Saturday, 27 February 2010

All you never wanted to know about punctures

To unacquainted rider punctures come as incomprehensible random events which generally follow Murphy law of happening at the worst possible moment. We will however try to put a trifle of rationality in it, hoping at the same time that vicious Puncture God doesn't take this as a heresy.

First of all - as heretical as it may sound - every puncture has its cause. This is the founding axiom of puncture science and the most important concept for puncture-free experience. If you have a puncture for which you don't know the cause, you will have lots more of them - and rightly so, since you didn't make an effort to understand it.

The causes of punctures can be separated into two types:
  1. external puncture is caused by an external object first protruding through the tyre and then puncturing the tube.
  2. internal puncture is one that is not external.
Within these types there are variations. We further subdivide punctures as to get a list of every possible puncture cause known to man, woman or child. The following causes are ordered from most likely to least likely cause, based on my experience.

  1. external punctures
    1. external object embeds into the tyre and punctures the tube after several wheel revolutions.
    2. external object protrudes through the badly worn tyre, punctures the tube
    3. external object is pushed through tyre and the tube deliberately.
    4. external object protrudes through tyre and the tube within one wheel revolution.
  2. internal punctures
    1. "pinch flat" or "snake bites" when a tyre presses against the tube, mostly when hitting a rock or a pothole.
    2. tube punctures near the valve facing the rim.
    3. tube protrudes through the valve hole and blows out.
    4. tube rubs against an object caught between the tube and the tyre (eg. piece sand, shreds of rim tape, etc.).
    5. tube punctured with a tool when installing it on a rim
    6. tube protrudes through the fault in the tyre and blows out.
    7. tube is pinched between the tyre bead and the rim and blows out or punctures.
    8. deliberately deflated tube (this is not a puncture, but looks like one).
    9. tube patch peels off
    10. tube patch cracked
    11. spoke is too long and punctures the tube from the inside
    12. tube rubs against a fault inside the rim, or against the rim tape.
    13. tube rubs against a fault on the inside of the tyre.
More often then not you can tell if the puncure is external or internal, or even its subcategory, by a
little forensic study. Use the following procedure:

  1. Before removing the tube from the wheel mark one matching point on both the tyre and the tube. This step is not necessary if you've installed the tyre and the tube correctly (see points 13, 14 and 15 below for installation).
  2. Before removing the tyre look if there are pieces of sand, etc. inside it.
  3. Look if the puncture is at the inside of the tube (the surface facing the rim). If it is, it's an internal puncture.
  4. Examine the rim and/or the tyre at the point that matches with the point of the puncture in the tube. You are likely to find the cause of the puncture at this point. If you don't find it there, go all around the tyre and/or the rim looking for one.
  5. Some punctures (a.2, a.3, b.1, b.2, b.3, b.6, b.7) have no physical evidence of the cause, but some
    can be identified by the position or appearance.
  6. Type a.2 punctures usualy occur on a very worn tyre thread (eg. so that tyre fabric beneath is showing or if there are a lot of cuts and cracks in the thread). The thread is so thin that the item that punctured the tube (a sharp piece of stone, glass) will not embed in the tyre and will fall off. Changing the tyre is the best remedy.
  7. Snake bites (b.1) are often in a form of double holes about 4 mm apart.
  8. b.2 and b.3 type punctures are at the base of the valve. b.2 punctures are frequently caused by high-pressured tube pressing against the nipple head on single walled rims. (See the solution below).
  9. Some punctures (b.2, b.3, b.6, maybe b.7) can happen when the bike is not in use (e.g. overnight).
  10. A minuscule, slow leaking puncture on the side of the tube is representative of a needle pinch (a.3)
  11. Blowout type of punctures (b.2, b.3, b.6) deflate the tube immediately. b.3 and b.8 leave large holes.
  12. External punctures of type a.1 are often slow leaking.
  13. Rubbing-type punctures (b.4, b.12, b.13) are also slow leaking, at least in the first stage when the tube develops small sracks in the rubbing area. Eventualy these cracks join into a bigger hole.
  14. If you can't identify the cause, record the circumstances for further analysis. If a puncture is particularly mysterious, send the details to me, I am collecting such data.
The good news about the punctures is that most of the type b punctures are avoidable. With little care, most of the type (a.1) punctures are avoidable too. Replacing the excessively worn tyre usualy gets rid of the (a.2) punctures. So only with the type (a.3) are we in the mercy of Puncture God, which in this case frequently takes the form of a common man. My advice to puncture-free experience is the following:

  1. Pump your tyres to high pressure. Check and keep the pressure high at all times. Recommended pressure on the tyre sidewall is high enough. If pressure range is indicated, go closer to the higher value. I don't believe in benefits of reduced pressure (e.g. higher traction, softer ride).
  2. Always find out the cause of the puncture and remove it.
  3. Check both tyres regularly for embedded bits of glass, wire, etc. Do this before or after every ride at home and at the start/end of the day on a tour. I always keep a safety pin on my jersey to pick out the bits from the tyre.
  4. Do the above check immediately if you ride through pile of glass, etc.
  5. Do the above check immediately if you hear the sound comming from the wheel at each wheel revolution.
  6. Do the above check always after a puncture unless you are absolutely certain it's an internal puncture.
  7. Don't use any tapes between the tyre and the tube - at least not as a permanent solution.
  8. Use rim tape.
  9. Change the tyre when its sidewall is cracked. As a temporary solution make a tyre boot (duck-tape worked for me).
  10. Change the tyre when its thread wears so much that the tyre fabric beneath is showing. As a temporary measure put a piece of duct tape over these parts.
  11. Few things are important when patching tubes: clean the tube with sand paper before applying rubber cement; let the cement dry before applying the patch!!; press on the patch for 3 minutes; remove the plastic cover from the patch after few hours.
  12. Inflate the tube slightly before puting it inside the tyre.
  13. As a protection against b.2 and b.3 punctures, cut a patch of an old tube and slide it down the valve, so that it covers the valve base (about 2 cm of tube left and right from the valve). Cut a notch on one side of this patch, let's say on the left side.
  14. Most tyres are marked by the manufacturer and brand names only on one side. Install your tyre so that the valve hole is at the middle of this mark and that the mark is on the QR side of the wheel (or on the opposite side, according to the drive direction marked on the tyre).
  15. Put the tube inside the tyre so that the notch mentioned in #13 is to the left when you are facing the tyre mark mentioned in #14. This fixes the relative positions of the rim/tube/tyre combination and hepls in finding the cause of the puncture.
  16. Refrain from using any tools for mounting the tyre on the rim. If this can't be done, it's time to think about different tyre.
  17. With the tube partially inflated go around the tyre and check that the tyre bead doesn't pinch the tube against the rim.

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