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Friday, December 29, 2017

Too much pressure increase

Reading an RV forum thread on TPMS usage. One comment jumped out at me.

"I have a serious concern with the G rated tires on my 5th wheel trailer. When I run the cold max pressure at 110 PSI, I get TPMS readings up to 134 PSI rolling down the road. Lately, I have been running 95 PSI cold and am getting 115-119 PSI rolling. The tires and rims are rated for 110 PSI, I can live with 120 PSI, but the 134 PSI concerns me."

I replied that inflation ratings for wheels are based on a "cold" pressure. Increase in pressure due to operation is considered by manufacturers and I would not be concerned as long as the proper cold inflation is used along with appropriate limits on load and speed.
Tire wording "Max Pressure" can be confusing but in reality the pressure stated on the tire sidewall is the cold pressure needed to support the stated load and that load is the maximum load the tire should be subjected to. I advised the owner that he should NOT underinflate his tires and plan on operation temperature to increase the pressure. "Cold" pressure is the only pressure you should be concerned with.

Tire pressure increases by about 2% for each increase in temperature of 10F. If you don't remember the Science from High School you can read THIS post.. If you are seeing a 21% increase in pressure (110 > 134) that means you are seeing about a 100F increase in internal tire temperature, which I would consider a bit excessive for normal tire operation.

If you are seeing a 25% increase in pressure (95 > 119) then you are getting a 125F increase in temperature which indicates you are working the tires even harder . This extra "work" that is generating a greater temperature increase is not good for long term tire life. You are "aging" the tire rubber faster. Some might want to review this "Key Point" of tire life as covered in THIS post.

I cover these points on Temperature, Inflation and Aging in various posts on my RV tire blog.

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading.

You need to confirm your pressure is 110 psi AND that your gauge is giving an accurate reading at that level.

The poster then responded:
"Today drove 250 miles and my 95 PSI tires were running 115-119 PSI and the tire temps were at 20F above out side temp, 50 outside and 71 tire readings. I still contend that 134 PSI is way to dangerous for tires to run on 110 rated tires."

So I responded back:
 Few people realize that the pressure increase as a function of temperature is based on well established and confirmable Physics and that a TPMS is not reading the actual tire temperature  but is actually reading the temperature of the brass valve stem and the metal base of the TPMS itself, which is being cooled by outside air.

Air is a very good insulator and if you think about it, you have a small column of air running up the inside of the valve stem which makes it difficult for the heat to travel up the center of the stem and past the valve core itself all the while the valve is moving rapidly around being cooled by the outside air.

I am aware of laboratory tests that goes against what "common sense" might indicate. That being that the air inside a tire is not uniform in temperature but it is always cooler than the hot spots of a tire and it is the hot spots that can result in tire failing if hot enough for long enough.

I have no doubt that the TPMS was indicating only 20F above the cool 50F outside air temperature. If you are still concerned about the hot pressure of 134 on tires that have a cold pressure rating of 110 for its max load capacity rating.

As a tire design engineer with 40 years experience, I trust the science of the "Gas Law" and knowledge that air is an insulator and metal conducts heat from a hot source to a cooler one.
I don't know what to advise other than to decrease the operating load and speed and to confirm your hand gauge is accurate and to always inflate the tire when cold to 110psi, as continued operation at current load and speeds will certainly result in pressure reading that are above the cold pressure of 110 psi.

Friday, December 22, 2017

Quick post on Dual tire air balance systems

Read a thread on an RV Forum on connecting two tires in dual position, with hoses to "balance" the inflation pressure. There were some claims made about performance improvements but IMO the conditions necessary to deliver those improvements are pretty extreme and improvements are tenuous.

The information on the function of this pressure balance equipment also ignores the potential for damage to the RV when one tire loses air but the driver is not immediately informed of the loss. There is even talk of serious air loss in excess of 25% of the pressure needed, only resulting in faster wear.

I found no mention of operating a tire that low possibly resulting in a catastrophic failure (coming apart and potentially doing thousands of dollars' worth of damage to the RV).

I can't understand why some would think that lowering the hot tire pressure (decreasing load capacity) would ever be a good thing to do.

