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I have a carbon fiber Giant Revolt that I keep in the hallway of my apartment, near the door. Opposite from where it's stored -- about 2.5-3 feet away, or one hallway width -- there is a hot-water type radiator, that is, one of those through which flows hot water pumped by a gas boiler. I've been keeping it off, because I wasn't sure what effect it might have on the bike frame: it's obviously not touching the radiator, but the radiator does put off a fair amount of heat.

I'm not sure whether the carbon fiber frame is basically immune to that kind of heat, or whether, over time, the continued heating and cooling will have some wear-and-tear effect on it. As said, I've been keeping the radiator off, but it makes that part of the apartment kind of chilly, so have been thinking of turning it on.

Any info from carbon fiber/materials experts would be appreciated!

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    I'd expect the tyres, bar tape/grips, brake/gear outers, and plastics of any sort to be affected by lower heat than the frame.
    – Criggie ♦
    Commented Feb 10, 2023 at 16:47

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Carbon fiber (technically carbon fiber reinforced plastic, or CFRP) is a composite material consisting of reinforcements (i.e. carbon fibres) held together in a matrix, which most often is some form of resin, both of which have vastly different characteristics and molecular structures. Therefore to understand the impact of temperature on carbon composite elements you need to understand how it affects the constituent elements.

Let's start with the fibers themselves. They are actually graphite, which is the stable allotropic form of carbon under normal temperature and pressure with a melting point of well over 3500 centigrade. This means that even if you heated it to a hundred centigrade, we are still well below the temperature at which negative effects such as creep or excessive oxidation can occur.

The matrix, however, is a different story. It consists of cross-linked polymer chains, which means that it does not melt, but rather degrades in high temperatures. The key is the so-called glass transition temperature, above which the resin is no longer in a glass phase, but transforms into a rubber-like state. This temperature is also the highest, at which resin can be kept theoretically indefinitely without charring.

Therefore as long as you keep your bike below it, no negative side effects should be encountered. Even more, most carbon fiber curing processes involve baking the components close to the glass transition temperature. How high it is, however, depends on the resin in question - it can be as low as 70 or as high as 400 centigrade, but most likely it is above 100 - though check with the manufacturer for the exact number.

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  • May be worth noting that for carbon rim brake rims, the structure needs to be compatible with rim brakes. These type of rims can be damaged by being next to car exhaust when put on a rack. I'd exercise more caution with these. I agree the frames are a lot less likely to be damaged.
    – Weiwen Ng
    Commented Feb 10, 2023 at 15:07
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    @WeiwenNg Engine exhaust gasses are far hotter than a typical home radiator. Even though the resin used for wheel construction typically has a higher glass transition temperature, it still won't be able to withstand several hundred centigrade from the exhaust, especially given the fact it's a stressed element.
    – Paweł
    Commented Feb 10, 2023 at 16:17
  • I suppose the temperature is around 40 degrees? Nice and toasty but not sauna level hot. This answer (from Pawel) is the most complete, but also most open ended. Do I really need to check with the manufacturer, or are the temperatures associated with home heating OK not only for the resin, etc, but the tyres as well? I’m not sure how I’d get a definitive answer in that.
    – Cerulean
    Commented Feb 11, 2023 at 17:41
  • The thing is that it's really impossible to give a definite temperature at which you should start to be concerned without additional data. Also, the temperature of the heater does not matter as much as the temperature attained by the bicycle frame, though the latter will undoubtedly be lower than the former. At 40 centigrade, however, I cannot imagine anything bad happening to your bike.
    – Paweł
    Commented Feb 12, 2023 at 9:29
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Probably should have bought a metal bike. You will never truly enjoy the benefits of carbon if you go as far as living in a cold apartment not knowing how robust Carbon Fibre really is (hint - Carbon Bicycle frames are way more robust than most cyclists think)

As far as heat damage, a radiator at 3 feet produces far less energy than direct sunlight (nominally 1000W/m2). Are you concerned about riding you bike on a sunny day?

As far as Carbon Fibres resistance to heat I found this paper.... "It will decompose, but reactions are typically negligible until the temperature exceeds around 700ºC" Reference

If you are still worried, simply cover the bike with a blanket to sheild it from the excessive heat.

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    Regarding "700 degrees", the carbon fibers themselves may survive that temperature, but the binding resin most definitely will not. I think the epoxy starts to rapidly break down around 200 degrees or so.
    – MaplePanda
    Commented Feb 10, 2023 at 0:56
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    @MaplePandea - yes, I have done further reading and found lots of studies. It looks like sustained high temperatures (over about 90C) are detrimental to the epoxies and degrades strengths by a lot (seen 40% - depends on study, which depends on exact epoxies and carbon composition/layup). How-ever 3 feet from a water based radiator the frame would be considered to be room temperature.
    – mattnz
    Commented Feb 10, 2023 at 3:10
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    "You will never truly enjoy the benefits of carbon if you go as far as living in a cold apartment no knowing how robust Carbon Fibre really is" What is this supposed to mean?
    – SirHawrk
    Commented Feb 10, 2023 at 8:24
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    @mattnz: Most water-based systems (at least here in Germany) run around 60C boiler temp, so even if the frame were to touch the radiator, I wouldn't expect any ill effects, especially since there are no big temperature shocks.
    – arne
    Commented Feb 10, 2023 at 10:46
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    @SirHawrk I came up with a better explanation for that line. “If you’re so worried about damaging your carbon bike that you’re willing to live in a cold apartment to protect it, you’re doing things wrong. You’re never going to fully enjoy the bike if you keep it all protected and out of harms way like that.”
    – MaplePanda
    Commented Feb 10, 2023 at 11:09

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