Condensed Answer: Shifters from the Tiagra 4600 series are compatible with 8 and 9-speed Deore rear derailleurs. The Tiagra 4700 series, however, is incompatible with Deore derailleurs.
Rear Shift Ratio and Cable Pull
In a modern indexed drivetrain, the compatibility between a rear derailleur and a shifter is determined by the rear shift ratio and cable pull.
The movement of indexed shifters is split into separate clicks. Each click equals an upshift or a downshift and pulls or releases a certain amount of gear cable.
The amount of actuated cable is known as cable pull. Its value depends on the number of gears, the bike type (road or MTB), and the component manufacturer.
The rear shift ratio indicates how much the rear derailleur moves per 1mm of cable pulled or released by the shifter. For example, if the rear shift ratio is 1.7, the rear derailleur will move 1.7mm per 1mm of cable pulled or released by the shifter.
Just like the cable pull, the rear shift ratio is dependent on the number of gears, the bike type, and the manufacturer.
The values of the cable pull and rear shift ratio are essential to the proper functioning of a modern index shifting system. If the numbers aren’t correct, the derailleur will not move to the appropriate location and will over or under-shift.
Compatibility Between Tiagra Shifters and Deore Rear Derailleurs
A rear derailleur and an index shifter are compatible when the following criteria are met:
- The derailleur is designed for the same number of speeds as the shifter.
- The shift ratio of the derailleur has the correct value for the drivetrain type (road or MTB) in question.
- The derailleur has the capacity to operate with the chosen cassette.
Tiagra is a 10-speed road groupset from Shimano. Consequently, the derailleur that we need has to have a rear shift ratio matching that of 10-speed Shimano road derailleurs.
The rear shift ratio of 10-speed Shimano road derailleurs is 1.7mm. Meanwhile, the rear shift ratio of Shimano’s 10-speed MTB derailleurs is 1.2. Thus, the 10-speed version of Shimano Deore cannot be mixed with Tiagra.
However, the rear shift ratio of Shimano’s 8 and 9-speed MTB derailleurs is 1.7mm. Consequently, one can mix 8 and 9-speed Deore derailleurs with Tiagra shifters.
Tiagra 4700 Is an Exception
The Tiagra 4700 series is not compatible with Deore derailleurs. The 4700 series are unique. The shifters are still designed for 10-speeds, but their cable pull matches that of 11-speed road shifters. At its core, the Tiagra 4700 series is an 11-speed system downgraded to 10 speeds.
For that reason, the derailleurs of the 4700 series have a different rear shift ratio too. If a Deore model is used instead, it will not move accurately across the cassette.
According to Shimano’s compatibility chart, the 4700 shifters are compatible with Tiagra 4700 derailleurs (RD-4700-SS and RD-4700-GS) as well as the RD-RX400 derailleur.