Someone will probably point out that some of the systems that connect the air between a set of duals limit the amount of air transferred, but that still leaves a problem.

If you have two tires set to 80 psi and one gets a puncture and starts to leak, the "good" tire bleeds off air to the leaking tire until 5 or10 psi has been transferred. Now both tires are at 70 to 75 psi and are just starting to be overloaded.

But the leak continues and the tire going flat transfers its load to the "good" tire, resulting in the "good "tire seeing ever-increased the level of overload.

This can continue until one tire is flat and one tire is 100% overload. Now you are driving down the road with no knowledge you have a flat and a tire at 200% of rated load.

If you are lucky, you stop and discover the problem before the "good" tire fails, too, leaving you with two scrap tires.

This system does make airing up tires easier in that you only have one valve for the pair of tires, but I'm not sure if that "benefit" is worth the cost.


Friday, December 15, 2017

Leveling Blocks - Only support one dual?

I received this question the other day.

"I have some questions about leveling with ramps, specifically on vehicles with rear dualies.

The obvious temptation is to use only one ramp -- and the obvious response is that this is a 2x overload condition and should be avoided.

HOWEVER -- since tires and wheels are clearly designed with safety factors, and since the overload is a very static situation with none of the dynamic variations of a moving scenario, what do you think are the real risks involved?

In weight-sensitive designs like aircraft, I know that a typical safety factor is 2.5x. But I don't know what's in tires and wheels.

It's easy to say "just carry four ramps" to level the rear of a dually coach, but for the smaller rigs, that kind of bulk is pretty impractical.

I'm curious, but I'm cautious too. I'd appreciate your thoughts.


My reply
First "Safety Factor".
I understand the concept for aircraft and for some other items but the concept doesn't translate to tires very well. Unlike the metal used in aircraft, there is a lot more variation in the raw materials and the finished products
Tires are designed and manufactured to meet and exceed a set of requirements. The requirements are such that all tires must be capable of meeting or exceeding the standards but there is no advantage to exceeding them beyond their design goals as you can't make a claim that some tires in a line might deliver x thousand miles if all tires in the line can't and with hundreds of thousands of tires in a production run it is essentially impossible.
In your aircraft example, there are thousands of subsystems that can each be replaced or upgraded individually and while there may be a goal of a 2.5 safety factor I doubt that each and every part manages to wear out at the same point in its life.

Tires don't have replaceable parts, so while some parts may last a long time and even longer than advertised, when any one component simply meets its design goal but doesn't exceed it, the tire's life is over.

Now to your question on ramps. The answer for ramps is the same as for basic supports used to keep tires out of water when parked for the winter months.

The #1 priority is to support the entire footprint (contact area) uniformly. Never just half the tread width or half the tread length. Even only supporting 80 or 90% of the contact area is not good for the tire structure. Technically the 200% load is not a serious issue but it will definitely stretch a sidewall greater than normal. This increased stretch could, in the long term, lead to localized sidewall cracking after long-term use. RV application can also add other issues such as the tire sidewall losing some of it's "stretch" properties. RV tires are not designed to ever be loaded more than 100% of the load stated on the tire sidewall and inflated to the level associated with that stated max load.

THIS post has some pictures of what you should NOT do. It also shows an example of a tire "footprint" after parked on grass for a couple days.  A 2x4 under part the tread is particularly bad. When you use a block or ramp that is not as wide or long as the tire contact patch the edge that is not fully supported will have localized bending that can overstress that part of a tire.

Yes, tires are built to tolerate abuse and overload and overspeed and under-inflation but in each case, there will be some damage done to the structure. The damage is cumulative and tires do not repair themselves.

I carry 3 leveling blocks in my small Class-C. Here are two set up for my dual tires.  I can use just one if I need to raise a front tire. These ramps can raise the tire(s) 1-1/2" or 3". If I need more I will move the RV to a more level location.

If I need to raise one side of the rear I always use two under the duals.  If that isn't enough, I move the RV to a different spot.  

Your tires are not likely to fail simply because you only supported one of the duals overnight but I can say that putting 200% load on a tire is not a good idea if you can avoid it.

