Two front tire "Blowouts" in one day

There was a thread started by the owner of a Class-A motorhome who suffered two front-end "blowouts" on the same day. There was not a lot of data provided such as actula load on each front tire or the tire age or even when the last tire inspection was completed. But here is one of the replies in the thread:
 "The reason that your coach is equipped with a heavier axle and bigger tires is exactly that the axle and tires on the earlier model Aspires were too close to what the coach was carrying. The close limits on the Aspire as well as the tire choice on OEM tires have been a well-known concern for years and many Aspire, owners, that frequent this forum have shifted their tires to tires with higher weight tolerances and inflation load-carrying specs and overall better safety record.

My 15 Anthem was similar. With me fully loaded for travel (which was still well below the coach's maximum rated weight, and tanks all loaded and us sitting in our seats, I was also overloaded on my front axle by 140# which was deemed acceptable. It was that way the whole time we owned her. A whole lot of weight is borne by these front axles on coaches this big and heavy..... within tolerances and specs but close."

I can't address the accuracy of Gary's comments on the Aspire or Anthem motorhomes but I have heard of other cases where the OE tires and even front axle in one case, was in overload when the RV was shipped from the MFG. added the following:

Individual tires don't care about the load on the other tires on a coach or the average load on all the tires. Only on the load on that individual tire. This is why "4 corner weights" is strongly recommended for all RVs.

GAWR, not GVWR is what is important for each axle BUT that is assuming a perfect 50/50% side to side split in load which is almost never the case. Some coaches have been fount to be unbalanced by 500# to over 1,000# yet still close to GAWR.

Also having a reasonable "Reserve Load" is very desirable, especially in the more critical front position. A reserve load (tire capacity minus the measured load on that tire) of at least 5% with 15% being a good goal may require changing the amount of "stuff" you carry or moving "stuff" around or even getting higher rated tires.

You can also stagger tire replacement if you have the same size tires in all positions. I cover how to do that in my post on spreading out the replacement cycle.  My post has a 1-year cycle but you could easily change that to 2 years and never have tires over two years old on the front axle.

##RVT915

Can I run my LR-E at 65 psi? or is this overloading the tire?

As trailer owners start applying the new Goodyear Endurance ST tire, many are discovering that for some sizes the Endurance tire is only available in a Load Range that is higher than their OE tires. Some are concerned about what inflation to run. I have even seen some claim that running a LR-E at LR-D inflation i.e not 80 but at 65 that the "tire will be overloaded, heat up and fail".

While I understand some of the confusion I do not agree with some of the concern or replies.

Tire load capacity is a function of the tire size and inflation level as long as you stay in the same "type" tire.  By "type" I mean P type or LT type of ST type or for large RVs "truck" type.

If you stay with the same type and use the same numeric "size" then the only thing left to change is the Load Range or "Ply Rating". While I do not like using Ply Rating as it is an old and discontinued nomenclature it may help for better understanding in this post for you to think of the old term.

Important Point. "It is the air pressure that supports the load NOT the Ply Rating." This statement is supported for every tire made by every tire company in the world through the use of Load & Inflation tables. These tables show a size and then for different levels of inflation the load capacity of that tire when inflated to that level. You will never see a tire shown where a LR-D at say 65psi can support 1,500# and for the same size the same tire when having a LR-E rating shown a higher load capacity at 65 psi. Not even just 1 pound more.

So a LR-E can support the same load at 50 psi as a LR-C or the same load at 65 psi as a LR-D at 65.

You will not be overloading the LR-E if you load it to the 65 psi rating shown for thet type & size tire and inflate it to 65 psi as you would for a LR-D. Since you are not overloading the LR-E tire it is not going to overheat at 65psi with the 65 psi load so the LR-E tire is not going to "overheat" at 65 psi any more than the LR-D will "overheat" if it is loaded to the 50 psi load rating and inflated to 50 psi.

When going to a higher "Ply Rating" you can then increase the CIP which increases the tire Load Capacity which means it will actually be running cooler because of the greater "Margin". The higher inflation will also lower the Interply Shear which may lead to longer tire life.

When making the change you do need to confirm the upper inflation level for the rim. The wheel manufacturer should provide that information. As an alternative the wheel will have a max load capacity stated. Looking at the OE tire size that comes on that wheel look for the inflation that corresponded to that load and I would consider that to be the wheel inflation rating.

When to replace tires? Can you drive on one "dual"?

Was reading a magazine aimed at motorhome owners and there was an item about a man that suffered a tire failure. It was one of his rear duals. The RV owner reported that he decided to drive to the nearest tire store where it was suggested that he replace all six tires. The tire dealer had to educate the RV owner about the life of tires in RV service being 10 years or less with many recommending that tires be replaced after six or seven years life.

