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Monday, June 23, 2014

Can I change from ST to LT tires on my Trailer or 5ver?

This question is a real "Hot Button" 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" 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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6 comments:

  1. The real problem is all rv tires are made in China and are substandard product if the RV industry would quit trying to cut prices and demand tires would be made in the USA the sub standard tires would not be used and you would have a better tire Thanks

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  2. No, any tire manufacturer, offshore or not, that sells ANY tire in the USA must meet the US standards for the tire. US manufactured tires fail at the same rate as those made outside the US. Most tire failures are due to overloading or under inflation. Invest in a TPMS to monitor what's going on with your tires and you'll save $$ in the long run.

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  3. I had a 2000 Avion 5th wheel that same standard with LT tires. I was very happy with them. I now have a 2007 Everest 5th wheel that came with ST tires. After a blowout I went back to LT tires considering both trailers were the same weight (just under 16,000 lbs). I also tried TPMS and within 300 miles had a blowout. I parked the trailer in Arizona (as a snowbird) and three months later when I uncovered my tires I found all were flat as the TPMS senders all leaked. Prior to this, in 28 years of rving, I had never had a blowout. I will stick with the LT tires as I have LT tires on my Ford Dually and all tires match which to me is very important.

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    Replies
    1. Sounds like improper install of the TPM sensors rather than tire design or manufacturing problem. Have you reviewed the analysis of "blowout" tires I have posted?

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  4. I am concerned with the sidewall flex built into the ST tires. If I change over to LT what will be the effect on the bearings. The scuffing that ST tires are designed to allow places a much greater load on the bearings when using LT tires. These side loads to the bearings could cause premature bearing failures.

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    Replies
    1. Not sure why the side loads would change based on your type of tire. The cornering forces needed to move the trailer in a turn are fairly consistent based on speed and turn radius. Otherwise the trailer would not make the turn.

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