Bottom Line
Anything you use to support a tire must be at least as wide and as long as the tire contact area or "footprint". If your "blocks" aren't wider than the tire or if you do not drive upon them such that they are centered you are doing some damage that could contribute to premature failure that you probably would never associate with the use of improper sized leveling blocks.

Friday, December 8, 2017

4-Corner weights and a "Meaningful" measurement.

I was following a forum thread on using closed state scales to learn your "4 corner weights" This is possible in WA & OR and some other states. 
Some commenters felt the practice was not wise as the area around the platform scale might not be perfectly flat. I posted my thoughts on the topic.

Not sure if I understand the concern over the accuracy of using a large platform scale to try and get a handle on your side to side loading.

I have a blog post on "Measurable vs Meaningful" that some might want to review.

When setting the inflation based on the published Load & Inflation tables, you should always "err on the high side." This means if your measured weight were 2,005# you should go UP to the first weight that exceeded 2,005# and maybe even higher.

Let's assume the table for your tire gave 90 psi at 1,900#  and 95psi at 2,210#. I would recommend you select 95psi as your MINIMUM inflation.  But what would you do if the chart for your tire showed an inflation for 2,010# or 2,005# or 2,000#? Would you cut things so fine as to select the inflation for these loads?  I hope not. What might happen if 50 minutes after getting the tire loads you top off the fuel and buy some groceries? You are now most certainly over the measured 2,005#. Are you gonna run back to the scales?  I wouldn't, and I would not need to as I have a built-in cushion of load capacity over the minimum needed to support my actual load.

We haven't addressed the question of your tire pressure gauge accuracy. Unless you have access to ISO Certified laboratory gauges as I do, you probably need to assume that your gauge is off by at least one increment reading of the pressure scale. For some truck type stick gauges that means 5 psi. What is your cushion that covers your gauge accuracy?

My "cushion" results from a few actions.
1. I always go up in the charts to the next inflation level and if my measured load is within 100# of a level, I will go up TWO levels of inflation which would be 10 psi.
2. I also add 10% to the selected table inflation and again round up to the next "0" or "x5" pressure

The additional 10% means you can avoid having to chase the Cold Inflation level every morning when the temperature drops and pressure drops a few psi.

When I checked my "4 corner weights" on the state scale was the area beside the platform exactly level? No, it wasn't, but it looks close enough that even if my weight was off by 100# my other adjustments would more than compensate.


Friday, December 1, 2017

Are you smarter than your tire salesperson? With Michelin LTX you may need to be.

As an RV owner, I believe that in most cases it is important for us to know as much as possible about our tires. Sometimes this means knowing more about our tires than the average salesperson.

Here is a real-life example:

An owner of an older Travel Trailer posted
"Please correct me if I'm wrong... The Michelin Defender LTX M/S 235/75R15 is a true LT tire and does not need to be derated?

Shopping for tires on my Airstream 2017 23FB. I'd love to just go with the 16" SenDel S02/Michelin combo but not sure if the clearance is there for them."

Just before the above post it had been pointed out  the correct designation is P235/75R15 XL
The "XL" is the tip-off as that stands for "Extra Load" which is only found in Passenger type tires.

LT tires have Load Range C, D, and E
P-type have Standard Load (no special marking)  and XL which is lower in inflation than an LT-C.

Your tire dealer should have made clear the type tire they are talking about.  Sometimes RV owners need to know more than the salesman if you want to get the tire you need or want.

I had also previously posted on that thread:
"Be sure you understand if your LTX tires are "LT" type or "P" type. If P you need to calculate the actual tire capacity when used on Trailer, SUV or PU-truck.

The adjustment is 
(Load molded on tire)/1.10 = Load capacity on the RV or truck or SUV application.

No load capacity adjustment is needed for LT type tires if placed on an RV, truck or SUV.
The LT in "LTX" does not make a tire an actual "LT type tire."

I don't know if the marketing folks at Michelin realized the confusion they were spreading when they came up with the name "LTX" and put that designation on BOTH LT type and P-Type tires. I would not be surprised if there aren't a good number of RV owners who think they have an LT tire when what they were sold was actually a Passenger type tire.

Friday, November 24, 2017

Are Nitrogen Molecules Really Larger Than Oxygen Molecules?

