There is insufficient or no air pressure in the tube of the tire.
The air pressure in the tube varies over time because the valve stem does not create an absolute seal. The tire will appear to deform and flatten where it contacts the ground. This problem can be simply fixed by inflating the tube with a bike pump.
A tube can be either punctured or popped. Obvious signs of a damaged tube are a visible object that is lodged in the tire and a rapid decrease in tire pressure. There are two options to address this issue: repair or replace the tube.
The bike does not change gears, skips gears, or does not reach all gears.
The tension in the cable of the derailleur may cause the assembly to be out of the correct alignment. When this problem occurs, the derailleur will not reach all of the gears or will proceed too far past the first or last gear in the cassette.
Derailleur hangers typically break due to stress or from impact. The derailleur hanger connects the derailleur assembly to the bike’s frame. The only way to fix this is to replace the derailleur hanger.
Interruptions in the chain following the spokes on a gear can most often be attributed to a slack or damaged link. A possible cause for this is a dirty chain that has accumulated build-up and can be remedied by cleaning the chain. Another issue may be with a damaged link, in which case replacing the chain will be necessary.
The brakes will under engage when actuated or over engage when not actuated
Hydraulic brakes rely on oil pressure to engage the brakes. When the oil pressure is too high or too low, the mechanism does not function properly. A sign that the brake is not sufficiently engaging is the bike failing to stop. A sign that it is over engaging is the wheel failing to spin freely. This common problem can be addressed by bleeding the brake lines.
Ceramic brake pads wear down every time they touch the rotor. An indication that the brake pads need to be replaced is a grinding noise when the brake is engaged. This problem can be solved by replacing the brake pads.
A wheel wobbles and does not follow a strictly circular path during rotation.
The tension in the spokes holds the rim in alignment. A broken or missing spoke is easy to identify either by the noise it creates when it breaks or the obviously missing spot in a spoke pattern. Replacing the spoke is the simple fix short of replacing the enitire wheel assembly.
The tension in the spokes of a wheel pull the rim in opposing directions causing a net force of zero in all directions. If the tension is not balanced correctly the wheel will wobble.
The front suspension is too stiff, too lose, or sticks when going through the range of travel
Suspensions rely on air pressure (in addition to oil and in some cases springs) to dampen forces exerted on the shocks. Adding or releasing air pressure will change the pre-load of the shock and the resulting rebound when engaged. Air pressure can be adjusted with a suspension pump.
Suspensions have a complexity of moving parts that wear with time and fluids that need to be replaced. A worn suspension could be indicated by the shock “sticking,” metal grinding, or the presence of fluid residue. In these cases, the suspension can either be rebuilt or replaced.
The rear suspension is too stiff, too lose, or sticks when going through the range of travel
Suspensions have a complexity of moving parts that wear with time and fluids that need to be replaced. A worn suspension could be indicated by the shock “sticking,” metal grinding, or the presence of fluid residue. In these cases the suspension can either be rebuilt or replaced.
The handlebars are not in line with the wheel
The headset screw holds the handlebars in a fixed position relative to the wheel. Tightening the headset will fix any alignment issues.
A cracked headset prevents the handlebars from being held in place. The solution to a cracked headset is simply replacing the headset assembly.
