I have quite intelligent friends who could write horribly complicated computer programs, but wouldn't even attempt to repair a flat bicycle tire. I think it's because there aren't enough HOWTOs on the subject.

Bicycles are at the cusp of mechanical complexity that can be understood without special training - mechanical watches are on the other side of the threshold. As such, everyone should be able to change a flat tire.

Below are instuctions on how to fix a simple flat, where the tire is still useable. If you have a bad flat, where you have a big gaping hole in your tire, you outta luck (unless you actually carry a whole spare tire, instead of just the inner tube...)

Stuff you need to fix a flat bicycle tire

Stuff that makes it easier to fix a flat

Instructions

  • First, get off the bike. Stop before you do. Continuing to ride your bike with a flat tire is dangerous. You'll lose control, and you'll damage your rims.
  • Remove the wheel on which the tire is flat. This is easy if you have quick-release hubs. Otherwise, get out the wrench. You may have to release your brakes in order for the wheel to come out.
  • Remove the tire. Take out most of the air form the tire, if it isn't completely flat already. This makes it easier to remove the tire. You don't need to completely remove it, just get one side off the rim, so you can get to the inner tube. This is where the tire levers would come in handy. If you have the kind with a hook on one end, you take the other end, lever it under the bead of the tire, bend it back, and hook the hook on one of the spokes. This keeps the lever in place, so you can work at another part of the tire. If you ever have to remove Continental Grand Prix tires off of a a Mavic MA-2 rim, you'll almost need the levers.
  • Look for what caused the flat. Run you fingers inside the tire to feel for rocks, glass, etc. You should be careful - you can get a little nick yourself. It's important to make sure there's nothing poking inwards through the tire, as you'll just get another flat if you don't get rid of it.
  • (optional) Patch the inner tube. Even if you have a spare inner tube, it's a good idea to patch the flat one, so you'll have a spare in case your spare gets a flat. Find the whole, scratch the rubber around the hole with the sandpaper that comes with the patch kit to remove the mold-release from the rubber. Smear some rubber cement around the hole. Let dry for a few minutes. Apply the patch.
  • Put the inner tube back in. Partially inflate the inner tube. Position the inner tube back inside the tire, starting from the stem (where you blow air in from). Make sure there are no kinks and the inner tube isn't being pinched anywhere.
  • Remount the tire. You may have to let a little air out of the inner tube. Reseat both beads into the rim, making sure you don't pinch the innertube between the bead of the tire and the rim.
  • Inflate the tire. Attach your pump or CO2 powered inflator. I've never used a CO2 inflator, so follow the manufacturer's directions. Otherwise, pump away. After the first few pumps, make sure the inner tube isn't trying to peek out from anywhere. Remount the tire if it is. You can't reach as high pressures with a frame pump as you can with a normal floor pump, so pump as much as you'd like. Once it's the tire is hard enough to ride on, you're done.
  • Pick up any trash you generated. Don't be a bozo who leaves his flats around. Flat inner tubes make wonderful luggage straps.

As someone who's just struggled with getting a set of Continental Grand Prix tires on Mavic MA3 rims, there's a few things I've learned to help the whole process go smoother if you're having trouble with the instructions above:

  • My tube blew up or has a leak right after I put it on!

    What's probably happening is the tube isn't seating in the tire properly; a small bit of tube peeking out from under the bead will either cause a puncture or explode when put under pressure -- a pinch flat. To help the tube move around in the very limited space between the tire and the rim, coat the tube in talcum powder before you put it in the wheel. The talc acts as a dry lubricant, and will help the tube seat properly.

    The other thing you must do when mounting a difficult tire is check the entire circumference of the wheel for places where it will pinch flat. I start at the label on the tire and, pushing the tire away from the rim with my thumbs, look down between the bead and the rim for the tube. If you see a bit of tube, try using a tire iron and lifting the bead up, then inflating the tube a bit. Ease the iron out of the bead, and hopefully the tube will be seated properly in the tire. Deflate the tube and keep checking for pinches. When you're done with one side of the wheel, turn it around and do the other side. Be thorough. An extra 5 minutes is worth it when you've taken half an hour to get a difficult tire on.

  • Aargh! I can't get these damned tires on these rims!

    The very first thing you should do is double check that everything is the right size. The tires could be too small for your rims. (An honest mistake; at times, it seems bike equipment manufacturers are picking sizes out of the ether instead of making and following standards.)

    If it is the right size, and you can't get it on, try different positions of wheel and iron. For me, holding the wheel between my knees with the tire iron facing out and pulling on it with both hands was the only way to get my Contis in the rims.

    If you really, truly, cannot get the tires in the rims, it's time to get messy. Go outside, and bring a bottle of dishwashing soap or shampoo. Coat the bead at the last section of tire you absolutely can't get in with the soap. The soap will hopefully reduce the friction on the system enough that you can muscle the tire into the rim. This is messy as hell, though, so use it as a last resort.


Albert Herring says re How to repair a flat bicycle tire: Using tyre levers to put a tyre on is just asking for trouble (pinched tubes). It is quite simple if you have any strength in your thumbs...
flamingweasel says 90% of the time you're right. Any rims wider than 3cm, or tires that take less than 120psi, you should be able to do with your thumbs. But I don't know if you've ever put 150 psi tires on 700x20C rims with no irons, but I sure as hell can't do it with my thumbs. I have no problems with my 80psi Conti Top Touring tires on 1-inch rims. These Grand Prixs on teeny little rims are totally impossible to do manually.

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