According to the "Get Nitrogen Institute" in their paper on N2 effusion

"The correct answer, with respect to 'permeation,' is yes."

So I imagine your question is why don't I support the effort to "sell" the idea of always inflating your tires with just Nitrogen. It comes down to effort and cost VS level of benefit.

Maybe one way to think of this would be to imagine dropping a penny as you walk away from making a small purchase at a store. If you dropped a number of coins you might stop, bend over and pick them all up. But what if you only dropped one penny and didn't discover the fact till you had walked to your car. Would you walk the 20' back to the store to look for the penny? I bet not.

There is no doubt that you would have more money if you picked up the penny but would you consider it worth the effort?

In general, tires lose about 1%  of their inflation pressure each month in laboratory testing. This is almost entirely Oxygen. It is also true that tire pressure changes about 2% for every change in temperature of 10F. This is true for Nitrogen or air.
I haven't tried to run a test but it is also true that every time you use a hand pressure gauge to check your air you let a little air out. How much air do you let out if you use a gauge to check your tires every day? Might it be 1% in a month's time? Might it be more?

Finally, what does it cost to inflate your tires with N2? Even if you have a deal with a dealer and can get your pressure "topped-off" for free, you still have to drive to the store location to get that "free" inflation.

Bottom Line:
IMO the small level of benefit of inflating with N2 just isn't worth the effort and cost. There is also the real negative of not checking your tire pressure simply because you believe that by inflating with N2 your tires will never lose pressure so you don't need to check. What about small punctures or leaking valves? If you don't check the pressure you will not learn about the leak till it is too late.

However, if you want to inflate your tires with N2 I see nothing wrong with doing that. After all, it's your time and money, not mine.

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Friday, November 17, 2017

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

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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Friday, November 10, 2017

DId your RV company or dealer follow Federal law?

Been reading a number of posts on recalls, violations of federal safety standards in the RV industry and on tire and fire extinguisher recalls.
If you have been a reader of my blog you know that I am a strong proponent of filing complaints on failures of safety-related systems and of tires to NHTSA.

You can read a portion of the Federal Regulations on what RV companies are supposed to do HERE as far as the regulations relate to identifying who owns what tire in case there is a recall.
Obviously, if the S/N ( full DOT serial including the date numbers at the end) of a tire is not recorded when the tire is sold there is no reliable way for a tire company to contact owners of tires that are under a recall order.

Selling dealers have the following responsibility
"The National Highway Traffic Safety Administration (NHTSA) requires that tire dealers must provide every tire purchaser with a pre-addressed completed tire registration form OR complete the tire registration process electronically on behalf of the tire purchaser (49 CFR Part 574.8). This is not voluntary, tire dealers must do one or the other"

When a vehicle is sold (car, light truck, or RV) the tires are "sold" with the vehicle. The regulation says "tire dealer" but obviously just because a company sells vehicles and doesn't think of themselves as a "tire dealer" they are in fact selling tires.

However, I have found that few people have received the appropriate forms. Did you receive a form with the purchase of your last set of tires at the tire store, or with the tires that were shipped to you if you bought them online?  How about when you bought a new or used RV?

You say you bought your RV used so you don't think the tire ownership needs to be registered? Well, I know of nothing prohibiting a tire being covered under a recall just because ownership was transferred. The entire reason for this registration is simply to let either NHTSA or the tire company contact the owner of tires covered by a recall. The information is not used for any other purpose.

IMO the odds are that very few of you received the pre-addressed form or if you did you may not have bothered to complete the information.

Well HERE is a website that you can fill in the information and submit it online and don't even need a stamp.

I started this post and mentioned Fire Extinguishers. If you have following the blog you know about the recall and free replacement of potentially defective extinguishers. You may have discovered you have one or more of the covered units. Unlike tires, fire extinguisher ownership is not registered so there will be millions of extinguishers that are not replaced simply because people do not know about the recall and Kiddy company has no way of contacting the owners.

Do yourself a favor and register all your tires RV, Car or truck. Who knows you may just end up with a new set of tires if the ones you have are covered under a recall.

Friday, November 3, 2017

Summary for Newbies

This is a summary of a discussion I had on RV Forum on tire pressure that started with a question and some comments.

