RobertMillar84

Looking for some second opinions... I'm going on a cycling trip in the Alps in a few weeks time. Was looking to give myself a slightly easier lower gear for climbing, without having to fork out for various parts. I'm running an 11-28 just now with a shimano 105 RD 5700 rear mech, on a compact front. But I notice I can now get a 12-30 tiagra cassette and was wondering if I could get away with running this cassette on my current rear mech - which technically states a 28t is the max. However, I've noticed my b-screw can still be turned in quite some way, which I'm thinking would give it some more capacity?

However, when I asked the mechanic at my LBS about this the other day, he said it wouldn't work, the way the rear derailler and hanger would need to be angled would mean there was less contact with the chain on the cogs and the shifting would be poorer. I can accept the shifting may be slightly poorer.. but was thinking it wouldnt be so bad that I couldnt live with it, if it gave me that lower bottom gear. I also appreciate some people will say the difference between the 28t and 30t is minimal and not worth it.. but I'm thinking over a week of climbing in the Alps every day it may just make a difference, marginal gains and all that!! Plus it would just give me that bit more piece of mind if there was a lower gear to go into if need be.

So, would do people think, is my LBS just covering themselves and going by the book? Or would this change more than likely be OK?

Thanks in advance.