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                                                                FIXING A FLAT TIRE

When fixing a flat or changing a tire your main opponent is the steel cable imbedded in most tire edges (beads) (see Figure 1 below for a cross section of a typical tire / wheel assembly).  This bead is what prevents the tire from stretching under pressure and popping off of the rim. 

There are two height distances to keep in mind when taking the tire off of the rim (see Figures 2 and 3 above).  The first, height A, is the distance you have to get the edge of the tire up to in order to get it off of the rim.  The second, height B, is the distance you need to take full advantage of in order to make height A possible.  Height B is the distance that most of the tire edge needs to get down to so that there is enough “extra” edge length to pop over the rim at your starting spot.  It’s a math thing: if the tire bead (cable) diameter is 24 inches it has a circumference of (3.14 x 24”) 75.36 inches; if the diameter of the bead space at the height B bottom is 23 5/8 inches the circumference is (3.14 x 23.625”) 74.18 inches; the difference of  (75.36” – 74.18”) 1.18 inches is now the “extra” length you have to get the tire edge over the rim. 

                                                                      REMOVAL

You will need 2 tire irons or other slim blunt edge tools to help you get the tire off of the rim (available at bike stores and bike departments in some department stores).  If you are changing the tire and have a still good tube, let the pressure out of the tube, but don’t deflate it entirely (see Figure 4).  This will help the tube push away from the tire irons to keep from getting pinched.

Once partially deflated start at the part of the rim opposite the valve stem, work the tire bead to the rim center (Figure 3 above) and push the excess bead toward the valve stem (if the amount of air in the tube hinders you, deflate it some more).  When you have worked the “extra” length to the valve stem section put the tire irons under only enough to catch the tire bead.  They should be about 4 inches apart.  Keep them in place with one hand and push the valve stem up into the tire to make sure the tube is away from the irons.  Pry the tire over the rim either one iron at a time or together depending on resistance.  Move further away with one iron and pry some more of the tire, while keeping the other iron in place.  Once the amount of bead tension decreases finish with just your hands.  With one side of the tire off of the rim, remove the tube.  Be careful of any sharp spots on the rim when removing or replacing the tube, including any you just made from prying.  If removing the tire entirely try a hands only removal for the other bead now that you have the tire to pull on. 

                                                                REPLACEMENT

Put one bead side of the tire on the rim.  Before replacing the tube check all of the tube contact surfaces to make sure there are no sharp points sticking through the tire etc.  Lightly inflate the tube to help it push away from pinch points, also to check for leaks, just because it’s new is no guaranty (see Figure 4).  Put the valve stem in first then tuck the rest of the tube into the tire.  Many tube problems come from over stressing the tube at the valve stem.  Make sure the tube is not pulling in either direction at the stem.  Start putting the rest of the tire on, again opposite the valve stem (the valve stem prevents you from getting to the low spot on the rim).  Work the tire bead to the center low point of the rim (bottom of height B, see Figure 3 above) stretching the “extra” bead toward the valve stem.  With good stretching you may be able to get the tire on without using tire irons, if not push up on the valve stem to keep the tube away from the irons while popping it on.  Check the valve stem to make sure it is straight, if not, rotate the tire slightly on the rim until it is.  Look for any signs of the tube showing at the tire beads and tuck under as necessary.  Inflate the tire to about 2/3rds of its normal pressure; let all of the air out, then inflate again to its normal pressure.  The 2/3rds and release helps even out any tube bunching and stretching inside of the tire.  In the time it took you to read this mini-novel you could have changed 2 or 3 tires.  Good luck.

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