"I'll give my take on maybe why, we have ----- Travel Trailer owners ----- Motor Home owners that never had a TT ---- TT owners that moved up to a MH ----- MH owners that still act like they still own a TT -- maybe there is any other I can not think of right now.


I have friends that own MH's and suffer blow outs all the time and their tires look good at the pressure they are using and not a Run Low Flex Failure that will most likely cause a blowout.

Maybe Tireman9 will answer some of the differences between TP's of TT's and MH's to clear up some of the misconceptions people have."

Here is my answer to the broad question

Yes, the proper inflation for MH and "tow-able" RV are different. There is actually strong science behind why there is a difference. THIS blog post is a short explanation. You can read the technical info HERE.

The other issue is that some folks just do not want to hassle with vehicle maintenance required with RV ownership. They are used to cars that have thousands of man-hours of engineering design, testing and development to make those vehicles extremely reliable.

Some of us are old enough to remember that when we learned to drive we were taught how to change a tire, as 10,000 miles was good tire life. Many probably learned how to change oil or adjust a carb. Some even knew how to set the engine timing and change a set of spark plugs. Nowadays it's hard to even see the plugs but with their life now at 50,000+ and the ignition computer controlled and many tires good for 40 to 60k miles who needs to know how to maintain a car?

RVs have a number of systems that do need maintenance. The tires on RVs are an outlier. While top tier tire companies use essentially the same rubber compounds and tire building equipment. This blog post explains the concept of "Tiers" in the tire industry. If you have a MH you will probably recognize the brands and names of the tires on your RV. 

However if you have a "towable" and by this, I mean both 5th wheel and more basic "Travel Trailers" many of you probably have tire brands that don't even fall into the 5th tier level. Some of these off-brand tires may not have a lot of up to date engineering built into them but IMO the main problem is the selection of tire size and load/speed capacity made by the RV company.

Again IMO many RVs are sold based on low cost and the unit has lots of "bling". I don't think I have ever heard of a salesman touting the benefits of the tires that come on an RV. I doubt that many even know the size or maybe even the brand provided on the various units they sell.

So the bottom line is that tires on RVs need more maintenance than the tires that came on their cars. Partially because of their usage but also because there is such a small margin of extra capacity provided based on the tires being undersized to keep costs down. Maybe if RV companies spent more, or even some time evaluating better options, i.e. had better quality for the tire sizes and type they offered, the incidences of tire failure on RVs would be less frequent just as they are less frequent on today's cars. 


Friday, October 27, 2017

What effect does speed have on tire failure?

"What effect does speed have on tire failure?
I have an 8000 lb trailer with 4 Goodyear Marathons. The truck is a Dodge 2500 diesel so I can cruise at 75 mph subject to wind and hills.

How much if any does speed factor into tire failure? I replace tires every 4-6 years regardless of what the look like, and keep them inflated at the PSI marked on the tires."

Basically, increased speed means increased temperature in the critical areas of a tire. Here is a graphic showing the relative temperature in different areas of a tire.
with Red being the hottest and dark blue being the coolest.
When you increase the temperature of rubber you are effectively increasing the rate the rubber is aging. I believe we all understand that old rubber is not as flexible as new rubber. If it isn't as flexible you end up with cracks or cracks that grow every time the rubber is flexed. More cracks and larger cracks are what result in tire failure.

For the above reasons plus others, the major tire companies suggest that in RV operation tires never be driven any faster than 75 and you can see this max speed stated in their literature for many of their RV tires.

But wait, you say your tires have a "Speed Symbol" that relates to 87 or 99 or maybe even 130. HERE is some information on speed ratings. Just because you have a tire with a fast symbol doesn't mean it can be driven at those speeds for 20 or 40,000 miles. About the only thing you can count on is that tires with a "higher" symbol will provide better steering response than a tire with a lower rating.

Now back to the question of how fast can you drive or tow your RV. I previously mentioned 75 as the upper limit but for some tires, there are other things to consider. With ST type tires as found on many trailers, we need to remember that the load capacity is based on a formula that originally specified a max speed of 65 mph. In the past, there were Technical Bulletins that advised that inflation needed to be increased (but not above the sidewall max) by 10 psi to go 70 mph. In addition, load needed to be decreased by 10% if you want to go 75. Still, there was that 75 max.

