Tire Pressure Basics Part One – Cold Inflation Pressure

Talk to any riding instructor or track coach and you’ll hear an alarming number of stories of riders who don’t know what their motorcycle’s tire pressures should be.  Perhaps just as alarming is of the riders who do at least know how much air to put into their tires, they fail to understand that this recommendation is based upon a cold inflation pressure.  That is, the manufacturer’s recommendation is based upon a pressure while the tire is cold and before you’ve ridden.  You can check out the video we’ve created below, or read on.

If you remember back to basic science at school you should know that when gas is heated it expands.  It just so happens that air is made up of a bunch of different gasses, so as your tires get warmer, the air inside them expands.  Because there’s nowhere for this air to escape from the tire, pressure increases.

A basic rule of thumb is that for every 10 degree change in temperature, air pressure changes by 1 pound per square inch. Now imagine that you only ever had to put air in your tires once – it never leaked over time.  If you filled your tires to 32 psi in the dead of winter, come summer your tire would magically have an extra 4 pound of air inside.

Similarly, if you put air in your tires before a ride your tires pressure will increase quite dramatically.  Even in winter it’s not hard for a tire to increase in temperature by around 90 °F just from normal street riding.  This can be even more dramatic in the heat of summer when roads are particularly hot or if you’re really punishing the tires at the track.

Cold Inflation Pressure Temperature and Pressure Change

Reading this, you may see a problem arise for a motorcycle rider and perhaps you’ve been in the same situation before.  You’re in the mountains and have been riding for a good few hours.  As you pull into the gas station to fill the tank, you decide you’d better check tire pressures because it’s been a few weeks since you last did it.  Connecting the pressure gauge, you’re surprised to see that it’s reading 45 psi in the rear tire, when you know it’s only should be 36.  You then deflate the tires down to 36 psi and continue your ride.

Once this rider has gotten home, his tires cool down.  If that rider then checks his tires before leaving the house for his next ride, he will find them to be grossly under inflated.  That’s because when he deflated his tires during a ride, it was at a hot inflation pressure.   His pressures when he checked at the gas station were actually correct – it was merely showing 46 psi because the original 36 pounds per square inch of air had expanded during the ride.

Letting pressure out meant he was actually riding under inflated tires.  It is for this reason it’s essential to check and adjust tire pressures before a ride – a cold inflation pressure.

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