mozad655

I'm using a set of 25mm Schwalbe Marathon Plus tires. Min pressure is 70 psi, max 115. The front one seems to hold air well and stays at around 90 psi for weeks which is perfect.

The back one on the other hand loses air almost over night. I pump it to 115. Next day its at 95-100. Day after it at 70 and its gets worse from there.

I checked the tube under water for any small slow leaks but found nothing at all. What could be the problem? Max total weight on the bike is around 150 lbs. I commute twice a day. I have not used the bike for a full week and its at 40 psi now. I use different tubes. Front is a cheap no brand and rear tube is more expensive Schwalbe tube.

tFUnK

Could be a slow leak, either at the valve or elsewhere. You mentioned the tube being more expensive; perhaps it is a lightweight tube (thinner walls) more geared towards racing? Super light tubes offer a more supple ride quality but may lose air more quickly than regular tubes.

jyl

The rear tube has a slow leak. You aren't finding it with the water because the tube isn't inflated to 100 psi during the test. Replace it.

2manybikes

It happens sometimes. Try another tube, they are cheap. Or just top off your bike before every ride. Another tube might do the same thing, you never know.

FBinNY 

You've just discovered the tube paradox, which is -----> The more you spend on a tube, the faster it will lose air.

Of course, this isn't always true, but it holds as a general rule. That's because the natural loss of air from tubes (bleed) is related to the starting pressure, the surface area, and the thickness of the tube's wall. (also the material, but most use similar butyl rubbers).

So thinner, higher pressure road tires tend to lose air faster than fat lower pressure mtn bke tires. The price paradox comes in, especially with road tubes, because pricier tubes are aimed at a more weight conscious customer, and so will tend to have thinner wall thickness. If you don't want to top off as often, buy the cheapest, heaviest tube in the right size and you should be fine.

The paradox aside, you may also have a problem because your rate of air loss is still very high. Could be a bad valve which leaks at pressure, but not when you submerge the tube. When I have a situation like this, I pump to pressure, and submerge the entire rim in the tub to a depth of 6" or so, and slowly check each section, especially the valve and base of the valve one at a time.

ltxi

Replace the rear's tube. It's leaking.

mozad655

Replaced rear tube with one identical to front tube. I pumped it up to 115-120 psi. Put the wheel away and left untouched. A little over 24 hours later and its down to 100 psi. Is this normal?

Andrew R Stewart 

Are you loosing air with each pressure check? (Of course you are but how much is the real issue.) If you pumped it up to 115/120 and left it for a few days, then checked would you indicate 100 again? In other words is the loss from time or from the pressure check. How are you checking the pressure on the first day and the second day? If you're using a hand held gage one can have more then a handful of PSI lost during the check with no obvious fumbles or other excessive booboos. The smaller the tire's volume the greater the loss with a check of pressure.

We find that many slow leaks (of 20/30 PSI a day) are from either bad valve sealing or tiny wires that are VERY hard to find in the casing. Another cause is the abrasions (from running too little air) that the casing's insides do to the tube.

There have been some flats I've serviced where the tire and tube replacement was the final fix, after many casing checks and customer returns. Sometimes getting the right result is more important then understanding the reason. Andy.

unterhausen

lots of tubes come with loose presta valves nowadays. I have a very slow leak in one of my tubes, it's annoying but as long as it doesn't go down in 24 hours, I'm fine

mozad655

I use my floorpump pressure display to measure the pressure. No air is lost when putting the gauge on but some air (3-5psi) is lost when I pull it off. I've kept this in mind. When I pumped it to 120 psi yesterday, I compensated so it would drop to around 115. Todays measurement was 100 psi. So at least 10-15 psi drop over night without use. Its a new tube identical to front which holds are really well at same pressure. I managed to sink the interesting end of the valve into a cup of water while it was still in the tire at high pressure. At least that part is not leaking at all.

What casing and wires are you referring to Andy? This tube is brand new and wheel is off the bike, so no abrasions from too little pressure.

So annoying.

trailangel

He is saying you possibly have a small piece of wire or glass stuck in your tire casing that punctures any tube you put in the tire. Have you thoroughly....! checked your tire casing?? And I'm talking about using a magnifying glass to look for the source. You've never had a flat from a piece of wire from truck tires?

Andrew R Stewart 

I think you have the air loss backwards. When you attach the air chuck from the pump and want to check the tire pressure you HAVE TO fill the pump hose and pump internals, up to and including the gage, with the air from the tire. This lowers the tire's pressure by the volume of air that's now in the pump hose et all. (If you instead use the pump's cylinder/piston to fill the gage/hose then no air escapes from the tire. But the resulting gage pressure showing is either real close to what's in the tire, assuming the valve hasn't opened by more then a tad, or slightly higher as some air DID go in the tire and now the pressure on either side of the valve).

