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Wednesday, March 30, 2011

Tire Loads & Inflations – A Heavy Topic – Part 1

OK so maybe the title is a bit too cute but it was suggested I try and balance my focus on highly technical stuff with a little bit of lighter stuff. Well air is light, isn’t it? Today’s key points: Know the minimum tire inflation based on manufacturer estimates. Check your inflation with a good gauge at least monthly and every morning before travel.

Tire inflation seems to be a topic that confuses some and has others believing in misleading or just plain incorrect information. Tire inflation is one item that directly affects the safety of your RV, truck or car as you travel down the highway. Many of my previous posts have been background information that you really don’t have to know or fully understand. The intent of these topics is to give you a better foundation of understanding more about tires but, if you only pay attention to one series of posts this is it.

Tires do not carry the load. They are just a container of air. It is the inflation air that does the work. Think for a moment of an impact wrench. It can’t do the work of loosening or tightening nuts on your wheels without the air, and a tire can’t do the work of carrying the load or provide the traction needed to turn, start or stop if it doesn’t have air. The load a tire can carry is based on the air volume of the tire and pressure of that air inside the tire.

If you want to get a feel for how little load your tire can carry without air in it, you can test this yourself. Simply take an unmounted tire and stand on the beads of the tire. I think you will find that the tire can support less than 5% of the rated load before it deflects more than an inch or two. Some people believe that tires with higher load range can carry more load at the same inflation. This is just not correct. You would be hard pressed to measure the uninflated load capability difference between a load range D and E tire or between a G or H load range tire.

Sidewall stamping information HERE.

If you are not sure, the "Load Range" is marked on the sidewall of your tires. If you don't see the words "load range" look for "LR" followed by a letter usually between C and G. "Load Range" may not be in large letters but you should be able to find them on your tires.
 

The simple answer to how much air you need is on your data sheet. Depending on the year your RV was made, this information is on a sticker on the side of your trailer or near the driver’s seat in your Class-A or glued to the wall inside a cabinet or on the inside of your entry door. Where ever it is, you should also have the information in the stack of owners manuals you received when you bought the new RV. Find this data sheet and write down the information for loads, minimum inflation, tire size and load range and place this info where you can easily find it. This inflation is the minimum based on what the RV manufacturer thought you would load into your vehicle. They may have guessed correctly or you may have gone way over that estimate depending on how much “stuff” you carry.

Our next blog will cover how to know the real load on your tires and what you need to do to be sure you are nor overloading your tires, wheels, axle or RV.

6 comments:

  1. Humour is good!

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  2. The tires on my Chevy Silverado say "Standard Load" rather than Load Range... What does Standard Load actually mean?

    Thanks,
    george

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  3. Roger,

    Will you please clarify? Here's a para from your post:

    "Tires do not carry the load. They are just a container of air. It is the inflation air that does the work. Think for a moment of an impact wrench. It can’t do the work of loosening or tightening nuts on your wheels without the air, and a tire can’t do the work of carrying the load or provide the traction needed to turn, start or stop if it doesn’t have air. The load a tire can carry is based on the air volume of the tire and pressure of that air inside the tire."

    My view: Technically, air cannot support the load. Here's why. The air pressure in the volume inside the tire is uniform; it is the same everywhere in the volume. The only mechanism for the air to support the load is by its application over a horizontal projected area. Look at a bare wheel. The horizontal projected area is the same whether you are looking down on the wheel or up from below. So the vertical component of force due to air pressure on the wheel is balanced.

    The only other mechanism for supporting the load is the net vertical force that the tire exerts on the wheel around the circumference, the bead. So, the force is supported through the sidewalls of the tire.

    The air serves to pre-stress the tire (make the tire's sidewalls more rigid). When the load is imposed on the hub of a wheel with an inflated tire, the sidewalls flex and take on load internal to the tire's sidewalls. The lower the air pressure, the more flex. When the tire is rolling, the tire's entire sidewalls go through a cycle of flexing during each revolution. The more flexing, the more "heat" in the sidewall. Hot sidewalls become weak, etc.

    Your view?

    Gene from Idaho

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  4. Tell us about nitrogen. I got quite a pitch from my tire dealer yesterday. If we run nitrogen and check pressure every morning of travel, very soon we will have to refill those tires since a bit is lot with every measurement. Can I carry nitrogen like I would air? This could get expensive. I pull a fifth wheel which means 8 tires to check and refill just from the measurement loss.

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  5. This comment has been removed by a blog administrator.

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  6. Final point. If you have a trailer, especially with multiple axles you will probably want to confirm and run no less than the placard pressure which in most cases is the same as the max for the tire.

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