I think I overshifted, going down from 3-1 approaching the lights. I have the Shimano Gripshift rotary shifters, not sure what Shimano call them. Now the shifter goes past 1. Can't get the biggest ring in the back at all. Is it a cable issue or a derailleur fix? ie, where's the issue - barrel adjuster in the derailleur or massive cable stretch or something dislodged inside the shifter on the bar?