The Shimano Total Integration (STI) 105 is Shimano's "workhorse" component group for bicycles. The system provides extraordinary shifting performance in any weather, even under dirty and muddy conditions. The group is purposefully overbuilt to be strong. It is designed to outlast even the bike frame on which it is installed. But to enjoy any of the benefits of such a sturdy component set, the system must be set up correctly. Learn to adjust the 105 shifters at home to avoid errant shifting problems.
Place the bike upside down on its handlebars and seat to allow you to spin the rear wheel. If you own one, a dedicated bicycle workstand is ideal for this kind of adjustment work.
Shift both shifters on the bike all the way down by pressing the black shift lever until it stops clicking. Spin the cranks on the bike; it will shift into the smallest cog in both the front and rear. Loosen the cable stop on both derailleurs using a 5mm hex wrench.
Look from behind the bike at the alignment of the derailleur pulley and the smallest cog on the cassette. If the pulley is perfectly in line with the cog, pull the cable tight using a pair of needle-nose pliers and tighten the cable stop. If it does not, adjust the high limit screw, marked with an H. Tighten the screw to move the pulley to the left. Loosen the screw to move it to the right. When it is in line, tighten and secure the cable.
Spin the pedals of the bike. Shift to the largest cog by shifting the silver lever until it stops clicking. Look again from the rear of the bike to ensure the pulley is aligned with the largest cog. Tighten the low limit screw (marked with an L) to move the pulley to the right. Loosen the low limit screw to move the pulley to the left.
Shift down into the second-to-smallest gear on the rear cassette. Inspect the chain where it enters the front derailleur. If the chain rubs on the derailleur when the pedals are turned, tighten the low limit screw of the front derailleur (marked with an L) using a Phillips screwdriver until it stops. Pull the cable through the front derailleur cable stop. Use a pair of needle-nose pliers to pull it tight and tighten the cable stop.
Shift into the largest cog on the crank by pressing the silver lever on the left shifter until it stops clicking. Shift into the smallest cog on the rear wheel by pressing the black lever on the right shifter. Spin the cranks and listen. If the chain rubs on the front derailleur, loosen the high limit screw (marked with an H) until it stops.
Take your bike out for a test ride. You should be able to shift into every gear quickly and easily. The chain should not rub on the front derailleur in any gear.