I just got a new ride, one with a Shimano 10sp Dura Ace 7800 x compact series groupo, and I'm wondering what is the max size rear cog I can mount on it.
It came with a 26T, but given that I ride extensively in a steep hilly area, I really want (need!) something like a 28 rear cog. I told the guy at the LBS that I wanted to swap out to the 28T cassette before I picked the bike up, but he came back and said the 7800 series RD has a max rear cog of 27T, and that he'd put that on there for me. He didn't. Now I'm struggling to get up prolonged steep grades in my compact's lowest, 34CR x 26T gearing; which seems way more difficult than my old steed's, 36CR x 28T setup. I'm not much of a math whiz, so I have no idea how much different the gear ratio or "inches" are between the two. Anyone know?
Question: is there any way to get the 7800 RD to accept a 28T cog; perhaps by going with tooth or two larger cog on the small end (like a 12 instead of 11), in order to keep the total chain "stretch/slack" in same "ratio", and maybe even install an extra link in the chain, if necessary?