Tires are critical components of off-the-road (OTR) vehicles and serve several functions that affect the productivity of your equipment and subsequently affect your bottom line. They support the load of the vehicle by retaining the inflation pressure, provide traction and braking, control the direction of travel, absorb road shocks to smooth the ride (on most OTR equipment, the tires are the only suspension for the equipment), and overcome road hazards.
The importance of tire shape
Tire designers use sophisticated tools such as finite element analysis and laser engraving to design and manufacture today’s OTR tires. A very critical design factor of all these tires, whether radial or bias construction, is the tire shape. Designers very carefully design the mold shape to produce the resultant inflated tire shape for maximum performance in the intended application. Tire shape has a large influence on many tire performance factors such as:
- Footprint shape – a critical factor in tire wear, traction, and handling
- Vehicle stability – critical in equipment such as telescopic material handlers and wheel loaders
- Ride comfort
Loaded tire shape – both bulge location and size must be planned for the desired load capacity, pneumatic cushioning and resistance to sidewall cutting and bruising from rocks.
Proper tire shape is critical to ensure maximum tire life and that a tire is able to perform at its best for its intended function. Optimal tire shape is maintained by proper tire inflation pressure.
Correct inflation for the applied load maintains the designed tire shape. The tire can deflect as intended under load and achieve the best possible footprint shape and size for maximum traction, floatation and wear life. Correct inflation will result in the intended sidewall shape under load, which will maximize ride comfort and best resist sidewall cuts, bruises, and punctures from worksite rocks and other hazards. Correct inflation will generate minimal heat and minimize stresses on the tire casing, leading to maximum tire life and increasing the potential for a retread of the casing.
Inflating a tire to even 110 percent of the needed inflation pressure for the applied load results in a distorted shape. The centre of the footprint will carry more of the load while the shoulders will be relatively unloaded. This will cause heat buildup in the tread centre and premature wear on the shoulders. Overinflation makes a tire much more susceptible to cuts and impact breaks. Overinflation also magnifies the stresses on the tire structure resulting in a reduction in original life and the potential for a retread. Overinflation will also reduce traction and flotation and compromise handling.
Underinflation = overloading
The most common occurrence is low inflation pressure, which is simply having less inflation pressure in a tire than required to properly carry the applied load for the application. Underinflation can also be called overloading because if the inflation is less than required by the load, then the load is more than that supported by the inflation pressure.
A tire that is underinflated is being applied at less than its optimal shape. Underinflation leads to overloading the shoulders and relative unloading of the centre. This will cause excessive heat buildup in tire shoulders as the rubber is usually thickest there. The tire centre will scrub, causing excessive centre wear. As well as negatively effecting wear, the poorly shaped and unevenly established footprint of an underinflated tire will lead to poor traction, flotation and handling. The excess stress on the overloaded shoulders may lead to belt edge separation or casing fatigue. Underinflation will shorten tire life, compromise handling and can be a safety hazard.
Zipper tires are dangerous, even lethal
A very important safety concern with underinflated tires, especially steel/steel radials, is the possibility of creating a zipper tire.
Tire cords work with tire beads and the correct inflation pressure to form a very strong yet flexible structure. When tire cords are repeatedly over flexed and reloaded through many cycles of running or sitting for long periods while underinflated, the cords become weakened or may even break. This is a dangerous phenomenon that is not visible from the outside, especially in an OTR tire due to the thick sidewalls. If a tire is run underinflated, allowing repeated over deflection to weaken or break cords, or parked with little or no pressure in it, the result can be a zipper tire.
Our first instinct when we encounter an underinflated tire is to immediately add the needed additional inflation pressure. In this case, that could be a very dangerous, even fatal mistake. Any tire that is found to be severely underinflated (SAE defines a tire that is 20 percent or more underinflated as severely underinflated) must be dismounted and carefully examined by an expert for signs of cord degradation. The tire should only be re-inflated after careful examination determines it is fit for continued service. As always, put the tire and wheel in a safety cage or, if the assembly is too large, use a clip on chuck with a long enough standoff to allow the individual, along with a pressure monitoring gauge and control valve, to stand away from the sidewall outside the tire “Blast Zone” which can extend 100 metres or more from the tire.
