EVERY TIME YOU FIX A PUNCTURE, THE NEW TUBE GOES FLAT
If the holes in the tube are on the bottom, the rim strip may be out of position, allowing the tube to get cut by the spokes. If they're on the top, there may be some small sharp object stuck in the tire. Find it by running your fingers lightly around the inside of the tire, then remove it.
FREQUENT PINCH FLATS
Put more air in your tires.
A REMOUNTED TIRE WON'T SIT RIGHT ON THE RIM
Let the air out, wiggle the bad spot around, reinflate to about 30 psi, and roll the bad spot into place with your hands. By pushing the tire in toward the middle of the rim you will be able to see if any of the tube is poking out. When the tube is fully inside the tire, inflate as normal.
BONUS: 12 Bike Fixes Explained
A PATCH WON'T STICK TO THE GLUE ON THE TUBE
Apply more glue and let it dry completely--about five minutes. (Don't blow on the glue to try to make it dry faster--this can leave moisture from your breath on it, which hinders adhesion.) When you apply the patch, avoid touching its sticky side with your fingers.
CREAKING SOUND FROM THE WHEELS
A spoke may have loosened. If tension is uniform, the sound might be caused by a slight motion of the spokes against each other where they cross. Lightly lube this junction, wiping off the excess.
CREAKING SOUND WHEN YOU PEDAL
Tighten the crankarm bolts. If the arm still creaks, remove it, apply a trace of grease to the spindle, and reinstall the arm.
THE LARGE CHAINRING FLEXES, AND THE CHAIN RUBS AGAINST THE FRONT DERAILLEUR CAGE
Check for loose chainring bolts.
AMBITIOUS , YOU REMOVED THE CHAIN-RINGS TO CLEAN THE CRANKSET, BUT NOW THE FRONT DERAILLEUR DOESN'T SHIFT RIGHT
You may have installed a chainring backward. Remove the rings and put them on correctly. Usually, the crankarm bolts fit into indentations on the chainrings. Sight from above, too, to make sure there's even spacing between the rings.
YOU'RE TRYING TO REMOVE A CHAINRING BOLT, BUT IT JUST SPINS
Hold the backside of the chainring bolt with a wide, flathead screwdriver or a special chainring-bolt wrench built for this purpose.
WHILE TRYING TO REMOVE OR ADJUST A CRANKARM YOU STRIPPED THE THREADS--NOW YOU CAN'T REMOVE IT
Ride your bike around the block a few times. The crankarm will loosen and you'll be able to pull it off.
BONUS: 16 Essential Bike Tools
SHIFTER HOUSINGS RUB THE FRAME, WEARING A SPOT IN THE PAINT
Put clear tape beneath the housings where they rub.
NOISY, SLOPPY SHIFTING CAN'T BE REMEDIED BY REAR DERAILLEUR ADJUSTMENT
The cassette lockring might be loose, allowing the cogs to move slightly and rattle around on the hub. You need a special tool to tighten the lockring fully, but you can spin it tight enough with your fingers to ride safely home or to a shop.
THE COG CASSETTE IS GETTING RUSTY
A little rust won't damage the cogs quickly, so it's not a major concern. Usually, using a little more lube will prevent additional rust, and riding will cause the chain to wear away the rust while you're pedaling.
IN CERTAIN GEARS, PEDALING CAUSES LOUD SKIPPING
There may be debris between the cogs. If you can see mud, grass, leaves, twigs or any sort of foreign matter trapped between cogs, dig it out. It's probably keeping the chain from settling all the way down onto the cog to achieve a proper mesh. If there's no debris, a cog is probably worn out. Most often this is a sign that the chain and cassette will have to be replaced.
FRONT DERAILLEUR WON'T SHIFT PRECISELY TO A CHAINRING
Check that the cage is parallel to the chainrings (when viewed from above), and loosen and reposition the derailleur if necessary. If it's parallel, you probably need to adjust the high- and low-limit screws, best done by a shop or experienced amateur mechanic.
THE REAR DERAILLEUR MAKES A CONSTANT SQUEAKING NOISE
The pulleys are dry and need lubrication. Drip some light lube on the sides, then wipe off the excess.
BRAKING FEELS MUSHY, EVEN THOUGH THE PADS AREN'T WORN OUT
The cable probably stretched. Dial out the brake-adjuster barrel (found either on the caliper or on the housing closer to the lever) by turning it counterclockwise until the pads are close enough to the rim to make the braking action feel as tight as you want.
BRAKING FEELS GRABBY
You probably have a ding or dent in the rim. This hits the pad every revolution, causing the unnerving situation. Take your bike to a shop.
ONE PAD DRAGS AGAINST THE RIM OR STAYS SIGNIFICANTLY CLOSER TO THE RIM THAN THE OTHER
Before messing with the brakes, open the quick-release on the wheel, recenter the wheel in the frame and see if that fixes the problem. (This is the most common solution.) If the wheel is centered but a pad still rubs, you need to recenter the brake. On most modern brakesets this is done by turning a small adjustment screw found somewhere on the side or top of the caliper. (There may be one screw on each side, as well.) Turn the screw or screws in small increments, watching to see how this affects the pad position. If you center the brake and the wheel, and a pad still drags on the rim, it probably wore unevenly from being misadjusted; sand the pads flat and recenter everything.
WITH EACH PEDAL STROKE YOU HEAR A CLICK COMING FROM THE SADDLE
The pedal may have loosened. Tighten it.
Wipe the rim to remove any oil or cleaning residue. If this doesn't work, scuff the pads with sandpaper or a file. Still noisy? The pads need to be loosened then toed-in, an adjustment that makes the front portion touch the rim before the back--an easy fix for a shop, a tortuous process for a first-timer.
Drip a tiny amount of oil around the rails where they enter the saddle, and into the clamp where it grips the rails. Heritage purists take note: Leather saddles sometimes creak the same way that fine leather shoes can. There's not much you can do about this.
YOU CAN NEVER REMEMBER WHICH WAY TO TURN THE PEDALS
Treat the right-side pedal normally--righty-tighty, lefty-loosey (clockwise to tighten, counterclockwise to loosen). The left-side pedal has reverse threads (which keeps it from unscrewing during pedaling) so it must be turned counterclockwise to tighten and clockwise to loosen. If that's confusing--and for many of us it is--think of this phrase: Back off. This can remind you that, with the wrench engaged above the pedal, you always turn it toward the back of the bike to remove the pedal.
YOU INSTALLED A PEDAL INTO THE WRONG CRANKARM--THE LEFT PEDAL INTO THE RIGHT ARM OR VICE VERSA
You can remove the pedal, but the crankarm will have to be replaced; its threads are softer than the pedal's and are now stripped out. Always check the pedals before installing. There is usually an "R" for right or "L" for left stamped onto the axle.
YOU PULLED APART YOUR HEADSET TO REGREASE IT, AND NOW THE HEADSET FEELS TIGHT NO MATTER HOW YOU ADJUST IT
The bearing retainers are probably in upside down.