Symox

Was riding lightly in my neighborhood in the morning. Later in the afternoon in my garage (which was about 100 deg) the rear tire had a flat.

"that's weird" I thought to myself

I inspected the tire inside and out, nothing unusual

I inspected the tube and there was a single pinhole on the inside (facing the rim) of the tube. I carefully inspected the rim bed (which has Velox tape completely covering it) and found and felt nothing unusual.

I'm truly perplexed on this one and wonder if somehow the heat caused a defect to turn into a pinhole. Weak theory, but I've got nothing else.

Ideas? I did take the bike on a rather rough gravel ride a few days ago, but like I said, the tube was ok this morning.

I'm using a tube meant for up to 25mm but I'm running it on a 28mm tire.


Thanks

frogman

Something caused the pinhole. Maybe it was a defect in the tube that finally opened up a pinhole, seems unlikely. Maybe there was a tiny piece of somenthing you didn't
find that caused the pinhole. Maybe one of the spokes is just barely pushing through velox rim tape, seems unlikely too. I have had flats where I never found the cause also. I go to the trouble of wiping and vacuuming out the rim when changing tubes.

3alarmer 

.
...I figured out a flat once for someone that was due to an aluminum shaving that had fallen out of the rim, and adhered to the Velox rim tape. (It was left over from the drilling during manufacturing). It worked its way over to the edge of the hole, and then whenever the tyre was fully inflated, after a while it would just lightly puncture the tube on the interior side. So if you haven't already, check the rim tape really carefully in the location of the pinhole.

I just had my first flat ever last week, where the high pressure air in the tube worked its way up the rubber exterior of a Schrader valve, and blew out halfway up the valve. Replaced the tube with a Continental, with an all metal, threaded Schrader stem.

cjenrick

mark the pin hole , put the tube back on the rim, mark the rim where pin hole is marked, look at the rim in that area,

lost track of orientation? you will have two places to look,

delbiker1 

The heat did not cause a pinhole. Something pin sized caused it. Getting a pinhole flat and not finding the pinhole cause is not unusual in any way. I double check the rim tape and tire, both visually and manually, to be better sure that I did not miss a small shard of something. That includes the outer side of the tire.

Iride01 

Pinching the tube can cause an immediate flat or flat at a later time. Pinch it between a lever and the bead while installing, or just between tire bead and bead seat while installing. Or rolling around the garage floor while flat.

Your idea of a pinhole might not be what my idea of a pinhole is. A pinch usually looks like a very very small cut, but if you don't have your strong readers on then it might look like a pinhole.

cyccommute 

I might have agreed even a few months ago. However, nearly every flat I’ve experienced over the last year have been on the interior of the tube…i.e. the rim side. I’ve changed rim strips from cloth Velox just in case there was something in the tape. I marked the tube with direction arrows and clocked my tubes to the label. Nothing seemed to work. I still experienced internal pinhole flats.

About a month ago, I went through flat hell in Wisconsin and Michigan while on tour. I experienced 8 to 10 flats of exactly the same kind including 4 in a single day. One of them was on one of the few really nice downhills in the U.P. in Michigan. I did a quick dodge around rumble strips to get out into the lane rather than do 20+ mph on a very narrow shoulder. The picture below is typical of Michigan’s rumble strips.


I experienced a blow out…which is very scary on a loaded touring bike at normal speed and petrifying at high speed. When I took the tire off, there was a rip on the inside of the tube about an inch long. After changing my bike shorts (), a light bulb went off. There is no way that I could have had anything inside the tire that would cause the tube to actually rip. I eventually decided that what had happened was that the quick steering to avoid the rumble strips had allowed the tube to be pulled too far in one direction. The rubber was obviously thin on the rim side and it tore due to the extra force on the rubber.

I have, in the past, been a proponent of using smaller tubes. They are lighter to carry and rubber expands to fill the space. I didn’t, however, take into account something that I’ve noticed when pumping up a tube outside of the tire. Tubes tend to expand more on the outside of the torus that is the tube than on the inside of the torus. If the tube were a straight pipe, the pressure expansion would look a bit like this (please excuse the extremely simple drawings). The pressure would expand equally in all directions and the tube (pipe, actually) would expand equally in all directions.



But in a torus shape, the outside edge expands slightly more than the inner edge like in the diagram below. The inner edge doesn’t have room to expand as much as the outer edge…it packs up a bit more. You can observe this when you pump up the tube outside of the tire.



Now think of the tube in the rim, especially if the tube is a smaller sized tube. When the tube is put in the tire and filling is started, the tube expands towards the tire first. Then as the tube fills, it expands into the rim channel. The outer part of the tube is trapped against the tire and the inner part has to expand into the channel where it thins. I suspect (but can’t really prove) that the tube thins a lot and, rather than the tube being punctured, the tube tears and creates a pin hole.



My solution was to replace my 23/28mm tubes with 38/44mm tubes. The wider tubes had more material in the channel and were less prone to tearing. The proof of this idea was that my inner punctures disappeared at about the 1/2 way point of my 1200 mile tour.

Going forward, I’m going to stop using small diameter tubes and use wider ones.

ThermionicScott 

Thought-provoking stuff, Stuart. Perhaps I won't be as cavalier in discussions about tiny tubes in large tires, going forward...

