Tires

Does over inflation cause blow-outs?

Greetings RV enthusiasts.  I’m posting a short article on the subject of cold inflation pressures and their potential contribution to blow-outs.  I’ll start by discussing the cold inflation topic in light of ambient temperatures, draw a few conclusions guided by a key engineering principle, and then the present a list of causes for of trailer tire blow-outs.  I’ll anchor the discussion around my own tires:  4410-lb, 75mph ST tires, with a “cold” inflation pressure of 110psi.

The “Cold Inflation Pressure” issue

Suppose you camp at Flagstaff, AZ in the month of May (where the low temperatures average approximately 34 degrees F), and you  dutifully fill your “G” rated trailer tires to 110psi in the morning (when it is “cold”).  Then suppose you travel approximately 150 miles down the hill to Phoenix where the high temperatures are in the 90s (the average high temperature in Phoenix, in May, is 94 degrees, F).  Have you properly inflated your tires for Phoenix?

The short answer is:  No, you have over-inflated them for Phoenix because the ambient temperature is significantly higher there.    The  (approximately) 60 degree temperature difference would itself produce to a 12% increase in tire pressure  (according to Tirerack.com, every 10 degree change in temperature results in a 2% increase in pressure).  And that doesn’t even account for increases in pressure due to rolling down the road!   In other words, by the time you reach Phoenix,  your spare tire (assuming it was also filled to 110psi in Flagstaff) would have more like 123 psi in it.  That means your running tires are over-inflated too.

But is that enough to cause a blow-out?  Probably not.  Classic Engineering “rule of thumb” guidance tells us that differences of 10% or more are significant, and that differences of less than 10% are probably not — that  means that our (theoretical) 12% increase in tire pressure is significant – but just barely. Considering that we don’t hear of many blowouts from trailers making the trek from Flagstaff to Phoenix,  my conclusion is that ambient temperature changes, in large part, are not a primary cause of blow-outs.  After all — if a tire cannot withstand a 12% change in pressure then you need a different tire!

I look at this from the perspective of the numbers written on the sidewall:  The numbers on the sidewall of my tires tell me that 110psi is required to support the rated load of 4,410 lbs.  That is clearly the minimum air pressure that the tire must see in order to work properly!  Go below 110psi and it won’t support the weight — its that simple.   The Sidewall also tells me that if I fill my tires to 110psi “cold” that the tire will support that weight while rolling down the road at 75 mph.  From this I conclude two important things:

  1. The tire expects to see more than its rated “cold” inflation pressure while rolling down the road.  We just have to figure out what that number is, so we can monitor our tires effectively and learn how to detect when something isn’t right.
  2. “cold” means “tire is at rest and acclimated to the current ambient temperature”.  This is why inflating in Flagstaff is not a good starting point for traveling through Phoenix.

So the  tire sidewall just told us the conditions under which the tire is designed to operate, including its (approximate) running psi in hot weather.  The lawyers have apparently stopped manufacturers  from saying “maximum pressure is X”, but we can figure this out for ourselves — with the use of a Tire Pressure Monitoring System (or frequent stops!).   All we need to do is the following steps

  1. Make absolutely certain that the tire is loaded properly, including side-to-side weights — you want to know how each tire is loaded — not just weight of the rig divided by the number of tires.   on that later.
  2. Fill tires to exactly the number on the sidewall before driving (so “at rest” or “cold).  Make sure to meet the definition of “cold”, which is “tire is at rest and acclimated to the current ambient temperature”.
  3. Drive normally, ideally just under the tire’s maximum speed rating, and see what your tires actually do.  This may take some experimentation — you might even be able to come up with a more accurate “temperature increase vs pressure increase” rule of thumb for your trailer tires.

The point of the exercise is to characterize your own tires and learn how they behave within the parameters for which they were designed!  Most of us, for example, who pay attention to our tires see pressure increases of less than 20% during normal driving, even in hot weather.  That means up to 130 psi for a 110psi tire, however — and as the expression goes:  YMWV  (your mileage WILL vary!).

A Tire Pressure Monitoring System is a godsend for monitoring tire performance on the road.  When things start to look funny you can react — typically by slowing down or even stopping.    Here’s some suggestions:

  • If one your tires is loosing air — you will know it and you can stop to install your spare before it heats up and you experience a blow-out
  • If the ambient temperature has risen considerably, you can still monitor your tires real air pressure to see if they actually exceed the pressures you see under normal circumstances.  If the pressure in all of your trailer tires has approached the highest value you have experience thus far, then its time to react — slow down until you can stop and let a few pounds of air out.
  • When you have the opposite problem — you travel from hot weather into cold — you must be very careful not to let your tires fall below that  minimum “cold” inflation pressure on the sidewall.  Remember:  if the tire doesn’t see the pressure listed on the sidewall, then  it won’t support the weight listed on the sidewall. 

