I have a Shimano 105 rear derailleur with a SRAM 12-26 cassette and triple crank. that I want to change. I have been out of cycling for a while and need alittle extra help with the hills until I get my legs back. What is the max size cassette i can use with the 105 Derailleur? I have been told 11-28 or 12-30. Any mechanics out there that can set me straight? Thanks.