Many trailer owners know that they need to replace their tires before they wear out. If they don't they are probably going to have some type of failure. But those same people will not replace the tires on their pick-up till they are significantly worn. Why?
The answer can be seen if you simply compare the load and speed ratings of an LT tire and an identical size ST type tire.

LT235/75R15 101Q LRC1985# 50 psi 99mph

ST235/75R15 110R LRC2340# 50 psi 106mph

So exactly what type of magical engineering has tire company X used to achieve both increased load capacity plus increased speed capacity when it is the air pressure that supports the load? If they have the ability to make that ST tire really perform at those speed and load conditions what is wrong with their LT type tires?

IMO what we are looking at is a marketing plan taking over the engineering reality.

Bottom line
You may be able to pull the trailer at speeds above 65 or drive the Class-A diesel pusher faster than 75 but you will be consuming the finite life of the tire and can expect a failure before you wear it out no matter how you maintain your tires.


Friday, October 20, 2017

Will confusion on how much inflation be resolved?

Hi Roger.

Do you think the tire pressure issue will ever be resolved to people accepting the values recommended by the tire manufacturers based on actual loading, or will some always be confused between the tables, the placard and quotes of Federal FMVSS Regulations from some on the forum? It seems so simple and logical, but some just don't get it. I still think a sticky written by you could settle the issue, but the moderators disagree. I suspect a concern for liability for the forum to advise something that contradicts the MH placard.

I have a question for you about my RV. I carry 5 psi above the minimum for the load on my steer tires on a 60* morning @ 1100' altitude. When traveling, I had a morning @ 5000' and temp in the upper 30's. This produced a pressure of 1-2 psi below the minimum. I was going to be traveling into a warmer climate (95*), so did not adjust pressures. This type of condition can happen when we are traveling. I don't want to chase the pressures, so if traveling into warmer weather, I just go and watch the TPMS. I don't feel this is a problem but wanted your thoughts.

Thanks for your thoughts and time.

Not sure if there ever will be a resolution to the Inflate to the placard vs Inflate to the actual tire load. I would liken this to the change oil every 3,000 miles vs change when the car's computer advises.

DOT has a goal of trying to make things simple and keep people safe. DOT also knows about the data that indicates that over half of the RVs on the road today have one or more tire overloaded (either too much load or too little air for the actual load) I really can't fault their approach as IMO many RV owners aren't willing to make the effort needed to learn the proper inflation and to then maintain it.

We have had tire inflation stickers in cars for many decades and there have been massive vehicle recalls because their tires were underinflated, I have read that many as much as 30% low. We don't hear anyone making a case that the accidents and fatalities were the driver's fault. That would be blaming the victim which isn't acceptable, even if true.

OK to your question. I am guessing your CIP is in the 90 - 110 psi range so IMO +5psi isn't enough to avoid the pressure fluctuation that results in your overload. This is why I suggest a +10% value for inflation margin over the table minimum number..
In my Class-C RV, with LT225/75R16 LR-E tires, I am lucky as my RV is rather light. I really only need 60 psi F & R based on my "4 corner scale readings. My certification sticker says 65 / 80 and my dealer delivered it at 64 psi all around. 

For my application +10% would indicate 66 psi and a  +15% margin > 69 psi. I run 70-75 
Even though I can be a bit "anal" about inflation I also wrote THIS post and I really do not mess around with my tire inflation.

My TPMS is set to warn before I would be in overload and I run across a scale once a year just to confirm I am not too far from my original "4 corner weights".

In 2014 I drove Ohio > OR > Calgary > Yellowstone >OH over a seven-week period with temperatures ranging from 90's to snow and elevation of 20' to 8,000' and never had to adjust inflation.

Thursday, October 12, 2017

Can Black tire "Covers" be used without causing damage?

Back in June 2011 I did a post that asked the question  "Tire Covers - Do they do any good?"  In that post I showed the numbers from a test I had run where I collected tire temperatures showing the effects of shielding my tires with white vinyl covers vs the tire temperature when exposed to direct sunlight for just a couple hours.

