I fix bikes as a hobby and have people bring me bikes that the gears wear out. Unfortunately the bikes that the Super Stores sell are mostly from China, and the gears use plastic parts. I remove the gears and make cruiser bikes out of them
Step 1: Removing the Pin From the Chain
You will need a few different tools depending on the bike.First you will need a chain pin remover.
Step 2: Removing Front Chain Positioned and Rear Chain Sprocket Positioned.
The front and back chain alignment brackets come off fairly easy,with a Phillips head screwdriver and some cases Allen wrenches. You can then cut the gear cables with a pair of wire cutter.
Step 3: Shortening the Chain and Replacing It.
When reinstalling the chain if possible mount the rear wheel forward in the mounting slot. This will give you room to tighten the chain later. When putting the chain back on you will need to measure the distance by putting the chain around the middle gear on front and rear sprocket. Make sure you have an inside link on one end and outside on the other you can push the pin back in with visegrips or use the pin remover tool to push it back in. Crank the pedals and see if you are comfortable with the speed that the gear is in and make chances accordingly.