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Saturday, April 16, 2011

Tire Load & Inflation part 2

In part 1 of Tire Loads & Inflation Gene from Idaho questioned how air carried the load not the tire structure.

Think for a moment about a standard balloon. If we increase the inflation pressure, the balloon does not become a square. In fact if we placed the balloon on a table and hold it down with our hand and then increased the pressure we would seen the side bulge out and be more curved. A tire section as seen in this picture (by Pirelli) is much like the cross section of our balloon held between the table (this case the rim) and our hand (the belts). Increasing the pressure will not make the sidewall go straight but in fact when measured will make the width increase due to more curvature. If we check Wikipedia for "Contact Patch" we find "The larger the inflation pressure, the smaller the contact patch".

I will leave you with some additional pictures. You can see how under-inflation will cause the shoulders to wear while over-inflation will cause the center to wear faster.

Here we see the results of a simple test where a chalk line is drawn on the tread. With an over-inflated and then under-inflated condition the tire is driven a few feet. It is easily seen that the center of the tread is supporting the load in the higher pressure (overloaded) condition.

I hope this answer Gene's questions.


  1. The chalk line test is the best visual aid I have ever seen. Great article!!!

  2. Does this test work with truck-grade radial tires as well? Such tires are so stiff, it's not obvious that the belts will flex much across the width of the tire, but rather extend/retract the length of the footprint with changes in pressure.

  3. The example was run on LT tires. This is only an approximation and is not a substitute for actual weights. While it is strongly suggested that all RV owners get actual corner weights the minimum that should be don is to get the numbers from a Truck Scale and learn what your real axle loads are.
    If your numbers end up being clost to the max for GAWR or the required inflation is near the minimum on your placard then this increases the importance of getting side to side loads as there is some level of unbalance on almost all units. The question is how much. 2% or 10% or more.

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