Starting with the last question first, YES, you should, ESPECIALLY if you are a motorcycle rider. And circling back to the first question, YES, unless you have been unusually diligent, you probably have SID. What is SID? Seasonal Inflation Disorder... i.e. under-inflated tires. Why "seasonal"? Because they lose pressure when the weather cools off. The change of seasons from summer heat to winter cold results in a massive wave of under-inflated vehicle tires. Every air-filled tire, whether on a motorcycle, car, truck, or wheelbarrow suffers inflation pressure loss when it cools down. Why? Because they leak when cooled? No. Because the air in them shrinks...or at least a container holding a fixed amount of air at a specific pressure would need to shrink in volume to maintain the same pressure as the air cools. So that's it. It's not your fault. It's not a design flaw. It's just physics. The pressure of the air trapped inside a container with a fixed volume goes up and down with temperature (higher when temperature rises and lower when the temperature of the air cools). Physicists have even worked out an equation that allows us to predict exactly how much the pressure will change as the temperature changes. There are other minor influences, such as changes in the Relative Humidity of the air and the inescapable fact that the container, even if made of solid steel, will certainly change size a little. But for the purposes of this discussion, the impact of those minor influences are small enough that we can forget about them and just consider air temperature and pressure. Let's consider a tire on a vehicle with a placard that specifies a "cold" inflation pressure of 42 psi (42 pounds per square inch) meaning, if that air is trapped behind a frictionless piston whose top is 1 inch by 1 inch, then it could hold up a 42 pounds weight against the pull of earth's gravity, without moving up or down. If the area of the piston cross-section were doubled to 2 square inches, the same pressure could hold an 84 pound weight, and a still larger piston, even more; which begins to explain why tires inflated with only tens of psi of pressure can hold up vehicles that weigh thousands of pounds...but I digress. The more interesting part of the inflation spec is the word "cold". In this context "cold" is shorthand for "parked at least 3 - hours" or something like that. Tires heat up when they roll, so the cold inflation spec refers to a tire that has not been rolled for a while, and is at the moment, internally at about the same temperature as the air around it (the ambient air temperature). It does not matter if the ambient air temperature is -20 F or 120 F. "Cold" just means "not heated up from use". So now you are rightfully thinking "Well then, how could weather changes possibly matter?!!". It is simply because the spec PRESUMES tire pressure will be checked regularly. Ideally, every day. Practically, every two weeks, at least...more if often if your tires seem to have a slow seeping leak, or you are about to embark on a long ride. The idea is that pressure being a little out of spec for a few miles isn't terrible...usually. But tire pressure being out of spec for lots of miles, or being badly out of spec for a few miles, IS bad, is probably dangerous, and particularly on a motorcycle, can be catastrophic. The ideal solution is a TPMS (tire pressure monitoring system). They are incredibly inexpensive and brain-dead simple to install on vehicles without any tools or wiring. Expect to spend less than $75 (perhaps much less) to get a continuous display of each tire's pressure and temperature AND the ability to set alarms for under/over pressure and/or temperature conditions. Getting a warning BEFORE a tire blows or rolls off a wheel due to under-inflation (perhaps you caught a nail 2 miles back while cruising down the highway at 70 mph) may be a life saver. If you see tire pressure going down and tire temperature going up at the same time, it is time to park...immediately. So back to SID. How much pressure loss are we talking about? Check this... Let's assume the last time this tire was inflated to 42 psi it was a bright summer morning and the air in the tire was all at 85 F. Those of us living in Texas are likely to recognize that as a conservative figure. However, this beautiful December day you are about to head out into a crisp 40 F day. Let's be generous and assume your tire has been perfectly air-tight and has not lost a molecule of the air you put in last summer, and that pressure changes have not changed tire volume and that we can neglect humidity changes.. What is your tire pressure today? 38.5 psi...and that is the best case scenario. And it's probably lower. What if you had filled those tires on a 100 F afternoon? Now they are at 37.5 psi. Granted, those numbers are not terrible, but a 4 to 6 psi drop is significant. It can take thousands of miles off your tire life. Bridgestone addresses the question this way...
Q: What if inflation pressure is too low or too high? A: Pressure that is too low can lead to internal structural damage of the tire. Pressure that is too high can allow for more external damage to occur to the tire. Both of which could lead to tire failure, a crash, serious injury or death. Under-inflation causes excessive heat build-up due to excessive flexing of the tire, and eventually causes internal structural damage to the tire components. Over-inflation makes it more likely for tires to be cut, punctured, or broken by sudden impact. These situations can cause a tire failure which could lead to a crash and serious personal injury or death. In addition to tire damage, improper inflation pressure may also adversely affect the ride and handling characteristics of the motorcycle. Under-inflation may cause the motorcycle to handle less responsively and to feel “sluggish.” Over-inflation makes your ride harsher and may cause changes in handling predictability. Both under-inflation and over-inflation can lead to rapid and/or uneven wear since the tire contact patch is not optimal. For instance, too little pressure can lead to rapid wear of the tread shoulders, while too much pressure can lead to rapid wear of the center. Follow the motorcycle manufacturer’s specifications for proper tire inflation pressure. Check the frame placard and/or the owners manual for that information. Still, given all this, why do almost all of the bikes that come into my shop have under-inflated tires, often in the range of 15 psi to 25 psi? Sure, part of the problem is often SID, but the degree of under-inflation I usually see can only be chalked up to plain old neglect. And as Bridgestone points out, it is dangerous. So if you start with a neglected tire and then throw SID on top, you are looking at real trouble. So let's be honest. Most motorcycle tires DO leak a little. One of the big reasons is the last guy to mount the tire did a poor job of cleaning the bead seating areas of the wheel. A schmutzy wheel is a leaky wheel. Those leaks are usually very slow, but after a few months and a few degrees of temperature drop, the rider can be in real danger. So...for all of the above reasons, if you do not conduct any other motorcycle maintenance task, keep your tire pressures up to spec. It is even more important than oil changes. Old oil will kill your engine. Low tires will kill you. Just a matter of priorities. How?
If you do get a TPMS, or check your tire pressure in the middle of riding on a hot day, do not be alarmed if the pressures you see are really high. On a hot day, sunny pavement may be at 120 F or more and the tires you are rolling over it may go to well over 150 F. So even if they started the morning at 70 F and 42 psi, you will see 48 psi or more. AND DO NOT PANIC AND LOWER THE PRESSURE IN HOT TIRES to meet the Cold Inflation Pressure spec. Remember, that is the cold spec. The manufacturers expect your tire pressures to go up as you ride. It is not a problem. Ride safe. Comments are closed.
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