Hi. I have a road-bike with a ‘standard’ road set-up: two-shift rings on front and nine cogs on the rear cassette. The largest rear cassette-cog currently is 25 tooth. The Rear Derailleur’s inner-cog (the jockey pulley?) fits closely to the 25 tooth cog (off about 8mm or ¼in) and there is an angle adjusting screw to adjust the distance of the jockey pulley from the largest gear/cog-on-cassette. I would be interested to climb some long-and-steep hills and saw a 30 tooth largest-cog in a cassette available at CRC web-shop. I was wondering if my Tiagra rear-derailleur would fit the 30 tooth cog? I have a new chain for the extra length needed and I have the tool needed for the lock-ring on the cluster . My question is whether the rear derailleur’s jockey pulley will clear the 30 tooth cog? Alternatively, I can also purchase a rear cassette with a 27 tooth largest-cog but I do not think that will get me into low enough gear for the really long-and-steep hill I want to train on. Does anyone know about how big-a cog you fit on the rear for a standard sort-of road-racing-bike?

Unfortunately the REAL read der cog max sizes are a result of trying and seeing what works. Published specs will show that a short cage Tiagra will fit about a 27T. Shimano usually says that their long cage versions fit the same rear cog size but with greater chain ring range (as in triple). I have found that the long cage rear ders handle a larger rear cog FAR ,more easily then the short cage ders. One problem that Shimano rear ders have is the low tension upper pivot spring. Many times, even on new bikes with 'within spec cogs" the "B" screw has to be all the way in and maybe there still will be some pulley knock. So going to a larger then spec cog diameter will likely result in the same. But until you actually try the components you won't know the real life situation. Andy.

Thanks for the reply, Andy. It seems my title for the post was not catchy enough as I only have you to reply and 77 hits at time of writing. You helped me enormously and further searching using some of your words led me to another post made about nearly the same thing in year 2007. I do not know how you get the Shimano spec's either, so thank you. I think I am going to play-it-safe and buy a 27t cog -- max size in the cassette -- instead of the 30t. I might get wobbly knees' from training on that hill but at least I will know the cassette will fit. Ta muchly, David.

You can go to Shimano's website and pull up their specs, although like any interweb search it can lead down dead ends easily. Also the docs included with the ders (aftermarket) state these specs. These max cod specs can be pushed with some awareness and experimenting. Since the limiting factor is the upper/guide pulley running up against the large cogs underside and that the rotational of the cage determines the gap between playing with chain length can get what you want. But the down side/concern is that the chain length might not be enough to handle the big/big combo. If you take this path and were to inadvertently shift into the big/big then you'll cause damage to the rear der and possibly frame. I have done this on my personal bikes in the past, but no longer. Andy.

You didn't specify which Tiagra derailleur you have. The 4400 (8-spped) and 4500 (9-speed) can handle a 27 tooth cog, the 4600 (10-speed) can handle a 30 tooth cog. The cage length makes no differance for this spec because both cages put the jockey wheel at the same location.

You still need to verify that you're not exceeding the capacity of the RD and will also likely need a new chain.

https://techdocs.shimano.com/techdocs/index.jsp