Tire Loads & Inflation - A refresher

 Just read another post on a FB page where more incorrect info was published....Soooo lets go over this topic again

 Today’s key points: Know the minimum tire inflation based on manufacturer estimates. At a MINIMUM Check your inflation with a good gauge at least monthly and every morning before travel. Better yet get, program and use a Tire Pressure Monitor system  TPMS to the knowledgeable.

Tire inflation seems to be a topic that confuses some and has others believing in misleading or just plain incorrect information. Tire inflation is one item that directly affects the safety of your RV, truck or car as you travel down the highway. Many of my posts have been background information that you really don’t have to know or fully understand. The intent of these topics is to give you a better foundation of understanding more about tires but, if you only pay attention to one series of posts this is it.

Tires do not carry the load. They are just a container of air. It is the inflation air that does the work. Think for a moment of an impact wrench. It can’t do the work of loosening or tightening nuts on your wheels without the air, and a tire can’t do the work of carrying the load or provide the traction needed to turn, start or stop if it doesn’t have air. The load a tire can carry is basically based on the air volume of the tire and pressure of that air inside the tire.

If the load was carried by the tire construction we would have Tire Load and Construction tables. But we don't. We have Load and Inflation tables because, for a given type and size tire the load is just a function of air pressure. More pressure gives more load carrying capacity.

If you want to get a feel for how little load your tire can carry without air in it, you can test this yourself. Simply take an unmounted tire and stand on the beads of the tire. I think you will find that the tire can support less than 5% of the rated load before it deflects more than an inch or two. Some people believe that tires with higher Load Range can carry more load at the same inflation. This is just not correct. You would be hard pressed to measure the uninflated load capacity difference between a Load Range D and E tire or between a G or H Load Range tire because almost any significant load would compress the tire to be flat.

The term Load Range replaced to old "Ply Rating back in the early 70's when tire construction materials became stronger and with the introduction of Radial construction.

If you are not sure, the "Load Range" is marked on the sidewall of your tires. If you don't see the words "Load Range" look for "LR" followed by a letter usually between C and G. "Load Range" may not be in large letters but you should be able to find them on your tires.

TireRack has a good information page on "Load Range" HERE.

The simple answer to how much air you need is on your Certification Label AKA Tire Placard that I wrote about last week. Depending on the year your RV was made, this information is on a sticker on the side of your trailer or near the driver’s seat in your Class-A or on older RVs it might be glued to the wall inside a cabinet or on the inside of your entry door. Yoyr car or Pick=up has the sticker on the driver door jam. Where ever it is, you should also have the information in the stack of owners manuals you received when you bought the new RV. Find this data sheet and write down the information for loads, minimum inflation, tire size and load range and place this info where you can easily find it. This inflation is the minimum based on what the RV manufacturer thought you would load into your vehicle. They may have guessed correctly or you may have gone way over that estimate depending on how much “stuff” you carry.

##RVT1038

Can you change tire size on your RV?

There seems to be a bit of confusion when it comes to selecting tires to replace the size/type that came as Original Equipment on your RV. What you can do and what you should do are not always the same thing.

Here are some statements collected and posted by a knowledgeable person who frequently responds to questions on some RV Forums.

"Goodyear: Never fit tires to a vehicle that have less load carrying capacity than required by the Original Equipment Manufacturer.

Michelin: Never choose a tire that is smaller in size or has less load-carrying capacity than the tire that came with the vehicle.

Cooper: The new tires must have a load carrying capacity equal to or greater than the maximum load carrying capacity specified on the tire placard on the vehicle.

Toyo: Any replacement tire must be of a size and load range that will offer equal or higher load carrying capacity compared to the original equipment (OE) tire on the vehicle."

I am not aware of any "legal" requirement that specifies what the RV owner is required to do. While it is a legal requirement from DOT that the vehicle manufacturer must select and specify tires and the inflation necessary to support the stated Gross Axle Weight Rating, as far as I know, this legal requirement does not apply to individual owners of vehicles.

HOWEVER, I doubt that you will find any company or responsible individual willing to state that it is good practice to select replacement tires that do not have a stated load capacity that is equal to or greater than the load capacity of the original tires.

All tires sold for use on public highways have a stated maximum load capacity at a given inflation pressure molded onto both sidewalls. This fact, along with the information in published Load/Inflation tables, makes it relatively easy to find tires with the needed load capacity.

There are a number of reasons to consider an alternate size tire (availability, cost, brand reputation, etc.) but in every case you should only select new tires that can support at least as much load as the OE tires.

The above is based on an assumption that the load on your tires is split equally 50/50 side to side on each axle. Since this is seldom the case, this is an extra bit of information you should consider when shopping for new tires.

I have numerous posts on this blog where I outline the importance of confirming the actual load on each end of each axle. This is because it is possible to have the load unbalanced to the point that one tire may be overloaded even if the total tire load capacity for all the tires on an axle numerically exceeds the total axle load measured on a truck scale.

RV tire load knowledge survey

Here is a quick survey on tire loading and weighing your RV.

I would like to assemble some facts about what people know about their actual tire loading rather than just what they think they know. This will allow me to provide more helpful answers to questions about selecting the proper cold inflation pressure for your RV.

Also I would like to do a few posts on things to consider when contemplating a change in tire size and this data will help provide guidance for those posts.

