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:

Dimensions

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.

 

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.

 

 

 

 

Should I run ST tires on my trailer or can I use LT type?

 When it comes time to replace your tires on your RV trailer you may have a few options. Sometimes you may also be confronted with a limited supply problem that limits your selection. What can you do? What are your options?

Now I am assuming you have followed the general instructions to get your RV on a truck scale and as a minimum confirmed the load on each axle when the trailer is loaded to its heaviest. I have numerous posts in this blog on scale weights and how to calculate and estimate the load on the heaviest individual tire

So knowing you need a tire that can support at least 115% (125% is better) of your measured or calculated heaviest load you are ready to start shopping.

I had one reader express some concern about what he called "Sidewall Shear" forces and thought a stiff sidewall would be a better selection. I have no idea how he was going to measure the inflated sidewall stiffness and pointed out that un-inflated sidewall stiffness was not a reliable predictor of the inflated stiffness.

Here is what I told him:

I would not worry about "sidewall shear" as the real culprit for belt forces is Interply Shear at the steel belt edges which can lead to a belt separation.

Sidewall failures, which some confuse with "blowout" is the result of running at highway speeds with significant (40% to 60% or more air pressure loss).

THIS post shows the forces that cause belt Interply Shear. Don't be mislead and think its the sidewalls that are being overloaded. It's not.

You can get warned about the Sidewall Run Low Flex with the use of a TPMS that is properly programmed to alarm as soon as air pressure drops below the level needed to support the measured load on your tires. This comes from scale measurement and consultation of tire Load & Inflation tables.

Belt separations develop over hundreds to thousands of miles. A TPMS will not warn of impending belt separation BUT sometimes it is possible to "see" indications of possible or probable belt separation with a close visual inspection to check for run-out. Either Radial or lateral. I posted a link to a video on a "Free Spin" inspection in THIS blog post.

The GY Endurance tires I have inspected included Nylon or Aramid belt cap in addition to the 2 steel belts. If you find an LT type tire that can support 115% or better of the scale load on your heaviest tire that might be an acceptable alternative if it also has Nylon or Aramit cap belt.  LT tires have to pass more difficult DOT testing than ST type tires so that should give us the confidence to use those tires in RV applications.

Finally, you need to limit your MAX operating speed to 75 mph with LT type tires and to 65 with ST type tires. Even though some ST type tires have a "Speed Symbol" letter that implies speeds higher than 65 we need to remember that the test for those speeds is only 30 minutes and in reality the Speed Symbol is really just an indication of high heat resistence. The load capacity of ST type tires is based on a formula that specifice 65 mph as the max operational speed.

--

##RVT1012

Another post & question of "Trailer tires vs Truck tires"

Original post and question on a 5th wheel RV forum I follow.
"Searched this forum (no, didn't search travel trailer forum) for any info on using LT (truck) tires instead of "trailer" tires on a 5th wheel.
Nope, didn't find anything - So,
Any problems with using a 6 or 10 ply LT tires on my '99' trailer instead of the replacement tires for trailers?"

He gave the dry weight and stated GVWR but no actual scale readings. He also did not provide information on his current size or type or Load Range of his tires. He did provide info heard around various campfires. In the forum, there were a number of replies. A few OK and sadly a number just plain incorrect.

In the'60's RV trailers were mostly smaller single axle and towed behind a car or 1/2 ton P/U. RV companies wanted less expensive tires. Speed limits and actual travel speeds were lower so Goodyear came up with "Special Trailer" tires with a 65 mph max speed as part of the load formula specification. This speed reduction and decreased tread depth theoretically offset the increased load capacity when compared to the same dimension LT type tires.

In 2002 after the Ford Explorer fiasco, new tougher standards for tires were implemented by DOT. RV companies fought to keep the old test requirements for ST type, the same as they were in 1968, while P-type and LT-type had to meet new, tougher standards for the 21st Century.

In 2017 China was accused of "dumping" cheap tires of all types into the US market. Trade restrictions by FTC (not tire safety standards) identified speed-rated tires as not having to pay the import duty. Almost overnight almost all ST type tires became speed rated. The SAE test for speed rating is stated as a "passenger car tire test" but was applied to ST type. It only requires tires to run for 30 minutes at the stated speed and to not come apart.

Back to the OP. "Dry weight" and GVWR are of no value when selecting tires. GAWR is almost useless as it is well documented that a majority of RVs have tires and /or axle in overload when actual loading is measured on scales.. The only weight number that really means anything is the actual scale reading for each tire, as RV weight is almost never exactly split 50/50 axle to axle or side to side. Some big RVs have been found to be 1,000# or more out of balance.

Reserve load is the extra load capacity above the actual load. Cars and P/U have reserve load in the 20% to 40% range while RVs have 0% to maybe 10%. That is a MAJOR reason for the difference in durability. There is also the shear forces seen in trailers that can be 24% higher than an identically loaded tire on a motor vehicle. This is a function of suspension dynamics. Tire Interply Shear was a complete unknown back in the 60's and not as well known or understood even in the 80's. You see the large tread distortion due to lateral loading when backing into a parking spot when the tire bends sideways. You never see this bending on a motor vehicle. The shear is always there in every curve or turns not just when backing into a parking spot. Even the normal sway observed when a trailer is towed down the highway is generating high Interply Shear. You can lower, but not eliminate the Interply Shear by increasing your Reserve Load.

