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Friday, June 21, 2024

RV tire types—Part 2: Changing from ST- to LT-type tires

 In Part 1, ST-type versus LT-type tires: Which are “best” for RVs?, we left off with having to do some calculations.

I will assume you have confirmed the actual individual tire loading and have moved some heavy items around to end up with a reasonable balance of loads.

I will also assume you still want to change from ST-type to LT-type tires. That means you must increase the Load Range and/or increase the tire size to get a load capacity in the LT to match or exceed the capacity of the original equipment (OE) ST-type tires.

Load & Inflation tables

If you compare the Load & Inflation tables for ST-type and LT-type in the tire Data books (ST here or LT here), you will see that there are no LT tires of the same size or Load Range as an ST-type that can support the same load. You can confirm this by also noting the “Load Index” which is a pair of numbers like this:


The dimensions are obviously 225/75R16 and the Load Index numbers are 115 for single and 112 for dual application. The “R” is the “Speed Rating” for this tire which, for RV tires, is a relative rating of the heat resistance of the tire. Tires in RV application are limited to 75 mph max, except for ST-type tires, which have a max of 65 mph, based on the Load Formula limitations. (I will cover speed in another post.)

Before we move on, you need to realize that “LT” is a designation used here in the U.S. In European and some Asian countries they have what they call “Commercial” tires. These Commercial sizes do not start with LT or CO but will probably look like 7.00R15, or for metric sizes, 205/75R16C. The “C” in this case is not the Load Range but stands for “Commercial”. The Load Range will be identified as normal LR-C, LR-D, etc., or possibly with R or XL for Reinforced or Extra Load. To make this post easier to read, I will limit my comments to LT-type tires. Just remember there are other options that may be better for those with 15″ or 14″ wheels that do not want to change rims.


NOTE: All of these letters and numbers are important when selecting a size, so be sure you record them all when doing your research.

So, on to the next step:


There are two key dimensions: Outside Diameter, or OD, and Width. I am confident that we all understand OD, but width can be a bit confusing. Depending on the wheel well contour, the overall maximum width or “Section Width” may be most important. Some tires may have a narrower clearance nearer the tread, so they will need some actual measurements at a number of locations.

It may be easiest to use the dimensions for OD and “Section Width” published for your current tires and just do a confirmation with your tape measure. Remember tire “width” is not the same as tread width.

You need to be sure the tires NEVER contact any portion of the RV frame wheel well or bodywork. This is especially true for the front position of a motor-home but since we are focused on ST to LT, there should be no ST tires ever placed on the steering axle of an RV. You should try to have equal or greater clearance with the new tires than you have on your original size, if at all possible.

The challenge

When moving from ST-type to LT-type, you will need to move up in Load Range or up in Size, or both.

Now comes the research to see what your options are

Knowing the target Load Capacity and the maximum OD and Section Width, it’s time to use the Internet to do some research. The objective is to find tires that meet your needs for the numbers and that have an appropriate tread pattern. You certainly don’t need Snow Tires or heavy traction tread pattern for normal RV use. I would suggest that the tread be identified for “All Position” or Steer for your trailer application. I run “HT” type tires which stands for Highway Traction.

You can go to websites from large dealers such as Tire Rack, Pep Boys, Walmart, NTB, Discount Tire, or similar. You might also just Google “Trailer Tire” + the name of a large city or town near your location. Once on their website, find the various possible tires that meet your needs.

Next, confirm inflation

If you are increasing the Load Range with the associated increase in inflation, you need to confirm the wheel can manage that higher inflation. The info may be marked on the back side of the wheel, or you may need to contact the wheel seller or manufacturer. Or you may need to get different wheels if your original equipment seller doesn’t know what the rating is for the OE wheels.

 Consider tire warranty

Finally, as I have previously suggested in any comments on “The Best Trailer Tire,” you need to make your purchase decision not just on lowest price, but you also need to consider the tire warranty, if there is one. Or check if there is a Road Hazard Warranty. Also, look into how easy it will be to get a replacement if your tire gets a sidewall cut or un-repairable puncture.

