Mister Scott

So last night I went for a wet ride on my road bike in the remnants of Hurricane Isaac. Along the way, I hear a rhythmic "sss...sss...sss...sss" that sounded like it was coming from the rear tire. I also thought a felt air blowing onto my right leg. So I figured I had a pinch flat a pulled over. The tire felt like it had plenty of air though, so I rode another 4 miles or so. Today, I did not ride, but I noticed the that the rear tire was flat. I took the bike out to the garage and aired up the rear to about 120 psi. It seems to be holding air for the time being.

Any ideas on what the problem could be?

10 Wheels

Take the tube out, air it up and place sections of it in water and look for bubbles.

himespau 

+1, then either patch it or replace it. I'd patch it just so you have practice when you find you need to do it late one night in the middle of nowhere in the rain when all your spares have flats too. That is assuming you carry a patch kit with you in addition to spare tubes.

10 Wheels

If you find a tiny hole in the tube, try to match it to the tire and look for glass or a tiny wire.

seedsbelize 

There is something in the tire, yes, and it only leaks while rolling.

himespau 

If you don't see a cause with your eyes, run a cotton ball on the inside of the tube in the area you see bubbles (assuming you have a way of lining the tire up to the tube (I like to put the valve and the tire label in the same spot). That'll catch things you can't see, but don't want to snag your finger on.

curbtender

I've had leaks at the base of the stem that have done that.

Mister Scott

Thanks for all the replies. I've never experienced a slow leak with a tube before (motorcycles and bicycles). They've either held or gone instantly flat. Evidently I do have a slow leak though. A pinch test a few minutes ago suggested the tire has lost some air over the last couple of hours or so. I'll probably take the lazy route and just replace the tube as opposed to patching it.

Flats are the bane of my riding efforts.

FBinNY 

leaks where the valve attaches are more common these days, especially with the new flangeless Presta valves many tube makers are using. I often get intermittent leaks with these valves. Wiggle the valve one way it leaks, wiggle it another and it'll be fine for a week, then leak anew.

For some reason I find that many of these will wold up fine for a long time, but after a hard ride in the rain will fail. I suspect that water is somehow attacking the glue that holds these together, and when I check defective tubes I find a decent amount of oxidation where the bond failed. I now punch a hole in a feathered patch and reinforce every tube before using it, or packing it as a spare. It seems to help, but I'm still very disappointed with the quality of tubes made in the last 10 years o so.

Valleybikes

Mark where the valve is on the side of the tire with a pen and on the valve on the same side as the tire take the tube out air it up find the leak (if you have to put it in soapy water do so then line up the mark on the valve with the mark on the tire and find what made it leak

curbtender

You'd think someone would have developed a product like the hole protectors for binder paper, Hmmm, wonder how they would work?

FBinNY 

Tubes have always been reinforced at the valve hole, so that wouldn't be new. It's just the way that valves are being attached to molded tubes that crates the problem. The new design has a taller, narrower base with the tube joined to the side, vs. the old design where the valve has a flange inside the tube. The older design has the virtue that pressure compresses the joint, while the new version is entirely dependent on bonding.

The reason for the change was mainly because the larger flanges caused issues on narrow rims, and there was a cost savings (after the tube molds were modified) in production.

The makers are aware of the problems with the new design, and I assume working to correct them, but it'll be a while before all the old stock flushes through the distribution system. My personal problem is that I have a decent stock left over from the early (bad) years of this design.

Mister Scott

So I took the tube off and it was still holding a little bit of air. I inflated it and it immediately lost most but not all of the air. I put it in the sink and found the leak - a tiny pinhole size puncture (I also happened to notice the corresponding hole in the tire later while replacing the tube).

I had a new tube that had been on hand for years. I installed it and aired it up to about 120 psi. As I went to put the tire on the bike - POW!!!! Scared the crap out of me as well as my nosey Siamese kitten who was "helping me". I removed the *new* tube and discovered that it had blown out within the section where the valve is attached to the tube.

I then took a trip to the new LBS nearby. They confirmed it probably was not user error that caused the failure and sold me a couple of new tubes. I rode a meager 6 wet miles yesterday and all was well.

Retro Grouch 

You'll get more punctures on a wet ride. Some say it's because the glass shards stick to the tires more, some cay it's because the water lubricated the shards so they can penetrate the tire more easily. Either way, that's what happened. If the leak is that slow it may be hard to find but, until you find it and fix it, you are doomed.

Mister Scott

That's interesting.