More on Tire Pressure

Boy oh Boy! My last article on tire pressures definitely raised responses from some of the readers on their interpretation of what I had said. I do not want to redo the entire article but I do want to clear up any misunderstanding so I will reiterate the intention in a different manner, and refrain from using the controversial “minimum” word.

On the sidewall of a passenger car tire, the maximum air pressure that tire is intended to have is moulded right into the rubber along with the word “maximum”. Along with this information is the maximum weight that the tire is designed to handle. And in some cases it even says that the pressure is cold inflation. In other words the pressure you would have before driving. Do not adjust tire pressures after driving or after being in the hot sun. And Ray, do not try to guess how hot a tire will get while driving and therefore put less air in at the cold inflation temperature. Less pressure may be okay but not based on anticipated increases.

In some cases passenger tires are being used on trailers and fifth wheels and in those cases if the actual weight that the tire is carrying is different than what it is rated at on the sidewall, then go to a competent tire shop and see if that tire is designed to run at a lesser pressure (the key words here are competent and lesser). The gentleman from Merritt who wrote me has done exactly that and correctly determined that the car tires he is using on his trailer do not come under my description of an RV tire and indeed are designed to run at different maximum pressures based on the various carrying weights the tire may be subjected to.

Now, the second type of tire I referred to was the one where the pressure is also moulded into the sidewall of the tire but the word maximum is not. This type of tire was never designed as a passenger car tire but was designed to carry a given load. Comfort and shock absorbing ability were not really big considerations. The sidewalls were 8, 9, and 10 ply with very little flexing ability. Using less air than recommended in these types of tires could cause sidewall flexing, which cause heat and damage to the sidewall, followed shortly thereafter by sidewall failure and pieces of tire along the freeway. It was common to carry a few more lbs. of air in the tire than stated on the sidewall just to insure longevity. Even today, if you go to a tire manufacturing company as I did, and speak to the designers they will say it is okay to inflate above the moulded rating on those tires with the heavy multi ply sidewalls.

Some tire manufacturers have taken into consideration the fact that on our class C’s, class A’s and bus chassis motorhomes we are looking for a better than harsh ride from our tires and have worked towards that means. These rugged tires could be carrying 1600 kg on the axle of one motorhome but over 5500 kg on the axle of yet another motorhome. Plus, we tend to load too many extras into our units which increases the actual carrying load of a given tire even more. There is not a single air pressure that will cover all the different loads that a tire could be subject to, so a given pressure (moulded into the tire) is given for the load that is also moulded into the tire. If your axle weight matches the moulded weight exactly and you want a softer ride you can decrease your tire pressure a few kpa or, if you want a stiffer ride, increase it a few kpa (both within reason). Of course to find out how much pressure you are supposed to carry in each of your tires you have to know how much load is on each tire. Go to a public scale and drive over the scale in such a manner that you will be able to weigh each corner of your axles, one at a time. Now you know the load on each tire, (single tire on the front axle and possibly duals on the rear axle). Take those readings to a competent, knowledgeable, tire dealer who can give you the actual (not moulded) pressure that each one of your tires should have.

One final way of determining if everything you have done is correct is to watch the wear patterns of your tires. If they are cupping or wearing on one side then... but that’s for another controversial article, stay turned because Bob’s Concerned.

Go Man Go

With the mild winter we had, a lot of people didn’t bother winterizing their units and instead they used them all year. During the Christmas holiday season I saw lots of them over the mountains and into Alberta. Lots of available camping spots and the scenery couldn’t be better.