Unpredictable tube deflation behavior

Question

I've recently had a full flat with hardly any air left in my rear tyre when my road bike was in storage. Storage means my storage compartment in my appartment building's basement, pretty constant temperatur and just a tad humid. The odd thing is that it wasn't "linear" like a slow puncture/leak or instant failure, the order events was something like that:

• Day 0: No problems on the last ride, bike was in storage with perfectly inflated tyres, they might have lost some pressure since the last ride but way enough to ride it, if I had to
• Day 1: Inflate to correct pressure, ride without any issues, clean bike and put it back into the stand
• Day 2: I just revisted the bike because I usually let the lubricant sit and remove excess lube some time after the actual clean job - Tube all fine
• Day 3: It is my storage compartment, so I just went there to pick some other stuff - Tube still fine (I nearly walk into my bike, so I would have noticed if it was totally flat)
• Day 5: I wanted to go on a ride and it was totally flat with practically no air in it

I still had some time before my ride, so I didn't instantly change the tube to be on the safe side and just reinflated it - bit of a gamble but it held air just fine, made a four-hour ride without losing any air and is still fully inflated when I checked today, more than a day after the ride.

Do you have any idea what could be the issue, here? I've had that in the past and think I can rule out a loose valve head/core that was slightly screwed out by accident until the point of failure.

I run pretty standard Continental Race 28 butyl tubes matching my 25 mm clincher setup, nothing exotic or lightweight, and usually they hold air pretty well, so they keep my 7,0-7,5 bar pressure when there's only a day or two between rides. The tube isn't very old, I just put it in 3 months ago fresh from the package a few days after I bought it.

If my memory serves me well, I've had this one or two times before and re-inflating always did the trick, so I think the tube itself isn't punctured and I always check that the Presta valves are closed tight.

What is going on, here? Is this just a common thing because tubes/valves are not meant to be 100 percent airtight, am I doing something wrong or is it a thing that Conti tubes (despite their brand's reputation) can be hit and miss depending on the batch?

I wouldn't be worried if it was consistant (because I check pressure before rides, anyway) but that kind of behavior confuses me.

Of course, I checked some old posts but most of them were related to sudden deflation on impacts such as speed bumps, ramps or potholes but no occurrences after serveral days.

Answer 1

Day 3: It is my storage department, so I just went there to pick some other stuff - Tube still fine (I nearly walk into my bike, so I would have noticed if it was totally flat)

What you don't notice is the difference between a tube that has 7 bar of overpressure (i.e. pressure above that of atmospheric pressure) or a tube that has 1 bar of overpressure.

A bike is light. It weighs maybe 15 kg. Besides, these days it's very trendy to have road bikes with weights so low that they can't be predicted to have a long life. A bike + rider weighs maybe 85 kg.

Note that when you ride the bike, it has maybe 60/40 weight distribution. So the rear tire has around 51 kg weight. Without a rider on the bike, the weight distribution is probably more like 50/50 so a tire has 7.5 kg weight.

7.5 / 51 = 0.147

So if you normally have 7 bar on the tires when riding it, and the tires don't become noticeably flattened by the rider's weight, without a rider on the bike, you can have 1 bar on the tires and it would still look the same as a bike with 7 bar and a rider on it.

Also, while normal air leakage through the butyl in the tube is a super-exponential process (the rate of leakage is roughly proportional to the pressure difference, but somewhat faster when the pressure difference is larger), it may be the case that with a very small puncture, the air leakage could be something else, for example more like a linear process.

So I suspect on day 3 you had around 1 bar but you didn't notice it. On day 1 you inflated to correct pressure (I presume 7 bar here). It's perfectly feasible that on day 5 the tube is completely flat. And even if my theory about linear process was false and the process would be exponential, then on day 5 you would have 1/7 bar = 0.14 bar, which for all practical purposes would be flat anyway.

It's a slow leak. So slow you probably need to inflate your tube way above its normal size and find air bubbles keeping it under water. If you still can't find air bubbles, check the valve (it could be a loose valve core). In extreme cases, you may need to ditch the tube completely, because obviously you don't want a tube that leaks slowly but so slowly that you can't find the leak and patch it.