Tires: number of plys, effect inflation, light truck tires

Question
I know the number of plys correspond to the load-carrying capacity of a tire. I've also always believed the number of plys helped protect against cuts and punctures (primarily from  rocks, etc.), but recently I was told by a tire store employee that the number of plys has nothing to do with resistance to flats/damage. I have a half-ton truck and usually not much of a load but we do work on remote ranches where we must travel on gravel/rock roads that can get pretty rough at different times of the year.

Thanks for your advice.

Answer
Herb,

I'll explain it this way.

The single most important feature when it comes to flats and damage is the ability of a tire to deform when it encounters an object - sharp or dull. And the more it deforms the less likely there is to be a penetration or damage.

The single most important property for tire deformation is inflation pressure - but this is just the opposite for load carrying capacity.  Put another way, it would be better to have a large load capacity tire and use less inflation.

The number of plies has very little to do with it - except to say that a thicker tire is less prone to penetration and damage - and more plies achieves more thickness - but that effect is small compared to the effect inflation pressure has.

Your first sentence contains a common misconception.  The number of plies does not equate to load carrying capacity - except to say that it is common for tire manufacturers to standardize the ply material they use - and the net result is that sometimes (and I emphasize "sometimes") larger capacity tires use more plies - but the clinker here is that there are many ways to achieve the same result without using more plies.

For example, Light truck tires generally have 2 polyester plies and 2 steel belts.  An over the road truck tire - which carries a much larger load - generally has a single ply of steel wire, with 3 or 4 steel belts - and one of those belts is there to protect the other belts, so it doesn't really enter into the load capacity picture.

I've answered this question assuming that you where using the term "ply" in the strictly technical sense - the actual number of plies in a tire.  However, many folks use the term "ply" in a very old and antiquated way when they really mean "Ply Rating" - a term that the tire industry specifically is trying to eliminate due to its confusing with the actual number of plies in a tire.

If you are using the term in this second way, then - yes - ply rating does have a relationship with the load carrying capacity of a tire - except to say so does the tire size - AND - "ply rating" does not affect resistance to penetration or damage - except to say like before, more plies implies a thicker tire.