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Monday, September 24, 2012

Tire pressure Increase

Russell G. said


"Hi This has been a question that I have ask and got a different reply as many time as I have ask the question. If a tire is rated for 80 psi as shown on the side wall is aired up to 80 psi. Then driven on a hot day the pressure goes up to 120 psi is that tire over pressure? If not then why? If so then what are we to do? Right now on my truck my tires are at 65 psi the tire shows 85 psi but when loaded pulling the camper it goes to 80 to 85 psi I asked the guys at the tire store and got no where with them. I like your blogs on tires and have got a lot of good info. off of them keep up the good work."

Thanks for reading Russ. This is a good question. I and others many times warn about not exceeding the max pressure of a tire or wheel but we are always talking about the pressure when the tire is "cold" and by that we don't mean you can cool your beer by setting it on the tire. What is meant by "cold pressure" is when a tire is at the same temperature as the surrounding air and not in direct sunlight. You know, the temperature in the shade.

Without going into the details of the "Gas Law" and looking at what is meaningful not just measurable, we find that  using 2% pressure increase for each 10°F temperature increase works out pretty well for real life. This brings us to the obvious question of what is a "normal" temperature increase. Tire temperature will increase as you drive faster, or as you increase the sidewall flex by lowering the pressure or increasing the load. Being in the sun or near the exhaust pipe can also increase the temperature.

The normal temperature increase over ambient is in the 30°F to 60°F range so I would expect to see a 6psi to 12psi increase in a tire starting at 65 to 70 psi. Now if you checked in the early morning while the day was still cool and then were driving in the heat of the day when it was 20°F hotter then we might need to add another 4% so we might see a 16% increase.

If your normal cold inflation is 65 psi you could see a 10 to 12 psi increase on a hot day. In your question you were asking about a 40psi increase which is 150% of the cold inflation. If you saw that level of increase I would suspect there is a problem and that you have a bad gauge or the tire is very overloaded or you have excess moisture inside your tire because the shop did not drain and filter the air from their compressor.

Your increase from 65 to 85 sounds high. I only see about half that in my Class-C. I run about 10 psi over the inflation required for the actual load and I drive at 60 to 63 mph and I see about 8 psi increase.
Have you confirmed your gauge and TPMS readings are accurate?
Are you sure 65 is not leaving you overloaded?  Remember you need to inflate to at least the minimum inflation shown for the heaviest tire on an axle and then I suggest at least a 10% increase for a reserve inflation factor.

BOTTOM LINE
Tire & wheel manufacturers account for a normal pressure increase as outlined above when they design tires & wheels, so you should not be concerned unless you are seeing a large increase in pressure. If you do see a pressure increase like 25% you need to find out why. Remember you should not be bleeding down your tires when they are hot.


5 comments:

  1. So many people use the "max" number on the tire as their regular inflation pressure, it's scary to think how many "rubber bombs" are out there.

    Folks, the number on the sidewall is the maximum pressure that the tire can sustain when inflated at standard atmospheric conditions (68F in the US, 60F in Europe, I think).

    The "proper" cold inflation pressure for your tires depends on the weight they're carrying. When you're on a trip, with all your stuff and critters loaded, and maybe with all the tanks 2/3 full, go weigh your vehicle. At a minimum, get front axle weight alone and total weight.

    You can the figure that the two front tires are each holding up half the front axle load, and the rears half the rear axle load. Tires on a dually obviously take 25% of the rear axle load. If you have a tag axle, assume each tire is taking 1/6th of the total rear axle weight. Subtract the front axle load from the total weight to get the rear axle load.

    Then, consult the tire manufacturer's pressure/load charts or tables and inflate the tires to the pressure recommended for the load they're carrying.

    If your load varies significantly during a trip, you might have to increase or relieve pressure, but it's probably in the righ ball-park if you check the weight soon after starting a trip.

    My engineering perspective doesn't agree with "adding 10% for a cushion" as this could over-inflate.

    ReplyDelete
    Replies
    1. Frank,
      Sorry but there are a couple of points in your comment that need to be cleared up so we can avoid confusing my readers.

      First off I don't know where you got the notion that there is some epidemic of "rubber bombs" out there. I am just not aware of any number of tires simply exploding out on the highway for no reason other than being inflated to a pressure higher than that marked on the sidewall. Tire engineers design tires to handle multiple times the inflation shown on the sidewall of the tire and tires must pass overinflation tests to confirm this before a design can be released to production. In fact the weakest link is usually the wheel or valve not the tire.

      Next the "proper" cold inflation is not some artificial standard test condition. Tire inflation is not to be adjusted to compensate for the difference between your standard temperature and the actual temperature. If you read any of the industry standard books such as Tire & Rim Association in the US or JATMA in Japan or ETRTO in Europe you will see that cold inflation is set based on whatever the atmospheric temperature and pressure is at the time and location the tire is inflated. This concept of adjusting the inflation to some theoretical conditions has been covered and debunked previously.
      Also the inflation on the sidewall is actually the inflation that allows the “Maximum Load” rather than a statement of the “Maximum Inflation” I think that if you read the safety warning and other wording on the sidewall of most tires this will clear up your misunderstanding. I know that many passenger tires do have a statement of Maximum seating inflation that is higher than the inflation associated with the maximum load.

      Your assumption that the side to side load balance is 50/50 is also not based on reality of RVs as manufactured or as driven. 45/55 is a more realistic level of unbalance for both front and rear axles when you are estimating the individual tire loads. Using this 5% variation will cover most RVs and help avoid tire overload for most applications.

      The recommendation to set the pressure at the minimum needed to carry the load ignores two facts. First that there will be day to day variation in the temperature and pressure outside the tire and if the owner is diligent and checks the air each travel day as they should they will forever be adding or subtracting a few pounds almost every day. This will certainly not be easy to do for few owners have an air supply plus this task does not add to the enjoyment of the RV lifestyle.

      Finally inflating only to the level needed to carry the static load will result is many trailer tires seeing structural shear forces significantly higher than seen on motorized RVs that do not have multiple axles close together. See my post of Sept 28.

      In closing the +10% is a reasonable way to avoid the daily hasle of adjusting inflation. I think you will see that most of the time I and others also include the advice of not setting the inflation higher than the inflation marked on the tire or rated inflation of the wheel. This also results in a tire size and load range and minimum inflation being selected that gives a cushion over the max load capacity for the tire.

      If you have questions please send me an email (address under my picture) so we don't bore the readers with too much "engineer talk".

      Delete
  2. I run my tires at the recommended pressures. That said, if my trailer tires are 2-3 lbs low in the morning I don't air them up (10-20 or more degrees cooler than when I'll be finished towing for the day). Once in Jan we were leaving camp early (overnight low in the 30's) and the tires looked a little low, but I just pressed on. For the first few miles my truck and fiver "wallowed" a little, but then all was well as the air temp rapidly rose. A guy I met once said when hauling heavy stock trailers in West Texas in the summer they only put 4-6 lbs in their trailer tires as they heat up so much and to avoid bursting. Was he kidding me? Seemed serious and truthful to me.

    ReplyDelete
    Replies
    1. No. Please DO NOT set your pressures low and expect the operating temperature to increase the inflation to the correct inflation. To do so means you are doing damage to the internal structure of your tires. Set your trailer tires to the placard inflation which for ST type tires is usually the inflation on the sidewall of the tires. My post of Sept 28 covers trailer tire inflation in depth.

      I think that guy in TX will also tell you it gets hot enough to fry an egg on the highway many days. Simply not true.

      Delete
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