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Monday, December 23, 2013

Do you need a portable air compressor

A number of people have wondered if they need to carry their own air compressor so they can maintain the pressure in their tires. While keeping the correct level of pressure in your tires is important for the safe operation of your RV, I believe there are a few preventative measures that can be takes so you can avoid the need to carry around a compressor.

There are a number of things to consider before you go out and spend $20, $100 or even $300 when it may not be necessary to have a compressor for normal inflation maintenance.

First, lets lay out a plan on how to manage inflation without owning a compressor. We are going to assume you are not trying to inflate a tire that has gone flat due to a puncture or faulty valve. If your tire has gone flat and the tire bead detached from the rim you need not only high pressure but high air volume which a small compressor simply cannot provide. There are also safety concerns when it comes to inflating a flat tire. This task should be left to the professionals.
Since most tires will loose about 1% to 3% air pressure every month due to permeation through the rubber and you might also see a pressure loss of 1% to 3% due to a significant drop in ambient air temperature, there is the potential of needing to replace 5% to 6% of your air every now and then. This translates to a pressure loss of 2 to 7psi which is not a large amount but could be enough to be under your minimum pressure if you haven't set your pressure with sufficient safety margin.

I suggest that motorhomes set their cold inflation for all tires on a given axle at the minimum needed to carry the actual load on the heaviest tire on that axle PLUS at least 10%. If you have a TPMS you will quickly learn your "normal" weekly or monthly air pressure drop so as you travel you will have advance warning of when you need to "Top-off" your tire pressure. With this knowledge you can get air when you stop for fuel. Even the largest Class-A will be able to get enough pressure at the truck stops when they fill-up and smaller Class-C or Class-B can replenish the 5% pressure loss at the regular gas station.

If you are going to be parked at one location for more than a couple of weeks I would suggest one of the last things you do before getting to the campground would be to inflate your tires to the tire sidewall max pressure. This way even after a month or two you should still have enough pressure to still be above your minimum cold inflation when you leave the campground and you should stop at the first location where you can get air to replenish the inflation to your 10% cushion.

Trailers have a different problem given that I recommend they always run the tire sidewall max. The good news for trailer owners is that they have smaller tires so even a small 12 volt compressor should be able to top-off their tires. I would just spread out the task over a few hours the day before planned travel, so as to not overheat the compressor.

If you decide you still want a mini-compressor so you can add 2 to 5 psi to your tires, I would plan on spending at least $50. One thing to consider is to get a compressor rated at least 20 psi higher than the max you need. You also need to look for "duty cycle" rating. This is the % time you can run the compressor without overheating the unit. You don't want a unit that can only run for a few minutes before it must be shut down to cool. The ultra low cost compressors I have looked at seem to get very hot. This could be because the "cooling fins" are simply molded plastic rather than heat conducting metal.

If you have access to 120 volts at your campground or with an on-board generator I think you can find a small compressor for less than $50 that can meet your pressure needs if you feel you want to be able to top off your tires and not depend on truck stop or gas stations for air pressure.

Safety note You should never exceed the max inflation rating of your wheels or valves when establishing your "cold" inflation level, Some wheels may be marked but it seems that most of you will need to contact your RV manufacturer or wheel manufacturer to get the specification, in writing. I would not accept the simple "match the tire inflation" as the answer. Wheel manufacturers have a specific number and do not depend on the tire pressure as they cannot be sure what tire is going to be mounted on their wheel.
Also I would not use basic passenger tire "snap-in" rubber valves in RV application. You can review the posts on valves for more detailed information.

Tuesday, December 17, 2013

How to match dual tires when replacing one new tire.

Had a question on my video on matching Dual tires.
"I get the importance of size-matching, but how are you going to get an adequate & accurate measurement of the remaining dual while it's still on the truck or coach? It'd be expanded under pressure, somewhat distorted where it presses against the ground, and would have some wear on it. Is one supposed to compare a tire under those conditions to a new, unworn, un-inflated tire & expect them to be within 3/4?"

Well I can understand the bit of confusion as "Dual Matching" can be a challenge when trying to replace one tire on an RV. Lets work through the process.
First off we need to be sure we are doing the correct thing in replacing just one tire from a dual pair. Normally people have one dual tire fail due to some form of damage and subsequent air loss. In the past, before TPMS, the driver would continue down the road till someone waved him over pointing out thr tire failure. This meant that for some unknown number of miles the remaining dual had been operated at 100% overload. The general guideline would be that you need to consider the remaining tire to have had its structure damaged and it should be replaced. There are many documented cases where only one tire was replaced and the damaged tire fails a few miles to a few weeks later. At that point the driver acts surprised there was a failure.

Today we have TPMS available so that in the case of a puncture or other slow leak the driver may get adequate warning so he can stop before the tire with low inflation suffers a catastrophic blowout because it was run with 20% or greater air loss. In this case the fully inflated tire may not have been overloaded for more than a mile or two as the RV moved to a safe place to pull over, so after internal and external inspection it may be safely retained and put back in service.

Now the RV owner is confronted with the challenge of how to properly match tires for dual application. The 3/4" circumference is the correct maximum difference for tires and tire growth and wear can make it a challenge to meet the goal.

