I’ve received a couple of questions recently about high pressure in tires. Most posts about tires and inflation warn about the problems of inflation being too low, but I recently read a post with the incorrect claim that “most tires fail because of too high of an inflation.”
This statement of “fail because of too high inflation” is not supported by data or facts, and I felt it deserved an explanation.
“High” pressure is relative
A tire pressure of 36 psi might be high or way low, depending on the type of tire, just as 150 psi can be too low or too high, again depending on the type of tire and application. The bottom line is that each “type” of tire and application has an inflation that we can consider as the target. With the exception of racing tires, the target inflation is given when the tire is “cold”. That means it is at ambient temperature and not warmed up by running or by being in the sun.
At the extremes, we might have a wheelbarrow tire or farm tractor tire which is designed to operate at relatively low inflation. Some farm tires are rated at 12 psi. At the other end we find aircraft tires needing 200 psi or more.
For our purposes we can limit our discussion to passenger (P) tires, light truck (LT), special trailer (ST) and truck/bus (TBR) type tires.
Passenger tires
Passenger tires generally have recommended inflation in the range of 32 to 36 psi, with some Extra Load or “XL” tires rated at 41 psi cold. If you read the sidewall of passenger tires you will see a “maximum inflation pressure” stated on the tire. None of these inflations are the hot inflation, so don’t set or bleed down the inflation when the tire is hot.
Many times the tire is stronger than the wheel. I have seen a few examples of wheels failing at a lower pressure than the tire when we put tires to a test of over-inflation and head toward 100+ psi. I have also addressed the tire sidewall “Max Inflation” number in this post, as there is much confusion on these words.
LT and ST tires have a number of different Load Ranges such as “C”, “D”, “E”, and some even go to “LR-F” (Load Range F).
The actual psi rating for each load range is not the same for all size tires, so you must consult the Load & Inflation chart for your specific size tire.
The cold inflation molded on the tire sidewall is the inflation that will deliver the “Max” load capacity for that specific tire. In general, you will see inflation range from mid 40’s to 80 psi, with the LR-F somewhat higher. As with Passenger tires, usually the tire is stronger than the wheel. However, the inflation is still “cold” and not hot inflation.
Truck/Bus tires have even higher Load Range and, accordingly, have higher inflation with some at the 120 psi range.
Safety warning
The one constant SAFETY WARNING is to not set the cold inflation higher than the rated inflation for your specific tire and Load Range, but also do not bleed down hot tire inflation. Tire Engineers know that tires will heat up. We test our tires at highway speed and above when the tire is fully loaded and design the tires to handle the hot inflation.
Over-inflation of tires make for spectacular explosions. A quick search on YouTube for “Tire Explosion” shows a number of examples.
Sometimes it is the wheel that fails.
Improper inflation can kill.
In general, we see that new tires are capable of handling from 200% to 500% of the cold inflation number molded on the tire sidewall when we do a burst test in the lab. With that large of a range there is no single number I can provide. The other thing to remember is that internal structural damage from potholes and curbs and road trash, as well as simply age, can reduce the maximum strength capability of a tire. So this compounds the problem of providing a maximum safe cold inflation other than that molded on the tire.
Bottom line
- When inflating your tires, do not exceed the maximum inflation identified on the tire or wheel.
- All inflation specs are COLD, i.e., “ambient inflation”.
- Confirm the max load on each tire will never exceed the load capability for your cold psi per the published tables.
- DO NOT bleed down hot tires.
- If unsure, let a professional tire service person mount and inflate your tires on the wheels.
Roger Marble
*****
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“The cold inflation molded on the tire sidewall is the inflation that will deliver the “Max” load capacity for that specific tire.”
Does this mean the “Max Inflation PSI”?
Every time I read an article about tires I get more confused.
Many folks read the tire sidewall and only focus on 2 words “MAX PSI” and almost never on the complete phrase. It Means the Maximum Load capacity of the tire is only delivered when you run the PSI stated on the tire. That is the COLD inflation. It is NOT the inflation that will cause the undamaged tire to explode.
“Confirm the max load on each tire will never exceed the load capability for your cold psi per the published tables.” I’ll bet very few readers have any way to weigh individual tires to check this. I sure don’t.
I understand the problem. If the best you can get is the total on the axle then I suggest you calculate that one tire is supporting 60% of the total on the axle.
RVSEF (RVSafety.com), Escapees, and most RV factory service centers can weigh each wheel position. There are others, you can Google Wheel Position Weighing near me. You may also be able to get wheel position weights if you call the state police to see where they will have a temporary weigh station set up.
I’ve mentioned this before, Bill. I’m not getting involved in any weighing stuff with the cops involved. If I’m overweight (and I’m guessing I just may be), they are the last people I want looking over my shoulder.
Thank you for another informative session on tires and tire safety. There is no substitute for a professional engineer when it comes to getting the correct information. I look forward to your articles every week.
Figuring out which tire pressure to use… listed on the trailer, the tire, or in the tables based on the weight each tire was carrying, I saved your advice in an August 13, 2019 article suggesting inflating 10-20% above the label. What am I not getting, when you say “When inflating your tires, do not exceed the maximum inflation identified on the tire or wheel” but you also say (please clarify):
“Having said all this I would suggest you run at least the certification label +10%. If you can run label +20% that should result in better durability.”?
Thank you, Roger! Happy new year and safe travels! 🙂