Hi all I bought a carrera vengeance from halfords at the beginning of the year,i knew it was a entry level bike but im only using it for towpath rides to get fit so figured it would be fine,however as time goes by i seem to be having one issue after another so thought i may start replacing some parts but better ones. this started a couple of weeks ago when i damaged the front mech in a wipeout (wet cobbles hurt btw) halfords said they couldnt even look at the bike for a fortnight so i took it to a LBS instead and they simply replaced the mech with a new better one the same day and its been fine since. so now im looking at possibly ordering some new parts and having a go at installing them myself or if installing becomes difficult then getting the LBS to do it. First things im looking at is a new rear mech and a new chainset/crankset,however knowing little about such parts i need to make sure i order compatible replacements,for the rear mech ive been looking at a shimano deore. http://www.chainreactioncycles.com/Models.aspx?ModelID=40542 I reckon i can remove my chain,remove old mech and fit this one then set it up,but ive read that different mechs have short,medium and long cages and i have no idea what type the sram x4 is on my bike will it make alot of difference ? or am i better making sire i get the exact same size ? and the front crank set can they be replaced by a novice ? or is this a job for the LBS ? i guess i know the answer to that but if the lbs does everything for me i'll never learn how to do it myself how can i find out what exact size my current bottom bracket is so i can source a replacement ?