Tire Pressure – Not alot of hot air
Although many riders spend a great deal of time discussing the virtues of various tire brands and rubber compounds, rarely does the subject of tire pressure enter the debate. Besides performing the essential function of holding a tire on a wheel, tire pressure affects a variety of factors. While most riders know that the wrong pressure can reduce a tire’s life expectancy, the amount of air you carry can also dramatically affect handling and stability.
For optimum performance while heeled over on the track, tire manufacturers usually recommend running pressures lower than those reserved for the street. Why? The lower pressure allows for a larger contact patch and therefore more grip in the corners. How low should the pressures be? They shouldn’t 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 the extra traction being sought will slip away. In the days of bias-ply tires, setting tire pressure was critical for keeping the tread in the correct operating temperature. Although radial tires have minimized some of the temperature fluctuations caused by pressure sensitivity, setting a tire to the proper pressure will pay off dividends measured in lap times.
While the most scientific means of determining if a particular pressure is working for a tire is the use of a pyrometer to assess whether the rubber has reached the manufacturer’s recommended temperature, charting the pressure increase of a tire after track sessions will give a good impression of how hard a tire is working. Dennis Smith of Dunlop’s Sport Tire Services recommends an increase of two to four pounds in front tires and six to eight in the rear. But he adds, like most of the tire representatives we contacted, that club racers should talk to their tire vendor at the track since their knowledge base will negate the need for a lot of trial and error.
Since maximum grip is a good thing, why not just run race pressures on the street? First, according to Michelin’s Claude Leroux, increased cornering grip comes at the expense of stability and feel. Lower pressures decrease straight-line stability, and regardless of how talented the rider is, most street bikes spend a high percentage of their time straight up. A side benefit of using the proper pressure is that the front tire will feel more precise and turn in quicker-a good thing during point-and-shoot sessions. Second, you can easily overheat your tires simply riding in a straight line by running the pressure too low. Remember, the lower pressure is to ensure a large contact patch, which is created by the carcass flex. The same process happens when you’re not cornering. Take your sport bike out on an extended interstate ride with too little air, and all that flexing of the tire’s carcass 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 on the street. While some manufacturers recommend running the same pressures listed in the owner’s manual for the bike’s OE tires, a significant number-such as Metzeler and Pirelli-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 for specific numbers. And check the pressure before every ride! Cory Johnson from Metzeler/Pirelli says between 75 and 80 percent of the tire warranty claims he reviews are caused by underinflation. When the cost of today’s premium rubber is considered, investing in a good tire gauge-and using it religiously-is cheap insurance.
This story was originally published as part of the tire test in the February 2000 issue of Sport Rider.