The problem of under inflated tires has become an issue big enough for the government to require many new vehicles to have tire pressure monitoring systems. Under inflated tires are inefficient and can lead to traumatic tire failure. This is why the Les Schwab tire store leaving me with tires below pressures specified on the vehicle driver door placard and below the tire manufacturer’s recommendations for the load on the tire were a concern.
Since tire pressure is to be determined when the air in the tire is at ambient air temperatures, taking the vehicle down to the tire store to get pressures checked is not usually a good idea. A tire that has been run a few miles or has had tires sitting in the sun should be considered a tire in use. About all you can do with tires that are in use is to realize that a hot tire is telling you it needs more air. Never let air out of a tire except when you can measure the pressure when you are sure the tire and the air inside it are at the same temperature as the air temperature in the shade. Manufacturers take into account diurnal temperature changes and other factors when they make recommendations for tire pressure so you should only adjust pressures with the tires at at ambient conditions and not service conditions.
On RV tire safety, Toyo says:
“The amount of air pressure in a tire determines the load that can be carried safely. Every Toyo tire will have the maximum load and inflation molded into the sidewall of the tire. This load and inflation ratio should never be exceeded. Tires for RV applications are subject to a variety of severe conditions when compared to automobiles or trucks. Underinflation of a tire can cause poor handling, irregular wear, and decreased fuel economy. It also causes extreme heat build-up within the components of the tire which can lead to failure. “
“Toyo Tire does not recommend an “inflate-to-the-load” policy for RV tires. Tires that are inflated to accommodate the vehicle’s actual loads do not have any inflation safety margin. Consequently, even a minor loss of air pressure will cause the tires to be under-inflated and overloaded. Toyo Tire’s policy is to observe (as a minimum) the tire pressure established by the vehicle manufacturer as indicated on the tire information placard.”
The base issue is heat build up. A tire has a foot print, a flat spot on the pavement, whose size depends upon the pressure in the tire and the load on the tire. As the tire rotates, there is flex in the tire as the flat spot moves to a different spot on the tire. That flexing of the tire causes internal friction and that friction causes heat. With additional pressure or less load on the tire, the flat spot is smaller and the flexing, hence the heating, is reduced. That is one reason why it is a good idea to check tire and hub temperatures when you stop during travels. There are two issues in heat monitoring to consider. One is how to measure tire temperatures and the other is how hot is too hot. Most RV tire web sites don’t really say anything on this. Dieter Motor Sports has this:
The best indicator of correct tire pressure is tire temperature. The air pressure acts like three springs inside the tire holding the car up. Overinflation causes the center to push too hard, creating excessive heat. Too little pressure causes insufficient push and reduced heat in the center. Our goal is to have the entire width of the tire share the workload evenly. If one section of the tire is overworked it will overheat and wear prematurely. Tire temperatures will never be perfectly even across the tire. But if a linear reading within 10 degrees can be achieved, far greater tire wear and performance will result.
Even more specific is a racing cars website.
Using an infrared thermometer, you will get a surface reading that may be 10-40 degrees cooler than temperatures taken with a probe type thermocouple.
Three readings are taken on each tire: inner tread, center tread and outer tread. Inner and outer readings are taken one inch from the tread shoulder. Write down the readings for evaluation. Check with your tire manufacturer to find the recommended operating temperature for your tires. A typical operating range for a DOT-R tire is 180 ˚F to 200 ˚F with a hot pressure of 37 to 43 psi. You want to see no more than 20 degrees difference in temperatures across the tread, with the inside being slightly hotter than the outside.
Another website with guidelines for a hot tire is the Bridgestone page on airplane tires.
A “Hot Tire” is defined to be a tire heated by aircraft operation (Braking) or by exposure to sunlight and having a surface temperatrure higher than ambient temperature by at least 30°C. A “Hot Tire” may also result from operating with an abnormally large tire deflection.
Tires are not recommended for further use when tire surface temperature exceeds 225°F (107°C), or when brake heat creates temperatures that exceed 300°F (149°C) at points where the tire is in contact with the wheel surface.
Tire wear isn’t usually an issue with RV tires. A bit of over-inflation is better than trying to get an ‘exactly right’ pressure according to some chart or whatnot. Trailer tires, especially, should be inflated to the max sidewall indicated pressure. Extra wear near the center of the tread from an inflation margin of safety is likely to be minimal over the life of the tire in most RV use.
A stop for a walk-around every couple of hours is not only good for avoiding DVT and muscle issues but also a good time for a quick scan of tires and hubs with an IR thermometer. You don’t need to get as specific as in the racing car website but you should look for anomalies – one tire significantly different from the others – and tires that are quite a bit warmer than air temperatures.
A good TPMS or tire pressure monitoring system will help you avoid problems. It will warn you before you start if any of your tires need air. On the road, you will get early warning of tire failures so you can pull off the road and minimize damage from a flat tire. Monitoring the TPMS readout will familiarize you with how your tires normally respond in service as they warm up and their pressure increases.
Start your RV maintenance and safety procedures with proper tire inflation.