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Commercial Tires

Hot & Cold: Ambient Air Temps Have Great Impact on ‘Correct’ Tire Pressure

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cause any flexing object will generate heat, proper inflation pressure also enables the tire to reach an equilibrium temperature where the amount of heat created is equal to the amount of heat dissipated through the sidewalls, shoulder and tread area.
Even though the tire industry has long reminded fleets and tire dealers about the importance of proper tire inflation, underinflation remains the most common cause of tire failures and early removal mileage. Fleets may well see the need for a proper inflation maintenance program, but many aren’t putting forth the effort or dollars needed to maintain their tire investment.
  Too often – and it seems to be growing more prevalent today ®“ “conscious” trucking fleets seem all too willing to trade dollars, figuring the cost of replacing tires that meet an early grave is less expensive than making sure its tires deliver maximum original and retreaded service.

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The Root of All Evil

   Incorrect inflation pressure alone usually leads to major short- and long-term problems. Inadequate inflation changes a tire’s footprint and causes excessive flexing in the sidewall areas, which creates additional heat that remains part of the “heat history” for that tire.
   As an underinflated in-service tire continues its pattern of excessive heating and cooling, the components of the tire break down and failure is just a matter of time. Since tires naturally lose anywhere from 1 to 2 psi per month anyway, the impact of a poor inflation maintenance program is compounded by other problems like slow leaks, bad valve cores and overloading.
   Some fleets and many fleet drivers and owner-operators still think tire thumpers and shoe leather (kicking the tire) are the most scientific and reliable means to measure air pressure. Unfortunately, boots and bats cannot be calibrated to determine actual air pressure. And inflate-through valve caps, dual tire inflation systems and high quality inflation gauges are a waste of money if they are not used on a regular basis.
When you look at the issue of proper inflation, it appears to be a fleet maintenance problem that cannot be solved by tire dealers. Given the current state of fleet tire inflation maintenance, what steps can a tire dealer take to help solve this problem?

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Defining Cold Inflation

Before we discuss tire inflation, a quick look at the words “cold inflation pressure” is necessary. By definition, cold inflation pressure represents the measurement of psi at the prevailing atmospheric temperature.
However, many manufacturer data books state that every 10ÞF change in temperature represents approximately a 2 psi change in the tire’s inflation pressure. What does this mean for technicians inflating tires during February in Michigan or in August in Arizona?
Since the cold inflation pressure is basically recorded at around 60ÞF, a tire registering approximately 88 psi at 0F will increase to 100 psi once the ambient temperature reaches 60F.

Other Influences

Operating the tire also generates heat and causes the inflation pressure to increase as the temperature of the tire rises. If a tire registering 88 psi at 0ÞF is inflated to 100 psi at the same temperature, it will become overinflated as the tire’s temperature rises.
On the other hand, let’s look at a tire with an inflation pressure of 110 psi when the outdoor temperature is 100ÞF. If that vehicle travels north into cooler climates, it will show an inflation pressure of around 100 psi if the tire cools to around 60F overnight.
Altitude impacts cold inflation pressures by changing 0.48 psi for every 1,000 feet of elevation change. A tire registering 100 psi on the coast of California at sea level will read 102 psi when it reaches 5,000 feet above sea level.

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How to Handle

So what does this important information mean to commercial tire service technicians?
®′ Technicians should never reduce the inflation of a hot tire – one that has just come off the road ®“ by bleeding it down to the recommended cold inflation level. A recently parked tire will usually show a build up of 10 to 20 psi as the direct result of normal operation on the road. Once the tire cools to the prevailing atmospheric temperature, the correct cold inflation pressure can be reached.
®′ A hot tire showing less than the recommended cold inflation pressure is underinflated and should be inflated to the recommended cold inflation pressure plus an additional 10 psi. After the tire cools, the correct inflation pressure can then be established.
®′ As far as calculating the exact cold inflation pressure with respect to prevailing ambient temperature, technicians are advised to inflate tires to the recommended level when the tire is at the same temperature as the atmosphere. Granted, a few tires may be slightly overinflated when they register 100 psi at 0ÞF, but damage to the tire becomes most severe when it operates underinflated at less than 80% of the recommended cold inflation pressure.
®′ Changes in altitude have minimal effect on inflation pressures, so there are few differences between tires inflated at sea level in San Francisco or a mile high in Denver.

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Gauge the Gauges

Technicians can provide customers with the best start for proper inflation maintenance by checking their air gauges against a master gauge while using them to achieve the exact recommended cold inflation pressure. Fleets complain about the lack of consistency in air pressures, often because technicians feel 97 or 98 psi is “close enough” to the needed 100 psi.
Quality control is obviously lacking when 16 tires with a recommended cold inflation pressure of 100 psi have a difference of 2 to 3 psi either way.
Finally, recommended cold inflation pressure stamped on the sidewall of a tire represents the maximum load and inflation for that tire. A trailer hauling corn chips on 11R22.5 load range G tires will probably have less than 23,360 pounds of load on each axle, so tires inflated to 105 psi are definitely overinflated.
Using the load and inflation tables from the 2001 Tire and Rim Association Yearbook – or in the tire manufacturers data book ®“ the technician would find that if the same trailer were carrying 21,000 pounds on an axle, the proper cold inflation pressure is 95 psi.
Of course, if the fleet cannot provide the actual weights on each axle, technicians should inflate tires to the maximum cold inflation pressure stamped on the sidewall.
Tire inspection isn’t as complicated as it seems. Sure it takes some extra time, but the time investment is well worth the cost savings if tires aren’t overworked and overstressed to the point of early failure.
Simply replacing boots or tire thumpers with calibrated air gauges is the first step. Second, technicians should make sure every tire is inflated to the exact pressure using guidelines and procedures outlined by OSHA, the RMA and the tire manufacturer. If these practices are followed, differences caused by ambient temperature and altitude changes will have minimal effect on tire performance or life. ®′

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Kevin Rohlwing is director of training for the International Tire & Rubber Association (ITRA), and this article originally appeared in ITRA’s Commercial Tire Service publication.

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