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I have a 10-speed 2013 Specialized Allez Elite with Tiagra componentry (and an 11/28 105 cassette) and would like to install my old Shimano 105 rear derailleur in the back to replace the Tiagra one. The 105 rear derailleur came off a 2015 Roubaix which was all-105.

Will this work? I assume the Tiagra shifters' different pull ratios will simply result in the rear derailleur being pulled at... a different ratio. Will this setup run cleanly and shift smoothly? Are the chain widths slightly different between 10 and 11-speed as well? Is this destined to fail?

  • Just try it out! It may work. The inner width of the chain is the same. The 11 speed has slightly thinner plates. – Carel Jul 1 at 8:35
  • If you were to add a photo of the components, it will allow a definitive answer, before making any changes. It depends on the series of each part but they look different enough to allow id in a photo – Swifty Jul 1 at 9:38
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TLDR: no that won't work. Tiagra 4700 was introduced in 2015 (see here), so your 2013 Allez will have 10 speed Tiagra 4600 series which is not compatible with any 11 speed derailleur.

Shifters pull a certain length of cable per gear, derailleurs have an actuation ratio. Cable pulled x actuation ratio needs to match sprocket spacing. All Shimano road groups up to 10 speed (excluding Tiagra 4700 and GRX RX400 series) used the same derailleur actuation ratio so derailleurs of different 'speeds' could be mixed and matched.

At 11 speeds the actuation ratio changed, so 11 speed derailleurs could not be used with 10 speed shifters and cassettes. The newer 10 speed Tiagra 4700 and GRX RX400 series groups are said to use the 11 speed actuation ratio so an 11 speed derailleur will work with them.

11 speed chains have slightly narrower external plates that 10 speed, so there is the possibility that a 10 speed chain will rub in a 11 speed derailleur cage.

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Assuming the 2013 Tiagra shifters are the 4600 series, it's not officially supported due to (indeed) difference in cable pull. I expect getting a smooth shift is going to be difficult if not impossible.

If for some reason the Tiagra shifters are newer (and 4700 series) then the 10 speed shifter will work with the 11 speed 105 derailleur. You will obviously only be able to use a 10 speed cassette as the shifts are limited by the shifter.

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