"Reserve Load" or Load Capacity Margin

Ran across a post on Reserve Load or Reserve capacity that suggested the RV owner had been given incorrect information. Here is the post and my reply.

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. :-) We'd then take them off and put them on a disk or trailer and use them until they sun-rotted.

I think someone miss-informed you about "Reserve Load".

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 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 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#

BUT the "Reserve Load" calculation is still  (Tire Load Capacity)/Measured scale Load).

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.

##RVT941

Response to some of my information and warnings on tire inflation

After posting on one RV Forum some steps that I felt if taken could result in longer tire life by lowering the Interply Shear Forces, I got this reply:
"Sounds like tires should never be used or stored in any configuration other than the 'as cured' state or they simply self destruct.
Gonna have to figure out how to make these R/Vs hovercrafts."

I offered the following:
A bit of an over-reaction. My advice is intended to offer a series of steps that can be taken to extend the life of their tires rather than actions or inaction that may shorten tire life.

One of the biggest problems is the inability to understanding the real "Root Cause" of tire failure. IMO too many simply assume that somehow, the zip code of the factory where the tires are made is a "cause" for a failure.
Have you read and do you understand the difference between the two major and different reasons (root cause) for failure as covered in THESE posts?


TPMS, when properly PROGRAMED and used  can essentially eliminate one of the two primary reasons for tire failure.


Having a good level (15% to 25%) Reserve Load is the second major thing people can do to get a more reasonable tire life. I would be very surprised to learn that tires with at least 15% actual Reserve Load didn't perform much better than the more normal 0% to 5% level.

Is the Goodyear Endurance ST type tire "Better Quality" than a Goodyear LT tire?

Had a couple of comments on my post on LT vs ST type tires for RV trailers

Alan said

"Now, I am VERY confused. For years, everyone in the RV business, its commentators, its experts and other blogger forums so-called experts “having used this or that for soooo many years without any problems”, etc, had me convinced in believing all the hullabaloo about the ST tire’s sidewalls being much better, much stiffer and much more resistant to the particular type of flexing that occurs when driving, turning and parking travel trailers or fifth wheels than LTs. After the appearance of the “china-bombs syndrome” a few years ago, people started to realize that practically all ST tires were manufactured in China anyway, whatever the
“US brand name” stamped on them, leaving the consumer with very few, if any, alternatives.
Now, coïncidentelly, we hear that LT tires are “just the thing” for trailering.
It also appears that a handful of RV manufacturers, like Jayco, have seemingly struck a deal with Goodyear, who now has resumed manufacturing ST tires in North America, to equip their new trailers with “US-made ST tires”. I guess their ST tire prices have dropped again enough to catch the big manufacturer’s attention and make it worth their while.
So I guess the real question should now be, are US-made Goodyear ST tires better, worst or equal quality than their US-made Goodyear LT tires, or why even bother to engineer ST tires at all? We shall see, in time, who wins between; Beta or Vhs, Plasma or Lcd, Android or Apple, etc, etc, etc."


Alain, The biggest issue is, I know of no way to do a direct comparison of ST vs LT tires other than an expensive tire test. We can't depend on the DOT test results. In all probability the tires all pass, simply because the RV industry and ST tire mfg companies were successful in getting ST tires excluded from the significant improvement in testing with updated standards in 2002. There is no question, in my mind, that the tests (FMVSS 571.119) for ST type tires are easier to pass than the tests (FMVSS 571.139) for new Passenger and LT type tires.
I have looked at the construction of a GY Endurance and it certainly looks better than the older Marathon design. Now remember I do not have access to the actual specification or material properties used by GY but in the Endurance tire line, they appear to have added a Nylon cap over the steel belts. You can confirm this by reading the material list molded on a tire sidewall.

"Quality" is a tough call when you have two different specifications to start with. Isn't Quality just a measure of a tire's ability to meet the specification for that product?

You can think of it this way.  If I had two pieces of chain. One rated at 500# and another rated at 550#. They both meet a strength test of supporting the rated load and if you tested 100 pieces of each and found that all 100 passes the rates strength test how would you rate the 'Quality"? You can't say the 550# chain is better quality than the 500# chain as they have two different goals, just as you can't say a 1-ton truck is better quality than a 1/2 ton truck because the 1-ton can handle more load.

Your example of Beta vs VHS is a good example of the confusion possible. Beta was judged technically superior to VHS but Sony made a marketing decision to prevent other companies from using the Beta specification in their video players. The result was that VHS units were less expensive so many people bought on price. Eventually, Sony lost out. Not because the "Quality" of that video format wasn't as good as VHS but because the market was "price-sensitive" and too many people selected lower cost over better quality product.


Gene offered
"OK, color me confused! I have always gotten the impression that ST tires and not LT tires are what I should be running on my RV. After reading this article now I’m not sure. Are you suggesting that all things being equal, LT tires of the same size, load rating, etc are better to run on my RV? I don’t mind the cost, just want to put best possible tire on my RV. Thanks for keeping us informed."

to which I replied

Gene, The problem is that "all things are not equal" as there are no LT tires of the same size, load range, and load rating, etc as an ST tire. There is always something that is different.
Sometimes the only option would be to go with larger size LT tires but in some cases, there is no physical room to run the available larger size tires.

IMO, what you "should" be running are tires built to the latest industry test standards (LT) that can offer at least 15 to 25% "Reserve Load " capacity

Is it against federal regulations to change tires on an RV?

I have been following a series of posts on RV Forums where people ask about changing tires [Size, Type, Load Range or cold inflation setting]. Occasionally I run across some people who have adopted a hobby of commenting on tires in RV application and with a little working knowledge, make pronouncements on the "legality" of making any change in tires, that I do not agree with.

Other times I see a question like this one;
 "My research (curiosity vs need) is that LT and ST tires are not sold in the same sizes, so changing RIMS would be required?"


Here is my reply

Some LT tires show the same "dimensions" for example (235/75R15) as some ST tires but the "Dimensions" are NOT the actual complete size "description" which includes the letters and numbers before and after the dimensions. i.e. ST235/75R15 110/105L LR-C  vs  LT235/75R15 110/107T LR-D.  In this example, the ST tire is rated for 2,340#@ 50 psi while the LT tire needs 65 psi tp support 2,335# (single load capacity shown) Also the ST tire is rated for a max of 75 mph while the LT is rated for 118mph operational speed

Yes there are some ST tires where the dimensions do not match any tire identified as an LT type currently on sale. This is where you have to do some research and learn some facts on what can be considered a reasonable and safe change in tire type or size.

The Key items to confirm:

1. Is the Load Capacity of the new tire equal or greater than to original tire when you consider your new intended cold tire inflation? This does not mean +/- 25#. It means "equal or greater"

2. Is the Speed Rating equal or better than the original tire when you consider your intended new cold inflation level? (yes some tire load capacities are a function of speed)

3. Are your wheels rated for the inflation level you intend to run with your new tires? This limit may not be easy to learn but wheels can fail from too high a pressure just as they can fail from too many pounds load. The pressure we are talking about here is always the COLD inflation pressure.

You may need to be smarter than the tire guy if you want or have to make a change.

Remember there are some who post on various RV forums who would tell you that you would be violating Federal Regulations if you change from a Goodyear Marathon made in 2016 to any Goodyear tire made in 2019.

While it is definitely true that some people make changes in tire "dimensions" or "type" or inflation level which IMO as an actual tire design engineer, I would consider unwise. BUT that does not mean you can not consider making changes as long as you follow the guidelines posted above that follow the published guidelines from major tire companies.