Want to hang something off bumper of your trailer? You need to do some math!

Been monitoring a forum thread about a guy who wanted to hang a 250# tool box off a mount from the rear bumper of his trailer. He just didn't understand the implications of changing the hitch load if he made such a change or the shift in tire loads between axles.

THIS video from Facebook shows the potential of not doing the calculations and for ignoring Science.

Distance from axle to tool box could be a serious issue when you consider how levers work.
I have also seen some Class-C Motorhomes with platforms attached to the trailer hitch receiver. Some have mobility scooters on them some tool boxes and some even have motorcycles. These items might be 100# but I am afraid that the total might approach 500# in some applications. While the hitch and platform might be rated for 500# or more the back end of the RV may have lower limits.

Here we see a 5,000# hitch on an RV with only a 250# load limit.


It is important to remember that just as with axles, springs, and tires, the load capacity is limited by the weakest link or component.For axles, the weakest component is many times the tires. You may have an axle rated GAWR of 3,000 and the tires may only be rated for 1,510#  but swapping the axle with a 4,000# unit will not raise the GAWR unless you also change the tires and probably also the springs, brakes and spring mounts too.

This same situation can occur for the hitch. Class-C Motorhomes are many times built on the "Cutaway" frames of standard vans. But the RV company extends the wheelbase by cutting the rear of the frame than welding in a couple pieces of steel to make it longer. This does not result in a longer but equal strength structure. In fact, it may mean the total frame strength is now lower.

Back to the question of what happens even if you don't have load limitations of the frame or receiver? Hanging weight off the rear bumper does TWO things. First, It obviously increases the load on the rear tires. Second it decreases the front tire loads and transfers that load onto the rear axle and tires. You need to be sure that this addition is not overloading any of your tires.

 

##RVT1016

My FIRST recommendation for a brand & size tire to buy

 If you review my blog posts and posts on various RV forums or have even attended one of my RV tire Seminars, you know I have tried to avoid making specific tire recommendations. Well this changes today.

If you use tires in a 225/75R16 dimension tire in  ST or LT or Metric type you might want to consider the Michelin Agilis Cross-Climate C-Metric tire.

The reason I am making this recommendation is based on the load capacity of their "C-Metric" tire.

Based on the data published by Michelin  HERE, Tire dimensions are the same.  What is different is in the "Fine Print" of the stated Maximum load capacity for their C-Metric when inflated above 80 psi.

The LT Michelin has a stated Maximum Load Capacity Single of 2,680# and Max Dual load capacity of 2,470 per tire when inflated to 80 psi   (LR-E).  This max load applies to the Michelin LT225/75R16 XPS-Rib and the regular LT225/75R16 Agilis Cross-Climate. The same max load is stated for both Bridgestone, Conti and Goodyear and Toyo tires and others. (If you find another company making the use of the C-Metric load capacity please let me know.

The Agilis C-Metric however has a maximum load capacity of 3,195# Single and 3,000# in Dual application when inflated to 83 psi.

The "C-Metric" can give you an extra 515# load capacity in single or 530# per tire in dual position.

For a Class-C Motorhome this works out to an additional load capacity of 3,150#. 

Now I would certainly hope that people not add another 3,000# of "Stuff" into their RV as in my opinion, you would be better off using the Michelin Agilis to give you a nice increase in the "Reserve Load" capacity of your tires to get you closer to a suggested 20% figure.

For you folks with RV Trailers your current ST225/75R16 LR-E tires are giving you a total load capacity of 11.320# for 4 tire applications. Switching to the Agilis could increase your capacity to 12,780# or better yet increase your reserve load from your current 10% to 24% if you simply resist the temptation to pack more "stuff".

It appears that Michelin has made some construction changes in their C-Metric line to provide more or better reinforcing materials to support the extra level of inflation and that is where you get the extra load capacity over LT type tires as it is the inflation that supports the load not the tire itself.

I can only assume that other tire companies have chosen to make the marketing decision to not build similar "C-Metric" tires. This option is available to all tire companies as there is nothing Magic involved here, just a marketing decision.

 

##RVT1015

How cold does it have to be to set tire pressure correctly?

With thousands of new RV owners out there, the answer to the question of "How cold does it need to be for me to check tire inflation" continues to come up. So this will be new information for some and a refresher for others.

 
Some people want to refer to a Temperature compensation chart and go through some calculations to learn the "correct" tire inflation when it is 82F or 62F outside. This is not what you should do.

Tire pressure is not based on any laboratory standard temperature (some claim 70 F or 68F) but is based on the tire not being warmed from either use, i.e. being driven in previous two hours, or from being in the Sun for previous two hours. Even partial sun can affect the reading.

Classical "Temperature in the Shade" is the "Ambient" we tire engineers are talking about. Not temperature in a theoretical laboratory.

