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Monday, February 24, 2014

Measurable vs Meaningful

It is important to understand the difference between these two terms. Sometimes people assume that if something can be measured it is important.
Over the years as technology has advanced it becomes more and more easy to detect ever smaller quantities of material or forces.  As an engineer I too often hear people expressing concern over some new discovery of x parts per Million or even x parts per Billion of some contaminant to air or water, or foreign material in some product. Some of these numbers are important and meaningful while others are not really significant so there should not be a cause for concern. The challenge is to know when some measurement is meaningful.

Some of you are asking yourself what this has to do with RV tires. Well if you have read many of my posts here in this blog or on various RV Forums you no doubt have seem me and others saying you need to know the actual load on each tire and not just guess. Also we tell you how important it is to have the proper inflation in your tires.

What made me think of a need for this topic is the occasional complaint I get from some that think we are suggesting a need to get their RV weighed every time they travel or add an item to their packing list.. They basically say they are not going to do that so ask "Why bother to weigh the RV at all?"

Lets step back and look at tire Load & Inflation tables. For highway use tires the tables have 5 psi increments and depending on the size tire that 5 psi increment can provide as little as 110# additional load carrying capacity for a small ST type tire to 320# or more additional capacity to a large Class-A tire. If we do some simple math that means on a single axle small Trailer you would need to adjust your inflation if you were to increase the load in the trailer by 220# but the Class-A RV with rear duals and a tag you might be able to tolerate (8x320#) or 2,560# additional load (assuming perfect load distribution).

If your hobby was going to yard sales and picking up cast iron lawn ornaments it might be possible for a couple good sized items at (115# each) to be meaningful so you would need to increase your tire inflation on your small teardrop trailer (assuming you were not already running the tire max). The owner of the 40' Class-A on the other hand could probably pick-up a couple dozen items that each weighed 105# and if the load was perfectly distributed still not be overloading the tires.
Now clearly we can count (measure) the difference in the additional load of the two 115# items and the load of twenty four 105# items. In both cases the additional load is easily measured but while 120# could be very meaningful to the small trailer owner an increase in load of 120# would certainly NOT be meaningful to the Class-A RV owner. Maybe you now have a better understanding of the difference between measurable and meaningful.

In a previous post, I provided information of the accuracy of pressure gauges, showing that I find that 10% to 15% of the gauges I have checked are off by 5 psi or more. While my personal gauges are certified accurate to +/- 0.5psi, that is way more accurate you you "need" if you are setting your cold inflation to give yourself a 10psi Safety Margin over what is needed for your normal full load. If however you are running loads that fall just below the load indicated for the cold inflation you are using, then BOTH your gauge needs to be more accurate than +/-5 psi and your scale reading needs to be accurate to less than half of the load increment in the table.

Example: Tables for a 255/80R22.5 show 4,805# @ 95 and 4,975@ 100 psi so your gauge needs to be at least +/- 2.5 psi from actual certified measurement and your load needs to be measured to an accuracy of at least +/- 85# if you are using 100 psi as your cold inflation.

Monday, February 17, 2014

How to avoid a tire Blowout and calculate your needed cold tire inflation


If you want to avoid blowouts you need to ensure you ALWAYS have enough air in your tires. If you start your day low on inflation, your tire will run hotter than desired and this will accelerate the rubber degradation. With enough degradation your tire will loose strength and this might lead to a failure.

You need to do two things to help avoid this problem. First you need to have at least the minimum inflation needed at the start of EVERY trip. Second you should monitor your inflation so you receive a warning if you have a puncture, cut or valve leak. This means having a TPMS.

Lets address the minimum cold inflation level needed.
Some people get different opinions on what inflation to use based on the idea that what others inflate their tires to is what they should use. Sometimes they depend on what the salesman at the RV store says and sometimes they depend on their brother-in-law. Well my job as a Tire Engineer is to give you the facts on tire loading and how to confirm you have at least the minimum needed tire inflation for your motorhome.

First it is important to tell people with Trailers that this information DOES NOT APPLY to you. Your multiaxle configuration places extra side loading on your tires so you need to read THIS blog post.

Second it is very desirable to know the actual loads on each corner of your RV as your loading is never exactly balanced side to side.  Now I know that it is not easy to get your actual corner loads. While some have used RVSEF and others have found truck scales where they can do the multiple weights needed to complete the worksheet HERE, a good portion can only get their total axle loads at a CAT scale.

The goal of this post is to walk you through the process of what to do if you only can get axle loads. You can use this until you learn your real corner loads.


