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I was using mild braking on a very hot day, a couple miles down the hill the back tire blew out the sidewall on a Continental Ultra Sport 700x23C inflated to around 100psi.

Continental bike tire blowout

The rim was quite hot to the touch from braking. The tire is a few years old, but didn't show other obvious signs of coming apart or excessive wear.

I'm going to order a replacement tire. Are there certain tire options that would be more resistant to this? Other than that, what else can I do to avoid having to get a ride home in a car from the middle of nowhere?

Edit: added more pictures after taking tire off the rim with plastic tool:

closeup outside

closeup inside

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    Silly question: When you squeeze the brake hard against the rim is the upper edge of the brake pad rubbing the tire?
    – Daniel R Hicks
    Aug 20, 2019 at 11:56
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    @batflaps I've changed the question's title from using "downhill" to "descent" to remove possible confusion with an MTB discipline (I was confused at first). I hope it is fine with you. Welcome to the website, by the way!
    – Grigory Rechistov
    Aug 20, 2019 at 12:29
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    The amount of energy that needs to be dissipated during any significant descent is pretty staggering - "a couple of miles" quite likely releases enough energy to literally boil away a good portion of a decent-sized water bottle. Constantly dragging your brakes to prevent your speed from building up dissipates most of that energy as heat generated at, in your case, the brake pad/rim interface. If you were dragging your brakes, be glad you had aluminum rims, and not pure carbon fiber rims
    – Andrew Henle
    Aug 20, 2019 at 13:16
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    You were applying slight but almost constant brakes to keep your speed from getting too high? That is just about the best possible way to overheat your brakes and rims or disks, and quite likely was the direct cause of your blowout. You probably didn't get to touch the rim for at least 20 seconds after the blowout, if not longer - and it was still too hot to hold for any length of time.
    – Andrew Henle
    Aug 20, 2019 at 19:01
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    @Criggie interesting idea, but it seems unlikely that any (modern) rim is too narrow for 23mm tyres
    – Chris H
    Aug 20, 2019 at 20:48

4 Answers 4

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The ideal gas law PV=nRT can be used to work out the increase in pressure, as there's no room for the air to expand/escape. Let's try some numbers. If pumped to 100psi at 17°C (290K) then heated to match a metal rim hot to the touch at 57°C (330K) the pressure would get to 114psi. The tyre should be able to handle that even if rated to 100psi. Even starting at 7°C and going to 77°C only gets you 125psi. Altitude adds a tiny bit to this if you pumped up at sea level - around 5psi for a 3000m pass.

Here's a plot of pressure vs. temperature assuming you pumped your tyres up at 0°C and ignoring altitude. The upper limit of the graph is approximately when your tyres and brake pads would be smoking; you couldn't get that hot because the pads would misbehave long before that.

Pressure vs temperature

Heat also softens the rubber, making it more likely to fail, but it sounds like you should have been within sensible limits.

I've blown out a sidewall like this because of a sticking pressure gauge. A subsequent test suggested I'd put about 50% more in than I should have. If you'd overpressurised the tyre a little, then the heating could have taken it over the edge.

Maybe it's going too far for you, but randoneuring I carry a spare tyre. A boot is all very well sometimes, but I've seen a few tyres fail like this one and that won't go back together with a boot.

Long steady braking is sometimes unavoidable, but causes problems (including my own recent crash*). If you can, it's better to let the speed get up a bit then brake just before each bend or when otherwise needed. Spreading the heat between both brakes is also a good idea.

* I was on disc brakes. The resin holding the ceramic together in my rear pads melted, one of the front pads jammed in a wrong position and only engaged the rotor with one edge. This may have been due to heat as well. This all happened when I released the brakes to allow them to cool on a long steep straight descent.

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  • Elevation around 2700 feet or so. Tire said max pressure was 120, so I figured 100 was safe. Using a Specialized Air Tool Sport floor pump about 2 years old, use it all the time, never had problems.
    – batflaps
    Aug 20, 2019 at 18:04
  • If the gauge reads a little under (but not enough that you'd netball notice) then a hot day with lots of braking could just about take you over the maximum. Combine that with a little wear and tear, and you'd still need bad luck to get a blowout
    – Chris H
    Aug 20, 2019 at 18:24
  • I've smelled my rim brakes on one long-fast roadbike descent, when they started getting a bit faded, but never saw smoke.
    – Criggie ♦
    Aug 20, 2019 at 19:59
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    @Criggie yes, I had to stop somewhere and I reckoned smoke would have got a mention. As for my autocorrect today...
    – Chris H
    Aug 20, 2019 at 20:38
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Are there certain tire options that would be more resistant to this?

Your tire was appropriate to your task. Any tire of similar quality would be no more or less likely to malfunction.

It's not perfectly clear from the picture but this does not look like a tire blown off the rim from high pressure. If a tire is over inflated - or heated to over inflation the tube usually blows out below the tire bead.

It does look like the tire casing failed near the middle of the tire next to the center tread. It's the tire casings job to contain pressure.

The flap with threads sticking out looks like tire casing that tore out from near the middle of the tire. There is an excellent question here on Bicycle Stack Exchange concerning bike tire construction. Either the tire was defective, dry rotted or weathered to the point where the stress of a high speed descent caused the tire casing to fail.

Other than that, what else can I do to avoid having to get a ride home in a car from the middle of nowhere?

An auto mechanic friend of mine says "Anything mechanical can fail at any time".
Some things are more likely to fail than others. Tires and tubes top the list for a bike. As Chris H suggests, carrying a spare tire and tube along with some basic tools can help.

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In general Continentals have served me well. The Gatorskin models have tougher side walls, and the Hardshell version is even tougher. I'm currently riding GP5000 tubeless and ride in very hot conditions (Including Egypt) with no issues. I'm 100kg and like to descend fast (so maybe not as much braking), have never had a tire blow like this that was correctly installed and maintained. The tire in your picture does appear to have cracks in the side wall, I suggest that the tire has not aged well if it is only a few years old. This type of fatiguing can be accelerated by certain degreasers/cleaners, and prolonged exposure to sunlight (among other things). May I also suggest moving up to a 25mm tire (or even a 28mm if your frame and brakes allow) which may allow you to run slightly lower pressures, and offer benefits such as; fewer pinch flats, increased comfort, and lower rolling resistance.

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Other than that, what else can I do to avoid having to get a ride home in a car from the middle of nowhere?

Try a change to a disc brake instead of rim brake.

I suppose braking that was Too good could also lead to flat-spotting your tires.

enter image description here

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    I'm not sure how relevant this answer is, you may as well suggest the OP buys a new bike.
    – Dan K
    Aug 20, 2019 at 14:00
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    It's impossible, unless you have disc hubs. You can replace the wheels, but a bigger problem is that you can't add disc brakes to non-disc frame either. Anyway, if you aren't into bikes, why did you even answer?
    – ojs
    Aug 20, 2019 at 14:06
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    You've also chosen to use a mountai bike for the illustration, while the question is clearly about road biking. Disc road bikes are becoming more common, but that's still unhelpful.
    – Chris H
    Aug 20, 2019 at 15:52
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    Disk brakes can overheat too - this is really an expensive no-fix.
    – Criggie ♦
    Aug 20, 2019 at 20:00
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    @Criggie having tested them pretty hard, I agree, but cable disc brakes with metal pads are as near heat proof as you're going to get; they're what I'm going back to very soon.
    – Chris H
    Aug 20, 2019 at 20:42

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