Originally Posted by
Lascauxcaveman
I'm surprised the 105 long cage can't work with a much bigger cog set out back. Gimme 10 minutes with a chain tool and such and I can dial it in for ya
If that pic in post
#3 is of the big/big combo, your chain is too long; that RD can stretch a lot more than that. As it stretches out, it makes more room for bigger cogs. Take out a link pair.
While precisely sizing the chain is critical for wide range applications, telling someone to just shorten the chain can cause a ride ending failure and destroy the rear derailleur. The picture is of no value, it's not even clear that it's on the biggest cog. If the chain is too short on the big-big combination and the cyclist shifts into that combination, the rear derailleur will fail catastrophically. While factory ratings can be exceeded with careful installation, the big-big combination must have sufficient chain length for use when selected.
I was unable to fit a 12-30 cassette with the 105 nine speed rear derailleur and a triple. I was surprise by this since I had been able to get that combination to work with the 10 speed models. Have you ever installed this derailleur and use it with the combination you are recommending? Even if you have (doubtful), the length of the derailleur hanger is critical and can vary from frame to frame, depending on the model.