Deore cassette on Tiagra derailleur
Amazon seems to sell an 11-34T "HG-50" Tiagra cassette. See here. But if you look on Shimano's website, such a thing does not exist! Once I add to cart, I see that this is really a Deore cassette.
Can I use it on my Tiagra rear derailleur? I'm talking plug-and-play, no modifications to anything. The Shimano website says the maximum sprocket for the Tiagra is 27T but I'm confused about the whole Amazon thing. There seem to be a lot of positive comments. Or should I just get one of the real Tiagra options?
Your current derailluer may not have enough reach to take up the slack when running small chainring/small cassette cog and/or your chain may not be long enough for large/large configuration. It might work ok if you are careful about never crosschaining when you shift, but I don't think dropping in a 34T cassette is recommended.
Bicycle Repair Man !!!
If you want to run an 11-34 cassette you will need a derailleur that can handle the larger cog and complete range of your bicycle's transmission.
Rating of the Tiagra is 27/22/37 which means it can optimally handle a 27 tooth cog in the rear, 22 tooth difference up front, and has a maximum capacity of 37 which is calculated by adding the difference between the largest and smallest chain ring to the difference between the largest and smallest rear cogs.
With that being said the Tiagra GS can probably handle a 30 tooth cog with no problems but think that it will be hard pressed to run a 34 as long cage road derailleurs are the equivalent of medium cage mtb derailleurs.
If you want Alpine gears a long cage mtb derailleur should be used.
It's more than just the wrap capacity. Shimano road rear derailleurs are limited in the size of largest cog will clear the jockey pulley. Shimano says 27T max (28T for the newest ones) and others have reported 30T usually clears, 32 sometimes does and nearly everyone agrees 34T is too large. An MTB rear derailleur is the only sure bet.
Originally Posted by reshp1