When measuring a tire for dual matching it needs to be off the vehicle and fully inflated. If the tire is brand new you can expect even radial tires to see some size increase after a couple hundred miles so it is difficult to match new and used and get correct numbers.

The solution if you are replacing just one tire would be to confirm your front tires meet the 3/4" difference and then place a new tire on one front and the old tire on the other front position.

Meeting the 3/4" max difference is critical as ignoring this figure can result is a failure down the road even if the tire has been properly inflated.

Monday, December 9, 2013

New Information on Loading and weighing your RV

Walter Cannon, Exec Director of RV Safety Education Foundation sent some information to me that will impact all RV owners that want to weigh their RVs and learn the individual loads on each end of each axle.

If the RV owner does not attend a large RV Rally or Convention where individual corner or tire loads are measured by RVSEF or similar companies, they face the challenge of finding a large scale that will allow them to measure not just individual axle loads but also allow the tires on just one side to be weighed while keeping the RV level side to side as outlined in worksheets such as found in this guide, or the worksheet on this site or at this site.
In the past, I and others have, provided links to CATscales and suggested that it might be possible to get the weight readings needed to allow calculation of individual RV corner loads. While I know that some CAT scales had guard rail installed to prevent trucks from driving off-center on the scales and this may prevent RV owners from learning the weight of one side of their RV, we believed that there were many scales that could still be used. However what I learned from Walter is that CAT actively discourages people from using any of their scales to learn the side to side load variation.

From CatScale web site we read "Our scales can give you axle weights and a total gross weight, however, they cannot weigh each corner of the vehicle. We cannot provide individual wheel weights and, to prevent damage to your vehicle as well as our scales, do not allow that type of weighing."
It seems they do not want to allow RVs to measure one side of their units. There are probably a number of reasons for CAT taking this position but the bottom line appears to be that you should not try and get any weights other than total axle loads if using CAT scales. This is probably OK if you are just confirming your total weight but certainly should not be used to try and get individual tire loads. This means you cannot use CAT weights to set your tire inflations.

Based on this information from CATScale, I can no longer suggest or recommend you consider using their scales to weigh your RV to calculate tire loading so you can then look up your inflation needs.

This presents a serious problem for RV owners that want to know the real tire loads. CAT seems to be the 8000# Gorilla in the truck stop scale market as it is quite hard to find a truck stop that does not use CATScales.

At this time all I can suggest to the consciences RV owner is to either check the RVSEF web site for their schedule and location for offering weighing. If that is not convenient then you will need to search for a local gravel yard or large farm supply company or grain elevator and see if it is possible to gets the weights needed to complete one of the above worksheets.

The one bit of good news is that once you learn the real weights of your fully loaded RV and identify the proper minimum inflation and add your 5 to 10 psi safety factor you should not need to re-weigh the RV corners unless you make some significant change such a adding a battery bank or changing furniture or maybe a residential refrigerator to replace the original fridge or adding new storage for your bowling ball collection. :-)

Sunday, November 24, 2013

Save money at the fuel pump

Tire Pressure.
 Ya, we tire engineers are like a broken record when it comes to tire pressure.Tire Pressure Monitor System. If you don't have a TPMS then we tell you to check your pressure every morning you are traveling.
We tell you you need to monitor it all the time by using a
We also want you to get your RV weighed then to consult some confusing tables to learn how much pressure is needed for your motorhome.
To make things worse we expect you to have an accurate gauge but then we don't tell you how to check your gauge to be sure it is accurate, or even why its important to have an accurate gauge.

Is that what's bothering you bunkie??

Well this post is going to help answer your questions, solve the problem of how to be sure your gauge is sufficiently accurate and to top it off tell you that its possible that many of you will end up saving more money than the "fix" costs.

To start off lets take a look at accuracy of gauges many RV owners are using today.
In a previous post I showed the results of a series of gauge tests conducted at an RV Rally Sept 2012 on 11 gauges, we saw that there was a 36% failure rate. This was higher than the failure rate seen on a similar sample from 2011 when 11% failed. This year my test sample was 24 gauges and we had a 12.5% failure rate. Failure is defined as more than 5% off when compared to a certified digital gauge that reads to the nearest 0.5 psi. For all these tests the test pressure was between 77 and 95 psi.

According to sources there is a range of fuel economy savings possible from keeping your tires properly inflated because tires are only one of the contributors to fuel consumption. Also different tires have different affects on fuel economy based on a number of variables. Some of these include different rubber compounds, basic tire construction features, tread design and tread depth. Without getting technical we learn that the impact of lower inflation on mpg ranges from  0.05% to 0.3% per psi. Now you may say this isn't much but lets look at how this hits you in your pocketbook

Assume fuel is $4.00 a gal when you do a fill-up. Lets look at the range of savings at -2psi and -10psi from our goal of 100psi (our cold inflation to carry the load with a built in +5psi safety factor). With a 30 gal fill-up the range of "extra cost" for being low on inflation is between $0.12 to $3.60. If you do a 70 gal fill-up running low on air is costing you an extra $0.28 to $8.40 for that fill-up. Remember this is from just being 10 psi low on your tires. A number of the gauges we tested had the owners running from 9 to 18 psi low so they were wasting some real cash.

With this much potential loss from your pocket each fill-up you certainly do not want your pressure gauge to be giving you wrong readings. Having an accurate gauge is easy and based on the above calculations you might save enough in a few tanks of fuel to pay for what you need.