So how do you improve your Reserve Load? An increase in load capacity would be a good approach. For most trailers, there are a number of options. Increased Load Range is one as long as you confirm wheel limits on load & inflation. Changing to larger tires. Even changing to larger diameter wheels might increase load capacity. I have seen some that increase all three specs, with reports of eliminating all tire failures other than road hazard or valve-related issues that can occur on any size, brand, or type tire.

If or when you replace tires the new tires should ALWAYS at a minimum have equa load capacity.

I have covered the above in numerous posts on my blog if you care to learn the facts from an actual tire design engineer.

Or you can listen to the guy in the camping space next door or the salesman at "Billy Jo Bob's Cheap Tire and Bait Emporium".

##RVT954

Is it against federal regulations to change tires on an RV?

I have been following a series of posts on RV Forums where people ask about changing tires [Size, Type, Load Range or cold inflation setting]. Occasionally I run across some people who have adopted a hobby of commenting on tires in RV application and with a little working knowledge, make pronouncements on the "legality" of making any change in tires, that I do not agree with.

Other times I see a question like this one;
 "My research (curiosity vs need) is that LT and ST tires are not sold in the same sizes, so changing RIMS would be required?"


Here is my reply

Some LT tires show the same "dimensions" for example (235/75R15) as some ST tires but the "Dimensions" are NOT the actual complete size "description" which includes the letters and numbers before and after the dimensions. i.e. ST235/75R15 110/105L LR-C  vs  LT235/75R15 110/107T LR-D.  In this example, the ST tire is rated for 2,340#@ 50 psi while the LT tire needs 65 psi tp support 2,335# (single load capacity shown) Also the ST tire is rated for a max of 75 mph while the LT is rated for 118mph operational speed

Yes there are some ST tires where the dimensions do not match any tire identified as an LT type currently on sale. This is where you have to do some research and learn some facts on what can be considered a reasonable and safe change in tire type or size.

The Key items to confirm:

1. Is the Load Capacity of the new tire equal or greater than to original tire when you consider your new intended cold tire inflation? This does not mean +/- 25#. It means "equal or greater"

2. Is the Speed Rating equal or better than the original tire when you consider your intended new cold inflation level? (yes some tire load capacities are a function of speed)

3. Are your wheels rated for the inflation level you intend to run with your new tires? This limit may not be easy to learn but wheels can fail from too high a pressure just as they can fail from too many pounds load. The pressure we are talking about here is always the COLD inflation pressure.

You may need to be smarter than the tire guy if you want or have to make a change.

Remember there are some who post on various RV forums who would tell you that you would be violating Federal Regulations if you change from a Goodyear Marathon made in 2016 to any Goodyear tire made in 2019.

While it is definitely true that some people make changes in tire "dimensions" or "type" or inflation level which IMO as an actual tire design engineer, I would consider unwise. BUT that does not mean you can not consider making changes as long as you follow the guidelines posted above that follow the published guidelines from major tire companies.

LT vs ST for RV trailers?

It's no longer 1970 with a National 55 mph speed limit with ST tires being introduced i.e. "pushed" by a large tire co, as an an alternative to the real LT truck tires of the day. I have been told that one of the "selling points" for this then-new type of tire was that "no one would ever pull their 15' trailer faster than 55 with their bumper hitch."

Well, times have changed. We now have '1-Ton" Diesel Pickups that can pull a 35' trailer up the side of a mountain and run 75+ all day long. Trailers now come with two bathrooms, residential refrigerators, and two AC units multiple TVs and other heavy equipment that we never considered possible in 1975.

What hasn't changed is the fundamental science that a tire's load capacity is still basically   "Load = Air Volume x Air Pressure".  In fact, the actual load formula still used for current ST type tires is identical to the one used in 1970 when the 65 mph speed limit for ST type tires was in the industry standards. I can find no mention of alternate materials delivering increased load capacity in those standards. Yes materials have improved and radials are better than bias tires but basically the only benefits all these "improvements" delivered is longer life and better fuel economy and tread wear. Some construction features such as the addition of Nylon cap strips or full cap ply have allowed an increased resistance to the heat from higher speeds but I haven't seen any increase in load capacity in either Passenger or LT type tires over the past 40+ years.

As I have posted in this blog, "There is no free lunch". If there is, why haven't tire companies increased the load capacity of P and LT type tires if all these "improvements" are available to tire engineers?

What "feature" is in ST type tires that gives then the +10% to +20% more load capacity over an LT tire of the same physical size?

IMO there is no reason why RVs could not be supplied with LT tires other than it would increase the cost of the tires to the RV company.
Remember it is the RV Co that is responsible for selecting the tires they provide. We all see numerous posts from some who are running truck 17.5 size or LT 16" tires with improved durability so I see no reason to believe that the entire market could not benefit from a switch from ST to LT type tires.