CLICK HERE TO TAKE THE SURVEY.
It will only take about 15 seconds.

Feel free to forward this post to friends with RVs (trailers or motorhomes)

I would really like to have a few hundred responses so the results are more representative of current real life loading rather than numbers based on historical (ten or more year old) data

We will post the results after we get a few hundred replies.

I hope to have a few more polls in the future.

Thank you for participating. The results should be interesting to all of us.

* * *

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When and Why some need to "De-Rate" their tires

Had a question from a TT owner about De-rating the tire load. He wanted to use P-Type tires on his trailer.

Can you explain why anyone would need to de-rate the weight rating just because a tire is used on a trailer? I don't understand why if the rating is 2510 lbs on one vehicle why wouldn't it be the same on another at the same pressure?

The quick and simple answer is that the service or environment seen by cars is not the same as the service seen by trailers. You don't design a home for northern Canada the same as a home in Mexico as the environment is significantly different.

Now if you don't like the simple answer here is a longer approximation.

When tires are designed there are certain things that can be expected. First off they are expected to pass all the regulatory tests. Some of these tests will be at or near the tire's max rated load. None of these tests are for 60,000 miles but may only last 2,000 miles at the max when running in 100F temperature on a curved drum (which is much harder than running on flat surface) and the tire is never be allowed to cool down as it is also run at say 50 mph. Outdoor testing may be at 100% load and 30,000 miles but the test car does come to a stop for 30 min or so every few hours. The test track however has no sharp turns or pot holes.

If a tire can survive the above in all probability it can run 60,000 on regular highways BUT occasionally at higher speeds and with occasional pot holes etc. We know that there is a "reserve load" of say 10% to 20% between the tire's load capacity at the placard inflation in normal car operation and there may only be 10% or less of the time the tire is asked to run anywhere near it's max load.

Now if that same tire is put on a small pick up or SUV the % of the time it may be required to run at or near it's max load may be 50 % or more.
It was learned many years ago with the introduction of station wagons that if we were to avoid premature tire failures something needed to be done. At the time there were only passenger and true truck tires so the only option was to reduce the load allowed when the passenger tires were placed in station wagon service. Later on, when SUVs were invented they replaced station wagons in the market place so the definition of "Multi-purpose" vehicles was coined and since pick up trucks were "civilized" to be more like cars and less like trucks and occasionally passenger type tires were applied to these trucks. Again the percentage of time the tire was asked to operate at its max load capability was much greater than when the same tire was placed on a car.
The "de-rating" was established at a factor of 1.10

Now in reality early RV trailers were single axle but would spend an even greater portion of the time at their max load. To counter that their service life was much less so the 1.10 factor was applied to passenger tires in trailer service.

If you look at the rated load capacity of a true LT type tire vs a P type at the same inflation you will see that the LT is rated at about 25% lower load capacity. This is because vehicles that are expected to be used as trucks ie a major part of their life fully loaded, the reality of the physics involved dictates that if you want to get 40 to 60,000 mile life you must reduce the load. This lower capacity is one reason there is no De-Rating for LT tires when placed on trailers.

It may help if you think of tire life as a formula in the form of

LIFE = 100,000 - (A x %of time at 100%load) -(B x % of time at 90% load) - (C x % of time at 80% of max load)-(D x %of time at speed greater than 50 mph)- (E x % of time at speed greater than 65 mph)- (F x number of 4" deep pot holes) etc, etc, etc.

If you think of A, B C etc as large numbers such as 50,000 or 20,000 I think you can see that it is easy to "consume" the finite tire life of 100,000.

Now in reality there is no such formula as it would have hundreds for factors, each of which is almost impossible to establish without hundreds of thousands of dollars research.

Hope this helps you understand why the De-Rating formula for P Type tires when placed in "mult-purpose or trailer service is  Load Max New = (Load Max on tire)/1.10


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Real life inflation calculations for Michelin XZA2 Energy on 3 axle Class-A RV

Had a post from the owner with the following facts presented.

Michelin XZA2 Energy 295/80R x 22.5 Load range H
Steer axle scale weight is 14200 (tire placard shows 110 PSI closest)
Drive axle scale weight is 17360 (tire placard shows 75 PSI dual closest)
Tag axle scale weight is 7800 (tire placard shows 75 PSI single closest)
Total gross weight is 39360 per scale

MFG front GAWR is 14600# max
Intermediate GAWR is 20000# max
Tag GAWR is 10000# max
GVWR is 44600# max


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OK as an Engineer I always feel Facts & Data are our friends.
Using info on Michelin web site, and assuming perfect 50/50 side to side balance (unrealistic)  I come up with F@110 psi which gives you a 2.8% Margin on load capacity
Drive & Tag 75 psi  for a 8.9% and 27% margin

However if we assume a 45/55% split which is more reasonable
F @ 120 we get a 0.3% margin
Drive at 80 psi gives a 4.8% margin
Tag @ 75 gives a 20% margin

Note I suggest that in a perfect world that once you know your actual individual axle end load, you use the tables to find the minimum cold inflation and then add 10% to that number for your morning cold "set" pressure.

For those smart enough to run TPMS, I would set the warning level to be the minimum cold inflation and not just the standard 20% below the morning "set" pressure. With the proper TPM system that gives not just the low pressure warning but also a rapid air loss warning you have a good chance of coming to a safe stop before damaging the tire or its mate from driving when officially flat i.e. having lost 20% from the minimum needed based on actual tire loading.

