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Friday, June 24, 2022

Do you run 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.

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 US 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 to their numbers will be close but not identical to the US standards.  
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 US 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 these tires directly from the company that made those tires. The differences are not great but they are not identical so you can be off by 5 to 10 psi or maybe a couple hundred pounds load.
RV companies are required to select tires that can support the GAWR but trailers have different "assumptions" than do Motorhomes. For trailers the US DOT assumes a perfect 50/50 split of the load between two axles or 33% each when there are three axles on a trailer and also a perfect 50/50 side to side load split on any one axle. Neither of these load splits are realistic as those of you that have had each tire positions 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 unbalance 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 &Inflation tables as "Single" load capacity is always different than when tires are is a "Dual" or side by side mounting as seen on most motorhomes.
Size and placement of water, fuel, propane, and holding tanks can result is 500# to 1,000# being in different corners of different model or different year motorhome 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 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 then known in-balance found on almost all RVs.

All the above ignores the advantage of having some "Reserve Load" capacity 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 normal engineering practice of not designing components to have some level of margin of load capacity. or for the occasions when trailers are towed at speeds above the tire design limit of 65 mph operating speed.
You can think of running at the max load capacity like running your TV 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 OK to run at engine redline for hours on end.


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