Originally Posted by
joejack951
Not all road rear deraillers and drop out configs will allow you to use a 30 tooth cog. 27 is the spec from Shimano and with anything bigger you are beginning to push your luck. While it works for some, it definitely does not work for every bike/derailler.
Only about 0.1% of the people who regularly state that shimano derailleurs handle more than 27t understand how it came about and why it can actually be *lower* than 27 given the right bike.