Originally Posted by JTownCitycycle

Hi everyone, I never had this specific problem in my 30 years of riding, last winter I replaced my chain/sprocket on my 1987 Schwinn Prelude road bike because of chain stretch. The rear derailleur is shifting perfectly but when I change from the big chain ring up front to the small one occasionally the chain will slide off the rear derailleur pulley as shown in the photo and I have to get off the bike to put it back on. any help please in what could be causing this would be appreciated.