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I'm stumped. For the last yew years I have been getting flats that look like this:


As you can see, it's a tiny, tiny hole that can only be found by inflating the tire to twice its size or so. I know these aren't puncture flats because there is never anything in the tire, and they don't seem like pinch flats because:

  1. They never come in pairs
  2. They happen much more frequently on my front tire even though my rear wheel has much more weight from loaded 40L Ortlieb paniers.
  3. I'm running between 110 - 125 PSI, and more pressure doesn't seem to reduce the rate of flats.
  4. They tend to show up when I go to get on my bike in the morning, rather than on the way or after work. (Though this doesn't really contraindicate a pinch flat, I just thought it was weird).
  5. They seem to mainly be on the outside, although sometimes they are on the inside of the tube.

Lately they've been happening more frequently, today I had three (I found my bike flat in the morning, then tried two more patched tubes with similar flats that I had missed).

There are the usual gotcha's that I usually try to avoid too:

  1. Watch out for lips/edges on the road.
  2. Only use tire lever for tire removal, not replacement.
  3. Put some air in the tube before putting on the wheel to avoid folds/bends
  4. Make sure rim tape is fully covering all spoke holes

I'm getting tired (pun not intended) of these, so before I buy a new (bigger) wheelset, I thought I'd call for help.


Some more details: Mavic CXP 30 700c wheels with Continental Gatorskin 700x23c tires, 4 mile commute each way over decent roads in hot conditions.

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Yeah, looks like a glass/wire puncture. You need to be very careful when inspecting the tire as a tiny sliver of glass can be embedded in the tire and not obvious. And worst case glass can get inside the tire. –  Daniel R Hicks Jun 4 at 0:50
And invest in a Quick Stick tire changer. Much better than levers and virtually impossible to damage the tube. –  Daniel R Hicks Jun 4 at 0:57
If you rub your thumb along the inside of the tire you may find an embedded piece of glass or wire that is otherwise invisible. You might be surprised at what you can feel that you can't see. –  Adam Davis Jun 4 at 14:56

3 Answers 3

up vote 16 down vote accepted

That looks like the sort of hole you get from tiny bits of glass working their way through the tyre, then slowly cutting into the tube. I'd check the outside of the tyre for tiny cuts, and the inside for barely-detectable points poking through.

This photo shows the sort of small cuts I'm talking about:

tyre with tiny cuts

Each of those had a little bit of glass or something in it once, but most of those work their way back out. The problem is that the ones that don't usually work their way in instead. If you poke in there with something like the file shown (a sharpened spoke is what many bike mechanics use) you'll sometimes find a ~2mm or smaller triangle of glass.

When those work through the inside of the tyre they don't punch a hole like a nail does, they slowly cut a little slit as you see above.

The way to avoid this is by going over your tyres every few weeks and digging out those bits of glass.

Note that even very puncture-resistant tyres will suffer this, just less often. I've had Marathon Plus tyres puncture this way, but only once (I use those tyres a lot). This guy had the same problem with tyre liners. Picking the glass out is the only solution.

Echoing @DanielRHicks comment: one annoying possiblity is that you have a little fragment of glass or something loose inside the tyre. So every time you check the tyre you don't find it because it slides around as you rotate the tyre checking it, and every puncture is in a different place. So check for that too.

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Spot on. And we need more bike lanes. –  andy256 Jun 4 at 0:24
I've had good luck preventing this with Kevlar-belted tires, since the Kevlar belt is too hard for most glass fragments to work through. Did have a tiny piece of wire get through last summer, though (first puncture in years), and it embedded itself in the belt so firmly that it basically wouldn't come out without destroying the tire. –  Daniel R Hicks Jun 4 at 0:54
I have gatorskin tires, which have a "dura-skin" layer in addition to another kevlar layer. I looked for things in the tire, but I'll give it a really good look tomorrow. Thanks for the help everyone! –  JKnight Jun 4 at 3:48
@JKnight If you're still having trouble finding anything, try flexing the tire, even turning it partway inside out. If there's something tiny sticking out inside, it'll be a lot more obvious that way. –  Jefromi Jun 4 at 6:01
@andy256 - round here the bike lanes are full of glass! The local council build them (presumably begrudgingly) and make absolutely no effort to keep them clear of debris. I avoid them like the plague and stay in the car lanes. –  PeteH Jun 4 at 14:00

That sure looks like puncture to me.

Are you sure you don't have something small in your tire?

You should rotate tires anyway so give this a try.

Pull the tires and rotate.
If you start getting flats on the rear the look hard at the tire.

If you get flats in the front then look hard at that rim.

Turn the tires inside out and inspect.
Wipe down the rim with a towel.

Are these tubes you bought as a set?
Maybe bad tubes.
Try a fresh set of tubes of a different brand.

Why are you saying bigger wheelset?
That is a decent wheelset.
That is not the first thing I would replace.
That rim will take larger tires.

Start with tubes and then tires. And if you replace the tire consider something bigger than 23.
You may be only able to go to 25 based frame and rim.

I commute on Marathon 32 and don't even get one flat a year.
Have Gatorskin 25 on another bike and like the speed but would not use that for commuting.

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I was thinking of buying bigger tires if they were pinch flats or different wheels if it was something about the gatorskin+mavic combo that was a problem. I'll definitely check for glass/fiberglass though. Inside/out and with a towel sounds like a good idea. –  JKnight Jun 4 at 3:50
A bigger tire is also more resistant to puncture flats –  Blam Jun 4 at 13:33
Specifically, a wider or fatter tyre. Changing from 700c/622 to 27"/630 will not make any difference. –  Mσᶎ Jun 4 at 22:10
@Mσᶎ In your opinion. I am not making this up. I have degree in engineering. If you want to post that as a question I will answer. And I said bigger than 23 (fatter). –  Blam Jun 5 at 17:06
@Blam here's your chance to show off that degree: Is a bigger wheel likely to get fewer flats? –  Mσᶎ Jun 5 at 22:10

gator skins are pretty tough, i rode a pair in san francisco for over a year without a puncture, i'm not saying they are completely impervious, a direct puncture is unlikely. the sidewalls are vulnerable.

tiny glass particles can work their way through the hole for the valve (i've also heard that the bead and the rim is another point of entry...), when the air is low that's when they can actually make it in there. particularly in less than dry conditions. i would run my fingernail along the inside of the tire casing and would find remarkably fine particles that i know weren't in there when i put them on my rims.

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