The answers given seem to range from "Sure, Why not" to "Absolutely Not, Never do it" and some will even offer that they think you are breaking some law if you make any change from the type, size, Load Range or cold inflation from the OE tire information shown on the tire Placard.
As we all should know by now answers about tires are never simple and straight forward and changing tires is definitely one of the more involved answers.
First off, I am not a lawyer but an Engineer and as such I form opinions based on data and facts. So here is the answer based on my 40 years experience as a Tire Design Engineer.
Yes you may be able to change tires
there are some things you MUST do to ensure that any change you make will actually improve your probability of having better tire durability.
My plan is to provide an outline of the steps you need to take before you make any change. I will try and include each step and each of the points of data you need to collect and evaluate. If you skip a step you may end up with a less durable tire selection which could lead to tire failure, RV damage and even an accident.
Before we start you need to consider that the most conservative approach is to make no change and to simply use the tire Construction (Bias or Radial), Type (ST or LT) and size and Load Range and inflation as specified on the placard and specification documents. This represents the RV manufacturers recommendation based on a number of assumptions as well as some legal regulations the RV MFG must follow plus in many or some cases a desire on the Mfg part to keep their costs as low as possible.
So if you still want to move forward here are the steps you need to take:
1. You need to know the actual load on each tire. This is important because A. we will be basing some decisions on the tire loading and B. It is possible that there is a significant unbalance in the tire loading which may be the cause of poor tire durability. With sufficient unbalance it may be impossible to provide a tire selection that would lower the probability of having problems.
To learn the actual individual tire loads you need to either find a company such as RVSEF or agency that has individual scales or to follow the steps outlined on worksheets such as This one or This one. I have hears some people say that they have been able to get the individual tire loads from their state police or state DOT.
2. Knowing the ACTUAL LOAD on each tire you need to confirm that no single tire is loaded more than the max load molded on the tire sidewall. This is an absolute rule. If any tire is overloaded you should not move the trailer until you either change the load or change the tire.
3. Assuming no tire is overloaded we want to make sure that all tires on en axle are inflated to the same inflation. For multi-axle trailers you can lower the internal tire structural shear forces (the forces trying to tear the tire apart) by running the inflation molded on the tire sidewall. Sometimes this is stated as the Max PSI and other times it is stated as the PSI for the max load. For our purposes we will consider this the proper cold inflation you should always run.
4. We should have "Headroom" or "Reserve Load" or "Safety Margin" on the tire loading. I suggest 15%. However I know that for many trailers 15% extra capacity above the actual load is very hard to do. Especially since some RV MFG manage to so under-size the tires that even when the trailer is empty they may not have 15% margin.
5. If you don't have at least a 10% margin I would strongly suggest you need to consider changing tires to something with more load capacity when inflated tot he sidewall PSI.
OK so you have some homework to do. We will continue this in the next post.
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