There seems to be a widely held belief that, when using rim brakes on long descents, the heat from the braking can heat the air inside the tire, increasing the pressure to the point of blowing the tire off the rim. As we will see below, this explanation for how such blowouts occur is very implausible; and the advice of releasing air from your tires prior to a descent actually increases the risk of bursting a tire.
Temperature and Pressure are proportional, right?
First, we will estimate how much pressure can we inflate our tire up to. After all, tires don’t blow off the rim if we inflate it 1 PSI over the maximum recommended pressure. There is a safety margin. Exactly how much over-inflation is tolerable could vary from one manufacturer to another, but a good rule of thumb is about double the maximum pressure on the tire.
This is where the misconception begins. Many people are familiar with the fact that when a gas is heated, it increases in pressure proportional to the increase in temperature. Some people make the mistake of thinking that, starting at 25 °C, heating the gas to 50 °C will double the pressure…
But then, what if we used Fahrenheit instead of Celsius? 25 °C is 77 °F. Doubling it gives 154 °F. That converts to 68 °C, which is quite clearly not the same as our previous result when using the Celsius scale. Does that mean that the Americans are able to overheat their rims more than us, simply by virtue of measuring temperatures in Fahrenheit rather than Celsius? Would we be able to instantly gain such an improvement for ourselves by converting to the Fahrenheit scale?
Of course, the answer to both is no. Because the temperature variable used in the gas equations is neither in units of °C or °F, but rather it is the gas’ absolute temperature; which is measured in kelvins (https://en.wikipedia.org/wiki/Gay-Lussac’s_law). So, back to where we were, 25 °C converts to 298 K. In order to double the pressure in the tire, the air would thus need to be heated to 596 K, which is a staggering 323 °C. To put in perspective just how hot this is, bear in mind that the melting point of lead is just a shade hotter at 327 °C.
Fun fact, If you don’t inflate your tires to the manufacturer’s maximum pressure rating, then the heat required to reach double that maximum rating is even greater. In order to increase the pressure by a factor of 4 (typical for tires inflated to half the maximum rated pressure) the air inside would need to reach 919 °C, which happens to be well above the melting point of aluminium (660 °C).
So, what really causes blowouts when rims are overheated?
The tire may fail due to overheating (not over-pressure). Although vulcanised rubber used in tires doesn’t melt, when it gets hot it does become softer, and loses strength. Aluminium is a very good conductor of heat, so will heat up the tire bead and also the inner tube (where it isn’t separated by the rim tape). Considering this would start to happen at between 150 and 200 °C, the heat would increase the pressure to an extent, but nowhere near enough to burst the tire alone. It is the degradation of the rubber due to heat that plays the most significant role.
I asked John Harland for his thoughts on this (because he has much more experience than I do), and he points out that overheating and over-pressure failures are also distinctly different in appearance. Over-pressure failures tend to happen along the center line of the tire, and may not be a catastrophic blowout: sometimes it can be a bulge where the plies have come unstuck, but haven’t torn. If this happens, you can perform a temporary fix by deflating the tire and wrapping some fabric around the section of tube that will sit beneath the bulge.
Overheat failures in contrast are almost always along the rim line, or sometimes the valve.
John also pointed out that another factor to consider is hysteresis. This is a property of rubber that when it flexes, it absorbs a certain amount of energy, which heats the rubber. As a wheel rotates, the section of tire in contact with the ground flexes, and as a result, there’s a continuous (but usually small) flow of heat into the tire. If the tire is under-inflated (for the load it’s bearing), it will flex more at the ground, which means hysteresis contributes more heat. When this is combined with braking heat, reducing the pressure in your tires at the top of a long descent may in fact make it more likely that your tires will fail due to overheating.
John had a direct experience of this when touring on a tandem. They were bursting tires (inflated to the recommended 85 psi) when the rims got hot, so he tried reducing the tire pressure. After that, far from solving the problem, they started having failures even more frequently! John had used 100 psi rated tires prior to that, and figured that the 85 psi rated tires should be able to tolerate 100 psi, so he tried over-inflating them. After that, they had no further problems.
Should you worry?
This would be a concern on descents that are fairly steep, and go on for several kilometers.
Thorn cycles have an interesting article on the topic here (https://www.thorncycles.co.uk/tandem-braking). It’s targeted more at tandems, but is still somewhat relevant for touring bikes.
Of course, using disc brakes does “solve” the problem in that the braking heat isn’t going to overheat the tires. However, disc brakes have their own set of problems when they overheat, primarily brake fade.