Lets look a a sample problem. Suppose you have a Class-A with a tag axle. You have received a variety of suggestions for tire inflation around the campfire but you want to know the facts. Well here's what to do


1. You should have a tire placard or spec sheet posted on the wall of your RV. It shows the suggested inflation based on what the manufacturer thinks you will be carrying. Use this till you get on a scale.

2. With your RV fully loaded with the fuel, food, clothes, people, toys and even your bowling ball collection if you normally carry that around :-) get on a truck scale to learn what your axle weights actually are.

3 Assume you have some imbalance so calculate that one side of each axle is loaded to 55% of the total.

Example for reading MI chart   on a 255/80R22.5 XRV LR-G tire
The "Dual" loads shown on Michelin charts are for the two tires. You can confirm this by reading the sidewall of your tires.  Other companies give individual tire capacities when in single (Front or TAG) or Dual (Drive) application
  Lets assume your Front axle gave you 8910# so 55% of that is 4900.5 and rounding up we would want the minimum inflation needed to carry 4901#.  According to MI chart that would be 4975 which gives 100psi as your minimum cold tire inflation. Add your 5 psi margin and you know you need to set the fronts at least to 105 psi when "cold"

Moving to your rear duals. With a scale reading of 15,010# we would find 55% of that or 8255.5 or 8256# for the TWO tires on the heavier side of the RV. Looking at the MI chart we find 8410 as the load number which indicates 90psi add the 5 psi margin and we have 95psi.

Tag axle Assume you have the axle loading set so the scale reads 7560#. 55% of that is 4158 and looking at the chart we see 80psi. Adding the 5psi margin you now know you need 85psi cold in the tag tires.

Remember ALL tires on each axle should have the same cold inflation. COLD inflation means not having been driven on more than 2 miles in the previous two hours and not being in any sunlight in the last two hours.

Hope this helps

Sunday, February 2, 2014

What minimum inflation should I run in my RV tires.

This question gets asked for a variety of reasons. Here is one example...

"My Winnebago was recently weighed on the four corners.  The weights confirmed previous truck scale results.  The Goodyear tire chart for my tire begins at 80psi.  The weights are well below those shown in the chart for 80psi at all corners.  Is it OK to run 80 psi or would you recommend say 85 so any slow leakdown would keep the pressure above 80?  I don't recall ever seeing you address this issue.  In other words, what is the minimum safe pressure for a tire even if the load is less than what the minimum will support?"

Simple answer first. Yes you need to run at least 80 and I doubt there would be anything wrong with 85psi.


Now if you want the more involved answer here it is......You didn't give the numbers but I think you will understand the following..

Lets assume the corner weight is  4,000#   If you have a Goodyear 255/70R22.5 according to their chart you see that tire is rated 4,190 @80 and 4,370 @ 85 or an additional 180# more capacity for 5 more psi
Now the load capacity is a direct function of inflation pressure. While the formula involves exponential functions, I don't think that we have to get to higher math to answer your question. So lets consider it reasonably linear over small changes in inflation so if you look at the 90 psi rating you see 4,550 which is 220# more capacity for 5 additional psi if we go up to 95 psi we see an increase in load capacity of 125#.  So we see a 180#, 220# and 125#  increase in load capacity for each increase in pressure of 5psi. I would suggest we use 200# capacity for 5 psi as a reasonable compromise.

4,190 - 200 = 3,990#  which is almost equal to the estimated 4,000# so this implies that 75 psi might be theoretically OK but we need to stay with the numbers published for your tires from the manufacturer of your tires so 80 psi would be the minimum cold inflation.

So 80psi would give you about a 5 psi margin over what is needed for the load. If you have read other posts you will see that I normally suggest a 5 to 10% increase in inflation so you are not chasing the pressure when it goes up or down a couple of psi as you travel across the country and the changes in ambient temperature affect your tire pressure or with the 1 psi or so normal leak-down seen in almost all tires.
Now if the corner load is actually 3,000# then in theory the tire might only need  55 or 60 psi to carry the load. BUT running 80 will not hurt the tire and all that might happen is you would have a little harder ride.

Again, run no less than the 80psi shown in the charts unless you can get something in writing from GY corporate, NOT A LOCAL DEALER but from corporate engineers in Akron, OH.

It may interest you to know that on my Class-C RV, I am running between 10 and 15 psi above the minimum pressure needed based on my actual corner loads and have had no problems.

In closing you need to be thankful that Winnebago selected tires with plenty of extra load capacity.

 Hope this helps.