I'm just as stumped as everyone else, but have another data point to add: I have LP18's on my UO-8 and I've never seen a problem like the OP's.

In case anyone finds it useful, here are more details re: my rims & tires:
Panaracer Pasela tires, 27" x 1 1/8" (~28.5 mm width)
Pressures between 90-100 PSI
Velox rim tape

I'd bet that this would fix the problem. Add enough layers to make the valley level with the shoulder. And NO TAPE on the shoulder. The tape up where the bead rests is a no-no.
[MENTION=429363]DQRider[/MENTION] I'd love it if you tried this because I'm 99% sure xenologer is correct. Personally, I believe the 28mm tire is a 100% non issue.

It appears it may remain a mystery, as the occurrence on new, unridden tubes negates my theory. Anything that does not explain the paired dimples is not sufficient either. I'm wondering, did any of the occurrences result in holes at both dimples?

I have a theory. The tape in the "Bead track" is not allowing the tire bead to hold onto the rim hook well enough. IMO that tape is occupying critical real estate that is a specific size for an important reason. The high pressure seeks the path of least resistance. Over time it pushes the "least well" seated part of the bead away from the rim enough to sneak past it and bursts, like a pinch flat from a bad install. That answers why it happens in random different locations on the same rim. A theory. Nothing more.

The OP 's snake bites are cause by two things , 1 , his rim tape is too thin allowing the tubes to touch the nibbles when air is added , he need to use a thicker rim strip . (it need to be thicker , not wider or use a plastic strip ) . Or the Tape is moving and allowing the tubes and move back when he remove the tire . 2. Is cause by mounting of the tire error by using tools ( tires levers ) . Best to do without when all is possible Try putting air into the tube to shape it , put into tire , mount tire onto rim without tool and bring up to pressure .

If [MENTION=135358]xenologer[/MENTION] is correct, I wonder if letting the air out of the tube after mounting, and then pumping it back up to full pressure would allow the tube to redistribute itself and avoid the stretched-out parts. This shouldn't cause a pinch if you were careful enough to avoid trapping the tube under the tire bead the first time around.

From the pictures, the bulged punctured areas look so much like the tubes were allowed to penetrate the spoke holes, that it's really perplexing for this to have happened with double rim tape. Hats off to you, [MENTION=429363]DQRider[/MENTION], for attempting to control as many variables as you can in this investigation.

To [MENTION=362748]SquidPuppet[/MENTION]'s point, a narrower rim tape like 10mm Velox should cover up all of the spoke holes completely and not interfere with the bead seating at all. Something to consider.

Perversely (?), I'm tempted to pick up a wheel or two with these rims and see if I can replicate the issue for myself.

He is using double rim tape that is too wide to expose the spoke holes on the rim, and too thick to allow the spokes to touch the spoke ends. Tire levers leave a tear when improperly used, not a dimple. Finally, neither proposed "cost" (cause) explains the consistent presence of the two dimples.

Please do. I really want to know what the heck is really causing it.

This is such a juicy problem. Nothing I can see in the specs or pictures jumps out and says "poorly designed" to me. They just look like budget rims that are sort of like a mini-CR18.

Man I really want to know.

The dimples LOOK exactly like there is no tape over a spoke hole. But he clearly has the holes covered. And well covered. And WHY THE HECK are there TWO holes? Equal distance? Every time? It's driving me nuts.


If I had the stuff I'd do the experiments in this order.

1. Yank the tape and start over with NO tape on the shoulder area.

If the flats still occurred...

2. Build up the valley with tape.

If the flats still occurred...

Nope, a burst outside the rim/tire confines results in a large split or a star-shaped hole from explosive release, and the paired dimples is still unexplained.

The dimples are sign of the tube touching the nipples because of the problem with the rim strip , now ask me why I know ?

The dimples are much closer together than are the spoke holes (or nipples) on the rim, so that's not a good explanation.

Alright, I started to do a multi-quote of all your observations and suggestions, but that would result in a novel that nobody would want to read.

So...

Those of you who understand scientific method know that I have done this in as disciplined a way as I know how. This is the thread where the pros hang-out, so I had better not enter here with any ignorant BS. I understand all the known failure modes of tubes in a clincher tire assembly, but I have never seen this before. I thought that by changing the rim-tape/tube brand/talc-lubrication variables in an organized experiment I would find a solution, but no.

My best take-away here so far is that I am indeed running too wide a tire for this particular rim. I'm comfortable with that conclusion, and have already replaced both wheelsets with same-hub & CR18 wheelsets. These are working perfectly for my applications so far.

But that damned dimple pattern - and no, there hasn't been a case where both dimples leaked - keeps mocking me. I've checked this with a digital caliper, and the c-c distance between the dimples in the tube is always at least 1-2 mm larger than the diameter of the spoke hole. If this tube were somehow poking into such a space, you would expect it to be smaller than that diameter when the tube was deflated.