Last step

After all this work, we want to do a first class job. Some might want to say you can’t change from ST-type to LT-type due to Federal Regulations. Well, that just isn’t true. The Certification Label on your RV provided some MINIMUM STANDARDS for Load Capacity and Inflation. There is nothing wrong with increasing the Load Capacity of your tires.

I hope these two posts have helped you understand the steps, calculations, measurements and research needed to make such a change.


Thursday, June 13, 2024

So what speed do you want to drive with your trailer?

 I see many folks claiming their ST type tires have "speed Rating well over 75 mph.

They are basing this on the "heat Resistance" test that runs tires at 88% of the max load for 10 minutes steps up to the "Goal" speed. If a tire can run 10 minutes at that speed the tire is said to be "speed rated" for that speed.

I do not recall seeing any advertisement for ST type tires that mention speed mentioning "at what load"

Here is the info directly from US Tire & Rim Association. 

Notice the adjustments for inflation and load used to allow a tire to run faster than 65 mph.

Sunday, May 26, 2024

ST-type versus LT-type tires: Which are “best” for RVs? – Part 1


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.





Wednesday, May 8, 2024

Why and how to take care of your tire valve cores


Tire valve cores are critical components to retain air in your tires.

Here are the components of a standard “snap-in” rubber valve. On the left is the rubber body of the valve stem. In the center is a standard valve core (I have a paper clip holding the valve open). On the right is the basic rubber valve cap.


Untold billions of these “snap-in” valves have been used for many decades in almost every passenger vehicle, light truck and RV trailer since the ’50s. They are cheap and usually work at holding the air in the tire. Normally, they were replaced whenever a new set of tires was applied to the vehicle, so the life span of this type of valve was only a few years.

Parts of the valve core

One feature of the valve core is visible in the above picture. The air is let in or out when the center pin is depressed and the “valve” portion, where the paper-clip is, is opened. The small red sieve is the gasket that sealed the valve core in the body of the stem. The valve core can leak, as seen in the below picture.







I read the questions in a number of RV forums, on what valve stem to use with an external TPMS sensor. This post will cover the “bolt-in” metal valve stem.

This picture shows two problems with cheap “snap-in” rubber stems. One problem is its flexibility, and the other is the fact that rubber gets old and can crack, which can lead to a leak. A standard 65 psi max “snap-in” rubber valve stem is very flexible. The weight of a tire pressure monitoring system (TPMS) sensor can cause vibration of the rubber stem and potentially a stem failure.








The addition of an external TPMS sensor can, in some cases, accelerate the cracking due to the extra weight out on the end of the stem.

Here is a rubber stem with a TPMS sensor. You can see the mark left on the wheel from the rubber stem bending.











Some people think the 80 psi max “high pressure” HP-600 rubber valve stem is OK to use with external TPMS sensors, but you can see the HP-600 is also flexible.

Rubber stem can fail with TPMS installed

Here is proof that an HP rubber stem can fail when a TPMS sensor is installed.

Leasking HP stem  video


This valve stem failure resulted in a tire failure.

In my opinion, staying with any “snap-in” type rubber stem is false economy, given the metal bolt-in stems only cost $3 to $5 each. Not all tire stores will have the bolt-in metal valve stems in stock, so check first. If they don’t have them, you can get bolt-in stems at AutoZone, O’Reilly’s, Advance Auto Parts, NAPA, or most any auto parts store or even on Amazon.

They are easy to install, too. Don’t let the service center tell you installing metal stems is a lot of work.  Watch and you will see.   HERE


Rubber snap-in stems

On standard “rubber snap-in” stems, like the TR413, if you look down the hole you can see the end of the brass part of the stem.







 These have been used for decades on hundreds of millions of tires. These can be installed by hand using a “puller” that stretches the rubber. That makes the diameter of the stem small enough to “snap” into place in the wheel hole. The “puller” in the picture is the tool with multiple notches that allows leverage to be used to generate the force to “pull” the rubber stem into the hole in the wheel.




