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I rode my road bike on Friday and the tyres were at the correct pressure then, around 100psi. On Tuesday, I jumped on without checking them and quickly realised something was wrong with the rear tyre. It wasn't completely flat but was fairly soft. I hadn't gone far so went home and took the car to work instead. When I got home later, I was surprised to find the tyre, while still soft, hadn't lost all pressure by any means. It was around 40psi and felt the same as when I left it in the morning.

I decided there must be a slow leak somewhere, but I couldn't find one anywhere, putting the inner under water and going around it methodically looking for bubbles. I've never failed to find a leak before that way.

I've switched it out anyway because I don't want to mess around with my commute, but I'm curious ... is there another explanation for that amount of pressure loss? Or must there have been a puncture somewhere that I missed somehow?

The tyres (whole bike) are pretty new and in excellent condition.

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How much did you inflate the tube when checking for leaks? Check for a valve leak? –  Moab Aug 4 '11 at 15:53
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"I road my road bike" -> "I rode my road bike" :) –  Hugo Aug 6 '11 at 13:05

4 Answers 4

up vote 8 down vote accepted

Could be a faulty valve that only leaks under high pressure. You won't be able to find this kind of thing by putting it under water because there's no way you'll get the tube up to 100 psi while it isn't on the rim with the tire on. It could also be due to very very small holes in the tube, again which don't make any difference until the tire is at very high pressure. You may be able to test this if you fill the tire up to full pressure and then leave it in the bath tub or other large sink. you should be able to see if air is escaping from the valve this way. Not sure if you would be able to see air escaping from very small holes in the tube though as they might not be visible due to the tire/rim.

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...or a bit of chalk in the valve. –  ʍǝɥʇɐɯ Aug 3 '11 at 13:45
    
Will give this a go, thanks! –  Luke Halliwell Aug 3 '11 at 16:19
    
I have noticed on my bike that the valves slowly loose pressure so I have to pump up my wheels every couple weeks if I want them to be high. –  runxc1 Bret Ferrier Aug 3 '11 at 19:13
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@runxc1 This is common on all bikes. No matter what. Tubes are nowhere near completely air tight. You'll notice it more on high pressure road tires, where many people recommend topping off the tires everyday. But even low pressure mountain bike tires will have this problem. However, going from 100 psi to 40 psi in one day, as per the question is excessive air loss, not normal air loss. –  Kibbee Aug 4 '11 at 2:16
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Looks like the valve did have a leak under high pressure. First time I've had one of those! :) –  Luke Halliwell Aug 16 '11 at 18:07

Rubber is porous, and tubes are cheap. It's possible to get a poor tube. If it keeps bugging you, don't waste your time, buy another tube, and carefully rub the inside of your tire wall when you change it, to feel for pokeys stuck in your tire wall.

I apply a lathery soap solution to the outside of the tube when inflated and watch for bubbles. I've seen my local bike mechanic hang an inflated tube on his wall for over a week looking for slow leaks. I don't invest in expensive tubes, I wouldn't do this.

If you do get repeated flats, pay attention to the alignment of the tube and tire, which side of the tube was on the left, and which side of the tire was on the left. This can help find pokeys in the tire wall.

If you ride the bike with very low tire pressure, it's possible to get grit under the bead of the tire and then you have bits of rock slowly grinding holes into the tube. I've found all kinds of stuff can poke thru tires: gravel, drill shavings, staples, thorns, glass. My commute was so dirty for a while, I regliously took a pair of needlenose to my tires ever weekend to reduce the rock count. I'd like to mention that those were some 2" Bontraget hybrid tires. On my current bikes, I have heavier Schwalbe Marathons, and they have not suffered a puncture yet (fingers crossed).

