Safety and Education Committee Report
The Correct Tire Pressure
The amount of air in one’s tires can greatly affect the performance, handling, and safety of their vehicle. I can’t tell you what your optimal personal tire pressure should be. Many factors must be considered when determining the correct pressure; changing any one of them will affect all the others. The tire manufacturer has an inflation pressure listed on the sidewall of its product. This is a compromise aimed to best meet the average expected conditions. Safety is usually the primary focus so the other factors are often compromised. If you choose to experiment with tire pressure; things could get “interesting” so use caution.
Rolling Resistance: Research has shown that a 10% sidewall deflection yields the lowest rolling resistance under most conditions. Excessive sidewall deflection requires more energy and too little causes vertical displacement of the vehicle. It is possible to have a larger (and softer) tire with lower rolling resistance.
Traction: To maintain contact with the road; the tire must be flexible enough to conform to the surface irregularities. Lowering the pressure increases the contact area (and rolling resistance) while raising it will decrease it.
Handling: Sidewall deflection can allow the tire to “wallow”. A large soft tire will allow the contact area to move relative to the rim causing imprecise steering. This is why “hot rods” use low profile tires. Hard tires will “skip” over irregularities and allow the vehicle to slide around at speed. Generally the very high pressure (140+) tires have very flexible and strong casings (Spectra?) coupled with very soft rubber.
Temperature: Cold tires are more difficult to flex and the rubber is less compliant. Changes in temperature can dramatically affect pressure. Winter roads tend to be rougher. Most people choose to use larger (softer) tires in the winter.
Reliability: Tire problems are usually either pinch flats or cuts. Pinch flats occur when the tire compresses enough for the rim to make solid contact with the ground. If pinch flats occur at the recommended pressure a larger tire may be needed. A hard tire is easier to cut; if you are exceeding recommended pressures you may experience more cuts. Hard tires have less contact area and the higher specific loading will cause the tire to wear out quicker. Soft tires can “squirm” around and this too will increase the wear rate.
No matter what pressure you choose to use it is critical to check it on a regular basis. All tires leak; the higher the pressure the faster it leaks. Air molecules migrate through the rubber so thin tubes lose air faster. Small tires hold less air so the percentage drop is greater for the same amount of air lost. I must pump my 700x23 (120 PSI) tires before every ride but I usually pump my 26x1.25” (80 PSI) tires about once a week.
“You can pump your tires before a ride or sometime during it.”
Most people prefer doing it beforehand.
Bill Fisk- Safety and Education
Safety and Education Coordinator
Archive Safety Articles
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Stopping in the Roadway
Encountering Animals
Why I Commute by Bike
Save Time Commuting by Bike
Your Bike and Knee
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Wrong Way Cyclists
Choosing the Right Tire
Don't Scare the Slow Guy
Using Your Front Brake
How Safe is Bicycling?
How Safe is Your Helmet?
Making Left Turns
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Impaired Drivers
Take Your Body to its Limits
HB-1203
Protecting your Bike
Helmet Mounted Headlights