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I'm planning my upcoming trip to ride real mountains in Spain on my road bike. So far I used Mavic Ksyrium aluminum wheels but mostly on flat areas.

I'm not confident with going downhill on this wheels because when I do downhill my wheels becomes hot from braking. I'm using clincher with tubes inside and standard Shimano brake pads. Should I be worried? Is it safe for inner tube?

I'm asking because I've heard a story about exploding inner tube from heat :(

I was thinking about going tubeless but never tried this on my current set and I don't want to experiment until its needed.

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    I'd consider improving braking technique before switching to tubeless for this particular reason. – Klaster_1 Jan 8 at 10:57
  • I was afraid that someone will suggest this as a solution. I'm not totally weak in case of technique but know my limits and don't want to act like Vicenzo Nibali while I'm far from him. I dont want to go like 80km/h and break before curve unless I'm in Torude de France with closed traffic. Dont get me wrong. – Dariusz Jan 8 at 11:05
  • No hard feelings, but somebody would have mentioned that sooner or later, lol. – Klaster_1 Jan 8 at 11:10
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    @Dariusz On long mountain descents, you can effectively manage your speed by sitting up taller and sticking your knees and elbows out. In this way you can generally keep your speed to around 65kph. On long mountains it is quite rare to get very steep sections that require you to scrub speed with the brakes – Andy P Jan 8 at 11:50
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    @AndyP that depends who else is on the road. I did a couple of passes (in the English Lake District) on new year's day, only a few hundred metres descent each but on single track roads with cars about. I could smell my discs and water sizzled on them. Cable discs and sintered pads are made for it though. Conversely they didn't get that hot coming down the Col d'Izoard on a hot day, because there I could let it go and brake just before a hairpin – Chris H Jan 8 at 14:15
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I'd say that it is more about the braking technique you are using and not the wheels themselves when it comes down to overheating. See the following thread where similar topic is discussed:
What is best for your brakes when stopping at the bottom of the hill?

Have a look at this article as well: Braking and blowouts
The author claims that aluminium wheels get heated up quickly but they also quickly cool down provided they keep spinning (airflow). Thus never brake hard to a complete stop, allow your rims to cool down.

I've ridden couple of Eroica events which are known for heavy climbs and long descends and I haven't noticed anyone suffering from overheat tire blowout (punctures on a rough terrain is a completely different story altogether).

To answer your question - master the braking technique I mentioned and enjoy your trip to Spain!

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When people talk about blowouts from braking heat, they're usually referring to latex tubes being used with rim-brake carbon clincher wheels. This combination can indeed lead to blowouts on long, fast descents if you aren't careful. I used to run latex tubes with my carbon race wheels, but after having two tube blowouts, I switched back to traditional butyl tubes and never had the issue again. If you're concerned about heat, just ensure that you're using butyl tubes, not latex.

  • Also heavy duty tubes maybe! – Carel Jan 8 at 15:31
  • The tube holds the air, the tire the pressure so the tube does not explode. Whats the failure that makes latex tubes cause a blowout? – mattnz Jan 8 at 23:49
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    They melt at a much lower temperature causing a huge hole which allows all of the air inside to escape much more quickly than a regular puncture. Since it only happens in high speed descents, it can be very dangerous. – Carbon side up Jan 8 at 23:58
  • OK, I now understand my problem. The loose use of the term Blowout - a tube leaking into the casing of the tire is a flat, but it happens relatively slowly. A 'Blowout' is an instantaneous loss of all pressure, and can only occur with catastrophic the failure of the container (tire or rim). The only question that remains is how long before a flat is noticed before the pressure is low enough to roll a tire off the rim (Technically not a blowout, but has the same result for the rider) – mattnz Jan 9 at 0:02
  • When a latex tube fails due to heat, it can certainly be described as a blow out. It will make a bang and go down almost instantly. Mine resulted in a 30 cm long tear in the tube, but I've seen some that were bigger still. – Carbon side up Jan 9 at 8:23
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There really is nothing to worry about. Ksyriums have been popular OEM equipment and mid range upgrades for at least 10 years and 1000's of people are riding them in the mountains every year with no issues.

Personally, I have been riding the same set of Ksyrium Elites since 2011 and have ridden the Alps, Pyrenees, Dolomites and Corsica on them with no issues.

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So far, I've witnessed one blowout with a rim brake in the alps. However, the cause was not heat but that the rim was braked through (i.e. a section of ≈ 120° split off). (Fortunately, nothing bad happened, and that guy was in a group that had a van alongside with them)
Take home message: take care that the rims are still sufficiently thick.

The one descent so far where we've been taking some extra caution was going down from the Vršič pass at 1611 m into the Soča valley at 780 m (≈ 9 % for a bit more than 9 km). The first part can be done at comparably high speed so air resistance helps. But the lower 5 km (ca. 500 m of the elevation loss) have one hairpin bend after the other (IIRC 19 in total). While there's decent asphalt in between, the hairpins tend to be done in cobble stone and may have loose sand on top. In other words, forget about using drag to get rid of excess energy. Our bikes were loaded with full camping tour gear (back panniers + low riders in front). On that descent we did stop several times to check temperature (and let the lined up cars and buses pass). While I don't have thermometer readings, we did reach "finger says ouch". We also employed water cooling: wet the rim surface - when you are around, the rim is dry again ;-)

(BTW: I don't recommend Vršič as there was a lot of traffic - everything from bikes over motor bikes, cars to fully-grown motor coaches)

You may be interested in these records of maximum rim temperatures together with whether they did have a blow-out or not.

I'm not at all sure that tubeless tires stand rim temperature any better than the tube variety.

  • Pay attention to the weight and ambient temperature in the linked temperature table. In all cases of blowout except one, they were over 120kg and 30C (i.e. a lot of energy from the mass and poorer cooling from airflow). – mattnz Jan 8 at 22:30
  • @mattnz: yes. Also, the bikes seem to be recumbent, so less air drag, more energy to be dissipated via brake. But ambient temp of up to 30 °C is something OP may encounter in the valleys in the Pyrenees as well, the weight will of course depend a lot on whether they want to do road bike day tours or carry on their luggage. But I'm pretty sure I've had total weight in the 120 - 140 kg order of magnitude during camping tours. OTOH, note that the blowouts didn't happen on descents of many 100 m but comparably small descents (525, 265, 50 (!) m). – cbeleites Jan 9 at 9:52

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