Greetings RV enthusiasts; this post is about inflation pressures and tire blow-outs. I’ll start by discussing the topic as it relates to ambient temperatures, draw a few conclusions guided by a key engineering principle, and then present a prioritized list of blow-out causes, obtained from my research. 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?
Answers from Physics
In High School, we all learned about the “Ideal Gas Law”, which tells us that the air pressure inside of a sealed container is proportional to the temperature of the air inside: When temperature drops, air molecules become less active and pressure drops; when temperature rises, air molecules become more active and pressure rises. This means when we stuff the required number of “cold” air molecules into a tire to achieve a certain pressure, and then we heat that tire up, its pressure will rise. Fortunately, a lot of people have studied the pressure-temperature relationship in real tires and we can predict what will happen with “good enough” accuracy. TireRack.com, for example, tells us that for every 10 degree (F) change in temperature, we can expect a 2% increase in tire pressure.
Back to our original question: Have we properly inflated our tires for Phoenix by filling them in Flagstaff? The short answer is no — in our example, the (approximately) 60 degree temperature difference would itself produce a 12% increase in tire pressure. In other words, by the time you reach Phoenix, you running tires would be over-inflated by roughly 13 psi. Put another way : By the time you (unloaded) spare tire acclimates to Phoenix, it would measure about 123 psi — assuming of course that you inflated it to 110 psi back in Flagstaff. By the way — if anyone reading this blog has made such a trip, please let me know your results!
Is 13 psi over-inflation enough to cause a blow-out? By itself, I doubt it, although I don’t recommend experimenting to find out! Classic Engineering “rule of thumb” guidance tells us that differences of 10% or more are significant, which means that our (theoretical) 12% increase in tire pressure is just barely so. 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.
How do I know if my tires are properly inflated?
I look at this from the perspective of the numbers written on the sidewall: The numbers 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:
- “Cold” means “tire is at rest and acclimated to the current ambient temperature”.
- 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 pressure is, so we can monitor our tires effectively and learn how to detect when something isn’t right.
So the 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 characterize this for ourselves — with the use of a Tire Pressure Monitoring System (or frequent stops!). All we need to do is the following steps
- 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 total weight of your rig’s axles divided by the number of tires. More on that later.
- Fill tires to the pressure listed on the sidewall before driving. Make sure to meet the definition of “cold”, above.
- Drive normally, ideally near the tire’s maximum speed rating (when it is safe to do so), and see what your tire pressures actually do. This may take time some experimentation, including multiple trips in hot weather. You might even be able to come up with “temperature increase vs pressure increase” data that is more accurate for your trailer tires than the TireRack rule!
The point of the exercise is to characterize your own tires and learn how they behave within the parameters for which they were designed — so that you can easily recognize when they are not. 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 — as the expression goes: YMWV (your mileage WILL vary!).
Here’s a real-world example: I recently filled my tires to 110psi when the ambient temperature was 70 degrees F. Returning home from the campsite, the ambient temperature was 85 degrees and my tires measured 128.5 psi. Tirerack’s rule of thumb accounts for only about 3 psi increase due to ambient temperature, which means 15 psi is due to heat from rolling down the road. Taking this analysis one step further — that 15 psi increase is about 13%, which suggests a temperature increase of approximately 60 degrees! That means my tires were running at about 145 degrees F.
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 — stop to install your spare before it heats up and you experience a blow-out, which will usually cause damage to a trailer
- 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 started to go higher than you expect, than its time to slow down or even stop to investigate further. I’ve been known to stop and let air out of my tires if I know that ambient temperature has risen considerably.
- 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: air is what supports the trailer’s weight — if the tire doesn’t have the pressure listed on the sidewall, then it won’t support the weight listed on the sidewall.
My “G” rated tires were inflated to 110psi, but after rolling down the road in 85 degree weather, they measured 128 psi.
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!
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 air temperature alone could be responsible for tire blow outs. In other words, it can certainly stress a weak or over-loaded tire, but it’s not the primary cause.
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 low pressure — not high pressure due to rising 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 previously), 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 itself, due to heat. In other words, a tire (usually) fails before it blows-out.
Heat is the 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:
- Overloading the tire. I’m putting this up at #1 because its so easy to do while remaining “blissfully unaware”. When was the last time you went to a scale capable of measuring each side of your trailer, 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, including the stove, water heater, and a large absorption type refrigerator, as well as kitchen storage. This produces a very livable floor plan, but it also puts more weight on the Kitchen side of the trailer. Moreover, the bathroom is often on the same side as well, and so you have 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 (they are not), but because my trailer’s manufacturer is transparent enough to publish side to side weights. 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.
- 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
- 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! I am told that older (than 2013) ST tires had no speed rating, but defaulted to 65mph. If your tires are old enough to have no speed rating, then your tires are too old to begin with.
- Changes in Ambient Temperature: I put this one last because I believe it to be the least likely cause of excess heat. As I discussed above, this factor is important; just not a primary cause.
In addition to heat, there are some environmental factors that can also weaken a tire:
- 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.
- 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.
- 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.