5

I'm fixing an old bike I got out of a dumpster. The tubes needed replacing and the tires had cracks all over the surface (picture 3). Since tires are held together by a weave and not just rubber I imagined it would still be somewhat safe to ride. About an hour after inflating it to 4.5bar (the maximum allowed pressure) the tire blew out the side. The rim appears to be intact. However some rubber sheared off of the bead of the tire.

enter image description here

A close up of the damaged bead.

enter image description here

Is it normal for the bead to rip off an old tire? Is the rim safe for me to put a new tire on?

1
  • I imagined it would still be somewhat safe to ride. vs after inflating it to 4.5bar (the maximum allowed pressure) Next time: if you have the slightest feeling something is not safe, do not push it to the limit, not even if the almighty soul of Garry the lawyer is telling you to do so en.wikipedia.org/wiki/Death_of_Garry_Hoy
    – EarlGrey
    Oct 12, 2023 at 12:36

4 Answers 4

11

Is it normal for a bead to rip out of an old tire?

Yes

When a tire reaches the age where the cloth core of the tie has dry rotted then it is normal for the bead to tear rip out even under normal tire pressure.

The cracks noted in the question are an indication of dry rot. Another is removing the tire and flexing it. A good tire will feel springy and make no noise. A bad tire will feel stiff and make crackling noises.

I've never seen a rim damaged by a tire blow out but check the tire bead seat in the rim. If the groove where the tire is seated is undamaged it's ok.

9

Maximum pressure in a rotten MTB tire at 4.5 bar. Why would it not go wrong? Pressure gauges are sometimes not perfectly accurate, so perhaps you went above the max pressure in reality? Max pressure is also at a certain temperature. If you filled it at 20C and then went into the sun heating the tire to 40C the pressure would increase. If you are a heavy/rough rider the stress is increased further.

If the tire gave in when riding on a straight path, and you stopped riding then the rim is probably in the same state as before the tire explosion. If it happened when landing a jump then the chances of the rim taking some damage is much higher. If the wheel is true, looks ok, no cracks/obvious damages and spokes feel ok then it probably is.

If it is a rim-brake rim, they sometimes have a groove along the breaking surface that is supposed to be seen/felt until the rim is worn out. Some have an indented-dot as indicator. https://road.cc/content/feature/how-do-you-tell-when-your-wheel-rims-worn-out-238960

3
  • If no brake indicator, you can hold a ruler up to the rim and look for significant concavity. Some info here: novemberbicycles.com/blogs/blog/how-long-do-rims-last
    – Weiwen Ng
    Oct 11, 2023 at 21:20
  • 3
    20°C to 40°C increases the pressure from 4.5 bar to 4.8 bar for a volume of 5 liters (~ 26"/2.2"). Oct 12, 2023 at 7:46
  • @Sakari: Yes, that is what you get using an ideal gas calculation. The increase in pressure with temperature is independent of size/volume of tire. Everyone should take temperature into consideration when inflating tires near max/min recommended pressures.
    – WornChain
    Oct 13, 2023 at 0:48
5

Clearly the tyre was unsafe, and a visual inspection would have showed it needed replacement. If you looked at that and thought it was okay, you need to reset and recalibrate your judgement.


Is the rim okay? Probably, because the tyre let-go first. However if it were me, I'd remove the wheel from bike then strip off the tyre, tube, and rim tape and look over the whole rim, and that area closely.

  • Work under a strong light or outside in sunlight. Defects can be harder to see in shadows.
  • Evaluate the rim all around. Compare flex on the blowout area with flex elsewhere on the rim.
  • Be brutal - you need confidence in the rim, and being gentle here may pass something marginal.
  • Wash the rim and re-inspect. Dirt hides things too, and a good scrub can reveal what's already hiding there.
  • Check the rim for running true - a wheel that is hard to straighten may have damaged/stretched spokes/threads etc.

If you have doubts or any concern, write the rim off. That may mean rebuilding onto a different rim or replacing the whole wheel.

On the other hand, if you can't find cause for concern, then its okay to ride.

Personally I ride things that I would call marginal to let someone else ride - my tolerance for personal risk is higher than I'd push onto anyone else.


Related: I had the opposite happen - a rim-brake rim failed and tore off about a quarter of the circumference, but the tyre was perfectly okay. I'm still riding that tyre on a replacement wheel and tube.

enter image description here

2
  • 1
    Wow, you had a rim fail? Was this a wheel with tens of thousands of kilometers on it, where it got worn by your brakes? I've worn out a couple of aluminum rim brake wheels, and stopped using them when the wear indicators said they were worn out.
    – Duncan C
    Oct 12, 2023 at 18:39
  • @DuncanC potentially - it was used when I got it, and on my "wet day" bike so exposed to more abrasive grinding dust than the good bike. However it didn't appear thin, and I have no idea if it ever had wear indicators. I was putting ~90 PSI where I'd normally put 80 but that shouldn't be enough to rupture the metal.
    – Criggie
    Oct 12, 2023 at 22:39
3

It's not surprising that a tire would be so deteriorated that the casing loses strength. Is the rim OK? It's difficult for a bunch of Internet strangers to say. If the aluminum is not visibly deformed or scraped, I'd guess it's OK. Rims used with rim brakes will eventually wear out because the sidewalls get worn down. This happens faster when they're ridden in wet climates, because the combination of water and road grit acts like sandpaper under the brake pads.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.