What is deflection and how does it affect OTR tyres?
To put it in simple terms, deflection is the change in a tyres radius that happens when a load is applied to the tyre. To find out what the deflection is in a tyre you must first find out the unloaded radius. To do that, you take the tyre and put it on a rim and then inflate it to recommended pressure. Arrange it in a way that the tread touches the ground but no weight is on the tyre, not even the weight of the OTR tyre itself. Then measure the distance from the middle of the tyre to the ground.
Next you need to find out what the tyre’s static loaded radius is. To do that you measure the distance from the center of the tyre to the ground with the weight of the truck and its payload applied. The difference between these two totals is both the deflection and the amount that the axle lowers when it is carrying a full load.
The reason that deflection is so critical to OTR tyres is because the engineers have designed them to operate within specific deflection ranges. When they have too much or too little tyre pressure or are carrying a load that is outside its limits, it will cause an OTR tyre to fail. This may not seem like a major concern, but OTR tyres are not like a standard tyre on your minivan. They are operating under extremely high stress and when they fail it can qualify as an explosion. In these situations death and property damage have been known to occur. There are also financial ramifications since OTR tyres are so expensive and the rising commodity costs have led to shortages of new tyres in the past. There are cases of mines having to mothball million dollar trucks simply because they could not afford to keep the tyres on them.
Overloading is the most common way to cause over deflection in an OTR tyre. In order to increase productivity or shorten travel time, some companies increase the vehicles payload past its recommended limits. To see what the carrying capacity of an OTR trye is, you can check the load index. It is usually right next to the speed symbol which tells a driver the speeds that each load is safe to drive at.