If shifting suddenly becomes erratic after a fall onto the bicycle’s right side, the cause may be neither the cable nor the derailleur adjustment. The small hanger between the frame and rear derailleur is often designed to bend under impact, helping protect the more expensive frame and derailleur.
A bent hanger changes the derailleur’s position relative to the rear sprockets. Several gears may shift poorly even though everything worked before the fall. If the derailleur appears crooked, a mechanic can use a purpose-made gauge to check the hanger against the wheel.
Turning the shifting screws alone will not restore that geometry. Many hangers can be straightened, but one that has bent repeatedly or is damaged may need replacement. Checking whether the mechanical foundation is straight before tuning the gears can prevent a frustrating cycle of poor adjustments.