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Sometimes the tires of your bike can lose air pressure over time from changes in temperature, try reinflating your flat tire with a simple bike pump as a first solution.
If after you try to reinflate your tire it still goes flat relatively quickly, you might have a hole in the tube of the tire and it will need to be replaced. To replace the tube, follow the Inner Tube Replacement Guide.
If you have replaced the tube and the tire still goes flat, the rubber outside of the tire may be defective and cause the tube to break. The tire will need to be replaced. To replace the tire, follow the Tire Replacement Guide.
Check the placement of the chain in the track. It may have become disconnected and needs to be put back in place.
A simple solution to fixing your chain if it has become jerky and loud is to clean it with some oil and grease.
If the chain is too stiff from rusting over time it may need to be replaced.
Reconnect the brake mechanism. To reconnect, refer to steps 1 and 2 in the Front Wheel Replacement Guide.
If the brake pads have become worn or damaged they will need to be replaced. To replace the brake pads, follow the Brakes Replacement Guide
If the rim has become bent or warped in some manner, it will need to be replaced for the bike to run smoothly. To replace the rim, refer to the Bicycle Rim Replacement Guide.
A broken reflector will need to be replaced. To replace the reflector, refer to the Front Reflector Replacement Guide.