recurrent mystery flats
 

I have had this problem multiple times and I have no idea what's going on...

I'll ride my fatbike (a cheap one from Bikes Direct, has 4.9" tires with tubes) and everything seems fine, the tires are still full of air at the end of the ride. The next day, one of them is flat . When I remove the tube and inflate it to find a hole, there is no hole that I can see or hear. The tube inflates and holds air. I'll usually put in a new tube anyway (thinking there was a tiny puncture too small to detect, or a problem with the valve.) I inflate the tires and everything seems fine. If the bike sits around in the garage for a while, I'll check the tires every day and they are still full of air. But the day after I ride it, one is likely to be flat again, again with no detectable puncture and tubes that seem to hold air perfectly well. This happens if I'm riding at low pressures on sand, or higher pressures on dirt.

Also, this may or may not be related, but the tires are really loose on the rim. They almost seem too large for the rim (they are the stock tires.) Not sure if this is just how super-wide tires always fit. When inflating the tubes, it's difficult to get the tire seated correctly and requires multiple rounds of deflating and reinflating while reseating the spots that have popped off the rim.

Make sure the tire is the right size for the rim. Is the tire tubeless? If so, they do not offer puncture protection. (I realize you cant find a puncture) Beyond that, idk

Not tubeless. They are the stock tires so should fit the rim, although they seem to be a poor fit to me. It's an 80mm rim.

Sounds like a small pinch flat. Remove the tube, pressurize it and dip in water or use soap. Let us know what you find!

Is the valve core loose?

OK, had to put the inflated tube completely underwater and squeeze it to increase the pressure as much as possible, but I finally found the tiniest of holes. It's completely invisible, and even with being underwater and squeezing it, only leaks the tiniest bit. Now I can at least patch it, but why does this keep happening repeatedly and can I prevent it? It must be a form of a pinch flat even though I only found 1 hole and not 2 like you usually see. I guess fatbikes must be prone to them because of the low pressure ... an argument not to use tubes, I guess. I always thought tubeless was primarily to save weight.

How low of a pressure are you running? I would replace the tube in question ASAP or better yet go tubeless.
Apply a good coat of baby powder to the inside of the tire prior to mounting. This will reduce friction and let the tube move inside the tire more freely. As you inflate the tube stop and bounce the wheel to let the tube straighten. Run the pressure well above your riding pressure during this process. Back off the pressure and ride.
I ride in the desert and am a huge fan of tubeless for the puncture sealing properties.

Assuming the OP has found the leak, which probably was caused by a small piece of road wire, there is no reason to replace it. A quality patch system, like Rema TipTop, will give a permanent fix. Why contribute to filling our limited landfills?

Do you really think that a pressurized tube can move within a tire? :foo: I consider Jobst Brandt to be the final word on most things related to bikes. He claims that talc is of no benefit and may actually cause air to be released more quickly from the tire in the case of a puncture.

The most recent mystery flat occurred at around 15psi. The earlier ones were at low pressures (<10psi) on soft sand (how much under 10, I don't know, because I don't have a gauge that can measure under 10.)

Anyone ever use foam inserts that go between the tire and tube, like Tannus Armour? It looks like a good concept, but they're kind of expensive for a piece of foam.

The OP said this keeps happening. Sounded to me as unsure if this fix will do the trick. For me personally, if I think a tube is suspect then I replace it. Tubes do get old. I have a couple things I reuse old tubes for. Rubber bands being one. Others go straight to the dump. :eek:

Yes the tube does move inside the tire with the greatest movement during first inflation. What do you think happens as you roll over bumps and the tire flexes. Not so much for a 23mm tire but a lot on a 4" plus tire. I have been doing it for years for all things two wheeled and will continue to do so. The larger the tire the greater the benefit. I disagree with the link completely for fat tires. There is a lot of tube surface area on a 4" tire. I have see the benefit first hand dual sporting small dirt bikes and getting or not getting friction flats. I'm not saying a bicycle will generate enough heat to cause a friction flat. But you are more likely to pinch flat if the tube is not evenly tensioned or worse still has a crease inside the tire. Reducing the rubber to rubber friction between the tire and tube is a good thing in my book. Rides better too at low tire pressure.

You are not getting pinch flats above seven or eight PSI. Five PSI is getting into the pinch zone. Accu Gauge makes a good low PSI gauge that's under $10 on Amazon. I like dialing my pressure to the half inch but I do like to ride at low pressures.

Can't comment on a tire mousse but have you considered tubeless? Going tubeless fixes most flat issues. I was surprised but the number of cactus needles I had in my tires when I pulled them off. Pulled the needles, swapped the tires front and back, filled with Stans and still running problem free. https://www.notubes.com/support-center/faq

It's not gremlins. Each hole has a cause, and it's quite possible there are multiple causes.

