Shimano is a brand that includes bicycle parts and accessories. Shimano 105 and Tiagra derailleurs, both front and rear, are part of a groupset of components that includes hubs, cranks, shifters, handles, brakes and other parts of the drivetrain. Bicycles are typically manufactured using all the parts from a common groupset, however, components such as derailleurs can be purchased individually to swap out or upgrade bicycle performance.
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Delicate or Rugged
Tiagra rear derailleurs manufactured before 2011 have only nine gear combinations, whereas 105 derailleurs have had 10 gear combinations for many years. The105 derailleur shifts cleaner and almost imperceptibly faster than the Tiagra derailleur when comparing derailleurs that are properly maintained. The 105 derailleur is lighter than the Tiagra, but the heavier materials of the Tiagra mean that it can take a beating and continue shifting if you're commuting or riding day to day on wet or gritty surfaces or applying excessive strain on the drivetrain. The 105 is more delicate than the Tiagra and can be prone to shift sluggishly if it or the cables that operate it get dirty. The differences in lightness and durability might seem insignificant to a novice rider, but to a serious cyclist they're noticeable.
The 105 derailleur is a mid-range component that can be used for racing or can be used by enthusiastic cyclists who want performance at an affordable price. The Tiagra derailleur is often associated with beginner or entry-level bikes and not typically seen at races. The 105 derailleur costs more than the Tiagra -- but the cost difference doesn't always mean that the overall cost of the Tiagra-equipped bike will be less. Groupsets are sometimes swapped out to lower the cost of the bike while at the same time maintaining performance. For example, to lower the cost of a fully assembled bicycle, high-end or expensive wheels can be combined with a Tiagra derailleur. In this example, the lighter wheels can make the bike equal to or faster than a 105-equipped bike with cheaper wheels.
The Cable Guy
When comparing Tiagra and 105 derailleurs, cable routing is an often overlooked albeit important feature to consider. The Tiagra has external cable routing, as opposed to taped-over cables on the 105 derailleur. The overall appearance of the Tiagra-equipped bike won't look as clean as the 105, but the cables are easier to change on a Tiagra and less likely to bind if they're dirty. The Tiagra cable routing allows the bike frame and handlebars to appear slightly thinner or more lean than the 105-equipped bike. Cable routing on a 105 is tighter, but can cause the bike to shift poorly if not properly maintained.
In The Eyes of the Beholder
The 105 derailleur has more flair than the Tiagra and is available in black. The Tiagra is available only in silver. If you prize the look of high-end bikes, the 105 is designed to appear similar to that of Shimano's more expensive derailleurs. The 105, with graceful curves, sleek lines and overall smaller stature, contrasts with the Tiagra, which is larger, appears more robotic or mechanical and can evoke the appearance of a less-expensive bike. If you're riding with a group of serious cyclists who are dressed to the nines with all the current gear and equipment, they'll notice it right off.