Every few weeks I run across someone on an RV forum that makes the claim that it is the stiff tire sidewall that supports the load. As “proof” of their belief, many times they present a demonstration of pushing on a couple of unmounted tires. One is identified as a “cheap China bomb,” while the other is usually a light truck tire that was made in the USA. In their “test” they show how the “China bomb” tire can be collapsed with minimal effort while the light truck tire does not collapse. This demonstration is supposed to be the “proof” that “China bomb” tires can’t support the heavy load of a big RV, so you should run right out and buy some “12-ply” truck tires. Of course, it just so happens that the person giving the demonstration has some good heavy-duty tires on sale this week.
The problem with this “demonstration” is that it has little to do with the facts of tire “strength” or “load capacity”.
Tire Load & Inflation tables
Since you are reading this post, I trust that you have used or at least heard about “Tire Load & Inflation” tables. Here is an example:
These tables are available for almost every size and type tire made. Consumer-type tires definitely are included in these tables. In the U.S., these tables are published for the entire tire industry. The data presented in the “U.S. Tire & Rim Association” yearbook covers all the tires sold at retail in the U.S. Many tire companies publish their own versions for their tires. The tire company may offer some additional information. But if you look at the data, you will see that the loads reported by a tire company for any given tire are identical to the loads in the industry publication and follow the format seen above.
You will note that for any given tire the load capacity changes with inflation. This is because a tire’s load capacity is basically air volume X psi. (Note that is not the actual formula. The formula is quite involved and complex and includes exponential function and other adjustments.)
No tables for tire load capacity relating to sidewall construction
There are no tables for tire load capacity as a function of sidewall construction. The load rating (aka ply rating) is just a method of ranking the ability of any given tire to retain a given inflation. If you look at any Tire Load & Inflation table you may see more than one load range for a given size.
In the example above, we can see that a 255/70R22.5 can support 5,205 pounds when inflated to 110 psi. If that size tire is inflated to 120 psi, it can support 5,510 pounds. BUT to hold 110 psi, you only need Load Range G tire, but you need Load Range H if you want to inflate to a pressure above 110.
A tire company may make a given size in more than one load range. Therefore, when replacing tires it is critical that you order and purchase a tire of equal or greater load range than the specified original tires.
Roger Marble
Check out my Blog www.RVTireSafety.Net
Read more from Roger Marble on RVtravel.com.
##RVT1140
Willing to bet that more errors involve tire pressure than anything else.
Well the data of tens of thousands of RV tires being checked for both their actual load and actual inflation has established that over half the tires in RV application are overloaded with most of the overload due to underinflation.
That is very interesting. So the tires are capable of carrying the load safely but due to the owner having them underinflated for the actual load, the result is, if I remember correctly, excessive sidewall flexing causing heat buildup, breakdown of the sidewall and eventual “failure because of a manufacturing defect”. Weighing each axle, if not each tire, having a tire inflation chart for the tires and a TPMS would prevent a lot of failures. Thank you Mr. Marble.
Excellent article Roger. Keep these articles coming. I wish RV Travel had more writers like you and Dave and less of the current opinionated writers.
You gotta’ be kidding me! They are all excellent and talented. Maybe you could offer your expert skills to the journalist community!
Blow a tire on a trailer and watch the results….$1000 a pop almost every time maybe more since that was a rule of thumb some years back.
One of the best tools to have on an RV is a Tire Pressure Monitoring System. It was the first accessory I bought for our TT and it proved its worth more than once. Now that we are in a Class A, I bought enough sensors for both the rig and the dinghy.
Thank you, Roger! Safe travels! 🙂
Please explain to those who don’t know about Load Range C, D or E, speed rating and load index.
Hi, Charlie. Roger has hundreds of posts on RVtravel.com as well as even more on his own website https://www.rvtiresafety.net/ I’m sure anyone who has any questions can use the search box at either of those two locations to find all of the information they need (and then some) to answer any questions they have regarding tires. Have a great day. 😀 –Diane at RVtravel.com
“China Bombs” Wow, sounds serious.. I’ve used them (Toro and Dynatrak) for 20 years or more and haven’t blown a tire yet on two different class A motor homes. I check pressure religiously and now have a TPMS and still check them every morning or every day before departure. Seems I have a better record than some Goodyear models. I also buy American when-ever possible, even at higher prices – but there is no reason to be scalped or sweat our recalls or unknown defects until you have one!
Another great, informative article. Thanks Roger!