Thursday, January 18, 2007

How To - changing a tube/tire

In Florida, on the first ride we did, I flatted riding on some glass. Although you should avoid sand, glass, rocks, etc., if possible, sometimes you have to ride over it, and that sometimes causes a flat. In my case, I was trying to be careful on a narrow shouldered road with cars passing at 50-60 mph and ended up riding for a while on debris on the side of the road.

Although inconvenient, the flat was not a ride-breaker because I had a tube, pump, and I knew how to change a tube. You might think that knowing how to change a tube is common knowledge but I've watched Cat 1's struggle for many minutes with a tube so it's not necessarily true.

You should virtually never flat bottoming out an uninflated tire (also called a pinch flat) - that is simply a reflection on poor bicycle maintenance. I check my tire pressure every time I ride. To balance riding comfort and pinch-flat resistance, I pump my tires to 110-120 psi (front lower than the rear). A vicious impact on your tire/wheel will cause a pinch flat but there is a technique I'll describe later that will enable you to avoid even those types of flats.

Tires, even relatively light ones, will last thousands of miles in the rear. Front tires last so long it's a good idea to rotate them to the rear so you wear them out. Tubes should never puncture.

Changing a flat actually starts with installing the tire properly. When you first install the tire, make sure you have a good rim strip (covers the spoke heads and protects the valve from the edges of the hole drilled in the rim), the right length valve (longer for deep rims), and a clean tire. It's a waste of time and money to flat because your rimstrip was insufficient or the valve ripped off the tube due to the rim edge digging into the valve base.

Note: electrical tape is NOT a viable rim strip. Go to the shop and buy a few and keep at least one or two spares in your gear bag. Inevitably one of your less educated cyclists will complain about puncturing every 5 miles, and when pressed, will admit to using some substandard material for a rim strip. Your spare rim strip will be a ride or race saver.

Line up the valve to the label on the tire and install it so that the label is on the right side of the rim (the drivetrain side of the bike). Pretend a photographer is taking a picture of your bike and that you want your tire sponsor to be visible. It might actually be true! Even if it's not, the label/valve position makes it easier to troubleshoot if you do have a flat. If there is no label, line up the pressure rating to the valve - this way you don't have to look around the tire to see if it's rated to 120 psi or 240 psi.

(Note: there is a "cheat sheet" summary at the bottom but it skips a lot of details)

1. If you do have a flat, stop off any busy road - a driveway, a side road, the sidewalk, etc. There is no reason for you to be replacing a tube in the middle of a busy road.

2. Next, remove the tire and tube using a tire lever or three. If the tire is very tight, insert one tire lever under the bead, then about 2-3" away insert another one. Move another 2-3" and insert a third one. The second one will fall out. Put it in 2-3" past the last lever and repeat until you can remove the tire and tube.

Note: ALWAYS start opposite the valve and finish at the valve.

3. If you've lined up the label and valve, remove the tube from the tire. If you haven't, hold the valve in place of the tire by pinching the tire sidewalls with your fingers. This is so you can track the puncture and check the tire in the puncture vicinity.

4. If there is an obvious cause of a flat (nail sticking out of the tire, etc.) skip this step. Otherwise, inflate the tube a bit and listen for escaping air. If it's escaping too quick to pinpoint the location of the hole, fold the tube in half around the suspected area (one fold on each side). This will allow air pressure to build up around the rest of the tube. You can move the folds closer together until you pinpoint the puncture. If you've held the tire in place, you can check the tire. Too many times there will be a piece of glass or something in the tire. If left alone, it will puncture your new tube right away.

5. Verify there aren't other punctures. Nothing would be worse than to replace the tube thinking the big nail sticking out of the tire was the only thing that caused the flat only to find a piece of glass elsewhere also punctured the old tube... as well as your new one.

6. Pump up the tire until it is holding its shape. Do not let it stretch. On a floor pump, it would be one or two strokes. For a mini-pump, maybe 3-6 strokes. You want it to fit inside the tire with no problems but you do not want it to "grow" so it doesn't fit in the tire anymore.

7. Line up the valve and the label as described above.

8. Insert the valve and seat the bead on the first side of the tire. For the sake of this tutorial, I'll say the left bead first, if looking at the wheel from behind. The left bead would be mounted from the right side of the rim. If necessary, use tire levers to pop on the bead, being careful not to pinch the tube. This is the only time you should use the levers.

