Shimano STX Rapidfire shifters are known as some of the most accurate, quick and reliable shifters on the market. However, all shifters, no matter how nice, will eventually go out of adjustment and need repair. Rather than spend your time and money at the local bike shop having a mechanic fix your shifters for you, learn how to repair them at home with tools you own. Knowing how to adjust your own bicycle is essential for anyone who rides routinely.
Spin the cranks of the bicycle forward and pull the upshift lever (the finger lever located on top of the shifter) of the right shifter until the chain sits on the smallest rear cog. Shift the left shifter into the centre gear, or gear two, using the thumb lever below the handlebars.
Loosen the rear derailleur cable stop, where the shifter cable is clamped against the derailleur, using a 5mm hex wrench. The shift cable should fall free of the derailleur. Spin the cranks around two or three turns to allow the derailleur to settle.
Stand behind the bicycle and inspect the alignment of the smallest cog, chain and derailleur pulleys. These three items should form a straight line as the chain moves through them. If the chain slants to the right, tighten the high-limit screw (marked with an H) of the derailleur using a Phillips head screwdriver. If it slants to the left, loosen the screw.
Pull the shift cable tight in the rear derailleur cable stop and tighten the stop using a 5mm hex wrench. The cable and derailleur limit system should now be tensioned correctly.
Take the bike for a test ride to ensure that all the available gears work correctly. If you are still having problems, have a certified mechanic inspect your Rapidfire shifters. They may need to be replaced.