Step 1: Remove the tire and old tube
The first step is to release the brake and loosen the axle nut so you can remove the wheel from the bike. Most bikes have a quick-release mechanism on the brake and a quick-release lever on the axle. If you’re not sure how to remove the wheels on your bike, check the manual or visit your local bike store and ask someone on staff to show you how. Do this before you have a flat so you’ll be prepared. For a guaranteed trouble-free fix, buy a new tube rather than patching the old one (see “Valve Types” below).
After removing the wheel from the bike, the next step is to take one edge, or bead, of the tire off the rim so you can remove the damaged tube (Photo 1). Pull out the tube. Then remove the tire completely from the rim, put it around your neck like a necklace and turn it inside out while running your fingers along the inside to feel for sharp objects. Be careful, though—there could be bits of glass lodged in the tire.
Tire Repair Kit
If you don't want to walk home after having a flat, here's a tip: Assemble a tire repair kit like this. You can put it in a resealable plastic bag and tuck it in a knapsack or buy a small pack that straps under your bicycle seat. Not including the seat pack, the kit cost us about $38.
Step 2: Install the new tube
After checking the tire, put one bead back on the rim, leaving the other side loose so you can install the new (or patched) tube (Photo 2).
Photo 3 shows how to push the second tire bead back onto the rim after the tube is in place. When the tire is completely installed, check all around on both sides to make sure the tube isn’t pinched between the tire and rim. To do this, push against the bead and look into the space between the tire and rim to be sure it’s clear.
Now you’re ready to fully inflate the tire. But there’s one last thing to watch for. Occasionally the bead may not seat properly on the rim, so stop before the tire is inflated to full pressure and rotate the wheel while you inspect the area where the tire and rim meet. If you see a spot where the bead isn’t seated in the rim, let a little air out and work the bead into the rim with your fingers. Then inflate the tire to the pressure listed on the side. Reinstall the wheel and spin it to make sure it’s centered between the brakes and rotates freely. If you’ve done an emergency repair using the CO2 cartridge, you may not have enough pressure. When you get home or to a bike store or gas station, fill the tire.
When you go to buy a new tube, take the old tube with you and match the valve. Presta valves have a nut on a threaded shaft that seals the air in and must be loosened to inflate the tube. You need a pump that’s compatible with a Presta valve to inflate these tubes. If you have Presta valves on your bike, keep a Presta-to-Schrader adapter with you so you can use a gas-station air pump to inflate your tire.