lineinthewater

So I haven't had a flat in a long while. In fact, the tube I'm using right now was installed (by me) when I purchased new tires about 6 months ago. I've probably put 1300+ miles on it since I installed both, riding every week. Anyway, I was riding yesterday and I noticed a slight repetitive bump in my rear tire. I pulled over, thinking I had something stuck to my tire. Nothing on it, but I did notice a slight deviation at one point in the rotation. I made a mental note to check it out when I got back from my ride. Anyway, about 5 miles later, I started to feel a much more noticeable bump, and then suddenly heard the rear brake rubbing. I quickly pulled over, bent down to inspect, and saw over the course of about 5 seconds: the tube grew out between the rim and tire, bulged, and then blew out like a shotgun! My ear was ringing for about 15 minutes.

So, I'm a little confused. I've never seen any flat like this before. If I had just installed the tube, I might be thinking it was an improper install ... but after 6 months? I checked the tire and it appears intact and the tire beads don't appear torn. In fact, that tire model has always been a real pain to get on/off, so it must have required a huge amount of force get between it and the rim. My memory says I inflated the tire to about 122psi ... the tire is recommended for 110psi, and 125psi max. I usually inflate to ~120psi. It was hot, about 92-95 degrees. Did I possibly over-inflate?

Any insight would be appreciated.

DiabloScott

Blowout/explosions happen when the tube gets outside of the tire - the noise is the very rapid expansion of the pressurized air... that's why you don't hear a bang when you just get a regular puncture.

Possibly a bad install that for some reason (temperature, pressure, whatever) didn't show up until yesterday. But it can also happen on a fine installation on a hot day on a long downhill with lots of braking (and higher pressure, heavier rider, etc). The corrective action is less pressure on hot days and better braking technique.

lineinthewater

Better braking technique? You lost me there.

bikinfool

I think you just installed the tube with a bit caught under the bead and it worked its way out over time, the heat assisting. Nice that your tire is still intact, just be more careful in getting the tube completely inside the tire next time is all you can do. Tire recommendations aren't for best results for you/your riding in most cases, go by size of tire (what is it, a 23mm?) and your needs rather than min/max "recommendations" from the tire mfr. I have no idea what the other guy means about using your brakes differently.

cny-bikeman

A well mounted tire with no defects will hold a LOT of overpressure. Theoretically a long, continuous period of braking on a steep downhill on a hot day might cause enough overpressure, but I would be the brakes would fail first. I would agree with a defective install.

DiabloScott

By bad braking technique I meant dragging your brakes on long downhills so the rims get really hot (and sometimes tires blow off); didn't mean to imply that I thought that's what you were doing since you don't even mention a hill (or braking for that matter). But that's how it typically happens - happened to me ONCE but I see it quite a bit on hot days on the local mountain.

Still, lower pressure on hot days is a good idea, or letting a little air out if you ever notice the "slight repetitive bump" again. Could be a tire or rim imperfection too.

Agreed - but other things happen too when things get that hot - they expand, they get softer, they stretch easier... it's not just a pressure-temperature effect and it REALLY DOES HAPPEN! (and again, I don't know that this applies to the OP).

lineinthewater

Perhaps it was a defective install, but I'm always very careful to seat the tube properly (and not pinch it) when installing new tubes. It's the 6 months that is giving me pause on calling it a poor install ... but I guess as everyone says the heat can do crazy things.

Didn't even think about that angle. I wasn't braking at all, but it's an interesting point.

Will do.

cny-bikeman

Have you remounted the tire with no problems at full pressure?

..and a slight repetitive bump would call for immediately checking both tire beads and tire surface for poor seating, split tire, broken cords (tread/tire will "snake").

lineinthewater

I just rode 15 miles on the new tube ... no problems so far.

dwr1961

Take a really, really careful look at the sidewall of the tire. A tiny hole or tear (that can't be readily seen) can cause the tube to push through and blow out in the manner you described.

The defect might be just above the bead (the edge that contacts the rim) so it might have looked like the tube was emerging from under the bead, when it fact it came through the sidewall. Dismount the tire and hold it up to a bright light as you inspect. You can sometimes use a tire boot to make a successful repair.

Good luck,

lineinthewater

Are you saying a small hole can cause the tube to come out between the rim and the tire, or through the actual hole? Because there was way too much tube exposed (before popping) for it to be coming through a small hole.

I didn't take the tire off, but I did just inspect it with the tube deflated. I see a hairline line/gash near where the rim meets the tire. But I don't think it is a hole (or torn). Probably just surface damage. Other than that, it looks OK.

I could always get myself a new rear tire and just not think about it ... I'm probably ready for one.

dwr1961

I'm saying that, yes, an amazingly huge, balloon-like section of tube can exit through a tiny hole in the tire's sidewall. 120+ psi will do that.

Strange, but true... Ask me how I know.

lineinthewater

Well, if that was the case, it was playing one heck of an optical illusion on me.

dwr1961

No. I think you saw what you said you saw... (say that fast three times).

IF there was a tiny hole in the sidewall, just above the tire bead, then the tube would exit the tire near the inside base of the wheel rim... Thus, the tube WOULD eventually squirt from between the tire and rim as it continued expanding. When the escaped section of tube finally stretched to its breaking point, then BAM!

Yellowbeard

That you say the tire was/is a real pain to remove and install makes the pinched tube seem likely.

DannoXYZ 

Yeah, most likely pinched tube. There's absolutely zero way you can heat a tyre up to the point where it will blow off the rim. That typically needs DOUBLE the max-pressure rating printed on the side. So a 130-140psi tyre would need to be heated up the point where pressure is 280psi inside the tyre. I'll let you guys calculate what temperature that is, but I can assure you that it's physically impossible to heat up a tyre that much using the brakes or leaving it inside the back-hatch on a hot day.

