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The How's and Why's of Proper Tire Inflation

By: Evans Brasfield

Tire pressure plays an important role on your motorcycle. First, the tire is an important part of the suspension. Air pressure can stiffen or soften the ride qualities in undesirable ways. However, the most important effect tire pressure has is controlling the size of the contact patch and, by extension, the life of the tire.

As a tire rolls down the road, it goes from being almost perfectly round to flat as it engages the tarmac. This flat oval of rubber constitutes the contact patch. To form this flat spot that moves around the tire with each revolution, the carcass actually bends in two directions. It begins by bowing out slightly before it bends back on itself under the force of the road. This bending back and forth creates heat inside the tire. Tire manufacturers use this heat build-up to their advantage by designing rubber compounds that reach their peak effectiveness at a particular temperature for the application for which the tire was designed-be it canyon carving, commuting, or touring.

You may have heard stories about all the tuning of tire pressure done at the track. This type of tuning plays a less important role with today's radials than it did back when bias-ply tires were the norm. Still, tire manufacturers do frequently recommend lower pressures on the track to create a larger contact patch and therefore more grip in the corners. However, they should never be so low that they allow the carcass to deform and cause handling problems. Also, if the pressure is too low, the tires could overheat and have the extra traction being sought slip away. Trackside tire vendors constantly watch what tires are winning and what pressures are working the best so that they can recommend the best options and keep their customers happy. They know what they're talking about.

Street riders need to remember that racers are willing to sacrifice straight line stability for increased grip. The truth about street riding is that street bikes spend the majority of their time straight up. If you run your tire pressures too low, you can easily overheat your tires simply riding in a straight line. Remember, the lower pressure is to ensure a large contact patch, which is created by the carcass flex. Take your bike out on an extended interstate ride with too little air, and all that flexing of the tire can cook the life right out of it.

Tire manufacturers spend a lot of time determining what pressures will provide the best compromise of performance and tire wear. Just look at the wide range of tires available-each designed to fit a certain performance/wear niche. You can buy tires for cruisers, dual sports, and touring rigs. Sportbikes even have different tires designed for street use, track day/street use, and track only. Honor these distinctions, and your tires will last longer and perform better.

Some manufacturers recommend running the same pressures listed in the owner's manual for the bike's OE tires, but a significant number have proprietary pressures that should be run on particular tire/bike combinations. Be sure to ask your dealer or check the tire manufacturer's product literature or web site for specific numbers-they do change as information is gathered about current-model bikes.

No matter what kind of bike you own, the key to maintaining proper tire pressure is making sure you have an accurate tire gauge. In the past, people said that the only truly accurate kind of tire gauge was one of those big ones with a needle that sweeps across a dial. Although many of those may leave the factory reading accurately, they are very susceptible to knocks and bangs and should be checked for accuracy frequently. Some people are strangely distrustful of digital gauges. While (as with almost anything) those with rock bottom prices may be of a questionable build quality, you don't have to spend a ton of money to find a reliable gauge. Since they have no moving parts, digital are also less likely to be knocked out of calibration through regular use.

The best digital gauges are constructed with full bridge strain-gauge technology. This is in the pressure die assembly where the valve stem and the gauge contact each other and the physical reading/electronic measurement of the pressure is made. Naturally, this kind of construction is more expensive to manufacture, which is why many "el cheapo" units only offer half or quarter bridge strain-gauges. Full bridge gauges deliver consistent readings that in a variety of weather conditions-including different temperatures, humidity levels and altitudes-and also deliver highly accurate measurements over a wider range of pressures than those lesser units.

Since representatives for tire companies stress that between 75 and 80 percent of the tire warranty claims are caused by under-inflation and when the cost of today's premium rubber is considered, investing in a good tire gauge-and using it religiously-is cheap insurance.

Evans Brasfield: Prior to setting out on his freelance career, Mr. Brasfield was Feature Editor at both Motorcycle Cruiser and Sport Rider magazines. In pursuit of his passion for two wheels, he has ridden everywhere he can (including all the way up to the Arctic Ocean) and finds pleasure out of riding in the rain. He club-raced for five years and has participated in two WERA 24 hour endurance events. Track riding is at the top of his list of favorite activities.

Evans
books include 101 Sportbike Performance Projects and How to Modify your Metric Cruiser.

If you would like to find out more about Evans, check out his website at evansbrasfield.com.

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