Hope this helps.


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Minimum or Maximum inflation or Load...I'm confused

Read a post of an RV forum from a motorhome owner who was a bit confused by the wording on his tires about Max Inflation and max Load when some people are telling him to learn the minimum inflation. He said
"Uhh.... So why does it say MAX PSI in front of it???  MAX means minimum???"

My answer:
Yes this is confusing.  The intent of the information is to let you know that when you inflate the tire to the pressure associated with the highest level for the Load Range it is rated to carry the stated load and no more.
Remember that "Load Range" is a replacement for the outmoded "Ply Rating"

When you look at a Load/Inflation table you will see a series of inflations in 5 psi increments. You will also see a series of load capacities.

I'm going to use a LT size but the same process would be used by other Motorhome owners.

Size LT225/75R16
Single  1790#@45  1940@50   2060@55  2190@60  2335@65 
2440@70  2560@75  2680@ 80  3000@95

Now the other bit of important information in the table is the Load Range limits for this size tire.  LR-C is 50psi  LR-D is 65  LR-E is 80 and LR-F is 95

If you had this size tire and it was a LR-E tire it would say 2680# at 80 psi Max or some such

Note some tire companies use the wording like 2680# Max at 80 psi Cold or some variation thereof.


The inflations in the table can be considered a Minimum when you start by looking at your measured load.  In the above example if you measured 2500# on the heavier front tire you would select the inflation from the table associated with at least 2500#  which means 2560# and you would then know that the Minimum cold inflation you should ever run was 75 psi.

Finally we know that pressure varies as the ambient temperature changes. This change is about 2% for every 10°F increment so to avoid having to chase around looking for a service station every time the temperature drops 10°F, I suggest you add 10% to the required inflation level which in the case of this example would be 7.5 psi

I wouldn't worry about getting the inflation to the exact 1/2 psi just know that if you add 7 or 8 psi to your minimum of 75psi you are good to go. You also don't have to worry about adding air if it gets cold and the inflation drops to 79psi as you have a nice margin built in.

Over time all tires loose air pressure so eventually your pressure will drop to the point that your day to day pressure variation will result in you having only 75 psi in the tire. I would not wait till your margin has been used up but would have added a bit of air once I got down to 77psi or so and bumped it back up to the 82/83 psi level.


Long explanation but I wanted to give you the background so you would understand why I say

1. Weigh the RV
2. Select the minimum cold inflation for your tires based on the table minimum inflation
3. Add 10% margin to that inflation to learn your "cold set pressure"
4. Inflate all tires to the inflation for the heaviest loaded tire on that axle. (all tires on each axle should have the same inflation)
5. Check your inflation in the morning before you travel more than 1/2 mile and before the tires are warmed by sunlight or monitor the inflation all the time with a TPMS.


Hope this helps.


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New Year Resolution on Tires

Happy New Year!

Well it's that time when we make Resolutions. Many make ones that they know, deep down they will probably not keep. You know the ones like "Go a diet and loose 25# or to start walking 5 miles every day.

Well I would like to offer some suggestions for resolutions that won't be too difficult to keep and could pay BIG dividends in improved fuel mileage or decreased chance of a tire failure or improved tire wear.


#1. Get a your own "Master" tire gauge. This should be a digital gauge that you keep packed away and not in the bottom of your tool box. You can use this gauge to check your regular hand gauge against once every few weeks.
My personal favorite is this Accutire gauge
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You don't need a $100 special truck gauge as you simply use this gauge to measure a front tire and compare the reading with the reading on your normal gauge. I have found this gauge to be accurate to +/- 1/2 psi or better, when checked against ISO Laboratory test instruments,.which is more than accurate enough. If your regular gauge is within a couple psi of this one that should be good enough PLUS you know if your regular gauge is reading high or low.

#2. On your first trip pull off the Interstate at a truck stop that has scales and confirm the current load on each axle. Now hopefully you have had each tire (for trailers) or each "corner" for motorhomes weighed in the past. Doing a quick axle weight check will confirm you haven't made a significant change in your loading as the axle total should match the total of the individual tires you previously had measured.

#3 At least once each year have your tires completely inspected by a tire dealer that sells your brand of tire. If you have a trailer you should have a "Free Spin" inspection to check for Radial or Lateral runout as seen in this video.  I covered the examination of this "Failed" tire in my post on how to inspect your tires.

#4 And now the tough one here. Go to my blog Archive (lower right side) and go back to oldest post and "Resolve" to review four posts each week. Now you don't have to remember everything but I do hope that when you are done you will have an idea of where to go when a question comes up and you will hopefully remember reading something on the topic of interest and you can then go back and re-read the specific post.

May you all have a Safe and Tire Problem Free 2015.
Roger Marble   aka Tireman9


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Goodyear RV tire Load / Inflation calculation

Had a request for assistance of how to calculate the proper inflation to run on a set of Goodyear  G670 RV 255/70R 22.5 tires

 on the Motorhome. The owner was a little confsed by the charts.

Here is what I did based on the AXLE weights provided by the owner. Maybe if you have been a little unsure on how to do the calculations this post will help.
While it is always better to know the actual loading on each corner of your motorhome, I know that not everyone can find a local facility that can weigh individual positions or they hav not attended a large RV event where RV Safety & Education Foundation provides the service so hopefully this will help until the time you learn your actual loading.

