Shimano STX is a shifter set made for bicycles with flat mountain-bike style handlebars. The shifters feature separate levers for upshifting and downshifting, and built-in cable slack-adjustment, making their use seamless and intuitive. However, in order to experience the benefits of the system, the shifters must be set up correctly with the front and rear derailleurs of the bike. You can adjust your STX shifters at home and save time and money over taking your bicycle to the local bike shop.
Clamp your bike by its seat post in a dedicated work stand. If you don't have one, you can set the bike in a stationary trainer or upside down on its handlebars and seat.
Loosen the cable stop of each derailleur (gear system) using a 5-mm hex wrench. Spin the pedals by hand to allow the bike to shift down to the smallest gears. Shift the shifters all the way down by pulling the finger shift levers until they stop clicking.
Inspect the alignment of the chain with the rear derailleur by watching from behind the bike as you spin the cranks. The derailleur should sit directly beneath the smallest cog on the cassette. If it pulls the chain to the right of the cog, tighten the high-limit screw, a small Phillips screw marked with an "H," on the rear derailleur until it is in line. If it pulls the chain to the left of the cog, loosen the high-limit screw until it is in line. Use needle-nose pliers to pull the shifter cable tight in the cable stop. Use a 5-mm hex wrench to secure the cable in place.
Spin the cranks of the bike and watch the chain where it enters the front derailleur. Listen for chain rub on the front derailleur. If the derailleur rubs on the chain, tighten the low-limit screw of the front derailleur, a small Phillips screw marked with an "L," until it stops rubbing. Pull the front shifter cable through the cable stop using needle-nose pliers and secure the cable in place using a 5-mm hex wrench.
Shift the left shifter up twice by pressing the thumb shifter lever twice. The chain should move onto the largest chain wheel. Tighten the high-limit screw of the front derailleur, also marked with an "H," until the chain begins to rub on the outside of the front derailleur. Back the screw out until the rub stops, then take the bike out for a test ride.
Bikes make noise by design. Many front derailleurs are not wide enough to accommodate the range of gears found on the rear wheel, so the chain will rub when in certain gears, such as when the chain is in the largest cog in both the front and the rear. There is no way around this, so unless it is excessive, the bike should still be in good adjustment.