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Actually, investing in expensive tubes can cause the problem. I recall a ride I was on some years back where one of the other riders, one evening, changed out the tubes in his bike and his wife's, installing new tubes that were supposed to be somehow "better" according to one of his cycling buddies. Two days later he was reinstalling the old tubes, since the new ones leaked down in 12 hours. –  Daniel R Hicks Aug 4 '11 at 11:06
    
valid point, that's a good example of increasing your chances of finding a bad tube –  memnoch_proxy Aug 5 '11 at 5:38

Road tires with no obvious defects can definitely lose 10 PSI per day--there just isn't a whole lot of air volume so the tiniest leak through the tube or the valve will drop the pressure in a hurry.

Moral of the story: check the air pressure every time you take out the road bike.

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As far as my experience gets, this can happen because:

  • Punctures in the tube are too tiny.
  • Valve is dirty, damaged or the seal is too old/has been overheated.
  • The tube is old or has been chemically damaged, so has been turned more porous than normal.

For tiny punctures, inflate the tube a lot more when testing and be more patient when testing, i.e. allow a little more time each time you sink the tube in water, and watch for bubbles that develop slowly and remain stuck to the tube. Don't be afraid to over inflate the tube, it won't blow up, but if it does, it was already too old or chemically damaged. You can safely inflate a tube up to three times it's normal "non stretched size".

If you ride without valve caps, dirt can get inside the valve or even the tube, later returning to the valve and getting stuck between the valve's sealing surfaces. This is hard to test without inflating the whole tire assembly and pumping it to maximum pressure. The valve can sometimes be cleaned by inflating to max pressure and deflating quickly a few times (2-4) IF this is attempted but does not solve the problem, look for a way of recycling the rubber!

Seals in bicycle tubes are usually made of very soft rubber, which looses flexibility with age, even in the shelves, so, if you ever buy a tube that leaks air by its valve, try to get it exchanged for a different brand or one that is from a different/newer batch. Schrader and Dunlop type vales cores that are easily replaceable and with luck can be cheaply available. (In my country I can get 3 to 5 Dunlop cores for less than US$ 1.00) Schrader Valves are the same used in car's tires so maybe you get used ones for free at tire shops. (AS for presta valves, I do not know of replaceable cores...).

I have had ruined dunlop valve cores due to rim brake overheating. As mentioned before, rubber seals are really soft and can be easily damaged. They become dry and cracked with age and heat. To avoid heating of rim brakes, try to keep your rims and pads clean and free of oil and crystallized debris and observe a good braking technique in long steep descents. Schrader and presta valves are much less prone to this kind of problem, however, rim brake heating can also damage the valve neck (the union between the tube and the valve stem) weakening the union, producing leaks that are worthless to repair and sometimes very difficult to detect with the usual sink-in-water test.

As for old or degraded tubes, it's fairly easy to detect the problem. Inflate the tube so it becomes stretched. The tube should stretch in a more or less uniform fashion, and keep almost the same texture all-round. A very old tube degrades naturally, but chemicals can also damage rubber, accelerating its aging. Be careful if you store, repair, or use your bike, tires or tubes in places with existence of powerful solvents, paints with very volatile properties and the such. If you ever suspect that a tube has been chemically damaged, carefully inspect the tire as well.

A tube that has aged out will stretch more in its weak parts. These parts would present a less smooth surface, showing cracks and pores which appear not to be leaky. Those parts of the tube won't have the subtle shine of a clean rubber tube. Another sign of a weak or aged out tube will be the appearance of inexplicable punctures, such as tiny perfectly round holes in the part of the tube that faces a good clean rim tape. These fine holes appear by themselves, not by any object damaging the tire or tube.

Finally, all that I have stated, comes from my experience, and I must say, that it varies somewhat due to quality, design, raw material and manufacturing processes used by various brands.

References:

http://en.wikipedia.org/wiki/Presta_valve

http://en.wikipedia.org/wiki/Schrader_valve

http://en.wikipedia.org/wiki/Dunlop_valve

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Great, thorough answer :) –  Luke Halliwell Aug 19 '11 at 15:58

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