Gear shifting on bicycles is done by cables. When cables stretch, bikes do not shift properly. Shifting becomes sluggish, makes noise or the bike shifts by itself. All bikes have built-in adjusters for this purpose. Most bikes have barrel adjusters located on both ends of the cable to tighten loose cables. The upper barrel adjuster can be operated as you are riding the bike. The lower barrel adjuster is used to fine-tune bike shifting.
Go to a deserted road or car park. Get on your bike. Begin pedalling your bike slowly. Shift the bike into the middle gear on the back.
Shift the bike into the next bigger gear on the back. If the bike does not shift, or it makes noise or stalls when shifting, reach over with your left hand and turn the barrel adjuster two or three clicks counterclockwise. The barrel adjuster is located on the right hand shifter. It is a black serrated knob with a hole through the middle. The cable passes directly through the hole.
Pedal the bike. If the bike does not shift to the next bigger gear, turn the barrel adjuster another click or until the bike shifts.
Ride the bike and shift the bike again to the next biggest gear. If it does not shift, turn the barrel adjuster one click at a time until the bike shifts.
Pedal the bike. Push the downshift button to shift the bike back down to the next smallest gear. If it does not shift, makes noise before it shifts or is sluggish, stop the bike and get off.
Turn the barrel adjuster on the rear derailleur clockwise one click. The rear barrel adjuster is located on the bottom of the derailleur. It is a black serrated knob with a hole through the centre. The cable passes directly through the hole.
Get on the bike and begin pedalling. If the bike still does not shift or is noisy when it shifts, get off and turn the barrel adjuster one click clockwise. Get on the bike and ride. If the bike still won't shift clean without noise, turn the barrel adjuster another click clockwise or until the bike shifts when you get back on and ride.
If the bike will not shift no matter how many times you adjust your barrel adjusters, your cable is probably dirty. Squirt some bike lube into the cable housings where the cable enters the plastic covering. Also lubricate the area where the cable passes underneath the bike frame.