Bicycle Mechanics - Slow tire leak question

This is more of an academic question, but my front tire has a slow leak. It looses about 60/70 psi in 24 hour period. I commute 8-10 miles each way to work, and before I leave I pump up to the 100 psi max, and the next day when i get ready to leave, it's down to 30-40 psi and I have to pump it back up. On my ride home it's low, probably 80 psi or so.

My question, could replacing the rim tape fix that? I replaced the tube and the new tube goes down in the exact same way. Replaced tire (Gotta love the kevlar tires at nashbar for $10!!) and the tube still deflates, so my only guess is the rim / rim tape. When I replaced the tube, I held on to the old tube, and have checked every inch of it, dunked in water and everything, and can't find enough of a leak to bubble.

The tube has to be leaking. If it isn't then what effect could the rim tape possibly have? The air is contained in the tube, for the air not to be in the tube there must be an opening between the inside of the tube and the outside.

torgrot

Replacing the rim tape will fix the problem only if the current tape has slipped and exposed a sharp spoke hole or valve stem edge.

Blow the leaking tube(s) up very firmly (they should look like sausages) and then dunk them in a bucket of water. The leak may be very slow but you should find it if thetube is presurized adequately. Then notice where it is relative to the rim and whether it faces the rim or the tire.

Welcome to the Forums, greenjeep.

+1 on properly inflating the tube before dunking it. Sometimes twisting the tube in water helps reveal slow leaks. Be sure you know which way the tube was on the rim, and also mark the valve stem spot on the outer tyre. Once you've found the leak you can then start to look for the actual culprit, in rim/tape surfaces or in outer tyre. Small shrapnels in outer tyre can be difficult to locate, even when you know exactly where to look.

But you should locate the leak first, everything else is guesswork.

--J

My question, could replacing the rim tape fix that? I replaced the tube and the new tube goes down in the exact same way. Replaced tire (Gotta love the kevlar tires at nashbar for $10!!) and the tube still deflates, so my only guess is the rim / rim tape. When I replaced the tube, I held on to the old tube, and have checked every inch of it, dunked in water and everything, and can't find enough of a leak to bubble.Try dunking the wheel under water. Inflating a tube only gives you 1-2psi and it may not leak. Putting it inside a contained vessel like a tyre and pumping it up to 100psi will open up small leak. Mark where you see bubbles coming out of your spoke-nipples or the side of the tyre and remove the tube and inspect the tube and wheel at that spot. Replacing or patching the tube will fix the symptom. Finding a thorn stuck through the inside of your tyre or feeling a nick in your rim or finding the rim-strip had moved sideways to expose the spoke-nipple hole will give you the cause of the problem.

If it is leaking that much over a 24 hour period, I can't imagine why you can't find the leak. Put more air in the tube and try again.

check your valve after inflating. spit works very well. valve may need to be tightened.

If the tube is not leaking replacing rim tape won't help.

Couple of things...

I would check the valve for leaks. Are you using the slime stuff? Some off that stuff can cause the valve to stick open a tiny bit.

What type of pump are you using. Are you sure you are not losing the air when releasing the pump nozzle?

Once you find the leak in the tube, and if it's not pointing at the rim-tape but rather the tire, run your fingers around inside the tire - carefully. You are looking for something sharp embedded in the tire. And you want it gone.

Sorry for the delay, if I could get a computer on my bike with internet over the weekend it'd be perfect...

I was thinking rim tape because since the rim and tube both hold in the tube under pressure, I was thinking maybe a small hole/deflection of some kind could be causing the tube to have a way to leak without actually popping.

I guess I should have mentioned... the tubes are bontrager 700c-18/25 (basic stuff at my LBS) using a floor pump bought at the same LBS. Can't remember the exact make, but it's a pretty nice one. Have had it about two years. I haven't ever used any slim or anything. Like I said, the tire was new, though I didn't check the rim when I was changing out. The old tube didn't show any bubbles when pumped up not in a tire. I like the idea of dunking the whole wheel, that makes sense.

If it's presta valves, how do I tighten them? Other then just turning the little stopper, I mean can they become loose like car tire stem?

Will report back.

Thanks!