A quick search for "Digital tire gauges" at Amazon. shows you could spend well over $300 for a digital gauge but that really isn't necessary. You can get a good digital gauge for less than $10. This becomes your "Master Gauge" Then you get a second gauge for every day use. When the gauges are new you compare them and they should give the same reading. If you notice a sudden change in tire inflation pressure you can dig out your master gauge and confirm. You should also do a comparison at least a few times a year. The chances of both gauges going bad the same amount at the same time is vanishingly small.

Finally don't just throw your master gauge in the bottom of your tool box. I suggest you keep it in a safe place that offers some type for protection.

Wednesday, November 20, 2013

"Interply Shear" and other Techno Babble

I have been putting this topic off for quite some time as I was worried about overloading you with too much "Techno-babble" but I find myself having to constantly repeat this information in individual posts on various RV forums I monitor so figured it would be easier to do a complete post that I could direct people that want to understand why the loading of some trailer tires is much more complex than the average person thinks.

To make this less painful I will give the Bottom Line info first, so those not interested, can stop reading before I put them to sleep.

When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. In addition when you turn a corner the forces generated to move the RV sideways have to be transferred through the tire structure.

This causes additional bending of the belt and body structure. The more the bending the higher the stretching of the rubber. With enough stretch, microscopic cracks form and existing cracks get bigger. Eventually with enough cycles and enough force the cracks may grow and join up with the possibility of tire components separating which could lead to a tire failure. You can lower the stretching if you lower the bending and you can lower the bending if you increase the inflation.

So now on to the Engineer Speak and Techno Babble

If you own a multi-axle trailer these forces can be much higher than those seen on a tow vehicle, motorhome or car, where the tires are not close together but at the corners of the vehicle.
I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.

Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes 24% higher loads than those seen in tires on non-trailer application. Initially tearing is at the microscopic level but with time and repeated cycles these forces grow which can lead to small cracks at the belt edges as seen here at the arrows.

 If not spotted these cracks continue to grow to almost the full width of the tread as seen below.

 If you are lucky you will see the bulge in the tread as seen here and now you know this tire has failed and MUST be removed AT ONCE as the separation can grow and  can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT you need to be sure you are not exceeding the max rating of the wheel.

So the best recommendation I can give to trailer owners is to run the inflation molded on the tire sidewall. For owners of a TV or motorhomes, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.


Friday, November 15, 2013

Should I jack up my RV?

I see this question come up each year when some people are parking their RV for the Winter. There is a quick and simple answer and a more involved answer,

Quick Answer:
I do not jack up my Class-C RV over the winter but I do inflate the tires to 80 psi (the pressure on the tire sidewall). I do cover the tires with white covers over the winter (and whenever I camp at a location for more than 1 day).

There are other things you can do such as keeping the tires out of contact with wet sand and dirt. I covered this with pictures in my post on Winterizing” 

More involved answer:
Jacking up the RV and removing the tire is one of those things that in a perfect world would be easy to do but in reality it isn't as many RVs don't have good jacking points or are so heavy you need very HD jacks so it may just not be safe to do..
I do understand the theoretical damage done from long term parking in one spot but again theory and reality clash. Lots of actions can theoretically prevent microscopic problems but if you extend the life of a tire by 1 month if you were to jack it up every time you were planning to park for more than 3 months, would it be worth the effort? Probably not.

There are actions that are easy to do and relatively inexpensive that can provide real "bang for the buck". I have covered these in various posts on my blog but will touch the big ones here.

1. Your actual static load should not exceed 85% of the max load capacity on any individual tire for the inflation you run..
2. You need to weigh and learn the actual load, when fully loaded and not simply take the total axle load and divide by the number of tires. You will NOT get the correct tire load by doing the simple division. You can download a worksheet on how to do the math HERE Or HERE

3. Get a TPMS so you will get a warning when (not if) you start to loose air due to puncture or leaking valve
4. Cover your tires with WHITE tire covers if they will be in the sun for better part of the day. A couple of hours in full sun does about same damage as a full day in the shade in Phoenix.
5. Replace the snap-in valves or rubber parts of your bolt in tire valves whenever you buy a new tire.
6. Get and use a digital hand held gauge at least once a month, even if you have a TPMS. You will probably be adding 1 to 3 psi each month anyway to maintain the tire inflation. This will also serve as a check on the TPMS.
7 NEVER drive on a tire that has lost 20% or more of its air. Structural damage will be done. Such damage is cumulative and this damage does not repair itself. (See post on Potato Salad)

8 Do not believe everything you read on RV Forums. Ask the poster for their actual training in failed tire analysis. Simply having owned tires from company XYZ does not make that person a knowledgeable source.
9. If you have a multi axle trailer your cold inflation should be the inflation on the tire sidewall

10 You might put on your To-Do list to read the posts in my blog (including the ones where I later point out my errors), then you could subscribe so you get a notice when I do a new post. I do about 3 a month so you will not be overloaded.
11.If you have a multi-axle trailer, learn how to do a rotating tire inspection and do your tires at least once a year. Watch this VIDEO  read the background in this POST to see what a failed tire looks like before it comes apart.

Wednesday, October 23, 2013

Breaking News - Max Speed on ST type tires

Finally chased down the answer. Getting to the bottom was a challenge because I ran into "customer service" people who could read from a script but had no idea why they were giving me the answer they were.