Is it possible that the tube is entering two adjacent spoke holes and expanding until one fails? Then when I deflate the tubes, does that distance between the "nipples" shrink that dramatically?

If that were the case, surely I would find evidence of that around the rim-tape.

(edit) I had written some nonsense here about sending wheels out to people, etc. What was I thinking? Do I really need that kind of hassle right now? No. So here it is, four-thirty in the morning, and I'm retracting that offer... sorry. Now maybe I can get back to sleep... Have a nice weekend everybody.

Has it been said.. are these dimple flats on the valve-rim side or tire side of the tube?

If you read the first post, you would know that this is on the rim side, always.

The dimples look suspiciously like the width of a tire lever,

are you using them to install the tires?

No. Surprisingly enough, the LP18 is very accommodating in that respect - and the tires go on quite easily. On other wheels, when I run into difficulties, I use a Kool-Stop Tire Bead Jack. But I didn't use that on any of my LP18s.

Someone else asked whether I index the tire label to the valve, etc... of course. I was trained by a very dedicated and skilled mechanic who owns an independent LBS - he let me build and repair bikes in his shop during the winter as long as I bought all my parts and tools from him. Best tuition I ever paid.

The thought of experimenting on one of your wheels had been crossing my mind yesterday but I didn't make it back to this thread in time to respond to the offer. Hope you slept well.

I love a good mystery, and this is one of them.

This might sound stupid, but I would install a 700 x 23 or 700 x 25 tire on the rim and see what happens.

No, that's not stupid at all. It says right in the rim specs: SIZE：700CX18C/23C <-- So I was using a tire that was two sizes larger than the specified max. The tubes were of different specs, but basically ranged from 25 - 32c, also larger than the Mfgr's rim spec.

As I said previously, I exceeded the spec because: 1) I have done this with other rims from Mavic, Araya, etc. without any problem, and 2) Sheldon Brown's chart has that disclaimer at the bottom.

This being the case, these are clearly not the right rims for my bikes. I want minimum 28c under me, and I prefer 32s when they fit. CR18 is a good, solid rim and has never given me any trouble. Not having any 23 or 25c tires around, other than dry-rotted takeoffs, I'm not going to buy tires I won't use just for an experiment.

I might try this combination of untested advice you all have given me:

1) On one pair of wheels, fill the channel with narrow rim strips, reducing the volume and eliminating the shoulders as a factor, but not extending out towards the bead track.

2) On the other wheels, fill the channel, then add 1 layer of hockey tape* to plug any gaps on the sides, and trim that carefully so it doesn't enter the bead track.

3) Use the next size smaller tube, since I have effectively reduced the volume the tube has to fill - there will be less material to get into little gaps and cause mischief. Or it will simply fail.

4) Inflate all tires to 90psi, well within the safety limit. Then let the bikes sit for the winter, one on the floor, weight-on-wheels, and one hanging up. Of course I'll check them periodically and "top them off" to 90, just to keep the pressure on - pun intended.

If they are still inflated come spring, I will ride test them and update this thread.

*(My mentor's answer to expensive rim strips: 1 layer hockey tape over 1 cheap rubber rim strip )

I'm personally not convinced the problem is related to tire width. I run 27" x 1 1/8" tires on LP18's -- slightly wider (in theory) than the OP's tires -- and I've never seen a similar issue.

I would either leave the size unchanged, or use the next size Larger.
It'll be harder to install, and you need to take care to avoid pinching or folding the tube.
but the larger tube will be closer to the final inflated diameter, thus the rubber won't need to stretch as far.
Thicker, unstretched inner tube is less vulnerable.

It's a trick I've used to good effect on repeat customers with chronic mystery slow leaks.

I'm gonna take a shot at this. I have a pair of LP-18's as well. I have used them with 13mm Velox rim tape with cross tyres up to "35c", but with the narrow rim, it ends up closer to 31-32c. I ran those tyres at 3.2ish bars or 50-55ish psi. I usually ride on "25c" aka 23c Paselas at 110 psi, but I weigh 140lbs. I know that is high, but I have rough roads where I live and pinch punctures are a common problem.I have never had any problems with the rims, but the '80s Weinmanns that they replaced had what I assume to be a defect. On the inside sidewall of the rim, the join/weld where the rim was joined to make it round was filed flat, but didn't follow the profile of the rim. The edges were sharp-ish. I think that the combination of this along with the wider than recommended tyres are a possibility. The aforementioned pear shape of the tubes also probably play a role. The wide rim strips may be hiding the bad filing.

I didn't recall seeing this mentioned or asked - how tight are the tires when mounting? Do you always mount tires starting at the same point on the rim? Is is possible that a tight bead is pushing the rim tape over and partially exposing the nipple holes? And when the pressure is removed (by deflation) that the rim tape returns to it's intended position, hiding the cause?

I realize that this isn't what rim tape is supposed to do, but with the lack of another explanation (so far) you have to look at outliers, no?