“The Rest of the Story” - Attention to details.

 Many of us have heard that phrase used as a preface to additional facts that sometimes do not make it into a story as widely published, but once the additional information is explained we learn that we do not always get all the important information  the first time around.

What brings this up was an item in an RV magazine I stumbled upon when cleaning out some old boxes of stuff. This is from Nov 2008 but the facts and information are still important today. The author is one of that small group of RV “experts” that make a living providing information on the Internet on just about all things RV. Sometimes they are answering questions about refrigerators, then they might be answering a question on holding tanks or plumbing. From my point of view it seems that they are quite knowledgeable about most topics but sometimes there are important details related to tires that don't always make it into their posts on tires.

The owner of a 5th wheel trailer had purchased a set of tires and offered that the tire store inflated the tires to 85 psi which was the “maximum” as molded onto the tire sidewall. The owner said he had heard that using the maximum inflation from the tire as the standard pressure was advisable and wanted to know the “rule of thumb” for the proper inflation.

He also wanted to know if he needed to reduce the tire pressure when he traveled to 8,000 ft elevations.

The “expert” correctly replied that the inflation on the tire was the minimum required to carry the load on the tire, he continued with the recommendation that tire loads be measured on a truck scale and then the inflation set based on Load & Inflation tables. There was no mention of ensuring that there was at least a 10% margin between the tire Load Capacity and the actual load on the tire. This recommendation to a trailer owner would address some of the unique side loading (Interply Shear) seen on multi-axle trailers which is significantly different than that seen on motorized RVs.

While I advocate that trailer owners get their rig weighed so they can confirm they are not overloading any individual tire, as it is not unusual for one tire or axle to be 500 to 1,000Ibs away from a theoretical 50/50 weight balance. I do strongly suggest that all multi-axle trailers run the inflation shown on the tire, which in most cases is the inflation shown on the RV placard as provided by the RV manufacturer. This will help reduce the side force  overload seen by trailers but not by motorhomes.

The expert did correctly advise that while the tire pressure will change with elevation, but said that unless the owner was checking his inflation daily, "as recommended", it was not necessary to adjust inflation when traveling to high elevations.

When I originally read this reply I was concerned and did send a letter to the expert pointing out the missed opportunity to educate RV owners about the importance of setting proper inflation, and that a stronger statement on more frequent or constant inflation checks by using a tire pressure monitoring system ( TPMS) was needed.

I do not consider daily inflation checks sufficient, especially for towables as too often the driver of the tow vehicle has no idea a tire is loosing air till it is too late to save the tire and there has been a "blowout" from a puncture or leaking valve which could cause thousands of dollars worth of damage to the RV. I also would not imply that when traveling to high elevations it isn't necessary to adjust your inflation whenever the inflation is checked.

As you can see the general RV expert provided mostly correct information, but he left out some considerations that I, as a tire engineer, do believe to be very important.

 

##RVT1031

What inflation should I run ? Motorized vehicles

 The question as asked is simple but of course I have to make the answer complex.

Not really. However there are two different answers. One is for "Motorized vehicles"  Class-A, Class-C Class-B and tow vehicles. The answer for trailers be they tear drop or triple axle 5th wheel trailers has some minor but important differences. This post will address the Motorized Vehicles so I don't have to keep switching back and forth.

Step 1: Learn the ACTUAL load on each end of each axle. This should be done when the Motorhome or truck is loaded to the heaviest you would ever load it.

Step 2: Using the heavier end load number for each axle, consult the Load & Inflation tables for your size and Load Range tire, to learn the MINIMUM inflation required.

Step 3: Add 10% to the number is step 2. This so you are not chasing inflation every time the ambient temperature changes.

Step 4: Set your LOW Pressure warning level on your TPMS to the pressure in #2 minus 2 to 4 psi. (Note this variation is to account for gauge and TPMS pressure reading variation). We want the warning to sound as soon as we might be overloading any tire.

Step 5: Using your hand gauge, that you have checked against your personal digital "Master Gauge", set the "Cold Inflation" pressure for all tires on an axle to the same pressure, which would be the pressure in Step #3. NOTE "Cold" only means the tire has not been driven on or been in direct sunlight for the previous two hours. This means the tire is at "air temperature" or the "Temperature in the shade".

Step #6: Get in your vehicle and you can now drive.


Information. The +10% is to give you "wiggle room" for day to day changes in air temperature. We want to "Protect" the "Minimum" inflation number we learned in Step #2 while at the same time not have to get out and fiddle with tire pressure every morning. Each morning when you get up, you can turn on your TPMS and make your coffee. After a few minutes you can then read the TPMS monitor and as long as there is no unexpected pressure change (drop), you can be satisfied that all tires are sufficiently inflated. You may see a slow drop in pressure over time even when there is no change in temperature. A drop of 1 to 2% per month is normal fol all tires, but once you follow this guide you can probably expect to only need to add air once every two to three months. For folks needing High Pressure ( 80 to 130 psi ) this means you can plan to "top off" your tires at the next truck stop where they should have plenty of air at the pressure need.  I will cover adding pressure to a hot tire in a separate post.

