Originally Posted by papapapaof4
Just bought a new road bike and would like to have a better really low gear for those long hills. Is there any reason why one cannot simply replace the stock 12-26 rear cassette with a 11-32 mountain bike cassette to go with the 52-42-30 front gears?
Don't see any reason why you couldn't do that.
FWIW, I had an 11-30 cassette, I found a scrap 34T cog and simply replaced it (Sheldon Brown has a great page explain what to do to break apart a cassette)
Folks can feel free to jump in and correct me but here's what I've learned about rear derailleurs:
They have a maximum cog size. I don't know all the factors this is affected by but assume arm length is one of them. A short cage Sora rear derailleur accomodates up to about a 29T rear sprocket. A long cage Deore goes to 34T.
Rear derailleurs will have a "chain capacity" also called "range" which does NOT denote the "range" of rear cogs.
IOW, a rear derailleur with a "max 14T difference" does NOT mean you can only have, say an 11 to 25T rear cassette. It means you can only have a 14T difference from highest to lowest FRONT CHAINRINGS.
The idea being, a longer cage rear derailleur "contains" more chain over it's rollers and through the cage and therefore has the ability to take up or let out enough chain to accomodate different sizes of front chainrings.
You're never worse off for having a long cage rear derailleur (unless ground clearance becomes a factor - likely not for road riding). Upgrade away!
One thing my recent drivetrain upgrade schooled me on was the interconnected nature of all components. Making a front chainring change meant I had to look at my rear derailleur to see if it could handle the jump between front rings. The new sprocket meant ensuring I had a long enough chain and a rear derailleur that could handle the 34T sprocket.