If it happens again, then when removing the tube make note of exactly where the tube is in relation to the tire and the rim. Use a sharpie or tape or something to record exactly how they were aligned. Then find the hole as before. Inspect the tire and/or rim where the hole aligned. Use your eyes, a magnifier, your fingertips to feel, a ball of cotton drug along the surface looking for something sharp. Or a hole in the tire where something came and left. Find it, fix it, repeat.

With such a small puncture, I suspect a piece of metal wire or thorn may still be in your tire. Did you do a thorough check on the inside of the tire?

totally agree on patching and not wasting resources, but as for creeping tubes, it's very common when people never add air to tires and ride with very low pressures. Common to see awkwardly slanting valves, tube creep. I've dealt with it with friends bikes numerous times --"sure I add air to my tires, I did it about 6 months ago, what? What? Every few weeks? This tire feels fine to me, thumb pinch" (tire at 25psi, should be at least 60)

caveat, I'm new to fatbiking, so I'm very curious to see how low pressures with fattubes goes for me. Previous low pressure stuff has only been up to 2.5 in tires, winter riding, with no problems, pinch flats nor tube creep. Can't recall pressures, would lower for snow conditions by feel, I suspect 20 psi ish. Light rider though and not jumping off curbs etc.

There goes my top theory...

I usually inspect the inside of the tire after getting a flat, but I didn't in any of these cases since the tubes seemed to still hold air perfectly well. So I figured it was not a sharp object, but maybe some kind of intermittent valve leak, or, you know, gremlins.
But could just be a very small sharp object.

I have another tube lying around from a past mystery flat, which seems to hold air, but I'm going to check it underwater and see if I can find the hole. I have had this happen both front and rear, so it's not something like a sharp spot on the rim.

I'm gonna get a low pressure gauge to see if I'm heading into pinch-flat pressures, and also try the talcum powder thing. Never used talcum powder before, but it seems reasonable that with a fat tube -- at low pressures and with so much more surface area than a regular tube -- that friction inside the tire could be an issue.

Pump that past mystery flat tube up, huge like a life preserver, and let it hang. It will tell you in a week or so if it has a hole. As you saw, slow leaks can even be hard to spot in water.

seems to me that you'd be unnecessarily stressing the tube for no reason doing that. After properly doing the submerged check, with enough pressure (teeny tiny holes have always shown up for me, with enough pressure and being very attentive and going slowly) just put tube in tire and monitor pressure with a gauge.

I am also curious what others run. But I also understand there is a lot of personal preference and conditions vary greatly.
I typically run 6.5 psi on 4.5" tubeless. My buddy thinks I run too hard. He rides tubes. Went closer to 5 for sand dunes and 10 for slab. Have not been on snow.

I think I may look for a good low pressure gauge to fine tune low pressures.

I have the Schwalbe digital gauge and seems to work well.
Lowest I go is 8 psi because being 220lbs and riding over rocky river beds I want to avoid snake bites

being a lot lot lighter then you, I'll be able to go rather low then. It will be interesting to get a feel for diff pressures and diff snow conditions.

Mystery Flats
 

Had one of these last year on a road bike. Had to partially submerge the tube to detect the microscopic hole. I patched, happened again. Figured my patch job was sub-par, so put in a new tube...rinse/repeat. Then, used a cotton ball to swipe the interior of the tire...found the culprit, a tiny tire wire. No visible on the outside of the tire, and just barely poking into the interior. It was enough that I could frequently get in a short 15 mile ride without the tire going flat during the ride, but would be flat sitting in the garage shortly thereafter.

Take a cotton ball and rub it around the inside of the tire and over your rim. If the cotton ball catches something along the way that area is most likely what is puncturing your tube.

Most has been covered already. Always note the tire to tube position when removing from rim so you can look at the specific tire area when you find the leak in the tube.

If you do that, sometimes you need to press against the outside of the tire so you can expose a wire on the inside. Wire sometimes withdraws into the rubber and only pokes out to puncture the tube when the pressure of the road drives it inward.

Also check the valve. A loose Schrader will leak.

Lastly, you can add some stans fluid to the tube if you get stumped. I had a persistent slow leak a while ago and doing this solved the problem. Pour some in and go for a good ride to swish it around.

Building on prj71 post, I had a similar issue with my gravel bike a few years back. I brought to the shop to get a new tube and the mechanic had time to look the issue. He took the tire, looking at the inside, rolled the tire bead-to-bead , working his way around the tire, and found a small piece of glass imbedded in the tire. It was so small you wouldn't feel it rubbing the inside of the tire with your fingers, and it wouldn't cause a flat right away ... but hit a bump at the right (or wrong?) time and the glass shard would put a small hole in the tube.

You can try the soapy water trick with the whole tire too ... spray soapy water on the tire and inflate the tire to max pressure, and the bubbles will show you where you have a hole in the tire, and that where you look for a small shard of glass or other tiny sharp object.