It is correct to say, "The BEST time to check cold inflation pressure or CIP is the FIRST thing in the morning BEFORE the day's temp has had a chance to increase and BEFORE the sun has had a chance to shine on the tires and BEFORE you have used the vehicle."

With many people installing TPMS that provides a temperature reading to the driver for the first time, some are surprised when they see that both temperature and pressure increase as they drive. Please remember it is NOT correct to bleed pressure from a "hot or warm" tire after you have started that days travels.

As I have covered in other posts, the tire pressure will change by about 2% for each change of 10F. There are some pages on the internet that say 1psi for 10F but those are talking about standard 36psi passenger tires not 80 or 100 psi RV tires.

Now if you are driving from the campground on top of Pike's Peak and stop for lunch for two hours in the shade in Flagstaff where it is 90 and check your air, you might find a change of a few psi. You could adjust your pressure before continuing to Phoenix, where it is 120 F, but I don't bother to adjust inflation by the 1 to 3 psi variation I observe day to day. In my mind that is too much work. This is one reason why I suggest a +10% "cushion" on the required inflation as that eliminates the need to chase inflation with every change in ambient  of 15 to 20F. We tire engineers know that tire temperature and pressure will increase and we have taken that into consideration when we design and test tires.

NOTE: My personal CIP is  75/80  F/R on my Class-C MH.  Both of these pressures are more than 15% above the minimum pressure needed to support the measured load on each tire so I have a  nice "cushion".

I usually wait till I am home and am getting ready for the next trip before I adjust my inflation to my personal CIP, so I simply monitor the running inflation pressure which goes up and down as ambient temperature, driving and Sun exposure changes the inflation. My TPMS will warn me of air loss, so all is good as I motor down the highway.

I hope this info helps some of the first time RV folks out there. If you are an "ol' timer" you might direct the "newbies" to this post when they ask about setting tire pressure.

 

##RVT1014
 

Consumer Reports on RV tires. Helpful or fear mongering?

In my opinion.....

I had someone point out that the Sept issue of CR had a couple of pages on RV Tires and wondered if I could offer my opinion.  I do not subscribe to CR, as I have been less than impressed with some previous articles over the years. But I went to the local library and borrowed a copy.

I just finished the six-page article and will start off by saying my opinion of CR and their information on tires has not changed. "Danger on wheels" might give some the impression that this expose is based on a broad investigation into tires in RV application, when in fact almost the the entire article is on a single line of Goodyear tires that was in production 20 years ago. The G159. There is a lot of information on lawsuits and lawyers, but I found no hard data on tire durability or test data or failure analysis from CR.

While I have no personal knowledge of the G159 or any specific failures of the subject tire I was disappointed that the only warning CR provided was that tires should be removed from RV service "at 10 years old from date of the manufacturer".

As an engineer, I would prefer to set aside claims and innuendos and focus on facts and data. CR did offer what they called "9 Tips for safer RV travel". Of these only three even mention tires. One is the recommendation to replace tires that are 10 years old. Readers of RVTravel.com and of my RVTireSafety blog already know that 10 years is the MAXIMUM tire age with shorter usage time often recommended. Some are as short as 3 years in certain applications.

The article did point out that "about 40,000 G159s" were placed in RV service and that 72 RVs suffered tire failures. Given that 22.5" wheel size would suggest Class-A usage with a normal 6 tires per RV those numbers suggest over 6,000 "sets" of G159 were applied and possibly as few as 1.2% of RVs suffered tire failure.  No mention was made of how many of the 72 failures were the result of under-inflation, overloading, puncture, or impact. I too often see that any tire failure is blamed on the tire yet very few of those claim include any forensic inspection or identification of the mystery "defect".

A second recommendation from CR was to invest in Tire Pressure Monitor System. I completely agree that every RV should be equipped with a properly programed TPMS. They said that "The better systems involve fitting a sensor inside the tire." I wonder what testing CR did to arrive at that suggestion. Where is their data that supports the notion that internal to a tire somehow performs better than the more normal external, valve stem mounted sensors? I have never heard of CR doing any long term, direct comparison testing and evaluation of the different TPMS application as I have been doing for the last three years.

It is a bit of a stretch to suggest that "balance the weight in your RV" to reduce tire wear is a strong safety recommendation. They do suggest that the weight in the RV should be based on the capacity of the RV but no specific mention of confirming that no individual tire should be measured to confirm its loading. They didn't even educate their readers on how to know the weight limit on their tires.

Is there a technical problem with the GY G159? I do not know and the CR article only uses inference and suggestions that the ire was "related" to or "associated" accidents but I saw no evidence of what the claimed "defect" might be or if there even is one in the G159. There is a small mention that some owners might have lowered the tire inflation but no mention if that act resulted in tires being overloaded.

In my opinion, CR might get a few additional sales of their magazine but the article certainly did not offer any insight into how to prevent tire failures in RV use or into the importance of never overloading or under-inflating tires.

##RVT1013

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.

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##RVT1012