Even got to an engineer at Tire & Rim Association and based on his answers it was obvious TRA had not thought about ST type tires or the unique nature of most RV trailer applications for many years so I believe he acted a little surprised when I identified the engineering analysis that indicated that the current loads too high for many trailers and that the load capacity should be decreased by 15% to 25% or more if the failure rate was to be decreased to a more tolerable rate rather than the 5 or 10% or higher some are reporting. This is not his or TRA fault as I understand it is the responsibility of tire company representatives to TRA to present and review technical matters.

Anyway here is the bottom line.

For Goodyear Marathon tires only:
There is a Goodyear  Tech bulletin   PSB#2011-13 that uses information from TRA.   
The tech bulletin says that the normal max speed for Marathon ST type tires is 65 mph. This speed can be increased up to 75mph only if the inflation pressure is increased by 10 psi. This new inflation can be 10 psi higher than the pressure molded on the sidewall of the tire that is associated with the tire max load.
The load does not increase beyond the load molded on the tire sidewall.
Goodyear does not support or condone operation above 75 at any time.
The trailer owner is responsible to confirm the rim is rated for the new inflation level.
While TRA indicates a further increase in speed might be achieved Goodyear has decided not to adopt this option.

The person I talked with at Tire Rack said they had already changed their web site to indicate this applied to Goodyear tires only and that they were attempting to learn if any other ST tire manufacturers were willing to make a similar change. As of Saturday Oct 19, 2013 they had not identified any other tire manufacturer willing to make an increase in max speed above 65.

Info added 10/26/13 10:51 AM EDT
If you have a GY Marathon ST225/75R15 LR-D it is rated for 2540# @ 65psi and a max speed of 65 mph. If you increase the cold inflation to 75psi the tire would now be rated for 2540# @ 75psi and a max speed of 75 mph.
The example currently only is OK for Goodyear Marathon ST tires as I have not seen a published document from any other tire manufacturer. 

The pressure increase must not exceed the max pressure capability of the wheel. You need to find a max pressure rating stamped on the wheel or get something in writing from the wheel MANUFACTURER.

My observation
This Max speed is like your engine red-line. While it might be possible to exceed this speed for short periods it will significantly reduce long tern durability.

I have sent a copy of this post to both TRA and Goodyear. If they ask for corrections or additions I will edit this post accordingly.

TPMS batteries, Changing tires, Tire Size info, ST type tire speed rating and RV control with tire Blowout

Few quick topics in this post.
Saw a few posts on RV forums on TPMS sensor replacement.
It seems some people have had their TPM systems long enough that they need new sensors
because their batteries are low. So some folks are confronted with spending $200 to $300 on a set of 6 sensors.  I suggested that they look into the TireTraker system as it has low cost watch batteries so the cost is only a few bucks for a new battery rather than $35 to $50 each sensor. Full disclosure. TireTraker is a sponsor of this blog but I did buy my TPMS from them at a rally a couple years ago based on their features.
I suggest you do a "Life Cysle" cost comparison based on 5 years. Include the initial system cost, the cost of replacing the batteries if your system allows that or the cost of replacing all the sensors if you can't replace just the batteries. You might also want to consider the length of the initial warranty.


Another person said he had 8 years on his Class-A tires and was asking "What brand should I buy"
This of  course immediately started a flurry of "I have had great success with Brand-X" or I had a failure with
Brand-X so will never buy another one of their tires, etc.
I asked the poster why he was considering changing brands if he had had 8 year  good service from the tires he had? When it comes time to consider new tires I suggest you make a list of the Pros and Cons of your current tires.

 Be sure not to include things like the puncture with the roofing nail as that can happen to any brand tire.

 I often tell people you are buying a tire company and their dealer network not just a set of tires. If you get a great price from Billy-Jo-Bobs Cheap Tire Emporium and Bate Shop but there are only a handful of dealers in the country where you can get a replacement and you have to pay shipping back to Billy-Jo if you want to make a warranty claim I doubt that the total price of owning that set of tires is as good of a deal as you first thought.


When asking a question on an RV forum, it helps if you include the complete tire size designation. Some folks say nothing about the size but want specific answers on load capacity. Others provide only part of the size such as 235/75R16 and leave off if they are talking about a "P" type or "LT" type or "ST" type. The answer to these questions will probably depend on which type of tire we are talking about. It also helps if you include the Load Range as in LR-D or LR-F or whatever is molded on the tire sidewall.
Sorry but indicating that you have a Mountain Top Rambler RV doesn't help as there are just too many makes and models of RV out there for anyone to know all the tire options that migh be used.

ST type tires have a normal Max Speed of 65 mph. You should consider this like the redline on your engine. While it is possible to exceed the red-line it isn't good for long term durability. When covering this topic some point to the Goodyear Tech Bulliten that indicates you may increase this max speed rating up to 75 mph if you also increase the tire inflation 10psi above the inflation associated with the max load on the tire sidewall. While Goodyear may be willing to stand behind the warranty of their Marathon ST type tires at this higher speed you should not do the same for other brands of tires unless you get something in writing from that tire MFG. Tire company Tech Bullitens only apply to the brand and line of tires mentioned in the bulliten.