When you remove the chuck from the valve and do so without bumping the valve NO air leaves the tire. The release of air you hear is coming from the pump hose et all.

This assumes presta vales are in use, not Schrader. Andy

ltxi

Then most likely something tiny wire like is puncturing the tube. Try different tire with a new tube.

jamesdak 

Rub a cotton ball all on the inside of the tire and the rim to see if it catches on anything that may be causing a possible leak. Of course, for such little loss over a 24 hour period I wouldn't worry about it. Of course I top off every day.

alcjphil

Try this: Pump the suspect tire up to the pressure you want and remove the pump. Then disconnect it and immediately reconnect as if you were going to pump it up again. My bet would be on a 10psi pressure loss. Every time you connect your pump your inner tube loses pressure. Higher priced lighter weight inner tubes lose pressure faster than cheaper heavier ones. The only fair test is to use identical inner tubes on both front and rear tires. You cannot expect similar results from dissimilar equipment

davidad

Mount the tire and pressure it up to 150 psi and test it in the bathtub. I guess that you have a fine wire in the tire
causing the flat.

cyccommute 

No paradox...i.e. something that contradicts itself...at all but a statement of Fick's Laws of Diffusion which were derived about 160 years ago. The rate of diffusion is related to the pressure in the tube, the thickness of the tube and time.

Yup, although pressure has more of an effect than surface area.

Yup again. But, as above, pressure has more of an effect that surface area. The surface area of a mountain bike tire is almost twice that of a road tire (2.2" x 29" vs 1.25" x 27") but, as you said, road bike tires will lose air more quickly than mountain bike tires. But there is no contradiction (or paradox) with lighter tubes. Of course they are thinner and diffuse more quickly. A lightweight mountain bike tube will lose air faster than a heavier one just as road bike tubes do.

However, even the lightest mountain bike tube won't lose air as quickly as the heaviest road bike tube because mountain bike tires aren't usually pumped up to the high pressure that even a wide road bike tire is. It's the pressure difference between the air in the tube and the outside air that drives the diffusion. The higher the pressure difference, the faster the air will escape.


Without knowing what kind of tubes mozad665 has, it is difficult to draw any kind of conclusion about what the problem is. A valve that leaks at pressure but not when the tube is submerged isn't likely...at least not in my experience. It could also be easily checked by pumping up the tube in the tire and submerging just the valve.

It is more likely that the tube has a flaw in it which allows for faster diffusion than other tubes. You couldn't see it under water, mozad655, but it is fast enough to leak down quickly overnight. FBinNY is right that you can replace the tube with a thicker tube or just live with the leak down. Losing 20psi (roughly) in 24 hours isn't that bad.

2manybikes

top off your tires before every ride and forget about it. It's not unusual. Or try a thicker tube.

practical

I had this issue this past summer. Finally and did a very thorough investigation of the tire and found a very small piece of glass embedded in the tire. Those things are very sneaky.

CNC2204

Tubes go bad.

$10 new tube.

Welcome to cycling.

Make sure you feel/check the inside of the tire for wire/glass in the rubber.

FBinNY 

The paradox has nothing to do with the tube itself but was exactly what I said in Boldface. The fact that contrary (the contradiction) to what one may expect, pricier tubes tend to have higher bleed rates.

msd3075

This.

I had this exact thing happen to me. My front tire would lose air significantly faster than my rear. Nothing topping it off wouldn't fix, but it still perplexed me as to why it was happening.

Then one day when I was about to air up my tires before a group ride, the valve core come out in my hand when I was unscrewing the presta valve. Put a new tube in and went on my way, but after the ride I went and bought a tire valve tool to make sure the valves were tight. It was crazy how loose they were. Haven't had the issue since.

ltxi

Thanks for this post.

rekmeyata

Typically a leak you can't find after water submersion is the valve stem, so how to check is to inflate the tire to 115 psi and put a mixture of water and soap on the stem and watch to see if it bubbles. If that fails you can you use the same mixture on the tube by blowing it up outside the tire and run the solution over the tube and look for bubbles. The soapy solution will find leaks better than water because it exaggerates it. More than likely it's the stem if it's not due to an ultralight tube.

My medium weight tubes I currently use lose about 5 psi a day, when I was using ultralight tubes they would lose about 15 psi a day, and latex would lose about 45 psi in a day; so tubes do play a part in how much air they will lose through the tube material itself.

cyccommute 

It is only contrary to what might be expected with more expensive tubes if you are expecting something entirely different out of the more expensive tube. The reason you buy expensive tubes is to save weight. Saving weight means less material. Less material means a thinner tube. A thinner tube means faster diffusion. There is nothing "contradictory" about that at all nor is there any "paradox"...unless you have unrealistic expectations.