The magazine did offer a brief explanation on how to "read" a tire DOT serial and learn it's age.

IMO the magazine missed an opportunity to further educate their readers with a warning of the damage that was probably being done to the mate of the tire that failed.

First off there is a good probability that the tire, being over 10 years old, failed from a belt/tread separation. We can't be sure, as the RV owner didn't have a TPMS, so we don't know if he could have avoided the problem of a "Blowout" or Run Low Flex Failure on the Interstate. We do know if there was a slow air loss, the tire that did not fail was being run with ever-increasing overload, for as the companion tire lost its air the load on that end of the axle was being transferred to the fully inflated tire.

In the tire industry, there are tables that provide information on how slow you need to drive as you increase the tire load above its normal load capacity.

Basically, you need to run no faster than 40 mph if you are running 107% of the rated load.
If you want to run 113% you can drive no faster than 30 and the max speed drops to 20 mph if the overload is +21%.

Since our RV owner was running at 200% load I would estimate that maximum speed he could travel without doing damage to the "good" tire to be no faster than 5 mph and even that is questionable as there are also distance limits for those conditions

BOTTOM LINE
If you have a tire failure, no matter the reason, you need to change out the failed tire and should not attempt to "limp" home on its companion. If you are concerned for your safety on the side of the road you need to be aware that driving over 5 mph means you need to have the companion tire also replaced. No matter its age. As always when changing tires in a dual position you must also match the pair as covered in THIS post.


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##RVT820

How are RV tires developed?

 By Roger Marble

If a tire is being designed for a specific vehicle manufacturer such as Ford, Chevy, Toyota, or BMW, there will be a number of tires submitted by competing tire companies all trying to deliver the best overall compromise in performance characteristics. Please note that all original equipment ("OE") vehicle manufacturers have slightly different requirements but all make similar requests for performance improvements in many areas. In the future I will use the term "OE" to include these car and pickup manufacturers.

Compromise: Now is a good time to talk about some of the various trade-offs the engineer is faced with when trying to meet conflicting goals and customer wants. I am sure we would all like an RV that has all the interior space and amenities of a 40’ diesel pusher but gets 25 mpg and can be driven down crowded city streets without knocking off our mirrors. Oh yes, it should also cost under $30k. Well, Bunkie, that just ain’t gonna happen in real life.

The same goes for a tire that handles like an Indy tire, is as quiet as the proverbial mouse, has great off-road traction, is good for 100k miles, and costs $25. One thing few people realize is that most if not all performance characteristics are a compromise. For example, if you improve wet traction you probably hurt fuel economy unless you use a special type of rubber that costs double per pound and is more difficult to process. If you improve handling you might hurt ride and noise. When you improve noise you can significantly increase the cost of making the molds used in manufacturing. The cost of a tire mold can be as low as $10,000 and can approach $100,000 each. Depending on the production volume needs, a tire manufacturer could need 30 or more molds. The list of trade-offs goes on and on.

The competition for a tire application might start three or more years before scheduled start of delivery with two to five tire manufacturers competing for the contract, knowing that only one or two will end up being selected to actually provide tires. The costs associated with building and testing special prototype tires can run in the hundreds of thousands of dollars and are absorbed by the tire company. The only way a tire company can afford this type of activity is by landing a contract for a few hundred thousand tires so the costs can be spread out.

Unlike “OE,” an RV manufacturer may only need a couple thousand tires, so a custom tire designed for a specific RV would be cost prohibitive. Since the RV manufacturer won’t be trying to get custom tires, it doesn’t have staff engineers working on developing specifications for such tires. The RV company will in all likelihood either take what comes already on the cut-away chassis or the bare chassis for Class-C or A vehicles, and in the case of trailers, may buy the tire with the lowest cost that can meet tire size requirements and expected delivery schedule.

For RV applications the one thing that is in the control of the manufacturer is “Reserve Load.” This is the difference between the load placed on each tire with the RV normally loaded and the load capability of the tires at specified inflation.

##RVT802

Quick Summary here

I have been following a number of RV forums and want to do a bit of a summary for those that don't have the time or interest to read a number of posts on this Blog: Remember more than half of the thousands of RVs that have been checked have been found to have one or more tire and/or axle in overload. So nect time you camp you can be pretty sure that one or both of the RVs parked next to you are overloading a component.