Most likely cause in this case is pinching the tube between the tyre & rim during installation. Happens all the time and is probably the cause of 99.9% of the spontaneously blow-outs presented here. Heck, I've done it myself even after having worked at a shop for 10-years and having replaced thousands of tubes. The most important step is to pull the tyre sideways after everything's installed and inspecting for a pinched tube. Yea, it does take some time to go all the way around the tyre and inspect every centimeter of bead, and then repeating for the other side. But it will save you the time sitting on the side of the road replacing a blow-out later.

conspiratemus1

+1 to that. It's one of my "always" points on the checklist when mounting a tire, even when fixing a puncture in the rain at the side of the road with cold hands. (Especially then, because the last thing I want right then is a failed repair.) Amazing how often I find a little sliver of tube just under the bead, and how hard it is to nurse and coax that tube back where it belongs. (And no, air pressure at this point to "round" the tube never works. Before installation, maybe, but not now.) Yes, *usually* the tire was hard to mount, which is what makes me extra sure that I'll find a bit of tube outside the bead, but not always.

I guess the part the puzzles me is why it took so long (6 months) before the OPs tube finally herniated and blew. When I've seen (heard) trapped tubes blow, it's usually as the tire is being inflated to riding pressure, or shortly after. Here's a theory, see what you think: At installation, a very small sliver of tube is trapped under the bead. But the bead of the tight-to-mount tire is compressing the trapped section so tightly that no (or very little) air can get into it, so it is protected. All goes well for six months, then something happens to cause the bead to loosen just a little -- a hot day causes the wire bead to expand and lengthen ever so slightly, maybe aggressive cornering, or air leaks slowly from the tire as normal. The grip on the pinched section now relaxed, air from the rest of the tube rushes in to fill it. Unconstrained by the tire casing the herniating tube section rapidly expands until it bursts.

Plausible?

DiabloScott

You just made up that statistic, right?

Knowing that this phenomenon is highly correlated with hot rims from braking on descents, can you discuss how a pinched tube behaves differently when it's hot? That is, how can the mismounted tire be fine for many miles and then blow out when the rim heats up? I'll note that normally a pinched tube makes itself apparent even at moderate pressures by uneven seating of the bead.

bikinfool

Where do you get information about how many tires have blown off from heat of rim braking? I've used rim brakes on road and mountain bikes for a long time, and some very long and hard descents, never had a tire blow off from the heat of braking...but then I don't use the brakes a lot on a descent.

cny-bikeman

It is possible that the OP allowed the tire to go flat at some point closer to the time the tube blew out and the tube was under the bead when reinflated. I suppose heat could be the last straw for a mis-installed tube, but I can't really see no "bump" at all till heat was a factor.

Also, the 99% is a bit high, but a very high proportion of the blowouts I have seen (hundreds) are due to mismounting. The remaining ones I could trace were due to failed beads or other tire defects or damage.

DiabloScott

I only said "highly correlated", I didn't mention any numbers. This happens when you drag your brake for long distances down steep hills on hot days. It happened to me once and I should have known better; it was a classic explosion of the rear, tire blown off the rim, I stopped and grabbed the wheel to take it out and the rim was still so hot it seriously burned my fingers. I see it occasionally on hot days coming down Mount Diablo with people who don't understand the effect and don't know how to prevent it. It's more common with heavier riders, loaded tourers, and tandems because obviously the braking heat is more intense, and it's more common when it's hot. I've also seen writeups that rim temps can exceed the boiling point of water (easy observation under the right circumstances).

So I don't see anyone denying that hot rims make the blowout more likely. Danno's claim is that the heat alone isn't enough and you need the pinched tube too; I don't know how you can have a pinched tube and not notice the bulge at normal operating pressure, and I contend that a perfectly mounted tire can blow off given enough heat. Maybe you could purposefully pinch the tube and then pump the tire up to see if you can get it up to operating pressure without seeing a bulge, then press it up to failure and note the blow off pressure.



How can you tell that from after the fact? Blowouts generally make a big straight rip in the tube, maybe a V-shape, and blow the tire off the rim. What's different with a mismount that would be evident from a post blowout inspection? Blowouts can happen with casing failures or rim/bead damage too but that's not what we're talking about here.

Aquaman5K

Just out of curiosity, I did some back of the envelope calculations based on the numbers people have been throwing around in this thread.

Assumptions:
1. Tires originally inflated at 25 degrees C, since the op did not state
2. Ideal gas behavior (I know air isn't an ideal gas, but close enough to get a rough idea)
3. Volume and number of gas molecules are held constant (not strictly true in this case, but close enough)

If it is true, as one poster suggested that a properly installed tire can take 2x its rated pressure before blowing off, in this case (starting at 120psi), a temperature increase of 298 degrees C would be needed to reach this pressure. If ambient temperature increased by 10 degrees C (because it was a hot day), that still leaves an increase of 288 degrees C that would have to come from somewhere else.

I find it hard to believe that any amount of braking could generate that amount of heat, and if it did, you might have other things to worry about. Anyone know the melting point of butyl rubber?

It would seem more likely that:

A: The tire was not properly mounted (pinched tube, tire damage, other reason)

or

B: Tires can't really take 2x their rated pressure before blowing out.

DiabloScott

or

C: Pressure increase is not the only effect of all that heat. When things get hot they expand, they get softer, and they stretch easier; all of which make blowoffs more likely.

fuzz2050

There is obviously only one solution to this debate, we each need to take a properly mounted tire, inflated to the maximum inflation pressure on the side wall, and stick in in our home ovens for as long as needed. If even a few of us can manage to blow off the tire, than we know it's possible.

I'm fetching my spare wheelset now!