Symox

Wow, great post. I think you just convinced me to go back to normal sized tubes

Geepig

I have always found the bicycle world a strange one - dire warnings about straightening your fork in case it breaks, and then happily recommanding undersize tubes to use in one of the most demanding environments on the bike

I liked your description of the pressures, you made some neat observations there!

Symox

I just changed my tubes to the "correct" size after reading that

Thanks everyone. Not sure this will prevent the issue I just had but it seems to make sense

delbiker1 

cyccommute , interesting read. Certainly knowing where the holes are located in relation to the wheel and tire is going to help with finding the problem. Where ever they are located, it is not the heat doing it. Your theory on why the tear happened needs more factual data to convince me. The small tube thing never made much sense to me. It just seems common sense that the larger tube has more material, fills the area in the tire with less stretch, so, more puncture resistance. I will also add that mechanics, physics and such are not strong areas for me, my opinion is definitely subject to fault.

I went through a spell in spring of 2020 with around a dozen flats in a 6 week time frame, with 3 in an hour one morning. I found the cause of the flat in all but one of the 12. The 3 in one hour; first a pinch flat, second and third a small piece of frayed wire that I missed after the first. That was my first try with latex tubes. Thankfully, I have done much better with them since, and have them on 3 bikes right now.

Naturally, with all this, good quality equipment in good, safe to use condition, with frequent inspection and maintenance, starts it all.

cyccommute 

Small tubes made sense to be since rubber expands. Thinning out the tube material by making it stretch shouldn’t make much difference to prevent punctures since tube thickness …at least in nonthorn resistant tubes…has little effect on preventing flats. But I have had lots of problems with pin holes on the rim side for quite a while (I can be a slow learner at time) on all my bikes. I’ve even seen the dimples you get when the tube expands into the rim when a rim tape fails. However my rim tape has always been in perfect shape. And the problem has persisted even when I change the rim tape.

The “eureka” moment for me came in that tube tear. I didn’t experience a conventional blowout. Even with riding on a flat tire to a stop, the tire was still seated securely on the rim. There was no damage to the rim tape. The only thing that was damaged was the tube and it was ripped open on the side towards the rim. There is no other possible explanation than that the action of a sharp turn of the wheel (with a touring load) caused the tire to flex more than normal and the tube was thinned enough in the rim channel to rip.

Rick

cyccommute

I run 2.3"-2.7" Continental downhill tubes in my Schwalbe 2" wide tires. I always put the largest tube I can fit with no ill effect in my tires. for many of the reasons you described plus I just got tired of having internal flats from crap tubes.

csport

I have just had a similar tube failure. It was a correct size (700x35-44 in a 700x42c tire). It had a puncture at the inner part facing the rim tape, I patched it, installed into a tire and let it sit to check that there is no leak. First I inflated it to 20-30 psi and let it sit for a day for the bond to form. Then I deflated, rotated the tube 180 degrees around the nipple and inflated to 60 psi. No riding. In 10-15 minutes the tube ruptured in a new place (no patch there), below is the picture:

Relatively new Bontrager tube ripped at the rim tape.

I have also had a similar failure with a Conti Tour tube, but that one was older.

Is the rim width a contributing factor? The rims involved in the failures I mentioned had 18-19mm inner width and were run with a 42mm tire.

cyccommute 

I can’t really say if the rim width is an issue. The widest rim I use is a Velocity Dyad which has a 18mm inner. The bike that had the inner blow out has Velocity Deep Vs on it. Those have a 13mm inner width. I was using 32mm tire on those rims which isn’t as radical as some of the bikes I use. I can see how it might be worse with very narrow rim because the tube would hang up more on the tire and have to expand more into the rim channel.

Bodgeitbob

Possible it was something as simple as the tube was installed slightly twisted, got a normal flat via the tyre. Then as it deflated the tube twisted back causing it to look like a rim caused puncture?

cyccommute 

Probably not. Tubes don’t twist that much in a tire nor would they spontaneously untwist. In my case, I’ve fixed thousands of flats and I pay attention to how the tube is laying in the tire. When I take the tube out, the pin holes aren’t randomly distributed like normal flats are. They are on the inner centerline of the tube torus.

csport

A thought: maybe it is useful to first inflate the tube to ~20psi and then go through each point of the rim squeezing the tire. This is usually recommended to check that the tube is not pinched between the tire and the rim. It can also allow the inner tube to move (normally it is pressed against the tire) and allow the stretched part near the rim to shrink making the relative extension equal. In both cases when I had the tube rupture at the inner centerline of the torus I installed the tube and quickly inflated it skipping this procedure.

letank

Interesting way to solve an issue, which seems to a practice : I bought a used touring bike which had touring tires 700x28 (a bit on the dry, caked side)... it will be used for local rides, so I switched to 700x25. When I removed the tubes the rear was 700x35/43c but had a large shriveled section where it seems that the tube folded over itself over a 3" length... I replaced with a new tube 700x18/25 and the front tube was a more appropriate 700x28/32.

SoSmellyAir

cyccommute Good logical hypothesis. Too bad I have hoarded a few tubes which will be used toward the upper end of their ranges due to the prevalent "use the thinnest possible tube racer" theory.

drlogik 

+1 mark the pin hole , put the tube back on the rim, mark the rim where pin hole is marked, look at the rim in that area

Find an old Lyra/Spandex bike shorts or swatch of Lycra and lightly wipe/rub the rim area where the pinhole is. The Lycra will catch on any burrs or objects.