 Once installed, the wheel “pinches” the rubber part of the stem to seal the air in. 

High pressure stems

Next, we have the “high pressure” stems, such as the HP-500.










The Diameter shown in the picture is for the the standard hole of 0.453" This is an industry standard so don't try to find some at 0.500". 

Bolt-in stem

Now, when we look at a “bolt-in” stem, like this TR416s, we see the location in the wheel hole at the arrows. We can also see the much larger brass body (nickel-plated in this piece) that goes inside the air chamber and expands to a broad base. This type of stem needs to be installed through the wheel hole with the rubber grommet sealing the air. An external washer is used and the nut is to be tightened to specification to prevent air loss.

Here is the metal part of the bolt-in stem without the rubber gasket. The arrows point to where the wheel would end up.










Here are a couple pages from the U.S. Tire & Rim Association yearbook, aka TRA, which publishes the “interchange and fitment” specs so all tire companies and valve manufacturers know what dimensions are required. This is the book where all the Load & Inflation tables come from and might be considered the Tire Engineer’s “Bible”. It is used by tire engineers around the world when they are making tires that are intended to be used in the U.S.

Here are the dimensional specifications used by valve manufacturers and some details on the smallest but potentially the most critical part on your RV, the valve core, that we covered previously.







They even include something as relatively insignificant as the height of the little pin that sticks out of the valve stem.

As you can see, there is a lot of engineering work involved in the valve stem system. If the proper stem is installed correctly, the system should operate with no problems for many tens of thousands of miles. But as with any system, if incorrect parts or improper method (torque) is used, you may have problems.

Wednesday, April 17, 2024

How to select a tire for your RV


If you want to know what tires to get for your RV, I can tell you that as a Tire Design and Forensic Engineer, the most important feature is that is it rated to support the weight you are placing on the tire when the tire is inflated to the level you set it for. 
The Max load is spelled out on your certification label affixed to your RV and is stated as "GAWR" which is the max load ever allowed on an axle. BUT it is important to remember that the tire load capacity depends on the "cold inflation" you set the tires to. If you do not have at least that level of inflation when you drive down the highway you will be doing damage to your tire. 
The MINIMUM inflation you need to achieve that load capacity is also clearly stated on the label. 
These inflations are the "cold" inflation you set your tires to BEFORE you start driving and you should never bleed down the psi after you start driving each day.
I hope this helps answer some of the questions I see in many RV forums.
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Monday, March 4, 2024

Do I need to balance my RV tires?

 I was recently asked about tire balancing.

Reasonable question but as always the answer starts off wit "It Depends"

Vibration can be because of a tire/wheel suspension component is out of balance. Vibration can also occur because the tire or tire and wheel assy is not "round". On a small (14' to 16") tire you can see and measure out of round when the tire & wheel are placed on a spin balancer.   I have shown that you can "balance" a square cement block

  so you need to be sure your tire & wheel assy is "round within at least 0.030"

The person asking the question, said that they had done some research and learned there were three methods. They wanted to understand the advantages of each and which they should use for their Class-A RV.

The basic method is static or "bubble balance". as seen in this  video

The mounted tire is placed on a balance and the heavy spot is counter balanced with weights.

This static balancing is lower cost but does not provide the best balance. This method is not usually done any more by full service tire stores as it only affects the "static" imbalance and with today's light weight cars the driver is more likely to feel even a minor imbalance. I once had a car with one front tire 1/4 oz out of balance and on a very smooth portion of the interstate on my way home from work I would occasionally got steering wheel movement. A re-check at the store solved the problem. The car was a small light weight sports car and it just happened to be sensitive at that level. My one-ton dually pickup was not sensitive at the two ounce level on the rear axle.

The next best method would be with the mounted tire on a "spin balancer", This rotates the wheel and tire at speed and electronically calculates where to place the weights.
When you buy a new passenger or pick-up truck tire, this is the method they are normally talking about.