Finally I had a question on what to do if you have a  blowout on your RV.  Michelin has a good video on this topic covering Motorhomes. Similar driver reaction if you are driving a tow vehicle would probably apply, so there isn't much I can add. The question however asked about blowout on an RV trailer. Now I have not tested this myself but I have seen more than one example where the impact on vehicle control was so small the driver never knew he had a trailer tire failure till someone flagged the driver down.
As explained in the Michelin video the forces would be drag on the trailer which would keep it generally straight behind the tow vehicle with only a little side offset.
If your trailer did start to sway I would use the manual brake control to slow the trailer down. This should quickly stop the side to side sway and allow you to bring the tow vehicle and trailer to a safe stop.

Tuesday, October 15, 2013

Will you increase load capacity with higher Load Range?

The question of load capacity and Load Range gets asked a lot, so it seems to be a bit of a stumbling block for a number of RV owners. Here is an example:
"I need to replace the ST235/75R15 tires on my trailer. Currently have load range C tires which is sufficient for the maximum trailer weight when inflated to 50psi but allows almost no safety margin. I want to change to either load range D or E tires. My rims are only rated for 65 psi so if I went to the load range E tires I would only be able to inflate them to 65 psi. My question is this. Is there any advantage to a load range E tire used at 65 psi vs a load range D tire at 65 psi?"

Now just because he is asking about a 15" trailer application and considering a change from LR-C to LR-D it does not mean the general answer does not apply to others, even if they are considering a change from LR-G to LR-H on a Goodyear 295/75R22.5 Class-A tire.

Let me explain.

As some of you remember, I previously covered the fact that it is the air not the tire that carries the load. For those that still have doubts, here is a picture from a special test machine that measures force and delivers the answer in color. In this case, the lowest force is Blue which increases to Green, then Yellow and finally Red at the highest force. It is clear that the tire wants to be round because of the high inflation pressure but when you press the tire against a surface it becomes flat with  the highest force in the center of the tire. If it was the sidewall that carried the load, as some want to believe, then the Red (high force) should be at the outside shoulders.

Now when you go up in Load Range you are actually looking at the capability of the tire to carry more inflation pressure. Remember to gain in load capacity you need to increase the inflation. Years ago when tires used cotton for body cord we might have 4, 6, 8 or more layers or ply of reinforcement to retain the increased air pressure. Those increments provided steps in the Load/Inflation tables. Since the late 60's and early 70's when materials improved the actual number of ply decreased and the term Load Range was applied to those steps.

So going back to the original question, when increasing the Load Range you gain the possibility of increased load carrying ability but if you don't increase the actual cold inflation, the change in Load Range will not give you any increase in load carrying capacity.

The answer to the question is;
 He will gain a safety margin if he increases his inflation to 65psi with LR-D but if he stays at 65psi there is nothing further gained by going to LR-E as there is no difference in the capacity at 65psi.
The same thing would apply to the 22.5 example. When we look at the Goodyear tables we see that at 110psi both the LR-G and the LR-H are rated for 6,175# single or in dual application at 100psi they both are rated as 5,675#.

Special note: It is important that you look at the table on the web site of the manufacturer of your tires as not all companies give the exact same capacity number at every inflation level, even for the same size.

Tuesday, September 24, 2013

The tire was "Defective"

I see this statement or somethnig similar in many posts on various RV forums. The problem I have is that too often the person making the statement offers evidence to back up their claim.

 The mere fact that a tire failed, simply does not mean it was defective. If someone wants to make that claim they need to provide a plausable theory on which material componenet was made incorrectly or which part of the tire manufacturing process was done incorrectly in such a manner that it would result in a tire failing in that specific manner.

Most radial tires made today have about 20 to 25 different components.

 This Wikipedia page identifies nine major components

 Watch this video and you will see a manual process used for a farm tire.
Hankook has a nice graphic showing more steps in detail for radial tires
Here is a promotional video from Michelin. The part about automatic tire assembly is very similar to what you would see in just about any modern tire manufacturing plant making Passenger, Light Truck or TBR Truck/Bus-Radials .
You can see that from manual to fully automatic the steps are very similar. It doesn't make any difference who makes the tire as all tires share very similar construction features.
So now that you have a little better idea of how tires are manufactured Try this exercise. Identify a step in the process that could be done incorrectly on one tire or a small group of tires. The out of tolerance step or material needs to be close enough to spec to allow the tire to finish the manufacturing process and to also allow the tire to function properly for a few hundred or thousand miles but then this error must somehow cause the tire to suddinly cause the tire to come apart but first provide no warning and second leave no evidence of the out of spec part or material being present in the failed tire.

I think you will find that it is very difficult to have a material out of spec enough to function acceptable for a short time but then to catistrophically fail a few miles later and to also leave no physical evidence.

I am not trying to imply that mistakes are never made but since the manufacturing process makes tires and even the components in batches if a mistake is made it should affect all the tires in that batch which would be from a couple hundred to a few thousand. If in fact those defective tires somehow managet to pass through final inspection unnoticed and make it to the tire store where they are mounted on your RV I think you can see that in all likelyhood all the tires with the same defect would fail in identical manner.
If a group of tires fail and if dealers or owners report the fialures to NHTSA then there is a high probability that action would result.

