Should You Change Your Bike’s Tire Pressure? Flying? At Races?
Let's have a look at the conundrum surrounding suggestions of tire pressure alteration in the low pressure environment of modern jet travel or at bike check in locales where the “afternoon sun will get the tires so hot they’ll burst.”
“Sir, you’d better take some air out those tires if you rack your bike early,” is heard frequently at venues like Kona, Hawaii for fear that one’s expensive tires may spontaneously explode. Or, more worrisome, that they may blow up and damage one’s even more expensive race wheels. Also heard at bike shops across the country, the time honored advice that reducing the air in one’s tires will reduce the potential of having them detonate in the reduced atmospheric pressure of that jet en route the your race site.
I was first confronted with this issue at Ironman Hawaii. My local bike shop had taught me their philosophy years before and in my experience I had no reason to doubt them. Inflate tires at purchase and “leave ‘em”, except to perhaps add air from time to time to correct for leakage. These guys raced a lot, all over the country, and in a tradition that works well in triathlon, if it works for them it will work for me. But here I am 6,000 miles from home, in what is likely the most important athletic event of my life, being advised to change my behavior or suffer dire consequences. Yep, dire consequences. By a volunteer no less. And it seems to make sense. Sort of. What, if anything, should I do?
Well does it make sense? No, not really. If you think back to high school chemistry class you might, if you were paying attention (no, I guess not), or might not remember that there are relationships between the temperature and pressure of a gas explained by Boyle’s and Charles’s laws. Basically they’re proportional. In other words, as temperature rises so does pressure but it’s the amount of change that concerns us. How much does the temperature have to go up before the tire goes kaplooey? We know that bike tires like many other aspects of our lives are regulated and have to meet a standard. That standard is blow off pressure, the force required to blow a tire off it’s rim. If the tire is Japanese made it’s governed by the J.I.S., the Japanese Industrial Standard. If it’s from Germany, the D.I.N., etc. This standard is typically double the recommended maximum inflation pressure. It tells you that the manufacturer will test an adequate number tires, correctly mounted on recommended rims, inflated to double the maximum printed on the tire itself, and the test tires won’t explode. If we can figure out the temperature needed to raise the tire on your bike to that pressure and stay below it, we’re golden.
Here’s where it gets complicated and some of you might want to skim to the next paragraph. Beginning with the equation of the ideal gas law, pV=nRT, where p is the pressure of the gas, V is the volume occupied by the gas, n is the absolute quantity of gas present, R is the universal gas constant, and T is the temperature of the gas, if we constrain the system of interest to have a constant n and V, the ideal law can be redefined and we are able to compare the impact of a change in temperature on pressure. I’ll be happy to share the calculations but suffice it to say that if we assume a 70 degree day when we rack the bike, and tires set at 110 psi, we calculate the temperature needed to bring the tire to 220 psi, the blow off rating. Still awake?
The answer is…..536.993 degrees Fahrenheit*, an unreachable result. That value is, of course, above the auto-ignition points of both gasoline and paper, and there are greater issues at hand if it is reached! Or, as the Chemistry Phd. I pestered for the math help here pointed out, “based on these values, it is unlikely the required temperature will be reached under the conditions of ambient sunlight.” An understatement if there ever were one.
There might be compelling reasons not to mess with your tires at the race, however. The relationship between a wheel, tube and tire is a complex one. I’ve been in T1 more than 30 times for Iron distance races and the biggest thing people fool with is their tires. Some folks know a lot less about their equipment than others and when there’s a misstep in this process they’re lost. We’ve all seen more than one racer come unglued in this situation. If the competitor has relatively new tires, has broken them in at home and is satisfied with their performance, does deflating/inflating them change their relationship with the wheel? Does this action allow even the smallest portion of the tube to get stuck between the tire bead and wheel leading to a predictable outcome once re-pressurized? According to Scott Paisley, owner of Blue Wheel Bike and Virginia Masters State Champion, regardless of flying or environmental temps, the only reason to ever reduce tire pressure is when packing the bike for shipment, “And it won’t fit into the box.”
So, when you say “Well I got my information from the Internet, it must be right” or you see the viral Jeff Gordon video on line (http://bit.ly/1bP0d1X) and wonder if it’s real – it’s not by the way – and it tells you that you need to be conscious of the late day sun cooking your tires into an explosion, it doesn’t hold air.