Replacing the gear shifting system on a Trek 4100 MTB
The mechanical parts on my Trek 4100 mountainbike were getting old and worn out, so I replaced the bottom bracket, front chainwheel, multiple freewheel, rear derailleur and bike chain.
It isn't that difficult, and the process is a lot of fun! On top of that you save a lot of money, in stead of letting your bicycle repair shop do it.