He said he had the coach weighted at a CAT SCALE facility in California. The axle weight is the only weight he has so far.

The front axle weight is    7,580 lb
The drive axle weight is  15,760 lb

1. Calculated individual tire loading
   7,580/2= 3,790

15,760/2= 7,880 per side

2. Applied 5% factor for not having actual axle end weights
  3,790 x 1.05 = 3,980
   7,880 x 1.05 = 8,510    Divide by 2 to get individual tire loading 8,510/2 = 4,255#

3. Go to Goodyear RV tire page
http://www.goodyearrvtires.com/tire-inflation-loading.aspx

4. Downloaded the Load Inflation Table.

5. Find the 255/70R22.5  (pg 3)

6. Look up the Single load that is equal or above 3,980 and find 4,190 at 80 psi

7 Add 10% to that inflation  80 x 1.10 = 88 psi. 

8 Looked up the Dual load equal or above 4,255 and found 4,275 @ 90 psi

9 added 10% to that inflation  90 x 1.10 = 99 psi

10 You now know the absolute minimum inflation to ever see on your TPMS  80/90 F/R

11. You know your "Goal" or "Set" pressure for each day of travel is 88/99 F/R

If you ever drop below 84 Front or 95 Rear you should add air. I would not let air out unless the COLD pressure exceeded 93F or 104R

The extra 10% inflation is so you do not have to chase your tire inflation around with day to day temperature & pressure changes.

Remember you need to be sure you do not exceed the cold inflation molded on the sidewall of your tires or the max inflation rating for your wheels. If you find yourself calculating pressures that exceed these ratings it means you need to put your RV on a weight loss program.

BOTTOM LINE:
You can use this step by step post as a guide for learning the correct Cold Inflation for your tires.

You can learn more by reading the posts on my blog with Temperature, Pressure or Inflation as "Labels" I do not expect you to remember all the information but if you review the posts you will probably remember that I wrote something on the topic of interest in the future so you can then go back and review to answer to whatever your questions is.

NOTE to TRAILER OWNERS
 This is NOT the method for you to use. The above is ONLY for MOTORIZED RVs.


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Picture from Goodyear website. I figured they would not mind since I am helping their customers.

Can I change from ST to LT tires on my Trailer or 5ver? Part Two

In Part 1 we left off with having to do some calculation.

I will assume you have confirmed the actual individual tire loading and have moved some heavy items around to end up with a reasonable balance of loads,

I will also assume you still want to change from ST type to LT type tires which means you must increase the Load Range and or increase the tire size to get a load capacity in the LT to match or exceed the capacity of the OE ST type tires.

Now before we move on you need to realize that "LT" is a designation used here in the US and European and some Asian countries have what they call "Commercial" tires. These Commercial sizes do not start with LT or CO but will probably look like 7.00R15  or for metric sizes 205/75R16C  The "C" in this case is not the Load Range but stands for "Commercial". The Load Range will be identified as normal LR-C, LR-D etc or possibly with RL or XL for Reinforced or Extra Load. To make this post easier to read I will limit my comments to LT type tires. Just remember there are other options that may be better for those with 15" or 14" wheels that do not want to change rims.

NOTE: All of these letters and numbers are important when selecting a size so be sure you record them all when doing your research.

So on to step
#6 Dimensions. There are two key dimensions Outside Diameter or OD and Width. Now I am confident that we all understand OD but width can be a bit confusing. Depending on the wheel well contour the overall maximum width or "Section Width may be most important. Some tires may have a narrower clearance nearer the tread so they will need to do some actual measurements at a number of locations.
It may be easiest to use the dimensions for OD and Section published for your current tires and just do a confirmation with your tape measure. Remember tire "width" is not the same as tread width.

You need to be sure the tires NEVER contact any portion of the RV frame wheel well or bodywork. You should try to have equal or greater clearance with the new tires that you have on your original size.

7. The challenge
When moving from ST type to LT type you will need to move up in Load Range or up in Size or both.

Now comes the research to see what your options are
8. Knowing the target Load Capacity and the maximum OD and Section width, it's time to use the Internet to do some research. The objective is to find tires that meet your needs for the numbers and that are appropriate tread pattern. You certainly don't need Snow Tires or heavy traction tread pattern. I would suggest that the tread be identified for "All Position" or Steer for your trailer application.
You can go to web sites from large dealers such as Tire Rack, Pep Boys, WalMart, NTB, Discount Tire or similar. You might also just Google "Trailer Tire" + the name of a large city or town near your location.
Once on their web site find the various possible tires that meet your needs.

9. If you are increasing the Load Range with the associated increase in inflation you need to confirm the wheel can manage that higher inflation. The info may be marked on the back side of the wheel or you may need to contact the wheel seller or manufacturer or you may need to get different wheels if your OE seller doesn't know what the rating is for the OE wheels

10 Finally, as I have previously suggested in my post on "The Best Trailer Tire" you need to make your purchase decision not just on lowest price but need to consider the tire warranty, even if there is a Road Hazard Warranty. Also how easy will it be to get a replacement if your tire gets a sidewall cut or unrepairable puncture.

