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Tuesday, July 21, 2015

How to avoid tire failure and RV damage. And I had a tire failure!

A general comment on tire inspection and how it may be able to prevent RV damage due to tire failure if done properly.

There are three basic types of tire failures.

- The least likely is a sudden large impact with some object in the road. This is the least likely and is actually difficult to do. By this I mean to have a tire that is OK with no damage having been done due to excess heat, high load, low inflation or improper repair. It could even be a new tire. You drive over something like a Railroad spike or into a foot deep pot hole or over a 10" chunk of scrap steel that fell off a truck. It has to be large and you have to hit is just right for the tire to suffer immediate failure. I know from personal experience (doing a special test project of tire "Rapid Air Loss") that even driving over a piece of 2" pipe sharpened at one end and standing 2" up in the road with sharp end up doesn't always cut through the steel belts of a tire.  Yes you might run over something that cuts the tire but you would see it in the road and hear it hit but most of the time the air loss is not immediate. The good news about this type of failure is that it has a very low probability of happening. I would guess that fewer the 5% and maybe less than 2% of RV tire failures are of this type.

- Next is the belt separation. This is when the tire tread and belts come off the body of the tire. This is usually the result of tire aging and long term cumulative heat related damage that reduces the flexibility of the rubber to the point that rather than bending the rubber develops microscopic crack which do not heal themselves but will grow. Excess heat and tire aging can come from many sources. Even parking in direct sunlight with "tire protectant" spray does not lower the temperature of the tire. Excess heat can accelerate the aging or the tire rubber properties and drastically reduce the tire life. I would expect that if properly diagnosed this type of failure occurs 25 to 40% of the time. The good news is that with proper and frequent tire inspection this can be discovered and the tire replaced before it comes apart enough to cause damage to the RV. I did a blog post just on this topic "How do I inspect my tires" back in Aug 12 2014. You can Google the phrase and find a number of web pages on the topic but many simply are telling you to look at tread depth but this is not sufficient if you want to do a complete and competent inspection. My link included a YouTube video showing the inspection of a tire with belt detachment that has not come apart and the result of the "tire autopsy" I was able to perform. You can even see the separation between the belts in the above post.

- Finally there is what is commonly, and incorrectly, called a "Blowout". This is really a failure of the tire sidewall due to excessive flexing from running with significantly under-inflated ( probably below 50% of the inflation needed to carry the load. For Polyester tires (mainly ST and LT type) This heat due to flexing can be enough to reduce the strength by half and in extreme cases even melt the cord. For Steel body tires the bending of the steel can result in a fatigue failure similar to bending a steel paper clip till it breaks. This type of failure may be 60 to 80% of the failures on RVs. The good news is that if you run a TPMS you will get a warning of the air leak and hopefully you will not ignore the warning as too many do with other warning indicators on their dash, and take appropriate action which is to stop and pull over as soon as safely possible. Amazingly some people, even when verbally warned that they have a tire that is significantly under-inflated simply choose to continue to drive off. This has happened to me a number of times. As the saying goes "You can lead a horse to water but you can't make them drink".

So how doe this information help you "avoid" failure and RV damage. While nothing is 100% I bet you would like to be able to prevent 90% to maybe 98% of tire failures you might experience.

To do this I suggest that you run a TPMS and to get plenty of advance warning I suggest you set the warning pressure to be the minimum needed to support the load. Then your cold inflation pressure would be about 15% higher.

Next do a "free spin" inspection with lots of light. This is relatively easy with a trailer but harder to do as your axle load increases. You may even need the HD jack from a truck tire center and the spin balance machine to get that big 295/75R22.5 tire spinning at at least 20 to 30 RPM to allow you to see if there is "wobble" similar to the video in my post on How to I Inspect my tires" above. While the tire is in the air also do a slow rotation to inspect 360° of the tire tread as well as 360° of each sidewall looking for cuts and bulges. If any are found you should get the tire to a service center for a more thorough inspection that may include de-mounting the tire.

I believe that if you don't see any bulges or wobble and don't have any localized irregular wear spots in the tread you can be reasonable confident there is no large separation.

Remember nothing is 100% but if you make an effort you can significantly improve your odds of avoiding a tire failure.

Information to show you how this can work. Three weeks ago I was doing my annual 360° inspection. My tires are 7 years old so even though I know they have always been properly inflated (4 corner weights + TPMS from 2nd week of operation), I knew I needed to be sure all was OK. I discovered one of my duals had developed stress cracking from long term parking. This was a real surprise for me but it shows that even with the best of care rubber can get old and tires do need to be replaced.

  This tire was scrapped (I cut two slices from bead to tread to prevent its re-use by a "dumpster diver") as it still had lots of tread but I felt it should not be on the road.

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Monday, July 13, 2015

When did you last check the air in your spare

If you carry a mounted spare, it won't do you much good if it is flat when you need it.

I have seen some studies indicating that most spare tires have lost more than 75% of their inflation pressure.

Amazon has a kit.

Camping World has similar.

Friday, July 3, 2015

When is Minimum inflation the Maximum inflation?