 I also covered the science of the damage that excess heat can do to our tires. The bottom line of that post was that white vinyl can help extend the life of tires by protecting them from both the affects of UV and the IMO more serious damage done to the internal structure of tires from excess heat.

I also did a rough check using dark trash bag covering a tire and in that case I found that the black cover actually resulted in a tire being hotter than when it was just in the Sun with no cover. Based on that limited data I have recommended against the use of Black or dark color vinyl tire covers.

On more than one occasion I have observed some Class-A RVs with what appears to be a mesh shield than hangs down off the side of the RV. This is different than the vinyl "bag" that hangs directly over the outside of my tires. I was able to collect a few data points while in Redmond , OR in 2014 at a large RV Convention, and that data suggested it might be possible to use this mesh material and not increase the temperature of the tires. Finally this Summer while at another RV Convention I struck up a conversation with a representative of ShadePro Inc who offered to send me a Tire Shade to test. In Aug & Sept i had some health issues and then I ran into difficulty with clouds here in NE Ohio but I was finally able to collect the data I felt comfortable with that would allow me to reach a conclusion of if this black mesh material could be used.
Here is a shot of my test set-up with a white vinyl on front, control sidewall in center and the black mesh shielding the rear. After 2 hours in the full sun 
In the shade a tire gave 92°F

In the sun the white cover was 126°F

The reference tire sidewall registered at 147°F

and the black mesh shade showed 136°F

Under the cover the front tire was at 114°

While behind the mesh shade the rear tire was only 101°F

The data shows that in this test the black mesh did a better job of keeping the tire cool than the white vinyl.

I can think of a couple of reasons for this.
1. The vinyl cover was in direct contact with the front tire so heat was being directly transferred to the tire.
2. The mesh allowed better air circulation around the rear tire.
3. The fact that the black plastic was also in direct contact with the tire probably contributed to the poor results.

I was wrong to suggest that all black shields were worse than white covers, as this test shows that data is better than opinion when it comes to facts. This is one of the wonderful things about Science. 

Friday, October 6, 2017

"Safety Margin"

Some people ask "How much Safety Margin should I have with my tires?" While this concept is simple, the reality is quite complex.

If you want to skip over the "Why" safety is complex just jump to the "Bottom Line" below.

In engineering it is more proper to talk about "Safety Factor" and Wikipedia covers the topic quite well. " Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load". For tires this can become difficult to establish for unlike many materials such as steel or aluminum, tires being made of a number of complex organic compounds both natural and synthetic that have properties that can vary from batch to batch. Even how the raw materials are handled and stored can affect the end product. Also the "strength" of the tire rubber varies with both time and temperature history and as I have previously pointed out the temperature history is not established by just considering the ambient temperature as tire load, inflation and operating speed as well as even storage conditions play a part in establishing the temperature of the more critical components of a tire. Some of these factors can be controlled by the vehicle owner while others can not.

Another part of the calculation concerns the consequences of failure. With some products, the consequences are just an inconvenience say as when a pencil breaks or the ink in a pen stops flowing. With tires the failure can range from an inconvenience if the tire wears out faster than expected or property damage may occur or in extreme cases personal injury can result.

Over the past decades the tire industry has developed a series of guidelines as they try to anticipate the variation in service the vehicle operator will subject the tires too, but even here outside factors such as changes in speed limits or legal load limit changes can affect tires made years before these operating conditions were contemplated.

Top line tire companies have staff of engineers, chemists and statisticians who constantly monitor variations in raw materials and in the finished product. Different plants have different requirements as even something as mundane as the water source can have an affect on the end product. Test labs at each plant are constantly monitoring the quality and consistency of the products that plant makes. not every tire plant makes the same type of tires so along with sales volume requirements plant capabilities are taken into consideration.

Ya but you are thinking "So what? I just want to know the Safety factor of my tires."

Basically I and other tire engineers have tried to consider all of these factors and are constantly looking at tires that have been run on both test tracks and by end users such as yourself. We adjust our specifications to allow our tires to meet and exceed a list of special tests that over time have proven very reliable at predicting the potential for tire failure. While we shoot for zero failures we also know that due to factors out of our control that goal is never possible given the constraints of real life tire use.