11. Last step:  After all this work we want to do a first class job. Some might want to say you can't change from ST type to LT type due to Federal Regulations. Well a friend Dave Gray has an excellent post just on this topic. He has even provided an example of what would be the appropriate label you should apply after making modifications to your RV.


I hope these two posts have helped you understand the steps, calculations, measurements and research needed to make such a change.



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Quick Summary here

I have been following a number of RV forums and want to do a bit of a summary for those that don't have the time or interest to read a number of posts on this Blog: Remember more than half of the thousands of RVs that have been checked have been found to have one or more tire and/or axle in overload. So nect time you camp you can be pretty sure that one or both of the RVs parked next to you are overloading a component.

- You should have a Digital tire gauge. I have posted the results of my tests of dozens of gauges that RVrs were using and the bottom line is that about 15% were off by more than 5 psi which  IMO makes that tire gauge a good door stop or tent peg. I have provided information on how to do your own gauge "Calibration Check" to confirm the accuracy of the gauge you use daily and how to maintain your "Master Gauge"

- If you follow the advice from the two Tire Engineers on this thread you will be setting the pressure based on the appropriate method here:
     - On your TV inflate to pressure based on the Door sticker and Owner's manual for "Fully Loaded" operation. BUT get on a scale at least once to confirm you are not overloaded.
     - If you have a multi-Axle trailer you should be running the pressure on the tire sidewall. I have written in my Blog and provided the technical justification and need to follow this advice. You should still go across a scale and calculate the actual tire loads to confirm you are not overloading one or more tires
      - If you have Motorhome you need to confirm your unit's "corner weights". Use one of the worksheets I have provided links to. Use Load/Inflation chart from your tire manufacturer, to confirm you are not overloaded and identify the heavier loaded tire on each axle. Using the pressure for that load add 10% (without exceeding the wheel max) and that is your Cold set pressure for all tires on that axle.
      - If you follow the above there should be no reason to be chasing your tire inflation around unless you have a major change in Morning temperature ( More than say 30°F ) as you would only need to "Top Off" the air once a month at most.

- Don't be too anal about inflation. Variations of 1 to 3 psi day to day are normal. You should not need to make adjustments unless the Cold pressure is 4 or 5 psi away from your goal. (Multi-axle trailers might try and get no lower than 3 psi below tire sidewall pressure)

- TPMS are a WARNING device and like Insurance we should all be using them. Also just like Insurance they cannot "Prevent" bad stuff from happening but with luck they may give sufficient advance warning to lessen the impact of the bad stuff that does happen.

- If you are parked for more than a day you need to cover your tires that will be in direct sunlight with WHITE tire covers. This protects against both UV and higher temperature damage which can shorten tire life.

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.

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

New Information on Loading and weighing your RV

Walter Cannon, Exec Director of RV Safety Education Foundation sent some information to me that will impact all RV owners that want to weigh their RVs and learn the individual loads on each end of each axle.

If the RV owner does not attend a large RV Rally or Convention where individual corner or tire loads are measured by RVSEF or similar companies, they face the challenge of finding a large scale that will allow them to measure not just individual axle loads but also allow the tires on just one side to be weighed while keeping the RV level side to side as outlined in worksheets such as found in this guide, or the worksheet on this site or at this site.
In the past, I and others have, provided links to CATscales and suggested that it might be possible to get the weight readings needed to allow calculation of individual RV corner loads. While I know that some CAT scales had guard rail installed to prevent trucks from driving off-center on the scales and this may prevent RV owners from learning the weight of one side of their RV, we believed that there were many scales that could still be used. However what I learned from Walter is that CAT actively discourages people from using any of their scales to learn the side to side load variation.

From CatScale web site we read "Our scales can give you axle weights and a total gross weight, however, they cannot weigh each corner of the vehicle. We cannot provide individual wheel weights and, to prevent damage to your vehicle as well as our scales, do not allow that type of weighing."
It seems they do not want to allow RVs to measure one side of their units. There are probably a number of reasons for CAT taking this position but the bottom line appears to be that you should not try and get any weights other than total axle loads if using CAT scales. This is probably OK if you are just confirming your total weight but certainly should not be used to try and get individual tire loads. This means you cannot use CAT weights to set your tire inflations.

Based on this information from CATScale, I can no longer suggest or recommend you consider using their scales to weigh your RV to calculate tire loading so you can then look up your inflation needs.

This presents a serious problem for RV owners that want to know the real tire loads. CAT seems to be the 8000# Gorilla in the truck stop scale market as it is quite hard to find a truck stop that does not use CATScales.

At this time all I can suggest to the consciences RV owner is to either check the RVSEF web site for their schedule and location for offering weighing. If that is not convenient then you will need to search for a local gravel yard or large farm supply company or grain elevator and see if it is possible to gets the weights needed to complete one of the above worksheets.

The one bit of good news is that once you learn the real weights of your fully loaded RV and identify the proper minimum inflation and add your 5 to 10 psi safety factor you should not need to re-weigh the RV corners unless you make some significant change such a adding a battery bank or changing furniture or maybe a residential refrigerator to replace the original fridge or adding new storage for your bowling ball collection. :-)

Will you increase load capacity with higher Load Range?