Sometimes the information on inflating tires can lead to some confusion.  OK a lot of the time it can be confusing to many.

As many of you know I try and follow a number of RV forums and offer comments. I try and focus the comments to correcting serious or significant errors or misunderstandings, especially when Safety Related.

Recently there was some confusion about the Maximum allowable inflation for a tire. Some wanted to co-mingle some information from PSR or passenger car tires with information about large TBR, truck bus radials. The discussion then went way off track. Rather than limit my audience to those following that thread, I decided a blog post would be more appropriate. Also I wanted to be sure to have all the information up to date and accurate I contacted an "old friend" from the tire industry and he sent me this nice summary. With his permission I re-print it here.

First some definition of terms may be appropriate as "tire engineer speak" may confuse some.
"Seating pressure" this is the inflation needed for the tire beads to "pop" home against the wheel/
"FMVSS" these are various Federal Motor Vehicle Safety Standards. If you need some reading that will put you to sleep HERE is a link offered by NHTSA.
"PSR"  Passenger steel belted radials or the normal tire you find on todays cars
"TBR" Truck Bus Radial  as found on most Class-A RVs
"LTR"  Light Truck Radials 
"CIP" Cold Inflation Pressure

OK here is the nitty-gritty
"First, the max pressure to seat beads as a matter regarding the technician’s safety is 40 psi, whether passenger, light truck, or truck-bus.  This bead seating pressure is totally independent of the tire maximum operating pressure.

It is important to clarify the differences in sidewall markings of the tires you bring up:

PSR & LTR Tires (load range E or less):  Since FMVSS 139, these tires have sidewall markings indicating maximum load AND maximum pressure.  Maximum load means max [static] load, and maximum pressure means max [operating] pressure (cold).  With respect to the minimum pressure that carries the maximum load there is a difference:

·         Passenger Tires:  These tires are usually marked with a maximum pressure that exceeds the pressure necessary to carry the maximum load marked on the sidewall.  For example, the tire may be marked with 44 psi max pressure, but only requires 35 psi to carry the max load.

·         Light Truck Tires (load range E or less):  These tires are usually marked with a maximum pressure that is also the pressure required to carry the maximum load.  For example, a load range E tire marked with max 80 psi would need that same pressure to carry the max load.

TBR Tires (and light truck load range F and higher):  On these tires the sidewall markings indicate maximum load AT a certain pressure (the word “maximum” is not used in regards to pressure).  Maximum load means max [static] load, but the pressure is not the maximum operating pressure (cold).  This marking just follows FMVSS 119.  Essentially, the pressure marking is informative, simply telling the reader the pressure that is required to carry the maximum rated load.

However, for most practical purposes, on TBR tires the pressure marking is typically considered the maximum pressure recommended in the tire while in ordinary service.  Certain situations may permit cold inflation pressure higher than the marking, usually in consultation with a tire manufacturer for a specific product, application, and service.

Regarding load-inflation tables:  As long as you are looking at the right table, this is where you find the pressures needed to carry certain loads for a given tire type, size, load rating, etc.  Note that for truck-bus, you might need to make sure the tire is a “T&RA tire” or an “ETRTO tire” since the tables can differ, even though the size codes are the same (such as 295/75R22.5).  Also, a tire manufacturer may have unique load-inflation table(s) associated with certain tire models, sizes, etc.

If an operator is running at max load, and the pressure to do that happens to be max pressure, then yes, they need to be diligent.  But it is manageable, and they owe it to themselves and to others on the road to do so.  Pressure loss through permeation requires minimal adjustment approximately once a month.  For predictable swings in temp, set the pressure when it is likely the coldest, and try to consistently check it during those times, such as early in the morning before setting out.  No one is saying that everyone in all circumstances needs to set pressure to +/- 0.1 psi every morning, noon, and night with three hours of ambient cold-soaking before taking measurements.  I agree with you about reducing load; not just due to the tire influence, but also drivetrain/axle and chassis wear and tear, braking performance, fuel economy, etc."

So we see there are similar but different words on the sidewall of tires. Some have a stated Max cold inflation others do not. This is one reason why it best to have tire service done at a store that has the appropriate equipment and training to handle safe and proper mounting and inflation of the type tires you are working with. This does not mean you can't add 5 or even 10 psi to your tires but IMO if you need more than 20% of the CIP there is something wrong and you really need to consider having a professional inspect and re-inflate your tires. Inflating an improperly mounted, improperly repaired or damaged tire can injure or even result in death if not handled properly.
OK now back to our regular programming.
One other comment I have is that many times some think tire failure "Blowout" is caused by too high a pressure but this is essentially incorrect. Unless you have damaged the body ply cords, be they Nylon, Polyester, Steel or Rayon by over flexing and running significantly under-inflated, tires are designed to tolerate the normal pressure increase seen when running highway speeds at the approved load.

But if you have damaged or run the tires in overload or under-inflated for the actual load or perhaps at a speed higher than the tire rating you may have damaged the cord sufficiently that it has lost a portion of its strength so in that case even normal cold inflation may be too high. This is one reason any tire that has been damaged must be rendered un-usable or if it appears to be OK then inflated in a Safety Cage.

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