Tuesday, December 6, 2022

# RV Tire Safety: Tire “reserve load” calculations

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with RV tire expert Roger Marble

I ran across a post on tire Reserve Load or Reserve Capacity that suggested the RV owner had been given incorrect information. Here are the post and my reply.

Post:
Personally, I’d run LTs, simply because of their higher “reserve” capacity; upwards of 30% over the stated load. Given that STs have, at best, 10% (used to be basically 0%), you’re still in ST load territory, with a much better tire. Hell, we used to run our old 1/2t trucks with massive loads and just air up to 60-65 psi and go. Yes, it wasn’t very far, or very fast, but those tires still lasted 50-60k miles, usually with steel cord showing around the edges. ðŸ™‚ We’d then take them off and put them on a disk or trailer and use them until they sun-rotted.

All tires have a stated load capacity, for example, “2,340# Max Load” molded on the tire sidewall at a stated inflation level such as”50″ psi.

“Reserve Load” is the difference between the actual applied load and the stated load capacity and is many times stated as a percentage.

Example: A vehicle is on weight scales and we learn that a tire has 2200# load on the tire. The tire has a load capacity of 2,750#. 2,750 minus 2,200 = 550#, which is 20% of 2,750#. It doesn’t make any difference what type tire we are talking about as the math is still the same.

Now, it is true that for a given set of dimensions, e.g., 235/75R15, the stated load capacity is different depending on the type tire and inflation level. P-type and LT-type and ST-type each have different stated load capacities at their stated inflation pressure. For this discussion, let’s keep inflation differences out of the picture.

Let’s look at a P235/75R15 at 35 psi that is rated to support 2,028#. (In a trailer application P-Type must be de-rated by Load/1.1, giving 1,842# capacity.) An LT235/75R15 is rated for 1,530# @ 35 psi and an ST235/75R15 is rated to support 1,870#.

The 10% margin for trailers is the difference between the GAWR and the total capacity of the tires on that axle at their max load. I have posted in my blog some actual margins showing that many cars have load margins of 25% to 35%, while some RVs made before Nov. 2017, when RVIA changed the “margin” to 10%, had margins of tire capacity vs. GAWR as low as 1%.

Hope this helps.

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

Â ##RVT941

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