The question of load capacity and Load Range gets asked a lot, so it seems to be a bit of a stumbling block for a number of RV owners. Here is an example:
"I need to replace the ST235/75R15 tires on my trailer. Currently have load range C tires which is sufficient for the maximum trailer weight when inflated to 50psi but allows almost no safety margin. I want to change to either load range D or E tires. My rims are only rated for 65 psi so if I went to the load range E tires I would only be able to inflate them to 65 psi. My question is this. Is there any advantage to a load range E tire used at 65 psi vs a load range D tire at 65 psi?"

Now just because he is asking about a 15" trailer application and considering a change from LR-C to LR-D it does not mean the general answer does not apply to others, even if they are considering a change from LR-G to LR-H on a Goodyear 295/75R22.5 Class-A tire.

Let me explain.

As some of you remember, I previously covered the fact that it is the air not the tire that carries the load. For those that still have doubts, here is a picture from a special test machine that measures force and delivers the answer in color. In this case, the lowest force is Blue which increases to Green, then Yellow and finally Red at the highest force. It is clear that the tire wants to be round because of the high inflation pressure but when you press the tire against a surface it becomes flat with  the highest force in the center of the tire. If it was the sidewall that carried the load, as some want to believe, then the Red (high force) should be at the outside shoulders.

Now when you go up in Load Range you are actually looking at the capability of the tire to carry more inflation pressure. Remember to gain in load capacity you need to increase the inflation. Years ago when tires used cotton for body cord we might have 4, 6, 8 or more layers or ply of reinforcement to retain the increased air pressure. Those increments provided steps in the Load/Inflation tables. Since the late 60's and early 70's when materials improved the actual number of ply decreased and the term Load Range was applied to those steps.

So going back to the original question, when increasing the Load Range you gain the possibility of increased load carrying ability but if you don't increase the actual cold inflation, the change in Load Range will not give you any increase in load carrying capacity.

BOTTOM LINE
The answer to the question is;
 He will gain a safety margin if he increases his inflation to 65psi with LR-D but if he stays at 65psi there is nothing further gained by going to LR-E as there is no difference in the capacity at 65psi.
The same thing would apply to the 22.5 example. When we look at the Goodyear tables we see that at 110psi both the LR-G and the LR-H are rated for 6,175# single or in dual application at 100psi they both are rated as 5,675#.

Special note: It is important that you look at the table on the web site of the manufacturer of your tires as not all companies give the exact same capacity number at every inflation level, even for the same size.

How to replace a tire in "Dual" application

First off if you have a towable RV and your tow vehicle only has four tires you can go get a cup of coffee as this post doesn't apply to you. However, the information here can still be important to know and pass on if you ever happen to be sitting around a campfire and hear someone talking about replacing a failed dual tire.

Now lets be sure everyone understands what is meant by "Dual" application vs "Tandem".

Tandem means one following or behind the other. Most trailers have either two or three axles in "tandem" and each axle has just two tires on it. This utility trailer has tandem axles.

Dual means two tires side by side on the same end of an axle. Most Class-C and Class-A RVs have Dual tires in the rear drive position. A drive axle with dual tires has four tires on it. This version of "Bigfoot" has dual tires on each axle.

 Ok we have definitions behind us. This post will address the proper way to match a pair of tires for dual application. This is very important, as mismatched duals can result in overloading and in extreme cases could lead to tire failure.

If you are buying new tires you should find that all the tires are almost identical in OD when first inflated. If you were to accurately measure the tires after a few hundred miles use you would find that the tires have "grown" or stretched out a bit as they were heat cycled, but again the tires would be very similar in dimensions.

If you were going to try and measure the OD of a tire you would also learn that it takes special tools to get a direct measurement. The best way to measure how big a tire is is to measure the circumference. You can do this with a standard tape measure. You don't even need to calculate OD from the measured circumference as I will give you the guidelines based on circumference. You just need to be sure you keep the tape at the center of the tire.

Here I have affixed one end of a standard tape measure to the center of an inflated tire using masking tape.

I next carefully roll the tire around, keeping the tape taught and in the center.


When the tape comes around I can get a fairly accurate measurement of the circumference. In this example I didn't use the end of the tape as the tang doesn't lay flat, so using the 3" mark I see 95" for an OC of 92.0 inches.

Now you should match tires +/- 3/4" of circumference in dual application. 


If placing new tires in dual application you do not need to do this measurement but if trying to match used tires or a new tire with a used tire this matching is important.


BOTTOM LINE:
If you do not match the tires the larger tire will be forced to carry an increased load and in extreme cases can result in failure.

Have you read a tire lately? Tire Marking

Have you read the information provided to you by the tire manufacturer? If you have, did you bother to record the important numbers in your log book so you only need to do this job once in the life of a tire?

There is a lot of information molded into the sidewall of your tires. Most of this is required by law. Most of this is important for you to know so you can look up the correct inflation for your tires or if buying new tires be sure to get replacements that can carry at least the same as the original tires.
Here is some information from one tire.

First the SIZE

This tire is a "Passenger" type as it starts with the letter "P" other types might be "LT" for Light Truck, or "ST" for Special Trailer. Small tires intended for passenger cars migth not have the "P" if made to European specs. Large tires like 19.5 or 22.5 rim dia have no letter and are "TBR" of Truck Bus radials and are found on Class-A RV. The "114S" is the Service Description which is a Load Index (from a published table but using the actual Max Load is better). The "S" is the Speed Rating or max operating speed. Like the Red Line on your engine. Not all tires have this description.

