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So what are "slow leaks", anyway?

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So what are "slow leaks", anyway?

Old 06-06-19, 03:42 PM
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MinnMan
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So what are "slow leaks", anyway?

I have repeated problems with slow leaks on one particular bicycle - my cx bike - and I can't figure out why. The tire will be more or less fine during a ride, but go totally flat overnight. Or it will slowly lose air during a ride. Of course, replacing the tube fixes it, but the problem soon repeats. It's only on this bike, and almost always (though not 100%) on the rear tire.

I generally assume that slow leaks come from the valve or the valve core. I further assume that the problem is greater in the rear b/c I pump up the rear tire to slightly higher pressure. But could it be something else? I wondered about the rim tape and replaced it, but the problem persists. Besides, I would think that a rim tape problem would just cause normal punctures, not slow leaks.

For the record, the rims are Stans Notubes ZTR, though I don't run them tubeless (obviously) with Stans rim tape and Clement X'plor Ush 35 mm tires.


What am I missing?
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Old 06-06-19, 04:01 PM
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There is something small embedded in the tire and it punctures the new tubes in the same way that it punctured the old one. Run you finger around the inside of the tire or rub a cotton ball around the inside of the tire to find the object. You may have to flex the tire or turn it inside out to get the offending object to expose itself.
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Old 06-06-19, 04:59 PM
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Your slow leak could be from a small piece of glass or thorn or a wire from a retread in your tire etc. It could also be from an over stretched tube. I have had this problem before. I know it weighs a few grams more but a larger tube can fit in your tire. I use freeride tubes in my 26X2 wide tires. Spec. 26X2.3 to 26X2.7
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Old 06-06-19, 05:57 PM
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In my experience, a slow leak could be practically anything that could also cause a big leak, but smaller. I call it a slow leak when I just can't locate it on the tube.
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Old 06-06-19, 06:04 PM
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I'm going to bet you've got a radial tire wire stuck in the inside of your tire. They're practically impossible to see and sometimes you can barely feel them with your finger but they're sharp enough to poke a tiny hole in a tube when inflated to full pressure.
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Old 06-06-19, 06:15 PM
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Try this: switch front and rear tires. If the condition moves to the front wheel you will be closer to an answer
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Old 06-06-19, 09:49 PM
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OK, I get the idea. I tend to think that even the smallest puncture in a tube will result in a more conventional "fast" leak - because I've seen so many normal punctures from tiny wires and objects. These would be punctures where the hole is nearly too small to see, but the tube won't hold any pressure at all because the hole aperture widens as soon as the tube is pressurized. So my intuition (which you all say is wrong) is that slow leaks aren't from tube perforations.

I've looked over the tire in great detail, turning it inside out under a bright light, and I don't see any protruding objects. But it's an old tire. Maybe it's time to replace it and move on.

Thanks for the input.
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Old 06-06-19, 10:06 PM
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Oh, I should have mentioned that I've inspected the rim tape carefully also.

Originally Posted by MinnMan View Post
OK, I get the idea. I tend to think that even the smallest puncture in a tube will result in a more conventional "fast" leak - because I've seen so many normal punctures from tiny wires and objects. These would be punctures where the hole is nearly too small to see, but the tube won't hold any pressure at all because the hole aperture widens as soon as the tube is pressurized. So my intuition (which you all say is wrong) is that slow leaks aren't from tube perforations.

I've looked over the tire in great detail, turning it inside out under a bright light, and I don't see any protruding objects. But it's an old tire. Maybe it's time to replace it and move on.

Thanks for the input.
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Old 06-06-19, 10:20 PM
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Originally Posted by caloso View Post
I'm going to bet you've got a radial tire wire stuck in the inside of your tire. They're practically impossible to see and sometimes you can barely feel them with your finger but they're sharp enough to poke a tiny hole in a tube when inflated to full pressure.
+1 And a really good reason to patch tubes. When the patches start showing up in the same place it's a dead giveaway both that the tire or rim has a problem and exactly where it is. (If you always locate the tire on the wheel in the same place. The "standard" these days is labels on the drive-side at the valve.)

