What should be used to fine-tune the derailleur system?

The barrel adjusters are crucial for fine-tuning the derailleur system because they allow for precise adjustments to the cable tension without the need for tools. When you turn the barrel adjuster clockwise, it tightens the cable, which can help shift the derailleur closer to the bike’s frame and improve shifting performance. Conversely, turning it counterclockwise loosens the cable, enabling better alignment for smoother shifting to larger gears. This adjustability is essential in maintaining optimal performance, particularly if the cables have stretched or the derailleur has become slightly misaligned over time.

In contrast, stop limit screws are used to set the range of motion of the derailleur, ensuring it doesn't move beyond the largest or smallest gears but aren't meant for precise tuning during ride adjustments. Pedal adjustments pertain to changes in the pedal mechanism or shoe interface and do not affect the derailleur system. Chain tensioners help manage chain slack but do not fine-tune the derailleur; they're primarily used in single-speed or internally geared setups. Therefore, barrel adjusters are the best choice for fine-tuning the derailleur system.