The heart of a bicycle's shifting system lies in the front and rear derailleurs. These devices move the bicycle chain between the chain rings in the front and across the range of cogs in the back. Shimano's Ultegra derailleurs are components that perform these tasks crisply and precisely. However, like with any precision instruments, it is essential to keep them properly adjusted. If you don't, the shifting will be jerky and unpleasant -- to the possible extent of causing severe damage to your frame or rear wheel. Fortunately, adjustment of both takes only a few simple turns of the limit screws.
Front Derailleur Adjustment
Turn the crank slowly and shift between the front chain rings by using the left shift lever on the carbon bike handlebars.
Adjust the low-limit screw if the chain does not shift crisply to the lower chain ring. If it goes off the inner chain ring toward the bike parts frame, use the screwdriver and turn the low-limit screw clockwise (tighten) a quarter or half turn until it lands cleanly without dropping to the inside. If the chain won't drop easily to the lower ring, turn the low-limit screw counterclockwise (loosen) a quarter or half turn until it does. The low-limit screw is generally the screw closer to the frame, although it may be labeled with an "L."
Adjust the high-limit screw if the chain will not travel correctly to the large chain ring. The high-limit screw is typically farther away from the frame, and it may also be labeled with an "H." Tighten the screw clockwise in small increments if the chain jumps over the ring. Conversely, if it is slow moving to the ring, loosen the screw until it moves quickly to the large ring without going over it.
Rear Derailleur Adjustment
Turn the crank slowly and shift the chain to the large chain ring in the front using the left shift lever and the smallest cog in the rear using the right shift lever.
Loosen the high-limit screw if the chain will not move crisply to the smallest cog. Use the screwdriver and turn it counterclockwise in small increments -- no more than a quarter or half turn at a time. The screws on a rear derailleur are typically marked "H" and "L," but the high-limit screw is generally the upper of the two screws on the body of the bike derailleur.
Tighten the high-limit screw by turning it clockwise in small increments if the chain tends to overshoot the smallest cog and tries to go into the frame. Remember to go only a quarter or half turn at a time.
Shift the chain to the small chain ring in the front and the largest cog in the rear. Do this by slowly turning the crank arms and using the appropriate shift levers on the handlebars.
Continuously shift between the largest rear cog and the next smaller cog. Do this gently to prevent the chain from possibly going into the spokes. Take note of whether or not the chain climbs to the largest cog easily.
Tighten the low-limit screw by turning it clockwise if either the chain tries to overshoot the largest cog or the derailleur bumps into the bicycle spokes. Work in small increments and reshift after each adjustment. The low-limit screw is usually labeled "L" and is typically the lower of the two screws on the derailleur body.
Loosen the low-limit screw by turning it counterclockwise if the chain does not move easily to the upper cog or if the derailleur cannot move the chain to that cog at all. Again, work in small steps and check the shifting after each change. Do this until the chain climbs crisply to the largest cog without overshooting it.
Shift up and down the full range of rear cogs in both the large and small chain rings in the front. Make small adjustments with the barrel adjuster located where the shifter cable enters the rear derailleur. It looks like its name -- a small barrel or fat nut with deep grooves in it. If the shifting is sluggish in either direction, turn the barrel in small increments in the direction you want the chain to go. This allows for fine-tuning the shifting.