Shimano's 105 Rear Derailleur is a sweet upgrade. Shifting is almost effortless thanks to its light-action spring and the top pulley stays a uniform distance from each sprocket so shifts resonate with the plunk of exact gear engagement. And, the wide outer-pivot design increases rigidity for precision shifts every time.
Largest cog this derailleur can handle: 28 tooth
The Shimano 105 Rear Derailleur is compatible with 10-speed cassettes.