There have been a few cases recently where thousands of tires were recalled because a small number had been found with a defect. What I haven't seen or heard about is a recall because a single tire failed.

When we see a single tire fail and it leaves evidence such as worn sidewall or melted polyester body cord

or fatigued steel body cord as seen on the right.

Tire engineers can be quite certain the failure was not caused by a manufacturing error but in fact the tire was operated under-inflated at highway speed for a few miles. Also when a tire has managed to perform satisfactorily for thousands of miles before failing it is again very unlikely because of a manufacturing error.

All to often those that jump to the conclusion that the tire must have been defective because they checked the air a few hours of days before the tire failed. These folk fail to realize that tires can fail with sidewall "blowout" in less than 5 miles of running at significant under-inflation.

Here is an example of a tire that had been run under-inflated in a controlled test of 3.9 miles at only 10 to 15 mph. You can clearly see the internal damage.
This tire still had 20% of the inflation needed to carry the load so it was not completely flat. That circumferential line is where the body cord would melt or fatique if the car had been driven at highway speeds and only a mile or two further. However dollars to donuts most people would say that since they had checked the air 100 miles prior to the failure the tire must be defective.

Thursday, September 12, 2013

TPMS Accuracy & I had an air leak!

I sometimes see people worrying about temperature or pressure readings they see on their TPMS monitor. Based on the reported numbers, I have never been concerned with the minor variation but I felt I needed data to see if my opinion was justified.

As you can imagine I can get pretty focused on tire inflation pressure so it may not surprise you that in addition to two digital pressure gauges,
which have been checked and found accurate to +/-0.5 psi against ISO laboratory certified pressure gauges, I also have two TPMS on my RV. I am probably the only individual in the US and possibly the world to have an RV so equipped. The reason for having two systems is so I can give you this test report as I have relatively high confidence that with my frequent inflation checks with my digital gauges and adjustments to inflation I feel I would identify any problems with my TPMS. More on that later.

I installed my internal TPM about two weeks after buying my RV in 2008. At the time I was quite familiar with internal systems as I had been spending a significant portion of my work time involved with the OE versions of TPMS sensors. Some like this one had been damaged by improper mounting practice.

 My internal system is actually a car based system as it has an upper pressure range of 75psi but since I set my tires to 65 psi which is about 10psi above the minimum needed based on actual tire load, I felt I would not be seeing a big rise in inflation pressure. I was of course more concerned with the potential of a puncture and loss of air pressure. I subsequently learned that I would occasionally get a "high press" warning on one or two tires but have confirmed that my tires only get up to about 77 psi max.

I started this blog in 2011. After that I also increased my reading of various RV forums where I started to see more and more comments and questions on TPMS accuracy, I decided I needed to do some testing.

Here are the results from a number of tests.
 This data compares the two TPMS with my accurate hand gauge with the tires all set to 65.0 psi cold

 As you can see that while there is measurable difference in the readings I do not consider these meaningful and would consider the +/- 3 psi acceptible as I consider a TPM to really be a warning system based on a change in pressure and not a substitute for an accurate pressure gauge.

Running pressure shows a simmilar range of variation as seen above.
Running temperature is a slightly different story.Since each tire has a slightly different load and there is a significant difference in air-flow around the fronts vs the dual rears as well as a difference between the inner and outer dual, I would expect more variation and we see this in the test results shown here.
I note that the internal temperature is consistently higher than the external temperature reading which is to be expected with the external sensor both further from the heat source and being cooled by the external air flowing over both the sensor and in the rears over my extender hoses.


I previously mentioned that I felt I would be able to see a failing TPM sensor and I can report that I was able to discover a leak of  about 8 psi a month because I pay attention to the range of readings of my TPMS. I do not write down the numbers every time but I do pay attention to the range of numbers observed. I can also confirm that my external system gave me a warning as it is set tighter than the internal system. The leak was at the TPM sensor itself but since my system has a three year warranty I have been able to get a free replacement sensor.

You can expect some variation in both pressure and temperature between different TPM systems. If you use a good hand gauge to set your pressures and just make a mental note of the range of pressure readings right after setting the tire pressures and on your first couple of trips, I think you will be able to recognize when there is a leak or an unusual reading that warrants additional investigation.

Wednesday, September 4, 2013

Why do tires fail?

There are basically two reasons that explain the vast majority of tire failures.
1. Over-load / under-inflation which are almost the same thing since it is the inflation air that carries the load not the tire.
Under-inflation: A tire operating at less than 80% of the inflation needed to carry the load, is considered to have been run flat and there is a good chance that there has been permanent internal structural done to the tire if driven on in that condition. Related to this is having the tire inflated to a level that just barely is rated for the actual load on the tire. Obviously you can be under-inflated because of cut or puncture or a valve leak or if you use an inaccurate gauge. If you run sufficiently low in pressure at highway speeds for a just a few miles you can have a Run Low Flex Failure or more commonly a "Blowout". Steel body tires can have "Zipper" failures due to fatigue of the steel body ply.

