globie

A minute ago, while I'm sitting in my office, a loud explosion nearby scared the stuffing out of me.
I went to investigate and in the next room saw that the rear tire of my bike was flat, blown off the rim.
On my way home last night, the same tire went flat. When I removed the tube, there was a shard of metal 1.25 inches long almost all the way inside the tube. I repaired it with a different tube, one that I had previously patched once.
When I filled the tube, I had a loose fitting on the presta valve so that when I reached the desired pressure, it would drop rapidly, but I stopped when it reached at 70 lbs., and it seemed to be about the right firmness, not overly hard.
I rode the bike all 6.5 miles to work this morning without incident. The bike had been sitting still more than an hour.
Any guesses about a cause for the explosion?
Thanks.

cny-bikeman

The tube escaped the confines of the tire, most often due to improper mounting, such that a portion of the tube is sitting under the tire bead. Less often due to tire hole or bad bead. Not due to tube, valve or tire pressure. If the tire was off the rim the only causes are mounting or bad tire bead.

wphamilton

Tube pinched under tire bead, and as it warmed up the pressure changed, the tube pulled on the pinch where it was weakened and then blew.

globie

I think you both must be right. The site of the shredding of the tube is where it would have met the rim. I've never pinched a tube before in many repairs, but there's always a first time. Thanks!

TimothyH

This is why I use brightly colored or fluorescent rim tape and visually inspect.

I pump the tube just enough to expand it into the mounted tire, slightly deflate so that tire bead can be pushed back from the rim and visually inspect all the way around the tire/rim on both sides. Fluorescent rim tape helps in low light.

Pushing back the tire bead and visually checking is the only way to be sure that the tube is not under the tire bead.


-Tim-

rumrunn6

hmmm, thanks. been tempted to do that but was afraid, in doing so, would actually encourage the tube to slide under the bead

Moose

Could be that the offending metal shard put a big enough slice in the tire casing that precipitated it's failure to hold pressure.

An inspection of the tube to confirm the proximity of the blowout would reveal where the failure took place... we will be awaiting the forensic details and summary of your conclusions.

globie

I inspected the tire casing pretty closely last night and saw no big tear or hole. I was having a little trouble last night clearing the new tube from the bead near the valve. I thought that I had pushed it back enough, but my conclusion is that some of it still got pinched. The extra inflation because of the warm office makes sense, because it was cold last night when I filled it.

dsbrantjr

It sometimes takes some time for the failure to occur, as air slowly leaks into the pinched area. I doubt that the increased temperature had much to do with it, since going from 50F (283.15K) to 75F (297.0K) will only increase the pressure by about 5%. Actually 4.94%: 297/283.15=1.0494

maddog34

temp. change will effect the TUBE... it will stretch easier. that pressure increase will have less effect than the now more pliable tube material.

wphamilton

Could be, but FWIW butyl rubber has minimal change in tensile strength for temperature changes in this region. We do know that it's pretty common, an hour or two after stopping and mostly reported after sitting in the sun or otherwise warming. The tire itself will also soften up, perhaps the shape or tensions change slightly, who knows.

More surely, it has to do with the temperature and most likely a pinched tube.

maddog34

all factors must be considered... blowouts are typically not a one cause failure.

as to temp of tires/tubes... a few minutes in the sun makes tire slip on much easier... i've done literally thousands of tire swaps over the years.... bicycle tire are easy compared to... oh,,, a Goodyear Rimsaver tire on a Sportster wheel... or a Michelin Rally tire on a BBS racing wheel... or a half rotted tractor tire on a rusty split rim coated with cow crap...... did a 3:20 time changing tubes in an M/C dirt bike front wheel, off bike, swap tube, back onto bike, air it up... fund raiser for the local USA ISDE team members that year... best time was a 2:10... one guy did 2:45 with a mousse tube! those tubes demand use of vice grips to mount up....

wphamilton

Yep, though we're getting a little off-topic I guess it's still pertinent since we're still discussing "why". The rubber, tires and butyl both, show a pretty good variation in elastic stretching and stiffness for that change in temperature, but I find it interesting that tensile strength, which is a different property, doesn't change all that much until quite a bit higher temperatures. So I'm inclined to attribute it more to a pressure effect which we know from the simple gas law could change 5 or 6% from cool morning to 70's. Radiant heat from the sun might roughly double that change.

rfmarotti

This is my SOP but had never thought to use flouro rim tape. Brilliant. Thanks for the tip.

ThermionicScott 

A good reason not to use those cheap black rubber rim strips, then. White Velox or blue Pacenti or yellow Stan's tape all make it abundantly clear if the tube is attempting an escape.

FBinNY 

Your tire will tell you.

If the tire plies are torn from the rim, or there's another large tear in the wall, that's why it blew. Probably related to the original cut, where you replaced the tube and didn't notice the damage to tire.

Otherwise, if the tire looks 100% OK, then it's on you. Specifically you failed to properly seat the bead, uniformly all the way around, and the high area was able to slowly creep upward until it blew over the side. Remember, tires are big enough to go over the side of the rim, which is how you mount them in the first place. So, it's critical that you seat them evenly all the way around so no area has the slack needed to blow off.

Retro Grouch 

Golly-gee-willikers! An objective, honest OP who admits that it's possible he may have made a mistake. I guess there really is a first time for everything.

leob1

I always get a good laugh when someone tries the "it blew because it was in the sun" excuse. It's a pinched tube. We all have or will do it, own it and move on.