I have seen some figures that show a failure rate in the range or 0.05% for many tires but I have also heard of some specific tires (brand, size, design) having a rate closer to 5% or even 10%.

The bottom line
The best I can do is to suggest that you obtain and read the product maintenance manuals for the brand tire you have or are considering of buying. You will probably find that the information across brands is pretty constant so I suggest you at least take a look at a couple different documents. Some of the top line tires have RV or truck application specific reference materials such as can be found from Michelin or Goodyear or Bridgestone or Maxxis or you can check some of the links on THIS post.
- If you have a Motorhome or pick-up slide-in camper you need to confirm the load on each tire position and using the highest loaded end of each axle and the Load Inflation tables from your tire company learn the MINIMUM cold inflation pressure.  I suggest you add 10% to the table number and use that for all tires on that axle for your minimum. 
- If you have a towable (trailer or 5th wheel) also confirm that no tire is loaded to more than 85% of the max load molded on the tire sidewall. AND inflate to the inflation molded on the tire sidewall associated with its maximum load capacity.
- Get and use a TPMS. I have written on how I would set the TPMS warning levels HERE.

- Inspect your tires. Motorhomes can have your tire dealer do the inspection. Trailer owners can follow THIS procedure at least once a year or every 5 to 7,000 miles if you travel that much.

- Never exceed 75 mph with any tire in RV application and if you have ST type tires with no speed symbol never exceed 65 mph.

In my opinion if you follow these guidelines I believe you will have a reasonable and realistic safety margin for your tires.

Friday, September 29, 2017

Who to believe on tire inflation?

I was reading an RV forum and found a number of folks offering their interpretation on what the proper inflation is for tires in RV application. Some insisted the only correct inflation pressure was what was shown on the "Tire Placard" or "Vehicle Certification label"
Here is my reply:

We have two separate issues to consider when selecting the minimum cold tire inflation pressure in RV application.

A vehicle MFG has DOT requirements they need to follow. They are to specify the minimum inflation needed in the tires they provide on the new RV. That inflation is required to be sufficient to support the stated GAWR divided by the number of tires on that axle. Basically this means the axle rating is used to establish tire inflation based on the published load & inflation tables from the tire company.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or either GAWR or the individual tire load capability.

You will immediately note that the RV company is making an assumption that is just not a reflection of reality, that being that axle loads are always split exactly 50/50 side to side. While many RVs may have the load split only a couple percent off of 50/50 some have discovered one end of an axle may carry more than 1,000# more than the other. Others have learned their unbalance is closer to 40/60 and some have discovered that their tires are overloaded as soon as they fill their fuel tank before loading anything else into the RV. Obviously a responsible owner needs to do a better job than some RV companies are doing.

While some feel it is sufficient to just read the regulations about the vehicle certification label I and other tire engineers know that policy is not always the best way to get the best combination of ride, handling and tire durability.

Lets see what Goodyear says in their RV tire application web page.
"Correct tire inflation is a key component in tire care. The recommended maximum inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the load on each individual tire.
Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard

Maybe you don't think Goodyear knows more than some RV owners. What does Michelin say in their RV documents Here on pg 4:
"The amount of inflation pressure required in each tire depends on the weight of the fully loaded vehicle, to include passengers, cargo load, fuel, and water."
" To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear
indication of how the weight of the recreational vehicle is distributed across the axle.

Also in their tire maintenance document on tire inflation:
"Unlike commercial light truck and truck applications, we recommend weighing motorhomes at each wheel position of the vehicle (right front corner, left front corner, etc.). This is the only way to get a clear indication of exactly how the weight of the motorhome is distributed. Otherwise, one wheel position may be significantly overloaded even though the G.A.W.R. has not been exceeded. Tire inflation for a given axle should be set according to the pressure corresponding to the higher of the two end loads."

Others are allowed to have and express their opinions and their interpretation of how the regulations are to be applied, but it was my job as a tire engineer to understand and work within FMVSS regulations for some 40 years.

I will leave it up to you to decide if the only acceptable inflation is what is on the vehicle certification label or that inflation is learned by reviewing what major tire companies say in their  published guides.
Retired Desi