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A poor braking system could lead to loss of control while riding, which could lead to accidents and serious injury. If you conclude that your brakes are not working properly, then do not ride your bike; take it to a bike shop to be repaired. If your brakes fail while riding, figure out a way to stop your bike (perhaps go up hill or steer onto the grass or other high friction surface). After you have come to a complete stop, get off your bike and walk home or call for a lift. Have the brakes repaired before riding again.
Tune-up task #3: Watch your wheels.
Wheels (rims) hold your tires in place and provide stability and smoothness while riding. Your bike would not move without rims because they allow consistent contact between the tires and road surface.
What to do: Clean the wheels with rubbing alcohol and a clean, dry cloth. Inspect the rims for nicks, scrapes, dents or other damage. Next, elevate one end of the bike and spin the wheel. The wheel should move smoothly, without wobbling. Repeat for the other wheel.
Damaged rims cause uneven wear to tires and brake pads, which can shorten their lifespan. In addition, uneven tire wear usually leads to flat tires or a blown tire while riding—a potentially dangerous situation. Replace your wheels if denting or other damage is excessive. A wobbly rim can be adjusted with a spoke wrench—a simple fix that a bike repair shop can handle better than most do-it-yourselfers. Adjusting spokes yourself can lead to more problems if you do not know what you’re doing.
Tune-up task #4: Inspect the drivetrain.
A bike’s drivetrain includes the pedals, chain, chainring, derailleur (the device that moves the chain to make riding easier or harder) and rear wheel cassette (all the little teeth in the center of the rear wheel). The drivetrain is important because it transfers the power generated by the rider's legs to the rear wheel. This transfer of power provides the force that moves the bike.
What to do: You will need a partner or bike stand to assist with this part of the tune-up. Raise the rear wheel and spin as you did when checking the wheels (task #3 above). This time, shift through all the gears. Shifting should be smooth and easy to perform. Inspect the chain, chainrings, derailleur and cassette for damage (excessive wear, missing teeth, dents, scrapes, etc.). Note that small chainrings wear out sooner than large chainrings and that chains are the most frequently replaced component of the drivetrain.