Nun and Cycco, this is what I was referring to (I cant believe you guys thought I was mixing up teeth and 9 or 10 speed systems...come on, Im a gearing nerd amongst a bunch of gearing nerds! ;-)
Plus, I dont follow bike magazines at all, so I wasnt sure if there are in fact cassettes with 10tooth and 9 tooth cogs, as disktrucker referred to when he said "some people were disappointed they didnt go down to a 9 tooth cog"
No matter, it just made me think how for me, I dont like being on a 11 tooth cog for the sound I perceive, and wondered if its just me?
ps, just read the responses after I saw Nuns post, as I hadnt read them. ya its probably derailleur alignment a bit, but I should clarify that I "feel" a difference through my feet slightly (and no, its not as blatant as the chain hitting a screw or whatever) and Im pretty sure I am feeling the chain being more "bunched" around a smaller diameter--but I admit I am talking about a small difference here.
I don't think you can fit a 9 tooth cog on a standard hub. There is a capreo cassette which only fits on a Shimano Capreo hub and is most commonly used on folding bikes to get higher top speed. Having said that, If you are having chain rub on the front derailleur, then it is not likely the rear derailleur is out of adjustment unless you have a Rapid Rise rear derailleur. All rear derailleurs mainly default to the smallest cog without cable pull, as opposed to rapid rise which defaults to the largest cog close to the drive flange of the hub. There are 2 adjustment screws on the rear H and L (high) and (low). Typically if the high gear is misadjusted, you will hear some clattering sounds as you pedal at the 42t 11t combo, but that would also affect the next cog up to as well. But before you do this, have you washed and clean your cassette and chain rings fromt dirt, mud and grit after the tour? It does make a big difference when these gears are cleaned ; like crunching noises as the chain meshes with the cog or chain ring if grit and dirt are lodged in between. But if drive train are cleaned, H and L adjustments on the rear are correct and chain ring teeth are not bent or shaped like shark fins and front FD adjusted then the only possibility is chain wobble with a slightly out of true ring.