- You should have a Digital tire gauge. I have posted the results of my tests of dozens of gauges that RVrs were using and the bottom line is that about 15% were off by more than 5 psi which  IMO makes that tire gauge a good door stop or tent peg. I have provided information on how to do your own gauge "Calibration Check" to confirm the accuracy of the gauge you use daily and how to maintain your "Master Gauge"

- If you follow the advice from the two Tire Engineers on this thread you will be setting the pressure based on the appropriate method here:
     - On your TV inflate to pressure based on the Door sticker and Owner's manual for "Fully Loaded" operation. BUT get on a scale at least once to confirm you are not overloaded.
     - If you have a multi-Axle trailer you should be running the pressure on the tire sidewall. I have written in my Blog and provided the technical justification and need to follow this advice. You should still go across a scale and calculate the actual tire loads to confirm you are not overloading one or more tires
      - If you have Motorhome you need to confirm your unit's "corner weights". Use one of the worksheets I have provided links to. Use Load/Inflation chart from your tire manufacturer, to confirm you are not overloaded and identify the heavier loaded tire on each axle. Using the pressure for that load add 10% (without exceeding the wheel max) and that is your Cold set pressure for all tires on that axle.
      - If you follow the above there should be no reason to be chasing your tire inflation around unless you have a major change in Morning temperature ( More than say 30°F ) as you would only need to "Top Off" the air once a month at most.

- Don't be too anal about inflation. Variations of 1 to 3 psi day to day are normal. You should not need to make adjustments unless the Cold pressure is 4 or 5 psi away from your goal. (Multi-axle trailers might try and get no lower than 3 psi below tire sidewall pressure)

- TPMS are a WARNING device and like Insurance we should all be using them. Also just like Insurance they cannot "Prevent" bad stuff from happening but with luck they may give sufficient advance warning to lessen the impact of the bad stuff that does happen.

- If you are parked for more than a day you need to cover your tires that will be in direct sunlight with WHITE tire covers. This protects against both UV and higher temperature damage which can shorten tire life.

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.


Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 14th year. Learn more or subscribe.

How RV tires are developed and why some are a compromise

If a tire is being designed for a specific vehicle manufacturer such as Ford, Chevy, Toyota, or BMW, there will be a number of tires submitted by competing tire companies all trying to deliver the best overall compromise in performance characteristics. Please note than all original equipment vehicle manufacturers have slightly different requirements but all make similar requests for performance improvements in many areas. In the future I will use the term "OE" to include these car and pickup manufacturers.

Compromise: Now is a good time to talk about some of the various trade-offs the engineer is faced with when trying to meet conflicting goals and customer wants. I am sure we would all like an RV that has all the interior space and amenities of a 40’ diesel pusher but gets 25 mpg and can be driven down crowded city streets without knocking off our mirrors. Oh yes, it should also cost under $30k. Well Bunkie, that just ain’t gonna happen in real life.

The same goes for a tire that handles like an Indy tire, is as quiet as the proverbial mouse, has great off-road traction, is good for 100k miles, and costs $25. One thing few people realize is that most if not all performance characteristics are a compromise. For example: if you improve wet traction you probably hurt fuel economy unless you use a special type of rubber that costs double per pound and is more difficult to process. If you improve handling you might hurt ride and noise. When you improve noise you can significantly increase the cost of making the molds used in manufacturing. The cost of a tire mold can be as low as $10,000 and can approach $100,000 each. Depending on the production volume needs, a tire manufacturer could need 30 or more molds. The list of trade-offs goes on and on.

The competition for a tire application might start three or more years before scheduled start of delivery with two to five tire manufacturers competing for the contract, knowing that only one or two will end up being selected to actually provide tires. The costs associated with building and testing special prototype tires can run in the hundreds of thousands of dollars and are absorbed by the tire company. The only way a tire company can afford this type of activity is by landing a contract for a few hundred thousand tires so the costs can be spread out.

Unlike “OE”, an RV manufacturer may only need a couple thousand tires so a custom tire, designed for a specific RV would be cost prohibitive. Since the RV manufacturer won’t be trying to get custom tires, it doesn’t have staff engineers working on developing specifications for such tires. The RV company will in all likelihood either take what comes already on the cut-away chassis or the bare chassis for Class-C or A vehicles and in the case of trailers, may buy the tire with the lowest cost that can meet tire size requirements and expected delivery schedule.

For RV applications the one thing that is in the control of the manufacturer is “Reserve Load”. This is the difference between the load placed on each tire with the RV normally loaded and the load capability of the tires at specified inflation.

Next time:
.
Before we cover tire manufacturing -- perhaps dull to you but interesting to me -- we will do our first post on tire loading. I say first, as this topic is the single most important performance characteristic for we RV owners. But few of us truly appreciate the potential safety issues involved.

Updated/reposted for RVT 802