My beater bike has 26x2.1 tires on a 3/4 inch rim. I repeatedly get flat tires, with the holes always on the inside of the tube. The spokes don't protrude, there are no flaws in the rim or anything else in the rim. I have put in two layers of the rubber protective strip, put in a layer of silicon caulking and tried a few other things (including heavy duty tubes) and nothing has stopped the punctures. Repeat: the punctures are always on the rim side of the tube and there is nothing there to cause a puncture. Any suggestions?

Do you get one hole, or do they come in pairs?

You should be able to use the tube to measure around where the rim is to locate the cause of the hole. Is it always in the same place, or in more than one place? Compare a few punctured or patched tubes.

Maybe a burr or something.

I've also heard reports of rubber covering spoke holes receding under pressure, exposing the spoke hole.

Another thing to watch is how you're mounting the tire. It is possible to get the tube entrapped when mounting which can lead to a hole in the tube. Or puncturing the tube with tools.

By definition, there is something there that's causing a puncture. Punctures do not appear by magic.

Are they always in the same general area of the rim? Are they snakebites? (i.e., two small holes adjacent to each other. )

Usually in cases like these the rim strip is moving, exposing the spoke holes. Running excessively low pressures can allow that to happen, as well as being a cause of snakebites.

Thanks for the attention, but: It's always just one hole, the location varies, and the spokes are well covered, (with a thick layer of silicon caulking) -- my thought is that the tube is getting pinched somehow in the narrow rim, but no matter how carefully I mount the tire it still happens (it also happened twice when the tire was fixed by a pro) and I do keep the pressure at the higher end of the recommended level. I agree something is causing the problem, even if it does seem like witchcraft --

Many flats are not punctures. Many flats are not snake bites. If the rim has a very deep spoke bed (WRT where the tire beads sit) and if the tire beads are not too far apart the tube can have challenges in it's want to expand down past the tire beads and fill the deep spoke bed. If the tube doesn't slide down in between the beads the tube will instead just balloon into the empty space. And too much ballooning will cause a too this a tube wall blow out. Andy.

Andrew R Stewart, interesting theory about deep pockets. I think I had a patch blow through at a spoke pocket, but never an ordinary tube.

Depending on the wheel, a double layer of rim tape could make mounting tires a bear.

I've been having troubles mounting tires on my RS-10 wheels. They seem a bit tighter than other wheels/rims.

Anyway, they came with a slick BLUE rim tape.

So, after mounting the tire, I can push the side of the tire in until I see the blue.... following that all the way around the tire. If I see blue all the way around (both sides), I know that the tube is not impinged somewhere.

I haven't used any tubeless tape, but I've heard it is good stuff.

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Oh, are the flats both front and rear, or just one or the other? Maybe try a new tire.

I'll bet you a beer or two, that the belly side of the tube also has stretch marks, or a pattern of stretched sections about 1/4' or so wide.

Here's what's happening. (theory and basis of bet)

With the tire on the rim the inside has two chambers connected by a narrows. You have the large chamber inside the tire, then the narrows between the beads, and a small chamber within the rim below the bead.

When you first inflate the tube it first fills the roundish tire chamber, stretching across the section between the beads. As you add pressure, the narrow strip spanning the beads, blows down to fill the gap in the rim. The tire doesn't equalize, so depending on the width of that spam, and the size of the lower chamber, that little strip of tube gets hyper stretched until it's almost ready to blow. After that it doesn't take much, and it lets go often with slit type tears.

Next time use plenty of talc inside the tire so the tube can slip and settle better. Then fill to 5-10 psi and give it some time to slowly stretch down pulling the area in the tire (why the talc). With luck the tire stretch will be more equalized and you won't have more problems.

Lots of wild guesses (and some nonsense) but the respondents love that. Why not post a detailed description and/or good photos of the holes?

The talc is a great idea, as is filling it gradually -- I'll give it a try, thanks a ton --- I've been really frustrated with this. (And the holes are always a slit of varying sizes)

Yes, but I'm the only one willing to put a beer on the line --- sight unseen. If he'd posted photos, and I saw the clues I was expecting, I'd have bet a 6-pack of a decent imported beer.

Do you have single-wall or double-wall rims?

If you look between the beads of a single-wall rim, in the channel where the rim strip or tape sits, the spoke nipples sit on the surface of the rim. If you look at a double-wall rim, the spoke nipples sit inside holes.

Rubber rim strips are meant for single-wall rims. They simply cover sharp edges of spokes and nipples. If you use those rubber rim strips in a double wall rim, the strip will stretch into those holes, allowing the tube to do the same. The tube can expand into spoke holes to the point it bursts from stretching, or it can get punctured by the edges of spoke holes that get exposed when the rim strip stretches and slides out of the way.

If you have a double wall rim, you'll need a rim strip or rim tape that doesn't stretch like rubber does. Cotton tape like Velox is a time-tested standard but there are plastic strips available, too. Veloplugs, plastic plugs that fill your spoke holes, are an alternative to tape or strips.

Kind of tough to diagnose over the Internet huh? Have you been using tire irons to mount the tire? If you have, then this may be your culprit.

I'll add this. I've had this same thing happen a few times over the years. In my cases it was often something in the tire. A sliver of metal, a thorn, something like that stuck in at an angle or just barely sticking through the tire; where a cursory rub-down didn't find it.

CAREFULLY and I mean carefully run your finger tip on the inside of the tire and do it slowly and methodically. Rub the finger tip all over inch by inch inside the tire. There could be something barely sticking through. If you don't find anything, mount a different tire, assuming you've really scoured the rim for burrs and the spoke ends. If you get another flat, it isn't the tire so it's the rim. The other post was correct, flats happen for a reason.

And if the talc and gradual filling doesn't work, I'll post again with some photos. Thanks again to all.

Another guess, but at least it has some 'logik.' With the rim wells clogged by two thick rim strips and caulking [!?!] it's probably impossible to mount a tire without levers -- even by pros. To the OP: regardless of everything else you need one layer of thin, strong rim tape and avoid the use of tire levers.

I'll post a follow-up in a couple of days, after I've given the suggestions a good try. Thanks all

This is what I was trying to describe but, as usual, Francis does a much better job then I do. His suggestion of using talc is a good one too. I use talc on my own tubes, I find that the tube tends to not stick to the tire insides as much as without. Makes for easier on road repair and, in theory, lower rolling resistance. Andy.

Thanks again for all the comments. The rim is single wall, and because of the work I had done, I was sure that the spokes or foreign objects weren't causing the flats. At one time or another both the front and rear tires had gone flat -- a couple of times both at once. I tried the talc powder and slowly inflating (actually I partially inflated the tires and bounced them a few times and then finished inflating them) --- that seems to have worked. Apparently the tube, when fully inflated, was stretching irregularly and that led to the flats. The talc lets the tube slide inside the tire and rim and that fixes the problem. Hurray! I can once again use the beater for short local errands.