• Finding the Right Tire Pressure

    Often, the question comes up, “what is the correct tire pressure for my car”. Aside from the rule of thumb of 1psi per 100lbs, this is a recent article from GoodGuys Gazette that helps with the question.

    The correct tire pressure will maximize traction while minimizing adverse wear. Changing that tire pressure can also be used to manipulate traction in order to alter the handling characteristics of a car. How can you know HOW the tire pressure is actually affecting those characteristics?

    Racers have used pyrometers for years to check tire temperatures. That’s really just a contact thermometer which can be found at race car shops and race supply catalogs, but can be fairly expensive. A modern alternative is the non-contact infra-red technology sensors gun. These have come down to the under $50 category and are an invaluable tool for temperature monitoring. We have discussed their use to diagnose engine cooling system and airflow issues, and now we find they are reallyhandy for tire temp diagnosis as well. The disadvantage of the laser thermometer is it reads surface temperature, which changes quickly, where a probe thermopile measures the internal tread temperature, which is much more accurate.

    Heat is one of the main enemies of a tire's useful lifetime. You may remember some problems experienced in the first decade of the year 2000 period. Large, heavy SUVs in particular were blowing tires with alarming regularity. The cause was determined to be overheating. It seems that drivers were expecting a softer ride than a truck based chassis with modern low profile tires could really deliver. In an effort to get better ride quality, the trend was to reduce the tire pressure so that the tire had more compliance, thus providing a better ride. The problem then occurred that the lower pressure caused the tire to run hotter, to the point where the integrity of the internal cord carcass was failing and blowing the tire. Those of us with experience with large trucks know well how critical tire pressure can be.

    What we need to find is a good compromise between ride quality, handling, and longevity. The result of the tire failures just discussed seems to be that tire dealers will recommend running the tire at the maximum load pressure listed on the tire sidewall. The difficulty with that idea is that different vehicle weights and individual tire loading will change the optimal pressure for that tire on that car. What we want is to find equal temperatures across the tire from inboard to outboard. That indicates that all of the tread is being used equally as a result of the correct pressure being used for the specific application.

    We can use the infra-red heat gun to see how the tire temps are changing as the tire pressure is varied. Again, we want equal temps across the tire tread. Since many hot rods use a tire that is wide for the hot rod stance we love, it may need much less pressure to properly load the tire tread on a hot rod that may weigh hundreds of pounds less than the 17 foot long crew cab truck that might use the same tire size. Similarly, a small front tire may have a widely different load on your hot rod than on the small car the tire designer had in mind. Once you achieve equal temps across the tire tread, you have found the ideal pressure for that tire on that corner of that car.

    Now we can become more clever by manipulating changing tire temps. As alignment settings peculiar to a high performance rather than road use, such as negative camber and toe out, are used, the effect of those settings can be monitored by watching the change in tire temps. Negative camber will allow the tire to do more work in a 'high performance setting by creating higher stress on the inboard edge of the tire tread. Tire wear will suffer, but that is the price of performance. In a similar fashion, going from normal toe in to a toe out setting will show up as greater heat in the inboard edge of the tire as it drags across the pavement. The effect will also be a tendency to "pull" the chassis harder into a turn. The hard part, and the "art" of it all, is to watch how much the tire pressure changes in order to find the right balance of how MUCH negative camber or toe out you want to gain performance before the tire overheats. Too much heat will still cause excessive wear and shorten tread life. Finding that balance will be affected by the track conditions, air temps, etc., etc. That is why you hear the NASCAR crew chiefs and drivers on TV talking about changing a tire pressure as little as plus or minus 1 psi.

    The next issue to tackle is tire growth due to temperature change. You hear comments about bringing tires to temp before running. You see the racecars swerving back and forth to heat tires. This is necessary for performance tires but detrimental to street tires. But heat will affect the street tire, causing it to grow in size. One way to deal with this is to use nitrogen rather than air as the inflation material. Air carries water vapor with it and heat causes that water vapor to expand and increase pressure. At temperature, the tire pressure may increase 5-10psi. A way to deal with this is to fill the tire with pure nitrogen. That eliminates most all water and the thermal expansion of nitrogen is negligible so the tire size (and tread contact area) remains relatively constant once the tire expands due to normal heat.

    Nitrogen can be obtained at almost any tire shop. Locally, Plaza Tire will purge and fill four tires in a vehicle for approximately $20. You will need to top off any pressure loss with nitrogen to keep your purity level, but that is easily controlled by just going back to the shop to use their dedicated fill hose. Those who are control freaks may wish to invest in a nitrogen bottle for their own supply. You can purchase a bottle of nitrogen from any welding supply house. Another method is to purchase an oxygen welding regulator and oxygen bottle from Harbor Freight. Change the regulator output fitting from that used for oxygen to one for nitrogen and then add your fill hose. That program will run you about $150 but solves the issue of where can I top off my tire that has lost some fill. Another benefit of the nitrogen fill is tire pressures remain much more constant. The nitrogen molecules don’t leak thru the rubber like oxygen does. One of the reasons you see car dealers filling cars with nitrogen is it makes pressure regulation more constant and cuts down on the call backs for a TPMS (tire pressure management system) fault due to low pressure.

    So, the bottom line – how much pressure should I run? I found that 22 was best for my car. I started with 24. To a purist, maybe he could tell the difference between 2psi. I think it is just one more thing to be anal about – especially as I drive to the local ice cream parlor on city streets.

    Need a source for a laser gauge?

    Harbor Freight #93984 or Northern Tools #38993 or #44445 And if you really want to do it right, LongAcre has a number of different pyrometers, starting at $109.95

    If you attempt the settings, make sure you take your temperatures after a normal drive along a straight path. Turning will elevate temperatures and give you a false reading. Once the tires are warmed up (10-15 miles) drive straight for a mile and pull off and measure across the tread. Be quick, the tires will cool within just a few minutes and you want the see the differences across the tread.