Example: Following the above over my travels from Ohio to Oregon to Calgary to Glacier and home over a two month period, I only needed to add air one time at Yellowstone and I only needed to add about 3 -5 psi to my LR-E tires to get back to my +10% number in Step #3.

Comment: After a while you will learn that the inflation numbers in your tires are reported as slightly different than seen with your hand gauge. This is normal as most TPMS are rated at +/- 2% for pressure accuracy. The primary job of a TPMS is to report a pressure drop Not to report extremely accurate pressure. I would feel that if your hand gauge reads +/- 2 psi from a reference gauge that is good enough. If your gauge is off by 5% of your tire pressure goals you might want to get a better gauge.

Please do not forget the above is specific for "Motorized Vehicles" and not for trailers or dollies you pull.

If you pull a dolly I might treat it more like a trailer with higher inflation than Minimum +10% but that is a separate topic.

##RVT968

Why not just inflate to the Certification label level?

I continue to read RV forum posts from people asking about what inflation to use. Tire Sidewall?, Sticker? Owner's manual? The inflation used by a neighbor? There is also continued confusion on what the sticker inflation is.

"Vehicle Certification Label" AKA "Tire Placard" only considers one thing. The max tire load capacity (molded on the tire sidewall) when the tire is inflated to the level associated with the original tire Load Range (Ply Rating) as shown in the industry load & Inflation tables.

Federal DOT Regulations specify the label indicate the tire inflation level needed to support AT LEAST  50% of the GAWR. NOTE there is no margin or reserve load capacity specified or required by the DOT Regulations.

Starting in Nov 2017, the RVIA (RV Industry Association) required that trailers have a 10% margin on tire load capacity. Motorhomes do not have this margin requirement from RVIA as far as I know.

This Reserve Load margin for trailers is more important than on Motorhomes due to the significantly higher Interply Shear imposed in trailer application.

Tire companies, do not know the exact loading that will be placed on their tires in RV application so you have to do a little work to learn the MINIMUM inflation needs for your personal vehicle. You could simply use the inflation on the Tire Placard but you still need to confirm, with scale measurement, that no axle is loaded more than the stated GAWR. It is also strongly recommended that you confirm your side-to-side load split is close to 50/50 as the tires do not "know" what the other tires are supporting, so you could be unknowingly overloading one tire by hundreds or even 1,000#.

I have other posts in this blog on how to learn the individual tire loads.

##RVT900

Optimum, Appropriate, Required Inflation ?

I have seen people use a variety of words to describe the inflation numbers on Vehicle Tire Placards.  These numbers are arrived at by different methods for different vehicles.

For cars and Light Trucks (1/2 ton), Tire pressure is almost always arrived at based on on-vehicle testing and evaluation by the engineers of the vehicle manufacturer in conjunction with the tire company engineers. There will be a number of evaluations done using prototype vehicles and requests will be made to the tire company to adjust ride, handling, noise and a dozen other objective and subjective qualities of the tires. Eventually, the desired compromise is arrived at with one of the variables being the placard inflation.

For most cars, the inflation is significantly higher than the normal expected GVWR. Many cars have reserve load 0f +20% to over + 30% of the normal vehicle loading. You will note that the tire placard for passenger vehicles do not show GAWR or GVWR.
Lightweight trucks (1/2 ton) will normally be treated more like a car but as you move the heavier vehicles Federal requirements kick in and by the time you get to HD 3/4 ton and certainly 1-ton units the GAWR & GVWR ratings become a higher priority.

RVs have different DOT requirements and also different Tire Placard requirements. Most RVs have the inflation specified based on just one criterion, that is load capacity.

DOT requires that the inflation specified to provide the load capacity of the tires when inflated to the specified level, be able to support half the GAWR.
RVIA in 2017 upped their requirement to requires to be capable of supporting 110% of the GAWR (IMO this is better but not enough).


So the bottom line is that for most RVs the inflation is basically the level needed to support 100 to 110% of the GAWR. But many RVs are not loaded to that level which means you can lower the inflation to achieve better ride as long as you don't end up with overloaded tires.

DOT doesn't trust the average driver to know the actual weight of the RV. They have done studies and discover that a significant portion of the population thinks they are OK but actually have tires in overload. So the DOT doesn't publish alternative inflation guidelines and RV companies follow what the government does.

It's just that simple.

##RVT886