Over-loaded: Few people realize that by design, most passenger vehicles have 13% to 20% or more "Reserve Load". That means that they are actually under-loaded by that much for a vast majority of the time. Most RV trailers on the other hand have tires selected that are at the tire max load and can just barely carry the actual load while motorhomes seem to have some reserve load but not as much as passenger cars.
Data on actual loads measured shows that over half of RVs measured (out of many thousands) have one or more tires overloaded based on actual inspection.
While at a campground last week I got into a discussion with the guy parked next to me with a 5 week old RV.  I bet him a beer that at least one of his tires was at least 10% low. We found all his tires were between 18% and 21% below spec. I did enjoy the beer. :-)

2. Heat. Heat damage occurs at the molecular level and degrade the ability of rubber to flex and stretch and not break the chemical bonds. Once cracks form the rubber does not repair itself, the cracks just continue to grow. If they grow enough eventually you may have components come apart. Heat comes from a few different sources. This heat is generated by the flexing of the tire with the hottest region being at the belt edges (edge of the tread) in radials, not the sidewall if the tire is properly loaded and inflated. Increased speed generates more heat. Sometimes the heat is generated faster than it can be transferred to the surrounding air. Over-loading generates more heat. Under-inflation generates more heat. Having 0% reserve load generates more heat than having 10% reserve load which generates more heat than being 15%  under-loaded etc.  This heat can soak into the structure of the tire and actually accelerate the aging of the rubber in the tire. As rubber ages, it looses it's flexibility so this contributes to the breakdown of the rubber at the molecular level mentioned above.

Part of Organic Chemistry is chemical reaction rate.
For every 18F increase in temperature the rate of aging doubles. Heat also comes from being in the sun when parked. So if the RV is parked with tires in direct sunlight you can see the tire achieve 36F increase or more which means it is aging at more than four times the rate it would have if in full shade.
If you want to understand the technology behind this accelerated aging due to heat I suggest you can read some of these sources if you have a few hours.

Here are some specific references on tires

I provided temperature data in my post on  white tire covers which protect tires almost completely from this heat damage. Many times the cumulative damage from excess heat can result in a separation of the belts and tread from the rest of the tire. Remember aging of rubber makes it less able to tolerate flexing.

Hope this helps others understand the causes of the vast majority of tire failures.

Thursday, August 22, 2013

Who Do You Trust

When it comes to buying tires, who do you trust?

I recently read two posts on an RV forum about problems people had with their new tires. They were the wrong size! 

One person said:  I have an F250 4x4. I got new tires put on almost a year ago and just realized they are not the same as was originally on it. The tire store told me they were the same.  I took their word for it. The new tires have been inflated to 75 lbs since buying them eleven months ago. I just read the sidewall.  They are P265/70R17, where my original tires were LT265/70R17 LR-E. The truck door sticker says 75psi  for LT tires.

The other person said:  I replaced the crappy OEM P275/60R20 tires on our 2012 Ram 1500 4X4 w/ LTX AT2 tires at 10,000 miles. When buying a new set of tires last month I assumed the tire dealer would put LT tires on. But nope, they put P series tires on and I didn't catch it until I checked the tire pressures. Was I surprised and mad at myself 'cause I sure can't afford to get a new set and it wasn't the tire dealers fault at this point. I should have insisted on LT tires, but they will only put what the factory put on unless requested. I really wished I had been more attentive at the tire dealer and got what I wanted!

Quite frankly I was surprised by these two accounts but maybe I shouldn’t be considering how little interest some people take in their tires. Now I hope that these people are the exception and the readers of this blog actually do take more than a passing interest in their tires. 

For many people, tires are simply round black things that they are forced to buy every few years but I find it amazing that people would pay more attention to purchase of a pair of $20 pants than they do to getting the correct size, type and Load Range of a $400 -$800+ set of tire. When buying pants you probably know and confirm you are getting the size you want.

From the examples above, it is clear you cannot depend on all tire dealers to know what you want from your tires. At some dealerships I am sure the “order takers” know more about washing machines they were selling at their previous job last week than about the tires they are trying to push because of over stock. You need to be an informed, educated and demanding customer.
Do your research BEFORE you get to the store.

1.  Be sure you know the loads on your RV and tow vehicle. 
2. Be sure you select an appropriate tire that has load capability that exceeds your maximum loading.   
3. Know how to determine the tires you need.
4.  Ask about the warranty, if any 
5. Consider the ease of finding a replacement for the brand you are buying. 
6.   Read my post on “Best Tire

Wednesday, August 14, 2013

Autoguard LT245/75R16 being recalled

I have previously written about tire recalls. Here is what can happen when it is confirmed that some tires do not comply with DOT regulations. 

 Some Autoguard LT245/75R16 tires are being recalled by the manufacturer.

Tires manufactured June 25th, 2012, through November 11th, 2012. These tires failed the endurance test standards of FMVSS 139 and contain incorrect maximum load load data on the sidewall. Thus, these tires fail to to comply to the requirements of Federal Motor Vehicle Safety Standard No. 139, "New Pneumatic Radial Tires for Light Vehicles."

You can get more information at NHTSA Campaign ID Number: 13T007

Owners may contact Tire Group International, Inc., BCT's U.S. contact, at 1-305-696-0096 extension 5538.  

Thursday, August 8, 2013

Why did my engine "blowout"?

Why is it we almost never hear the above question on various RV forums?
Could it be that we don't hear this question because people know it is not wise to exceed the RPM limit or "Red-Line" specification of their engine so they simply don't run at and above the engine speed limit?

