By Roger Marble
It appears I messed up in my previous post “More on tire cold inflation vs. ‘set pressure’.” I failed to properly define the terms I was using, so if you bear with me I will try and clear things up.
“Cold inflation pressure” – This is the pressure number you see published in tire company “Load & Inflation” charts. It is also the pressure you see on the Certification Label sticker aka tire placard that vehicle manufacturers apply to all cars, trucks and RVs. It appears that some folks have a little difficulty with the word “cold.” This does not mean the tire needs to be refrigerated or that inflation pressure needs to be “adjusted” by calculating the difference between some theoretical laboratory standard and the current air temperature. “Cold” for tires simply means at ambient air temperature and not warmed by either being driven on or being in sunlight for the previous two hours.
When I am discussing tire pressure I am always referring to the cold inflation unless we are specifically discussing the pressure increase due to sun exposure or due to being driven on and reported by the TPMS, or if the driver checked the “hot” pressure at a rest stop with their hand gauge.
Finally, the pressure number molded on the sidewall of tires is the cold inflation pressure required to support the load that is also molded into the tire sidewall. The load number is the maximum load capacity for the tire and so the cold inflation would be considered the minimum cold inflation required to support that load.
The wording on tire sidewalls does vary a bit. If you look at a variety of tire types from different manufacturers you will see some variation in the wording and IMO this contributes to some of the confusion. One fact that many do not think about is that increasing the tire cold pressure above the number on the tire sidewall WILL NOT increase the load capacity number molded on the tire sidewall.
It is also important for people to understand that tires can tolerate a significant increase in pressure due to operation under load or at speed. While I can’t provide information on the specific design limits used by different tire companies, what I can say is that in my personal experience many new tires are capable of tolerating inflation increase of upwards of 100% or more over the number molded on the tire sidewall. So the idea that an undamaged tire will suddenly explode due to an increase in inflation due to operational heat is not justified.
Tire pressure increases. In my post of March 3, 2014, I covered the science and math of pressure change due to temperature change. You can read that post HERE, or just accept the rule of thumb that pressure changes by about 2% for each change in tire temperature of 10° F.
While we are talking about pressure change you can review my post of July 8, 2011, where we pointed out that driving from Death Valley to Denver, CO, will only result in about + 2.5 PSI but the change (drop) in ambient temperature will probably decrease the pressure by more than that increase due to elevation. This is why we tend to ignore tire pressure due to changes in elevation.
Tire load is important information. You know the GAWR, or Gross Axle Weight Rating, is on your certification label. The problem is that the actual load is almost never split side to side to give a 50/50 split. While many RVs may have a 48/52%, or similar, side-to-side split on an axle, the actual scale readings have confirmed some RVs have as much as a 1,000 lb. unbalance. So without actual scale readings we could only guess which tire is loaded more.
A tire on one end of an axle has no idea about the load on the tire on the other end of the axle, so simply dividing the axle load by two is not sufficiently accurate to be confident that you “know” the actual load on your tires. The other problem is that many people simply estimate the load on their tires. The reality is that a majority of RVs (10,000+) that have actually checked the tire loads have been found to have a tire or axle in overload.
This data demonstrates the importance of learning the actual load on your tires. While learning the load on each tire position is not easy, at minimum RV owners need to confirm the load on each individual axle and this is easily done with a visit to a local Truck Stop. This needs to be done with the RV loaded with as much “stuff” as you ever carry. With the axle loading known and until you can get individual tire position weights, I suggest you assume one end had 53% of the axle load.
My final point for this post is reserve load, and this is where we get to the “set pressure.”
First, we need to remember that reserve load is the load capacity of the tire at its cold inflation pressure that is in excess of the measured or calculated load of the RV on the tire. Some use the term “safety factor,” but as an Engineer, this term is not really appropriate.
In general, it is suggested we have at least a 15% reserve load. Most new cars come with 20% to 30% reserve load, and this is a major reason why we seldom see tire failures on cars. An exception was seen in the ’90s when one vehicle manufacturer provided for less than 10% reserve load and a number of tire failures occurred and even made the TV news.
Many motorhomes may have less than 10% reserve load even if the inflation pressure and the loading shown on the Certification pressure are followed. IMO this is a major reason for the relatively high failure rate of tires in RV motorhome application. RV trailers have it worse. In addition to having 0% to 10% reserve load, the suspension design contributes to high interply shear due to being dragged rather than steered around corners.
So what should an RV motorhome owner do?
1. Learn your actual loads on your tires by getting on a scale for each tire position.
2. If you can’t get individual axle end loads assume the heavy end has 53% of the axle load.
3. Use the tire Load & Inflation tables to learn the minimum cold inflation needed to support your actual (or 53%) load.
4. Consider applying a +15% to the load figure to give yourself a reasonable reserve load and consider that your minimum cold inflation. You could also consider adding 10% to the load table pressure if that is easier for you to calculate.
5. Consider adding 5% to the inflation in #4 and use that as your “set pressure.” This gives you a cushion for day-to-day temperature variation which can change inflation pressure 2% to 5%.
So what should an RV trailer owner do?
Do 1, 2 and 3 above.
4. If you want to try and lower the interply shear, I recommend you increase the inflation to the number on the tire sidewall and use that for your set pressure. If you have increased the tire load range from, say, a LR-D to LR-E, you can use as an inflation number the 65 PSI for LR-D and the 80 PSI for LR-E tires, and use that as your set pressure.
5. Try and learn the wheel max pressure rating and do not exceed that number.