What is the correct inflation pressure?
Since we have established that the proper inflation pressure is very important for optimal tire performance and safety, we need to understand what correct inflation pressure is and how best to maintain it. For example, what is the correct inflation pressure for a 20.5R25 * OTR tire? The only correct answer is: “It depends.”
First of all, we must know what the tire is designed to do. That is indicated by the tread code. OTR tread codes are usually one or more of the following: G-Grader Service, E-Earthmover Service, L-Loader/Dozer Service.
Correct inflation pressure depends on the specific application
The chart on the bottom shows that the design inflation pressure is 35 percent higher for loader/dozer service than for grader service, but the corresponding load is a whopping 205 percent more. The load and inflation for earthmover service is in the middle. Obviously, it is very important to select the proper tire and to inflate it to the needed inflation pressure.
From the chart, the 20.5R25 1* L tire is designed for a maximum loader service load of 20,900 pounds at an inflation pressure of 73 psi. The Tire and Rim Association allows up to 15 psi additional inflation pressure for front-mounted loader tires due to the very high loads temporarily encountered on “breakout.” Most tire manufacturers, including BKT, recommend adding 10 to 15 psi to the front tires of a loader. Rear loader tires have a lighter load than front tires and should be inflated for the actual load in operation (with empty bucket) being careful not to go so low as to allow rim slippage, which can damage the tire bead area. Such things as operating cycle speed and distance, as well as material density, should be considered when optimizing tire inflation pressure. Your service dealer or tire manufacturer can help you customize inflation pressure to your application.
What does inflate to 65 psi really mean?
You may at first think that is a silly question, however, the answer is not simple. First of all, stated inflation pressure targets, unless otherwise labelled, are for cold inflation pressure.
Cold inflation pressure is the pressure contained in a tire/wheel assembly when the tire, wheel and inflation medium are at ambient temperature. That is true even if ambient temperature is quite high. Inflation measured under any other condition is commonly called hot inflation and is definitely not to be confused with cold inflation.
So who cares? Sometimes it’s hot, sometimes it’s not. In 24-7 operations tires are seldom, if ever, “cold.” The important point to remember is that gases (air or nitrogen) expand when heated. Therefore, inflation increases just from the hysteretic (the lag of an effect after its cause) heat generated by tires as they operate. Since this operational increase in inflation pressure is variable and depends on many factors, ideally you should set, check and adjust inflation pressure when the tires are cold.
A good plan is to check and adjust inflation pressure, if possible, before restarting equipment after your weekly shutdown.
It may take 24 hours or more of rest for large mining tires to come to equilibrium with their surroundings; therefore, you may have to monitor and adjust “hot” inflation pressure.
Hot inflation will generally be approximately 7 to 15 percent higher than cold inflation but may, in some cases, exceed 20 percent depending on many operational factors. If a tire is inflated while hot to a cold inflation target of 100 psi, then it might only be 80 psi cold inflation pressure and thus “severely underinflated.” Your service dealer and tire manufacturer, along with your own operational historical data, can help you determine the needed hot inflation if you cannot measure cold inflation.
One more note on the effect of temperature on inflation is in order. When inflating tires during cold weather, you must consider the temperature effect on inflation pressure. A tire and wheel stored overnight in a warm tire shop, inflated from an air or nitrogen source also inside the warm shop, then put into service in a colder environment, will be underinflated unless initial inflation pressure adjustments are made.
There are charts available from manufacturers which are helpful for such a situation. For example, a tire with a target inflation of 100 psi that will be used for an application outside at 21 degrees F, should be inflated to 111 psi in a warm 65 degrees F shop. These charts are available from your service dealer or tire manufacturer.
Final wordThe correct inflation pressure in an OTR tire will result in optimum tire performance and longest possible tire life. It will maximize your return on your tire investment and reduce the risk of tire related accidents. An educated and planned approach to inflation- pressure maintenance is a page to put into your operations manual. Whether you train and equip your personnel to manage inflation internally, or contract your service dealer to do so, inflation pressure maintenance is an essential management practice.