But truck/bus size tires can also be balanced using a heavy duty version of this type of machine.

Spin balancers measures the up-down imbalance and the side to side balance and tells the operator how much weight to place on both the inside and the outside of the wheel to counteract forces in both directions.

 Finally there would be "On-Vehicle" spin balance this would give the balance for the tire, wheel and the brake drum and hub of the vehicle so if the drum was slightly out of balance it would be included and weights would counter balance all the spinning components. If you get this type of balancing done it is important to mark the wheel position on the hub if you ever remove the wheel to check brakes and to re-mount the wheel in the exact same orientation. A downside to this method is that it can't be done to tires on drive axles.

Many drivers of Class-A do not balance their tires as they do not feel the imbalance. Some others always balance the fronts because the driver & co-pilot are sitting almost on top of the tires. On-vehicle spin would probably give the best results but this would be for the front only. Here is a video showing the process on a Corvette
 but RV tires would be the same process but with HD bigger equipment.

I see little reason to balance the rear duals on a Class-A as you will not feel the balance problem unless something was very out of balance.

Some additional info in another post.

For Class-A I think you can just take the RV out for a quick test drive on a nice section of smooth Interstate. If you feel shaking either through the steering wheel or floorboards then you would go and have the front tires "on-vehicle spin balanced".

For Class-C and smaller vehicles using 16" diameter LT type tires I would spin balance all six assemblies.

Saturday, February 24, 2024

How much air pressure do my motorhome tires need?


Tire loads and proper inflation for motorhomes

Today’s key points: Know the minimum tire inflation based on tire industry guidelines. The basic instruction for your minimum RV tire maintenance is to check your inflation with a good gauge at least monthly and every morning before travel.

Tire inflation seems to be a topic that confuses some and has others believing in misleading or just plain incorrect information. Tire inflation is one item that directly affects the safety of your RV, truck, or car as you travel down the highway. Many of my posts have outlined information that you really should know and understand. The intent of these posts is to give you a better foundation of understanding more about tires. But if you only pay attention to one series of posts, this is it.

Tires do not carry the load

Some people have been led to believe that the load is carried by just the tire sidewall. This is not correct. Tires are just a container of air. It is the inflation air that does the work. Think for a moment of an impact wrench. It can’t do the work of loosening or tightening nuts on your wheels without the air. A tire can’t do the work of carrying the load or providing the traction needed to turn, start or stop if it doesn’t have air. The load a tire can carry is based on the air volume in the tire and the pressure of that air inside the tire. You can see this if you look at any Load & Inflation chart, as found HERE.

 it is important to understand that these tables are essentially identical across all tire companies that are making the same size tire.

Load Range vs. Ply Rating

Some people believe that tires with a higher Load Range can carry more load at the same inflation. This is just not correct. Don’t forget that the term Load Range replaced Ply Rating with the introduction of radial tires to replace bias tires. You would be hard-pressed to measure the uninflated load capability difference between a Load Range D and E tire or between a G or H Load Range tire.

If you still think the load is supported by the tire construction, I would challenge you to find a Load vs. Construction table. Here is an example of a Load & Inflation table for a 255/70R22.5 and it covers both LR-G and LR-H tires in that size.



 Sidewall stamping examples

Here are sidewall stamping aka “marking” examples from a large P-type tire showing the dimensions, along with Load Index and Speed Rating. An LT-type tire would look similar except that LT-type tires and Commercial Truck/Bus (22.5″) can be applied as single or dual (side by side). So they would have two sets of numbers for the Load Index (single and dual application). Commercial truck tires are not speed rated, but Michelin and Goodyear and others indicate a 75 mph Max operating speed in RV applications no matter what the tire rating is.


 Here are the tire reinforcement materials, both type and quantity.



Next, we see the Load and Inflation ratings for this standard load P-type tire.



Nowhere do we see a statement on construction or Load Range for various inflations and different load capacities for different constructions.