wilfried

A couple of nights ago I got a rear flat. I replaced the tube with a new one, inflated it with CO2, and rode 13 miles home. When I got home, I deflated the tire and reinflated it with a floor pump, as I was told one should do. A short while later, maybe an hour, I heard a loud PSSSSSSST!, and the tire was flat again. Once again, I removed the tube, and found that it had split wide open, with a foot long gash running along the inside of the tube, facing the rim. I patched the first tube that had the pucture, reinstalled it, and so far it's holding up fine.

So my question, what could have caused this? Did I do something wrong, did I make some noob mistake? Or perhaps the tube was defective? I think something similar may have happened to me some time ago. I'm a bit mystified, so I'm trying to figure out what might have happened, and maybe try not to let it happen again. Thanks for any insight.

SkyDog75

If the hole's facing the rim, it could be because of a problem with your rim strip or tape. But my first guess is that you might've gotten a bit of the tube pinched between the bead of the tire and the rim.

You can usually detect a pinched tube by looking closely at the tire after it's mounted and inflated. Most tires have a subtle reference line on the sidewall that should be equidistant from the rim all the way around. If it's not, or if you spin the tire and detect a bump or a dip, the tire may not be seated correctly and one possible cause is a pinched tube.

FBinNY 

When I see a long gash as you describe on the inside of the tube, I suspect one of two things, both user error (sort of).

1- if it goes with a low bang and the tire ends up off the rim for a length, then it's usually that the tube was trapped under the bead, and eventually jacked the tire over the side of the rim, or the tire was simply seated badly and didn't hold. The key elements are the bang, and the tire ending up OFF the rim.

2- it blows quietly, with a muted bang, or the psssst you describe, and the tire is found on the rim all the way around, and the slit is long vs a small puncture, then I immediately look at the width and interior profile of the rim.

To understand the following, please reference this sketch of the tire and rim cross section.

When the tube is first inflated, it wants to assume a round cross section, so it fills the upper section within the two beads and spans the gap between the beads. (sorry, the sketch is inverted, so above and below will be reversed). As you put more air in, or with time, the thin section panning the gap blows down into it and fills the void below. This causes the narrow strip on the rim side to hyper stretch, and eventually pop or tear like a child's toy balloon. The rubber tubes are made from is a bit funny and once a tear starts it can propagate down a decent length which is how you get the long slit. The narrower the rim the narrower the gap, and the more you get this hyperstretch down the rim side.

The preventative is to give the tube a chance to equalize and spread the stretch more evenly. I inflate to only 10-15psi, inspect for even seating and sometimes sort of massage the tire to help the process. I've also had good luck using talc on the tube to lubricate it so it slides within the tire more easily.

See if mt theory matches your situation based on these points.

1- narrow rim
2- long slit
3- tire NOT blown off
4- no or muted bang
5- evidence of over stretching, on either side of the slit, or elsewhere along the rim side of the tire (not always present, but if so is solid confirmation).

The sort of in my saying it's your fault is the issue is an inherent problem of narrow rims, but it's your job to manage past it.

digger

Just a question for my understanding please.

This void you speak of is between the bead and rim; that V shaped void between numbers 3 and 4 in the diagram?

FBinNY 

No, that is greatly exaggerated in the sketch and rarely if ever a factor.

Imagine the tube initially forming a circle within the tire and running an almost straight line between the 2 red beads (4). Then as the pressure increases, that line blows out to assume the deep U you see in the sketch. ONLY the straight section panning the beads will balloon out, like when you blow a bubble with bubble gum, so that small area stretches much more than the rest of the tube that stopped stretching when the initial circle was formed.

So the narrower the gap between the beads and/pr the deeper the well in the rim, the more that narrow zone stretches.

The powder helps the tube slide along the tire and spread the stretch more equally around the entire periphery of the tube.

Homebrew01

No need to remove perfectly good air, just to pump it back up. Unless you need to reseat the tire, or checking for a pinched tube.

Bill Kapaun

The OP initially used CO2 which leaks out much faster than "straight air".

FBinNY 

True, but that had zero to do with the tube failure. Also there's no need to let the CO2 out and replace it at any point. The CO2 will bleed, and be replaced in stages as the tire is topped off in the normal course of service. The frequency and amount of topping off will be greater until the air fully replaces the CO2 over the initial span, then all will be as if it was air all the time.

wilfried

Thanks for the detailed reply.

There was no bang, more like a loud hiss, and the tire did not blow off the rim. So it wasn't option 1.

All true except for number 1. I have 20 x 1.50" tires, so the rim isn't particularly narrow. I had another look at the tube, here's a photo:



I hope you can see the dimple above the gash, where the tube was stretched. It looks like the tear started sideways, and then turned to follow the grain of the rubber. So now I'm guessing that the tube got caught on something on the rim side, stretched as it inflated, and eventually tore, causing the gash. The thing is, I don't know what that something on the rim is. The nipples are recessed, and there's rim tape. I'd have to take the wheel apart yet again to see, but for now the tire seems OK, so I'd rather leave well enough alone. I rode to work today, and so far so good. The mystery is partially solved, so thank you. I'll have something to look for next time.

BTW @FBinNY, Chain-L is my go-to lube. We met very briefly once at the Philly bike show.

Bill Kapaun

I didn't mention anything about the cause of the failure.
Just why the OP "changed the air".
I simply assumed he didn't want to top it off "more frequently" until the CO2 had reached its normal concentration.
Seeing now that it's a 20X1.5" tire, it probably wouldn't make that much of a difference since it's relatively low pressure vs something like a 622-23mm.

wilfried

I've only used CO2 a couple of times, so I don't have much experience with it, and I re-inflated the tire simply because it was what I was told to do. Had I known it would cause the tube to pop, I wouldn't have bothered. Maybe that's another lesson, that is, leave well enough alone.

FBinNY 

It didn't cause the failure, nor did reinflating the tire. Looking at the tear, it might be that the tube was pinched under the bead at one point, not enough to blow the tire off, but enough that the tube couldn't equalize a twist, or spread itself equally around the tire.

All of this in another reason to inflate to 10-15psi, and do an inspection that the tire is evenly seated, then go to full pressure. Also tube sizing might have an effect. In order to not have too many sizes, the tubes are sold in ranges, ie 20 x 1.25 - 1.75, or 20 x 1.50-2.125, or whatever. The tube has to have an unstretched size based on the narrowest size it'll fit, then being rubber stretches for the larger one. That's well and good, but stretching makes the walls thinner, and more prone to issues if not positioned evenly all the way around.

-----------------------------

One question, which you don't have to answer. Did you put air in the tube so it was an unstretched donut before installing, or did you just unroll it from the box and install without air?

Either method is OK, but installing flat increases the chances of installing twisted, or with a part trapped under the bead. So, if you installed flat, remember to add some air firt next time, and it'll not only be easier, but you'll reduce the chances of error.

wilfried

I lightly inflated the tube before installing. It think it was still floppy, so perhaps I should inflate a little more next time. The tube is labeled 20" x 1/1.25/1.50".

FBinNY 

I inflate to where it's a soft sausage and sometimes then have to deflate a bit to get the last of the tire on. But the 1"-1.5" size is a piece of the puzzle, since it's stretching to it's design limit in your tire. They can stretch even more than that, but like a balloon the more you stretch, the easier it tears.