Note: ALWAYS start at the valve and finish opposite the valve when mounting a tire (the opposite of removing the tire).

9. After getting the left bead on the rim, push the tube into the tire so it sits inside the "U" of the rim. Again, start at the valve and finish opposite it.

10. Now your tire should look almost mounted from the left side. The right side is still hanging outside the rim. Pull the bead onto the rim at the valve and about 8-12" on either side of the valve. Push the valve in and push the tire down so both beads seat solidly on the rim.

11. Pull the bead onto the rim from the left side, using your four fingers. Don't use your thumbs and try and push the bead on - it's a lot harder. You'll probably have to push the tube in place every now and then as it has a tendency to pop outside of the rim. It's absolutely critical that you keep the tube inside the rim.

12. At the opposite end of the valve, when you have about 6" of bead left, hold one hand (say your right hand) with your fingers anchoring the bit already on the rim and using your other hand (left) immediately next to the right hand to pull the bead over the rim. It takes effort, especially on new tires, but you should be able to pull the bead onto the rim without using tire levers.

13. After the bead is all the way on, pinch the tire on both sides so you can see the rim strip under it. You should not see the tube at all - if you do, you'll need to carefully push it under and past the bead. If you do not, you are assuring yourself a spectacular blowout with immediate pressure failure at some point in the future.

14. Now you're ready to inflate the tire. Do one final push on the valve (push it up into the rim, not all the way, push the tire into the rim, then pull the valve back out) and inflate the tire.

15. DO NOT use the little threaded nut to "hold" the valve in place. It is sometimes useful to hold the valve high enough if the valve is a bit short, but if you use it that way, remove it as soon as the valve has enough pressure in it to mount the tube without it.

The main reason you should not use the threaded nut is that it hides a sliding tube. When you ride an under inflated tire, the tire and tube will slip along the rim. A great early warning indicator is the valve starts to tilt relative to the rim. If this happens, you need to move the tire/tube unit around the rim until the valve is perpendicular.

If you use the threaded nut, the valve remains perpendicular even though the tire and tube are slowly rotating around the rim. You only learn about it when the valve rips right off the tube. Using the threaded nut is like disabling your car's warning lights - you never see the oil pressure light go on and only learn you're low on oil pressure when your engine seizes. Not good.

Let's review (this can serve as your cheat sheet):
1. Always remove opposite the valve, and always finish opposite the valve.
2. Line up label with valve. If no label, line up PSI rating with valve. If one label, put on right side of wheel.
3. Pre-inflate tube till it just holds its shape and insert into tire before mounting tire.
4. Put one bead on, then the tube, then pull the other bead on.
5. DO NOT use tire levers for the second bead (i.e. to finish the tire)
6. Before inflating, push valve stem a bit into the rim and allow the bead to seat around it.
7. DO NOT use the little valve stem locknut.
8. Before every ride, check tire pressure.

If your labels are not lined up with your valves or you have never changed a tire before, you should try it in the luxury of your home. It certainly beats figuring it out while the sun is setting, you're freezing cold (or bonking or whatever), and generally miserable.

Congrats, you're all done now.

If requested I'll post a vid on this.


GMF said...

Funny. I always use the little threaded nut thing on the valve to keep it from rattling around in the rim. On some carbon rims it can really cause a racket. That's one reason I prefer the threaded stem tubes to the smooth ones. (Also because they grip the pump chuck better).
Is the sliding tube problem common?

Aki said...

I was talking to an unnamed teammate about two weeks ago and he mentioned he just flatted due to this exact problem.

There is a "user" element necessary - unless you exert stump pulling torque, proper inflation will prevent this from happening. If you habitually forget to check air pressure before riding, this will be a common problem.

The exception is in mountain biking, where commonly used pressures (15-30 psi) will allow the tire to rotate on the rim. It's imperative not to use the nut in this case since the tire will rotate a bit on every ride.

Rich said...

One other thing to include is that you should keep your tubes in plastic bag seperate from all the dirt, mini-tools and bits in a saddle bag that might cause a small leak before you ever get to use it. Throw some baby powder in the bag and it will be easier to move the tube around the edge of the tire as well.

For Item 6. you could also add a quick way to partially inflate your presta valve tubes for installation is to use your mouth. You can get just about the perfect amount of air in the tube without stretching it. This might not be a palatable option for some.