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Monday, October 5, 2015

What is the optimum tire pressure?

Load inflation tables identify the minimum inflation for a number of specific load placed on the tire.

Tire sidewalls tell you the maximum load capacity for a given tire when inflated to the maximum pressure for that load range in that size tire.

Optimum implies we have common agreement on which performance requirements we feel are most important. Optimum also implies that there is agreement in the inherent trade-offs of the numerous performance characteristics.

If the only performance we are concerned about is load capacity and if we want the maximum capacity possible for the size and load range of a specific tire, then we have a situation where the optimum inflation is the maximum for the load range, which is also the minimum inflation specified in the tables.

If, however, we do not need to support a load that corresponds to the tire's maximum load, then it is possible that other performance criteria may be considered and as a result there will be trade-offs to arrive at a new optimum.

If we are talking about motorhomes, there are normally performance characteristics other than just load capacity. Some might be fuel economy, noise, ride comfort, tread wear, steering response, etc. Inflation pressure will affect each of these characteristics — some positively and some negatively. So clearly the "optimum" depends on the clearly understood and agreed on priority of these and other characteristics.

Bottom line
As a tire engineer I suggest that people select an inflation pressure that will provide at least 15 percent extra load capacity over the heaviest loaded tire on an axle. All tires on an axle should run the same cold inflation. The above will still provide acceptable ride and provide improved durability and fuel economy.

For multi-axle trailers I would consider improved durability, i.e., reduced chance of failure, to be of primary importance. So in this application, the "optimum" inflation pressure would be the pressure on the tire sidewall associated with the maximum load capacity. Even if you are not loaded to the max load you want to lower the "interply shear" forces as much as possible, as trailers induce much higher shear forces than seen in similarly loaded tires would if on a motorhome.

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  1. A lot of this 'tire-speak' makes my head spin. So, I read the side wall, find the suggested air pressure for the tire, and pump it in until my tire gauge and the side wall agree.

  2. Tommy, That's not an unreasonable approach.

  3. When discussing optimum tire pressure the discussion always seems to assume a near static, steady state condition (ie: constant speed on a smooth road). That is often not the case. When going around a curve the tires on the outside of the curve will have a higher load than the tires on the inside of the curve. A tire that hits a bump (especially a chuck hole) will momentarily have a higher load than normal. When braking heavily the load on the front tires will increase significantly.

    Will your suggestion of 15% above minimum take care of these conditions?

    Signed Anonymous due to lack of choices.


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