Shimano Ultegra RD-6870-SS Rear Derailleur / Rear Mech

This is the manufacturer’s claimed average weight; actual weights may differ.

Gear shifting can be mechanical or electronic. Mechanical systems use cables to move derailleurs, while electronic ones rely on battery-powered motors. Wired electronic setups connect shifters and derailleurs with wires; semi-wireless systems use wireless links from shifter to junction box, then wires to the derailleur; fully wireless systems link shifter and derailleur directly.

Number of cogs on the cassette.

Number of chainrings on the crankset.

The cage length refers to the length of a derailleur’s arm that holds the jockey wheels, influencing how much chain it can wrap around the gears. Even so, cage length by itself doesn’t specify the exact numerical capacity, so you should check both the total capacity and cassette capacity to ensure everything fits together properly.

The total capacity of a rear derailleur refers to the maximum amount of chain slack it can take up. This determines the maximum difference in teeth between the largest and smallest cassette cogs pluss the largest and smallest chainrings that the derailleur can handle.

Cassette capacity is the range between the largest and smallest cassette cogs that a rear derailleur can support. Unlike "total capacity", it excludes chainring differences. Always verify the "low sproc" and "top sproc" specs to ensure compatibility between the derailleur and cassette.

The max front diff is the largest tooth count gap between chainrings a rear derailleur can manage. A greater difference requires a longer rear derailleur cage to handle extra chain slack. On a single chainring system, the difference will be 0.

Low sproc max is the maximum number of teeth of the largest cog on the cassette that a rear derailleur can accommodate.

Low sproc is to the minium number of teeth of the largest cog on the cassette that a rear derailleur can accommodate. Selecting a cassette with a low sprocket below this value may result in poor shifting performance or incompatibility with the derailleur.

Top sproc max is the maximum number of teeth of the smallest cog on the cassette that a rear derailleur can accommodate.

Top sproc min is the minimum number of teeth of the smallest cog on the cassette that a rear derailleur can accommodate.

Pulley teeth refer to the number of teeth on the rear derailleurs jockey wheels (also called pulley wheels). A higher number of pulley teeth gives a marginal improvement to the efficiency of a drivetrain.

A clutch maintains chain tension, thereby significantly reducing chain slap and minimizing chain bounce on uneven terrain. This feature improves shifting reliability and reduces chain drops but may slightly lower drivetrain efficiency when riding on smooth surfaces.