In reply to johnboybuchan:
ive only recently changed from 3x9 to 2x9, simply because I needed a new crankset and derailleur for my frame (new frame is braze on old was band on).
I was a bit hesitant to make the change as I did use all three rings on a regular basis, however I bit the bullet and got a 10 spd crankset and 10 spd front derailleur but stuck with the 9spd shifters and 9spd cassette. This works fine as a combination but I found I had a bit of chain rub so changed the chain to 10spd also and this works smoothly - the 9spd chain is going on my mtb when it needs it so no waste there.
Ive found it much nicer not having the three rings at the front with less shifting needed on the chainrings which always seems clunky even with shimano 105 groupset.
All you really need to decide on is what gear range youll likely need - I went for a compact i.e. 36-50T and what range youll want on your cassette. There are a lot of choices of gearing on cassettes, Ive got a 12-26 which seems to work ok for me and the hills I do, then again im quite light and dont often carry any luggage, If I were to be doing some cycle touring Id probably get a larger range such as 12-28 or if my derailleur will allow it a 12-32.
Thats another limiting factor the length of the rear derailleur as not all will be suitable, e.g. short cage mech I think is up to 28T.
The main differences between a 9,10,11 speed cassette is the amount of 'jump' there is in between gear shifts. You can use a spreadsheet or an online calculator to work out what the gear ratios will be and see how much gap there will be between changes. I manage fine with 9 cogs on the back between 12-28 but the more spread out they are the less easy youll find it to get the 'right gear' to keep your cadance.