But I do see questions of what has to be done to run 75 mph with a standard ST type tire when the RV manufacture specifies the pressure on the tire which is one of the factors limiting the tire speed capability.

The interaction of tire types, load capacity, inflation and speed "rating" are complex, as are almost all topics about the various performance trade-offs with tire selection

I did a post just on the topic of "How fast is safe to drive on your tires?"   I have also pointed out that if you are going to deviate from US industry guidelines, such as published by Tire & Rim Association, you must follow the published guidelines for your specific size, type and brand. You cannot use Goodyear document to learn the appropriate load & inflation for your Maxis tires. It even means you cannot apply specifications, including speed, load & inflation from a Goodyear Marathon with a Goodyear G614.

Now as we all know, an ST type tire has a higher load capacity rating than a similar sized LT type tire.
ST235/75R15 LR-C is rated 2340# @ 50 psi (min) and 65 mph (max).
LT235/75R15 101/104Q LR-C is rated 1985# @ 50 psi (min) and 75 mph (max).

We have also established that load capacity is nominally a function of tire size (air volume) and its inflation pressure. If however we decrease the tread depth and the lower the maximum operating speed we can increase load capacity slightly. In my post of Oct 12, 2011 I identified a Michelin trailer tire with a higher load capacity but many want to ignore its max speed rating of 62 mph which is even lower than ST type tire rating of 65 mph.

Now to the specific question of how to operate a specific tire at 66 to 75 mph. This is accomplished by increasing the inflation pressure by 10psi according to a document published by the tire manufacturer. This is not as simple as many assume.

If you select the 65 mph max speed for an ST245/75R16 LR-C you will find a load capacity of 2,600# with a minimum of 50psi if you stick to 65 mph max. But if you want to drive at speeds up to 75 you need to increase the inflation pressure in the industry standards as published by Tire & Rim Association. So now to carry 2,600# you need an inflation of 60psi but this exceeds the max inflation rating for this size @ LR-C so you either need to get new LR-D so you can run 60psi or to limit your load to 2,270# and 60psi which is the published load capacity for 50 psi inflation of the subject size tire. This higher inflation may also exceed the rating of the wheel, but again many choose to simply ignore that rating too.

One obvious item many simply ignore is that the max speed for a tire is much like the red-line rev limit for their engine. It clearly is possible to run an engine with a 6,000 red-line at 6,300 RPM and possibly higher but what about long term engine life?

There are hundreds if not thousands of posts on RV forums from people complaining about tire failure but I have to wonder why they seem to understand the effects on engine life of high RPM operation but fail to understand that running a tire faster than its design specification will also contribute to shorter tire life.

Saturday, August 3, 2013

I read it on the Internet

With apologies to the "French Model" guy in the TV advertisement, I want to talk about the reliability of tire related information as seen in many RV forums.

 I subscribe to or monitor about 10 different RV forums. I know there are more but I do have other things to do than just read RV forum posts.
 Many times I use the "search" function to look for threads that are about tires. It appears to me that a significant percentage of the people asking questions, have not made an effort to seek out answers already posted before they ask a question. Too often what follows are answers from other RV owners who clearly do not have a good engineering background on the topic of tires. I would like to post a few examples and point out the problems I see in both the question and answers provided by others. 

An owner said “I have a 1992 Ford Four Winds motor home. 29 feet, I think. What kind of air pressure would I run in the tires. I have 60-70 but have read where some run around 100. Please help me decide".
Well first off the poster of the question left of much important information out of his post. What was his current tire size or the original tire size, including Load Range and what are the actual loads on the individual tires.
Clearly he had not noticed in numerous other threads that there is no “rule of thumb” for providing the proper inflation other than following the vehicle tire placard information. I would not be surprised to learn that this owner doesn’t even know he has a certification sticker.
What followed were 4 posts telling him the proper inflation for a motorhome, is a function of the real load and there are too many variables such as model, kind & size of tires to provide an answer. Another person replied that he should read other posts about tires and review the tire mfg “Load Charts”. Another asked for the tire loading but also mentioned the need to know the tire age by reading the DOT serial. Finally a more knowledgeable person suggested the tires need to be checked for sidewall cracking and pointed out the tire “birth date” is part of the DOT serial.
As of a week later the OP has not replied with any of the requested information. I think you can see that if he had made a little effort and read a few posts, or my blog :-) he would have been able to learn the answer to his question right off.

In another forum a person posted complaint about “blowout”.  Here we see the picture he posted.

 In this close-up you can see the tread on the right side is much more worn than on the left side.

 This is evidence of the detachment being in the tire for many hundreds of miles.
This thread has over 20 posts with lots of complaints about junk tires etc but not one properly identified that the subject tire did not have a blowout as it still held air. At that point I jumped in and provided a number of pictures of other tires showing the wear was not due to brake lock-up and the tire did not have a “blowout” but had a belt detachment and that if the tire had been more thoroughly inspected the failure and RV damage could have been avoided.
I suggested that if you have an RV, especially a multi axle trailer, an in the air rotation to look for tread wobble is something I would suggest at least once a year starting at year three.
This video  shows what a bad tire can look like.

Bottom Line.
If you have a question First spend a little time and see if the question has already been asked. Second, was the answer based on a lot of complaining and ranting or was the answer based on facts with pictures, links and hard data.
Remember that just because you read it on the Internet doesn't mean the information is correct.