 

 

 

 

 

 

 

I always carry a high-volume portable air compressor that is capable of adjusting the pressures in my trailer tires or my truck’s tires. While I haven’t yet been through a dramatic change in ambient air temperature that requires such action, I’m ready!

In Summary — maintaining tires includes monitoring them while rolling down the road to make sure they are operating within their design parameters of load, speed, and pressure.  While it is important to “babysit” tires in this way, and even to make an air pressure adjustment in response to extreme circumstances changes in outside air temperature,  I am not convinced that such air temperature changes are really responsible for tire blow outs.  In other words, environmental conditions (heat) will certainly stress a weak or over-loaded tire, but are not the primary cause of blow-outs.

 

So its  not that…

With the above, it’s clear to me that over-inflation due to environmental or local real-time weather conditions is not a likely cause of tire failure – including the familiar stories of  so-called “china bombs” that pop up on the forums from time to time.  In fact,  according to one tire expert I talked to (from a respected West Coast chain), tire blow-outs while driving down the road are more likely due to an loss of pressure —  not a rise in pressure due to a natural rise in outside temperature.

At first, it seems counter-intuitive to suggest that loosing air would cause a blow-out.    After all, most of us (especially having seen the damage done by such an event), naturally perceive of a blow-out as an explosion.  A tire is just a container of air — and aren’t explosions caused by too much pressure in the container?  I’ve been able to interview a number of tire experts (all from the West Coast chain mentioned above), and have received some very practical and consistent advice on this topic.  It turns out that the explosion we call a “tire blow-out” is caused, not (primarily)  by rising pressure inside the “container”,  but (more likely) by weakening the container itelf, due to heat!   In other words, the tire (usually) fails before it blows-out.

Shooting at the right enemy

Having identified heat as the primary root cause of tire failure, it is good to consider what causes a tire to heat up . Here are the most important factors,  in order of their importance:

  1.  Overloading the tire.  I’m putting this up at #1 because its so easy to do, while remaining “blissfully unaware”.  Its not enough, for example, to know the total amount of weight that is carried by all of your trailer tires because thats just an average that does not account for side-to-side weight differences or the way the trailer is loaded. When was the last time you went to a scale capable of measuring each side separately? Consider the Classic “rear living” fifth wheel, where the kitchen slide is opposite the living room slide.  All of the kitchen appliances are on one side, such as the stove, water heater, an a large absorption type refrigerator, are all on the same side of the coach, as is kitchen storage (and who wants kitchen storage on the other side?).  This produces a very livable floor plan, but it also puts more weight on the Kitchen side of the trailer.  Add to that the fact that the bathroom is often on the same side as well, and you have a recipe for a “heavy left side” trailer.  Add to this the possibility of  the supplied tires running to close to their load ratings,  and you have a recipe for too much heat!  I feel fortunate, not because my trailer weights are even side-to-side, but because my manufacturer has published (dry) side to side weights, and I know that I am 700 lbs heavier on the the kitchen side of my trailer even before the first bottle of ketchup goes in the refrigerator.
  2. Loss of air pressure.  An air leak, either from a puncture, a bad valve stem, or even trauma to the bead (like spinning your trailer around in a very tight circle on dry pavement), can result in air loss “going down the road”.  Left  un-detected, this causes tire temperature to rise dramatically which weakens the tire
  3. Driving too fast.  Recreational trailers with “ST” tires will have a speed rating that is probably either “L” (75 mph) or “J” (62mph). That speed rating is very important because the tire will heat up quicker if you drive it faster than it was rated to go!  “Special Trailer (ST)” tires use to arrive on U.S. shores without a speed rating, and in this case the default was 65 mph. Based on the fact that most trailers pass me on the interstate while I’m doing 65mph, I’d say that there’s a lot of trailers out there exceeding their tires’ speed ratings!
  4. Changes in Ambient Temperature: I put this one last because I believe it to be the least likely cause of tire failure/blow-outs.  As I discussed above, this factor is important;  just not a primary cause of blow-outs.

In addition to heat, there are some environmental factors that can also weaken a tire:

  1. Cumulative damage:  Over time, if tires are subjected to pot-holes, road hazards, etc., but do not “fail on the spot”, they can become weaker over time and just give up suddenly on the road.  In addition to monitoring pressures, frequent inspections for such things as un-even tread wear, cracks, blisters or bulges will help detect these factors before they product a catastrophic failure on the road.
  2. Time and weather:    I’m told that the expected life of a tire is about five years, but if “sun rot” has degraded the rubber (trailer is stored outdoors for example) the tire’s life can be substantially shorter.  Whenever possible, I cover my trailer tires and when they are not covered they are protected with “303 Aerospace Protectant” — its like sun block for tires.
  3. Manufacturing defects. As much as some of us dote over our tires and take good care of them — sometimes the manufacturing process hiccups and we premature tread separation or steel belt separation, for example.  Inspection is the key here:  Left un-attended, a manufacturing defect will weaken the carcass and produce a blow-out on the road – potentially destroying any evidence that the real cause was a manufacturing defect.

-Doug

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