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Monday, June 24, 2013

Had a question about High Pressure

Most posts about tires and inflation warn about the problems of inflation being too low but I had a question about high pressure and felt it did deserve a few lines.

"High" pressure is relative.
35 psi might be high or way low, depending on the type tire just as 150psi can also be too low or too high again depending on the type tire and application. The bottom line is that each "type" tire and application has an inflation that we would consider the target. Most of the target inflation are given when the tire is "cold" which means 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. Some Drag Race slicks are also designed for pressure below 24 psi.
At the other end we find aircraft tires needing 200 psi or more.

For our purposes we can limit our discussion to Passenger tires, Light Truck, Special Trailer and Truck/Bus type tires.
Passenger tires generally have recommended inflations in the range of 32 to 36 psi with some Extra Load tires rated at 41psi cold. If you read the sidewall of many 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 and 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+.

LT and ST tires have a number of different Load Ranges such as "C", "D", "E" and some even go to "LR-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 is also molded on the tire sidewall. In general you will see inflations 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 but the inflations are still "cold" not hot inflations.

Truck/Bus have higher Load Range and accordingly higher inflations with some at 120 psi range.

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 tires. Tire Engineers know that tires will heat up and 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 tires make for spectacular explosions. A quick search on YouTube 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 700% of the cold inflation 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 pot-holes and curbs and road trash as well as simple 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.
Do not exceed the maximum inflation identified on the tire or wheel.
All inflation specs are COLD i.e. Ambient inflations
Confirm the max load on each tire will never exceed the load capability for your "set" cold inflation 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.

4 comments:

  1. Roger, suppose you have upgraded the RV tires and wheels to a higher load range, say from LR E to LR G. You look at the manufacturer's inflation chart and it might say at that weight to inflate to 80psi although the tire and wheel will go up to 110psi. How big a problem is it to inflate to max cold sidewall pressure when the load doesn't require that much?

    Dan M

    ReplyDelete
  2. I see nothing wrong with running LR-G tire at LR-E inflation, based on the load charts.
    BUT
    Lets say you really need 80 psi in the size tire you have, to carry the actual load. You can run +5 to +10 psi and gain a bit of safety factor and avoid having to add air as soon as you drop a psi or two.
    If however you need 80 psi to carry the load, are you sure you are not exceeding the load limit (GAWR) for the axles? GAWR is on your certification sticker and should not be exceeded.

    FYI My small Class-C came with LR-D and based on actual loads I only need 55 to 60 psi. The tires I wanted to run did not come in LR-D but were available in LR-E and are rated for 80 psi.
    I run 65 to 67 psi cold so even with day to day pressure change due to ambient temperature change I don't have to worry about "topping-off" the tires if my cold infl drops a couple of pounds.

    ReplyDelete
  3. Your example of running 65-67 with higher rated tires where you only need 55-60 to handle the weight answers the question.

    It's not about being overweight. Just thinking about how to put more distance between me and a blowout, without causing center wear or some other problem.

    Dan M

    ReplyDelete
    Replies
    1. An extra 5 or 10 psi when looking at a base inflation of 60 or so is not going to be a big issue especially considering that you will probably "age out" before you wear out.

      Delete

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