One thing is that the LT tires that have to meet the new DOT standards (FMVSS 139) that were introduced in 2002 are probably more durable than the ST tires that are still only required to meet the standards on the 1970's.

I am not saying that some of the newer ST tires with  newer construction are not significantly better than the same size tire from the 90's but IMO there is still a limit.

Should ST tires be outlawed?

A post on an RV forum caught my eye
"In 2014, when I bought my RV Trailer, I immediately went to Discount Tire for 6 Michelin XPS Rib truck tires. I don't baby them, I barely pay any attention to them, they've never been at 80 psi, and they have been wearing fine for 5 years. Been all over the US with no issues. Yet I see new trailers coming into the dealership where I work with blown tires just coming from Indiana. Chinese bombs need to be outlawed."

It's unlikely they will be outlawed given the price sensitivity in the RV market. With so many folks buying when the quoted price is just a monthly amount that is so low they will always be "underwater" on their purchase. The RV industry seems to only focus on making the sale and as long as it feels it can only make the sale by offering the lowest possible price the industry will fight tooth and nail to not have to spend an extra $100 on a set of tires that can provide better durability.

In 2000 after the Ford Explorer recall both Passenger and LT type tires were forced to meet newer, tougher quality & durability standards as required by the "TREAD Act" but I believe that because of pressure from the RV industry, ST type tire requirements were excluded from the new requirements as complying would have increased the cost to the RV company a few bucks and they simply didn't feel they could stay in business if they had to increase their prices.

So we are stuck with tires built to 1970 quality and durability levels.

IMO Until or unless the RV community in large numbers, demands an improvement in the tire quality with an update & upgrade in the performance standards required by DOT there will only be improvements on a small number of models that offer ST tires with more Reserve Load or offer actual LT tires. How many RV owners have made the minimal effort of filing a complaint on a tire failure to NHTSA? Or written a letter to the  Administrator of NHTSA?     U.S. Department of Transportation,    1200 New Jersey Avenue SE.  Washington, DC 20590

When was the last time you heard someone not ask about buying tires at the lowest price? Or walking away from a sale if the dealer didn't provide better tires OE?  or the dealer didn't offer a multi-year warranty on tires that came on the RV?

A review of some comments on this forum shows the truth in what I am saying. The new Goodyear Endurance seems to be providing a significant improvement in tire durability for ST type tires but we see several people stating they are not willing to pay the price.

If you are only willing to pay Harbor Freight prices for your tools why would you expect SK, Milwaukee, Proto, or MATCO quality?

From day one ST type tires were introduced as a low-cost option to higher cost Light Truck type tires when comparing Pounds load capacity per dollar cost. We also had the 55 mph National Speed limit so offering tires with a 65 mph max wasn't a deal breaker

Why are ST tires better than LT type?

Following some posts on an Airstream forum of tire inflation and type tires. Some are complaining about rivets "popping" when they increase tire pressure. Many questions in the thread and some confusing replies.  I thought some readers might find this information interesting.

Some general observations and comments.
On this RV forum, there are two Tire engineers with significant Forensic Tire Inspection knowledge, myself "Tireman9" and "Capri-Racer" Our focus is on getting a better tire life and seeing fewer structural tire failures.
Out there, I would guess there is a pop rivet expert who would ask why the size, number or type of rivet being used is failing at a high rate on Airstream trailers. Airplanes are riveted together but I don't hear about "poped" rivets in that application, so maybe there is room for some improvement in the rivets called for.

Very few RV owners know the actual load on their tires. Simply dividing the scale weight by two or 4 (the number of tires on the RV) doesn't provide the correct answer, as it is easy to have one tire position to be hundreds of pounds heavier than another and they only respond to the actual load on that tire and not some mathematical average.

Also, I don't know the loaded weight of each year/size/model TT so can't provide an informed estimate on the inflation that TT should run without knowing the actual measured load on each of your tires.
I do know that having a MINIMUM of 10% Reserve Load (tire capacity - actual load) is a new requirement (2017) from RVIA (Recreational Vehicle Industry Association). DOT has no margin requirement ( Zero%). Capri and I are on record of suggesting at least 15% margin. The Interply Shear numbers would suggest that TT need to have a reserve closer to 25% if you want tire life more like a Motorhome gets (5 - 7 years).

Since it is the air pressure that supports the load and not the tire, simply going up in Load Range but not changing the inflation will in all probability not gain you anything in actual load capacity.

The pressure on the tire sidewall isn't really the "Maximum" you can safely use but is the MINIMUM inflation needed to support the MAXIMUM load that is marked on your tire sidewall. Personally, I do wish the wording on tires was more technically accurate but I can only guess that the lawyers and bureaucrats decided to use "max" in relation to the pressure for some reason.

Here is a question I wish someone could answer. Why are ST tires rated to support 10% to 20% more load than the same size LT tire? In the past, the reason was that ST tires were only rated to 65 mph to compensate for the increased load. Now ST and LT tires have almost the same speed rating but ST still carry the extra load capacity? If tire companies could all simply improve their ST tire Speed Symbol, almost overnight in 2017, why don't they use the same "magic" tire construction to increase the load rating of their LT type tires?

Anyone?


##RVT911