Question: I have an 11-27 cassette but wondered if a 12-30 would fit my standard Ultegra Di2 rear derailleur? I have read some advice that says ‘yes’ and some that say ‘no’. From: Ryan
Answer: My short response is that both answers you’ve heard are correct. The long response is that Shimano does make a derailleur specifically for 30-tooth cassettes, so the official answer is ‘No, your standard SS cage derailleur will not work.’ If you have 11-speed Ultegra 6870, you’ll need a 6870 GS long cage derailleur, which can handle up to a 32t cog. For 10-speed mechanical groups Shimano makes a 6700-A version of their derailleur to work with 30-tooth cogs. Unfortunately for Ultegra Di2 6770 there is no 6770A SS version like there is for the mechanical group.
But, plenty of folks have successfully used 28-tooth limited SS derailleurs on 30-tooth cassettes. The way to make it work is to use a longer b-tension screw or to install the b-tension screw backwards so that the head of the screw presses against the derailleur hanger. The geometry of your derailleur hanger will play into the compatibility question. On some bikes derailleur hangers sit lower or further back or have their b-tension stop further aft on the bike. Derailleur hanger variances in this direction will help with 30-tooth compatibility.
If the upper pulley on the derailleur bounces as each tooth on the large cog passes over it, then you know that it won’t work on your bike. Setting up your chain a tad on the short side can help to create improved clearance by moving the upper pulley further away from the large cog to solve this problem. With that said, you need to make sure that your chain is long enough so that your derailleur won’t be ripped off of the hanger when you are cross-chained in the big chainring and biggest cog.
I also have one more bit of warning for you: there are sacrifices to be made when using a big cassette with the wrong derailleur. Using excessive b-tension to make the rear derailleur clear a cog that is larger than intended will move the derailleur further away from your smaller cogs than it should be. The extra distance will mean slower shifts, more chain and cassette wear, and a greater likelihood of having the chain skip over the top of the cog’s teeth during hard efforts. If you are unwilling to make these sacrifices, you’ll need another solution.
For 10-speed Shimano mechanical road groups you can substitute a 9-speed mountain rear derailleur. These will handle an 11-32 or 11-34 cassette perfectly and even an 11-36 cassette with a little extra b-tension. For 11-speed groups, just go for the GS mid-cage derailleur that is made for 11-30 and 11-32 cassettes.
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