Thursday, October 6, 2022

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RV Tire Safety: Correcting misinformation about tire inflation and load limits

Do you run your vehicle at or above engine redline? Why not? OK. As a tire design engineer and someone with RV tire experience, let’s see if I can clear up some of the partially correct and partially misunderstood information we see posted almost daily on one or more of the internet RV forums about tire inflation and load limits.

If you have a Load / Inflation chart for “ST”-type tires, you will find that ALL ST-type tires follow the same information. A chart published by company X may not have all the same sizes and Load Range tires as company Q. But when both X and Q make the same size and the same Load Range, the load capacity and the required inflation for the tires is identical.

The same applies to “P”- and “LT”-type. Now we need to pay attention to some other details on tire “type.” Some tires made outside the U.S. are not marked with a P, LT or ST before the size numbers. This means that they were made to some different standards. They may have been designed to comply with European or Asian standards, so their numbers will be close but not identical to the U.S. standards.

Inflation number in chart is MINIMUM required

The inflation number in the chart is the MINIMUM inflation required to support the stated load. Or, to put it another way, the LOAD in the chart for your tire is the MAXIMUM load the tire can support when inflated to the specific inflation found in the chart.

Some larger sizes with 19.5 or 22.5 size wheels make things a bit more difficult, as some follow the U.S. standards while others follow European standards. But there is no leading letter to help you know which. So, if you run tires of these sizes, it is more important that you learn the load capacity for the different inflation for those tires directly from the company that made them. The differences are not great, but they are not identical. So you can be off by 5 to 10 psi on maybe a couple hundred pounds load.

DOT load split assumptions are unrealistic

RV companies are required to select tires that can support the gross axle weight rating (GAWR), but trailers have different “assumptions” than do motorhomes. For trailers, the U.S. Department of Transportation assumes a perfect 50/50 split of the load between two axles, or 33% each when there are three axles on a trailer. They also assume a perfect 50/50 side-to-side load split on any one axle. Neither of these load splits are realistic. Those of you that have had each tire position measured know that many axles have end-to-end load splits of 48%/52% to 40%/60%. Individual axles on multi-axle trailers can also have significant variation away from a uniform load split.

Motorhomes also have similar problems with the imbalance of each end of each individual axle. Engine placement and dual tire position on the rear axle require paying attention to other details when consulting Load and Inflation tables. That is because “single” load capacity is always different than when tires are “dual” or side-by-side mounted, as seen on most motorhomes.

Many factors contribute to weight differences

Size and placement of water, fuel, propane, and holding tanks can result in 500# to 1,000# being in different corners of different model or different year motorhomes. So that is why you can’t simply ask the RV owner parked next to you about proper tire inflation numbers.

The inflation number on most Certification labels is the MINIMUM inflation needed to meet the above requirement.

A complicating issue is that in 2017, the RV Industry Association (RVIA) (gold oval sticker on the side of most RV trailers) added a requirement that the tire inflation be sufficient to support 110% of the GAWR. This partially addressed the known imbalance found on almost all RVs.

All of the above ignores the advantage of having some “Reserve Load” capacity. That is needed to allow for variation in loading of the RV that can occur if you carry more water than normal, or load more “stuff” than normal, etc. It also ignores the normal engineering practice of not designing components to have some level of margin of load capacity. It also doesn’t account for the occasions when trailers are towed at speeds above the tire design limit of 65 mph operating speed.

So … Do you run at engine redline?

You can think of running at the max load capacity like running your tow vehicle right at the engine redline. If you think that is a good practice to run with no reserve load, then I guess you also must think it is OK to run at engine redline for hours on end.

(click to enlarge)

Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT1058

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