Temperature/Heat and Tire Pressure Formula

I did the Milkman Triathlon in Dexter , NM and it was quite hot. My tires say that maximum pressure is 120psi. I've heard that tires can expand and blow due to the heat. What should I inflate my tires to if it is really warm? What should I inflate them to if the temperature is moderate? I usually put about 100psi in, but sometimes a little more.

Also, how often should tires be replaced. How do I tell if they are wearing out?

Thanks for any info,
Karen Weatherby
kmweath@msn.com

Dear Karen,

Tires, more specifically the tubes inside the tire, can expand and explode due to temperature rise, so it is important to be aware of how temperatures change during a ride or race. Changes in the pressure of a tire initially inflated to 120psi are relatively small for each 10 degree change in temperature – about 2psi. So, what you really need to watch out for is dramatic changes of 20 degrees or more, which could cause a 5psi change in pressure or more. This type of a change is easier to create than you might think. An example:

You initially inflate your tires to 120psi on a cool morning (or in an air conditioned room or car) that is 65 degrees. You arrive at the transition area and the temperature is 80 degrees, a 15 degree change, and your pressure rises to slightly above 123psi. By the time you get to your bike from the swim, the temperature has gone up to 90 degrees and your tires have been in the direct sun for ½ an hour or longer, raising the effective temperature to 110 degrees and your tire pressure to 130psi. By now, your tire that was initially inflated to 120psi may have exploded. Pavement temperature, road friction, heavy braking and additional temperature rise could drive the pressure up even more and could lead to a dangerous blow out on the course.

A quick ballpark way to figure out what the pressure change will be on a tire is to add approximately 2psi of pressure change to your starting pressure for every 10 degrees of temperature rise. For the technophiles that want to figure the pressure change more exactly, use Guy-Lussac’s Law [at constant volume, pressure is directly proportional to absolute temperature (P1/T1 = P2/T2)]. Absolute temperature is figured by taking the current temperature and adding 460 to it. Using the example in the paragraph above, the solution would look as follows:

Initial Pressure (P1) – 120psi

Initial Temperature (T1) = 65 degrees + 460 = 525 (absolute temperature 1)

Final Temperature (T2) = 110 degrees + 460 = 570 (absolute temperature 2)

Final Pressure (P2) would therefore be found by solving for P2 in the following: 120/525 = P2/570 (0.228571429 = 0.228571429 * 570 = 130.3psi).

You should check your tire pressure before each ride. If you are concerned about blowing a tube or tire because of temperature changes, inflate the tire as close to the ride, and in the same environment as the ride, as possible. For a triathlon, it would be a good plan to let your bike sit in the transition area for awhile to adjust to the ambient temperature and then inflate the tires. If you know that the temperature of the tire is going to rise, because of an actual change in temperature or from heavy braking, etc., subtract out the amount of expected temperature change and do not inflate to more than the recommended max psi minus this amount. In the example above, the maximum you would initially inflate to is 110psi.

What is your optimal tire pressure? There is not simple or direct answer to this as your weight, frame stiffness, tire width and model, and the riding conditions for the day are all influential and important variables. Some general guidelines are as follows:

Lighter riders, especially those riding stiffer frames, will want to run slightly lower tire pressures than a heavier rider – between 100-115psi range with most clincher tires.
Heavier riders will usually benefit from slightly higher tire pressures than lighter riders – often in the 110-130psi range with most clincher tires. Bigger riders may want to also consider riding a slightly wider tire (say a 700x25 instead of a 700x23). This can increase comfort while actually decreasing rolling resistance.
While some tires, especially tubulars, have very high maximum psi ratings, riding a tire pumped to 190psi, will not only create a harsh ride, but often will actually increase rolling resistance and decrease traction. Stay within a reasonable range, regardless of what the tire can handle, and never exceed the maximum psi.
Do not under inflate. Under inflation leads to more flats than over inflation. You would be hard pressed to find a situation where running under 100psi or over 140psi on a performance oriented 23c tire would be beneficial.
If the conditions are wet, consider running a tire pressure about 10psi lower than normal.

How often should you replace your tires? Tires wear out for two primary reasons. The first is road wear and the second is environmental wear. Road wear will vary markedly depending on the tire’s rubber compound and the rider’s weight and riding style. Because there more weight on the back of the bike, rear tires usually wear quicker than front,. Inspect your tires regularly, and if you notice that your tire is developing a long flat section to it, consider replacement. If you see threads from the casing or large slits or holes, replace immediately. Heavier riders usually wear through tires quicker than lighter riders, so choose your brand and model of tires accordingly. Environmental factors also influence tire durability. Ozone, oil, road salt and direct sunlight are all rough on rubber and can lead to cracking and sidewall deterioration. To extend tire life, make sure that you do not store your bike in direct sunlight or too close to ozone producing equipment such as freezers or air conditioning units. If you ride in wet conditions, especially early season in the snow belt, clean your tires with water when you are finished to get rid of any petroleum based products or salts that could dry out or eat your tires.

If in doubt, have a good bike mechanic review your tires periodically.

Have a great season and train hard.

Ian