighten the shaft in place.

    • 5

      Rotate the DAG to the 9 o'clock position on the wheel. Turn the wheel so that its valve is also at the 9 o'clock position.

    • 6

      Adjust the DAG indicator so that the tip touches the side of the wheel. Tighten the indicator knob to lock the indicator in position.

    • 7

      Rotate the indicator knob and wheel valve simultaneously to the 3 o'clock position.

    • 8

      Note the proximity of the indicator to the side of the wheel in the three o'clock position. The indicator should be within 4 mm from the side of the wheel, indicating a proper horizontal alignment of the derailleur hanger.

    • 9

      Use the DAG arm as a lever to gently and gradually bend the derailleur hanger left or right as needed. Use the DAG indicator to check that the adjustment places the tip of the indicator within 4 mm of the side of the wheel.

    • 10

      Rotate the DAG and wheel valve to the 6 o'clock position. Adjust the indicator so that its tip is touching the side of the wheel. Tighten the indicator knob to lock the indicator in place.

    • 11

      Rotate the DAG and wheel valve to the 12 o'clock position. Note the proximity of the indicator tip to the side of the wheel. The tip should be within 4 mm of the side of the wheel, indicating proper vertical alignment of the derailleur hanger.

    • 12

      Use the DAG as a lever to bend the derailleur hanger so that the gap between the DAG indicator and the side of the wheel is within 4 mm.

    • 13

      Unscrew the DAG shaft from the derailleur hanger once all measurements and adjustments are complete.

    • 14

      Thread the derailleur mounting bolt back into its mount of the side of the derailleur hanger. Use the 5 mm Allen wrench to tighten the bolt firmly.