Wednesday, December 7, 2022


RV tires: “safety factor” versus “reserve load” – What’s the difference?


RV Tire Safety

with RV tire expert Roger Marble

The dictionary offers this definition of “safety factor”: “The ratio of the maximum stress that a structural part or a piece of material can withstand to the maximum stress estimated for it in the use for which it is designed.”

While that sounds reasonable, it really only works when talking about pieces that fail from simply increasing the load placed on the component.

Items like tires do not really have a “safety factor,” as tires generally do not fail from simply increasing the load too much. In a non-rolling situation, I would not be surprised if we could load tires to 200% or maybe even more than 300% of the load marked on the tire sidewall. However, as soon as you introduce rolling or time or operating temperature, the maximum load before failure is much closer to the max load molded on the tire sidewall. The exception to max speed is affected by temperature, time and load. With zero load many tires can probably handle 200+ mph but, again, for how long and at what temperature?

Since tires are basically a structure made of “organic” components, time and temperature can have a significant impact on the maximum load capabilities of the tire.

If we think of non-organic items like a steel girder or maybe even a stone block as used in the pyramid, we can see that time and normal atmospheric temperatures have essentially no impact on the long-term maximum strength. The exception would be if we were to allow steel to rust or stone to be exposed to water and freeze/thaw cycles.

Tire engineers prefer to use the term “reserve load” when talking about the load capacity of a tire. Here we would find the tire engineer definition as the difference between the tire’s maximum capacity when inflated to the stated level for the specific application (the inflation on the tire placard) and the actual load to be placed on the tire.

Here are a few comparisons:

First, some normal car and truck applications:

Next, a larger 5th wheel RV:

When you compare the reserve load percentage of the different groups you can easily see the different level of reserve load.

What should the reserve load be for your RV? Currently, RVIA considers 10% to be the minimum reserve load. However, the few actual tire engineers that are posting on RV forums are suggesting a MINIMUM reserve load of 15%, with more being desirable.

I know that on my Class-C I am running closer to 20% reserve load based on actual “four corner” weights, i.e., individual tire position scale readings. What is your actual reserve load?

Read more from Roger Marble on his blog at



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2 years ago

Regarding the trailer example: why use the GVWR rather than the actual weight of the trailer as it is typically loaded. In my experience, few people load their 5th wheels to the GVWR. In addition, when (or if) it is loaded that much, I would be very confident that the pin weight would (and should) be way more than 2,210 lbs. The pin weight would likely be closer to 3,000 and likely even more…as it should be (20% approx. pin weight).
I would figure that there would be closer to 12,000 lbs. on the tires which would drastically change the calculation.
15k trailer weight minus 3k pin weight would be far more likely, hence the 12k on the tires. That would make the calculation 12,000/(4×3520) or just over .85 or a reserve of approx. .15 (15%).

Michael L Johnson
3 years ago

I am at 38% with my PrimeTime 325RST.

Bill Lampkin
3 years ago

Always informative, Roger, thanks! Off topic, but not by much; any data on how big a risk to park motorhome on asphalt while in storage? That’s what I have to do, and I’d like to know if there are any objective data or studies that can quantify this risk? A lot of trucks and cars park on asphalt for extended periods. I don’t want to diminish the life of my $800 ea motorhome tires.


Roger Marble
3 years ago
Reply to  Bill Lampkin

Sorry but I don’t know of any clean studies on parking on asphalt. It’s just one of those numerous items in life where there are indications that suggest the oils in tar can do microscopic damage to the tire rubber. How much would depend on how fresh the tar is and the time and the temperature. It’s just not the best practice. Maybe think of it as eating burger & fries. If you did that every day for every meal I think you would agree that’s not good for your heart or weight. Occasionally is not so bad. Where do you draw the line?

2 years ago
Reply to  Bill Lampkin

I’m not sure why you feel you must park with your tires directly on asphalt. Park instead on pieces of wood (boards) which are wide enough and long enough that the entire tread area is on the wood, not on the asphalt below it. Or am I misunderstanding your concern?

Roger – any comments?


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