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Old 06-07-19, 02:20 AM
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Originally Posted by MinnMan View Post
I have repeated problems with slow leaks on one particular bicycle - my cx bike - and I can't figure out why. The tire will be more or less fine during a ride, but go totally flat overnight. Or it will slowly lose air during a ride. Of course, replacing the tube fixes it, but the problem soon repeats. It's only on this bike, and almost always (though not 100%) on the rear tire.

I generally assume that slow leaks come from the valve or the valve core. I further assume that the problem is greater in the rear b/c I pump up the rear tire to slightly higher pressure. But could it be something else? I wondered about the rim tape and replaced it, but the problem persists. Besides, I would think that a rim tape problem would just cause normal punctures, not slow leaks.

For the record, the rims are Stans Notubes ZTR, though I don't run them tubeless (obviously) with Stans rim tape and Clement X'plor Ush 35 mm tires.


What am I missing?
Most likely there is a sharp object embedded in the tire or a crease in the rim tape, repeatedly puncturing the tube. You need to sort that out. - And learn to patch a tube. No need to discard a tube for a small hole. A patched tube is every bit as good as a new one.
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Old 06-07-19, 03:19 AM
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Same here, I own an old mountainbike (rebuilt to commuter) which still had the original rim tape. It had become all rigid, a small part of it had detached on the side and was piercing through my inner tubes. Always worth checking (run a rag over the rim, inspect the inside of your tire, repeat if necessary
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Old 06-07-19, 04:49 AM
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Take the tube out (mark the valve placement on the tire beforehand), inflate it, submerge in water. You don't even need to submerge all of it at once, if you don't have a tub large enough - you can go around the whole tube submerging it bit by bit. A leak like the one you described would show as small air bubbles forming and escaping one by one (larger leaks would cause a continous flow of bubbles), so take your time and pay attention. There will probably be some tiny bubbles sticking to the tube where some dust particles are, so brush them away and observe if new ones aren't forming in the same place. After you find the leak, the location of the culprit shouldn't be too hard anymore.

In my experience, the submerging has always worked; if there is a leak, it shows.
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Old 06-07-19, 06:12 AM
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Originally Posted by caloso View Post
I'm going to bet you've got a radial tire wire stuck in the inside of your tire. They're practically impossible to see and sometimes you can barely feel them with your finger but they're sharp enough to poke a tiny hole in a tube when inflated to full pressure.
My guess as well. Sometimes you just can't see them from the inside. I think they retract then re-emerge when the tire is pressurized and ridden on. The key to success is accurately matching the location of the tube puncture to the tire. Then your search is narrowed. Look both inside and outside of the tire while flexing. Bright light and good magnification help.
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Old 06-07-19, 06:45 AM
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I mark my tires and tubes with a sharpie, a gold color one seems to work best. The specific markings I use are not the point as long as you are consistent. On the left side of the tire I put a F or R and an arrow for rotation right at the valve stem. Also an arrow for rotation on the tube at the stem. With that I can always correlate relative position. Back when I was mainly running tubes and always getting flats, this helped a lot, easy to spot and less chance of putting a directional on backwards on the side of the road too.

Back to the original issue.. Either a festering piece of small wire or glass will cause that, also a hardened rim strip. Water in the tub should pinpoint that. I've had some file thread tires that would cause a lot of flats that way. Small sharp debris would fester in the tread gaps and slowly work it's way to the tube, as the tire went flat the small debris would fall off or sit there waiting for the next tube. Tire liners solved that nagging problem. The thing though was all of those holes were easy to find and patch but I often could not find the source.

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Old 06-07-19, 08:15 AM
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Years ago I had that problem with my commute to work bike. Repeated slow leaks in the same place on the tire but, no matter how hard I tried, I couldn't find the culprit. My temporary fix at the time was to cut the valve stem out of a used inner tube and use that as a tire liner. Eventually I found a customer who wanted to replace his Continental Town and Country tires with something else. His cast offs fixed the problem permanently.
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