The DOT Serial

This has important information used in determining the tire age. Other information such as the location of the tire plant that made the tire is part of this code ( first two characters  8X in this example). If there is a recall, this code is used to identify which tires are covered by the recall. NOTE that the last portion, the 4 digit date code, 3908 in this example is only molded on one side of most tires. Every tire sold for use on the highway in the USA must have a full DOT serial including the date code molded on at least one side. This tire was made the 39th week of 2008.

The Load & Inflation information

This is the maximum load capacity for the tire when the cold inflation is set to this pressure. In this example (2601 lbs) when the inflation is set to( 44 psi max press) when the tire is at ambient temperature. NOTE for LT and TBR type tires there is a second lower load limit for dual application (tires side by side on the same axle as on the rears). If you have two axles and ST type tires I personally suggest you not exceed the "dual" tire load as you need a safety factor in your tire loading due to extreme side loading unique to tandem axle trailers.

Tire Materials

This is really just FYI and is more like truth in advertising to let you know the materials used in the sidewall and center of the tread of your tires. In this case there are two ply of Polyester in the sidewall and in the tread there are two ply of polyester + 2 ply of Steel + 1 ply of Nylon. Most TBR tires will only have 1 ply of steel in the sidewall.

Safety Warning

This is for the person mounting and inflating the tires for the first time. Do not confuse the inflation number here ( 40 psi) with the inflation number associated with the load (44 psi). This is the max inflation to seat the bead. If you have ever watched a tire being inflated it is the "Pop" or "Bang" first heard. If the tire doesn't seat by this inflation then it should be deflated, re-lubed, re-centered and re-inflated. People can die if they ignore this warning.

Bottom line
If you get new tires and they match the Size and Load & Inflation information you are good to go. If ANY of these numbers are different you need to be sure you completely understand why and that you are not getting a tire with lower load capacity.

"Danger, Will Robinson, Danger"

I think a good number of us remember those iconic words of warning from B9 the robot on the "Lost in Space" TV show from the 1960's. I repeat it here for I feel that just like young Will, many RV owners simply continue on and do not heed the warning. My message is of the importance of having adequate inflation based on the real tire loads. Without knowing the real load you can't use industry guidelines on how much air you need to have in your tires.
Too many RV owners simply and incorrectly assume that if they get on a scale and learn the axle load they can assume they know the tire load because they simply divide by 2 and think the result is the tire load. In reality few motorhomes are balanced side to side and even fewer trailers are balanced between axles so towables tend to be worse off as they start down the wrong path by taking the total load on all the trailer axles and divide by the number of axles and assume equal load distribution. They then compound their error and further assume equal side to side loading.

Now these assumptions might be tolerable if there is adequate (12 to 20%) "Reserve Load" ** but most RVs have no room for error and many have so little safety margin that they end up with one or more tires and/or axles overloaded.
Recently I did some research on a couple of forums to see if I could learn what the real load balance was.
Looking at 14 different Class-A motorhomes I found side to side variation on front axles ranged from 50/50 to 47/53 with no trend showing which side was heavier. The rears ranged from 50/50 to 54/46 but again there was no consistency as to which side was heavier. Here is a detailed breakdown of a group of Class-C motorhomes.

So here we see that while a majority seem reasonable balanced in the range of 48/52 or better we do see that some people are out with a split of 45/55 and this is on a vehicle that probably started life very near 50/50 which is what you would expect from a major mfg like GM or Ford.

For 5th wheel units,
I reviewed over 500 posts to a thread that asked what the "Real Weight" was but only found two owners know the actual tire loads. Searching some other threads I found a very small sample. I found that the best trailer had 50/50 on the front axle and 52/48 on the rear axle. This sounds good until you also look at F-R axle to axle split and find 61/39. If we look at the trailers with 50/50 balance between axles we see some as bad as 41/59 side to side.

The only trend I think might exist is that Class-B and smaller Class-C without slideouts might be a bit better for side to side balance than the rest of the RV community.

Important note. All of the above is simply looking at balance and not if any tires are overloaded. RVSEF has done over 35,000 RVs and recorded the individual tire load measurements. They point out in their seminars they have found some 57% of RV have an overloaded component.

While preparing this post I learned from an RV repair company that they find $7,000 worth of damage on motorhomes is normal when there has been a tire failure so clearly making the effort to either get your RV weighed at big RV events or finding a scale where you can get both individual axle loads and one side at a time is worth the effort if you want to avoid this unnecessary expense.

If you want additional information on how to get your individual tire loads you can check this site. If you have a facility that can weigh individual axles and one side of the RV at a time you can download a worksheet to help you calculate the individual tire loads. Once you know the actual individual tire loads simply select the load on the heavier side, look up the minimum inflation needed to carry that load and inflate ALL tires on each axle to that level.  If you exceed the max for a tire you MUST move stuff around or out of the RV or you can expect to simply be another statistic.

 BOTTOM LINE Hopefully I have convinced you that you can't assume the weights on your RV are either balanced or under spec. The odds are that you have one tire carrying at least 5% if not 10% more than another but you can't guess which.

** Reserve Load. The difference between the tire load capability at it's cold inflation pressure and the actual load on that individual tire.

