Can I use with 11-34 Tiagra cassette?
I have a Giant Defy with compact chainset 50/34, shimano 105 shifters, and Tiagra 12-30 rear cassette. The bike has a short cage 105 rear mech with quoted max 30 teeth capacity. By my calculations the overall no. of teeth is right on the quoted capacity of 34 teeth, but it works perfectly.
I would like to fit a Tiagra rear cassette 11-34 which exceeds the official 32 max largest cassette ring size for the 105 medium derailleur (I am getting on a bit and live very near the steepest of the Surrey Hills). However I believe the overall no. of teeth would be 39 and the mech is quoted as having a 40 tooth max capacity.
I know the Tiagra medium/long cage derailleur works with 34 teeth but would rather not downgrade from a 105 mech.
Can you advise? Would I notice any difference if I had to downgrade to a Tiagra mech to fit the 34 tooth cassette?
Unfortunately, this derailleur would not be suitable for you. The largest this sprocket this mech can handle would be a 32 tooth option.
In this instance, if you wanted to lower the gearing to the 34 tooth option, you would need to consider swapping to the latest Tiagra 4700 groupset, but this would require a complete overhaul of your drivetrain unfortunately.