ST-type versus LT-type tires: Which are “best” for RVs? This question is a real “hot button” topic on a number of RV forums and blogs. People ask this question because they want a “better” or alternate selection of brands, or they want to improve the durability of their tires.
The answers given seem to range from, “Sure, why not” to “Absolutely not, never do it.” 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 original equipment (OE) tire information shown on the Certification Label.
As we all should know by now, answers about tires are never simple and straightforward, and changing tires is definitely one of the more involved answers.
First off, I am not a lawyer but an Engineer. As such, I form opinions based on data and facts. So, here is the answer based on my 45 years of experience as a Tire Design Engineer.
Yes, you may be able to change tires. If you do make a change, there are some things you MUST do to ensure that any change you make will actually improve your probability of having better tire durability.
Steps to take before changing tires
This post offers an outline of the steps you need to take before you make any changes. I will try and include each step and 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.
First, we need to be sure that everyone understands that the “type” of tires we are talking about have the letters “P”, “ST” or “LT” in front of the dimensions numbers. “P” stands for Passenger, “ST” stands for Special Trailer, and “LT” stands for Light Truck. If your wheel size is 14, 15 or 16 and does not have P or ST or LT in front of the size numbers, then it is probably a European Metric tire.
If you have passenger-type tires, you need to know that the load capacity marked on the tire sidewall of that type MUST be reduced by 10% based on tire industry standards. There are not many RVs being delivered with P-type tires except for very small trailers such as “teardrop” or similar-size trailers.
Be sure you’re using correct table
ST- and LT-type tires have their own load and inflation charts, so be sure you are following the correct table. We need to be careful here as many times you will see a post on an RV forum where the person does not include the “type” letter in their post.
Finally, we have European tires, which look similar to U.S.-type tires. They have different load and inflation tables and even have both passenger- and commercial-type tires. I will cover the application of “Euro-Metric” size and type tires in a future post.
Special care is needed when choosing tires, as mixing up the type can result in tire failure due to improper inflation or load.
Before we start
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), size, load range and inflation, as specified on the Certification Label and specification documents. This represents the RV manufacturer’s recommendation based on a number of assumptions as well as some legal regulations the RV manufacturer must follow, plus, in many or some cases, a desire on the manufacturer’s part to keep their costs as low as possible.
Here are the steps to take
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 RVSafety Education or an agency that has individual scales, or to follow the steps outlined on worksheets such as this one. I have heard some people say that they have been able to get the individual tire loads from their state police or state DOT. RV event conventions run by Escapees or FMCA also offer the service of individual tire weighing. While learning the load on each tire is best, an alternative is to get individual axle loading and assume that one end of each axle is supporting 52% of the total load. This is just an approximation, but is better than nothing.
Make sure no tire is loaded more than maximum load number on sidewall
2. Knowing the ACTUAL or calculated LOAD on each tire, you need to confirm that no single tire is loaded more than the maximum load number 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 an axle are inflated to the same inflation. This means the inflation psi required for the tire with the most load is the inflation psi for all tires on that axle. 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. That is especially true since some RV manufacturers 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 to the sidewall PSI.
OK, so you have some homework to do.
I will have more in Part 2 next week.