Quick post with Real Weight numbers

Just got these numbers from a 5th wheel owner
Trailer R Front  3,230
Trailer R Rear   3,185
Trailer L Front  2,840
Trailer L Rear   2,780

If the owner did what many do if they bother to weigh their trailer he would simply assume 50/50 axle to axle and 50/50 side to side. With that calculation he would believe his tire load was 3009 but he would be wrong by 7.4%. This may not sound like much but many RVs are operating with less margine than this.

 This is a real life example of why "tire guys," myself included, tell you to get the load on each corner of your RV an not just a single weight and then dividing by 4 (or 6)

How to get your tire questions answered correctly

I have offered to answer specific tire questions on this Blog. I respond to questions on four different RV web sites that have forums where tires seem to be a hot topic. One things is a constant with many who ask questions is that they seldom provide enough information with their question such that I am not forced to ask for more information or guess.
I and the other tire experts on the web have a request.


When asking about specific loads, inflation, alternate size, or specific tire problems it would really help if basic information was provided.

While I and the others are more than willing to consult the standards books or provide an opinion we really can't spend hours trying to first solve the problem of knowing what tire we are being asked about. One thread I am following has over 275 posts but at least 25 - 40% are replies that show the need for more accurate information is needed from the poster.

Please provide the "Complete" size. That would include the Service Description part that comes after the rim diameter as well as the type tire which is the letters up front if any.

235-15 isn't correct but many times that is what we get.
Technically an LT235/75R15 107/110Q Load Range D
is a different tire than a LT235/75R15 Load Range D.

Also concerning the load on the tires. As a minimum you should have a real scale measurement of the total load on all the tires. While you may feel you know a lot about your "Death Valley Special" RV not all of us know if it has one, two or three axles.
Ideally you should have at least once had your RV weighed with certified scales where you can get the individual loads on each tire.
Almost ALL trailers are out of balance axle to axle in addition to side to side with a good portion (usually the ones with tire problems) discovering that they are at least 5% off and sometimes as much as 15% out of balance.
So in addition to the complete tire size how about:
"I guess the 4 tires on my trailer carry 10,000 pounds"
or
"The tires on my front axle carry 5500 and the rear axle 4500"
or best of all
"RF 3300 LF 2200 LR 2700 RR 1800"

Motorized RVs are a little better but still have some out of balance.

Finally if you really want to do a First Class job then the tire brand and design as molded on the tire, the max load at what inflation as molded on the tire and picture -in focus please- of the placard would be of great help.

I know that if given the choice of who to answer first I would answer the person that supplies the most information first.

OK I am stepping down off my soap box now.

Why should I weigh my Trailer tires individually?

Many times when I give a seminar or class to people from the RV community on proper application of tires, I get asked the above question by trailer owners.


Motorhome users seem to understand that knowing the actual load on each corner of their RV will allow them to select the tire pressure they want to run as they balance Ride, Tire Durability, Fuel Economy and Steering Response.
Owners of trailers say they have been told that all they need to do is to run the max inflation on the sidewall of their tires. Almost all the time, that is the information on their placard per the trailer manufacturer’s guideline so that recommendation seems reasonable.

Recently I have been having a discussion via email with an owner of a large 5th wheel trailer.
He even believes that because the load changes on every trip and almost every day it could be “dangerous” to get the trailer weighed as that could mislead the trailer owner to think it OK to lower the pressure to just match the minimum needed to carry the measured load. I would like to offer a different view on why it is important to get the trailer weights on each tire at least once.

More than half of the trailers that have been weighed over the years have been found to have a tire or other suspension component exceeding its maximum load capacity. We all know what happens to people who “Assume” something. When you get the trailer weighed, it does need to be with all the stuff you normally carry on your travels. Knowing this load you will have the facts to see how critical it is to re-distribute the load side to side or front to rear. You will also know if your unbalance has overloaded one or more tires even when the total load seems OK. You will also know if you have proper loading on your pin. I have heard that some axle manufacturers consider having one end of an axle loaded to more than 50% of the total axle rating as overloading the axle. This means that even if you have changed wheels & tires to not overload those components you might still be overloading the axle if you are not careful.

Another thing to consider is the “Reserve Load”. That is the “safety factor” between the actual load and the calculated maximum for a component. Most cars have a 12 to 18% reserve load. One of the other benefits of having a good sized Reserve Load is that it will allow you to occasionally bring home that big load of stuff you just had to buy at the flea market and not overload a component. One way to identify a minimum cushion would be to look at the load capacity of your tires when inflated to a pressure of at least 15 psi below the max inflation and to be sure your load does not exceed that level. A 20 psi cushion would be even better. Reserve load helps compensate for the unbalance that occurs due to side loading because of road crown and wind side loading.

You are probably asking yourself why trailers can’t lower their tire pressure the way motorized units can. The reason has to do with the mechanics of axle spacing and tire side loading whenever you turn a corner. Trailers with two or three axles put enormous side loading on the tires, wheels, and axles whenever they turn corners. A quick look at the tire distortion when you see a big twin axle trailer park will confirm that the tire needs to be as stiff as possible when side loaded. Since we all know it is the air that carries the load we also now realize that it is the air that stiffens the tire and as a result will lower the side deflection and loading.

BOTTOM LINE: Next time you are fully loaded please go to the effort of weighing your trailer and go the extra mile to do the multiple weighing and the math needed to calculate the actual load on the individual tires. Remember a blown trailer tire can lead to substantial damage and financial loss.