Why are air pressure requirements different for Free Rolling, Low Torque, and High Torque Applications? Why are VF Single Tires allowed minimum inflation of 12 psi, whereas Duals/Triples are allowed minimum inflation of 6 psi? What is the reason for weight carrying capacity reduction of 12% for Duals, and 18% for Triples?
As discussed on a previous article, tires face different forces that act upon the structure of the tire to carry a load from point a to point b. Every AG tire faces three forces to help move your tractor forward or in reverse:
- X Force: Drives the tractor forward or backward.
- Y Force: Stabilizes the tire from side-to-side movement.
- Z Force: Supports the load or weight being carried
As we shared in previous articles, tires must be able to transmit the engine torque or power which overcomes the rolling resistance also known as tire slip, providing the forward or rear motion to move the machinery. In the case of agricultural radial tires, 80% of the weight is carried by the compressed air in the tire chamber which is adjusted based on three given working conditions:
- The axle load or weight being carried
- The desired speed to achieve the optimum productivity
- The desired tire contact patch or footprint at a given air pressure to reduce soil compaction.
Each of the above conditions will dictate the capability or limitations of the tire to achieve its purpose of moving a given load from point to point.
The AG industry has termed three different air pressure possibilities that a tire may face when operating in dynamic work conditions:
- Free Rolling Applications: When an ag tire is on towed wheel position where the tires are only subjected to axle load applied and the pulling force of a powered tractor or unit – the only limitation is the weight on the tires for a given maximum speed.
- Low Torque Applications: When an ag tire is on a powered wheel position with both load and lower torque applied due to reduced power applications to pull an implement across a field for a given work application: plowing, disking, planting etc.
- High Torque Applications: When an ag tire is on a powered wheel position with very high torque or power applied to pull an implement or load at very high speeds mostly due to transportation requirements or movement from one place to another.
The working requirements for the tire will be based on the vehicle purpose or tire configuration, closely followed by the work applications to determine which air pressure conditions to consider. In all instances, the criteria to determine what air pressure is required will be determined by:
- What Machine is the tire going on? For example: MFWD Tractor, 4WD Tractor, Harvester, Floater, Sprayer, Grain Tank, or Implement etc.
- What tire size is being considered for which wheel position? Front Axle Tire, Rear Axle Tire, Trailer Axle Position?
- Is the tire going to mounted as a single, dual, or triple tire fitment?
- What is the maximum axle load expected? Per tire?
- Is this a cyclic load condition that includes work conditions fully laden and then fully unladen?
- What will be the working or highest speed that tire will experience to complete the work requirements?
In an ideal world, all agricultural platforms would be equipped with vehicle-mounted central inflation systems. These systems would allow tire air pressure adjustments at the touch of a button, switching between lower air pressure for low torque field applications and higher pressure for faster high-torque operations when moving from field to field.
As a rule, when operating at lower torque (lower speed) in the field, an Ag tire can use lower air pressures to carry significant loads allowing the tire sidewall to deflect as needed and increase the tire tread contact area reducing soil compaction while increasing the tread footprint delivering greater traction with less tire slip. Conversely, when faced with the need to move a machine or platform from one field to another, air pressure must be increased as the higher transport speed will require more air pressure to allow the load carrying capacity to adjust to the higher heat generated by the increased velocity. If the platform in question has a central inflation on board system, this air pressure change can be applied immediately, allowing the optimal air pressure to be added to the tires to achieve the desired load carrying capability for a given speed. Although this technological solution is becoming more accessible and popular across the Ag industry for the majority of grower and farmers, the current accepted working solution to this inevitable problem is to select the highest air pressure necessary based on the heaviest implement or trailer weight required per axle load. The next step is to adjust to the highest air pressure required for the maximum operating speed and load.
The above rules are still pertinent even for all operating conditions to include tires in dual or triples fitments on the same axle. When operating Ag tires in duals (four (4) tires on an axle), the weight carrying capacity is reduced from 100% down to 88% or -12% lower carrying capacity. This industry standard is applied to maximize the load capacity of the four tires across an axle operating through an open field where the ground conditions are neither level nor perfectly flat. The -12% weight reduction increases the air pressure on all four tires to reduce or minimize the impact of load transfer from one side of the axle to the other as the working platform travels its way across a given field or farming operation. This air pressure adjustment helps to improve the performance and endurance of all four tires on the axle through greater lateral stability, traction, and weight carrying capacity despite the changing or uneven ground surface conditions inherent to any farming operation or field.
When moving from duals to triple tires on an axle using six (6) tires across one working axle from side to side, the weight carrying capacity is reduced by -18% or 82% from 100% of the original load. Therefore, the air pressure must be increased by almost +35% to carry the required load using all six (6) tires. For example, let’s assume the tire load is 7000 lbs. per tire in a single tire application operating at 10 mph will require 20 PSI, that same tire now in a triple fitment loses 18% of its load capability at 10 mph unless the air pressure is increased by +35% to 27-28 PSI to allow for the inevitable field working conditions.
In addition to the axle load adjustments for duals or triples impacting the air pressure needs, you must also consider front or rear axle requirements caused by the type of platform in use due to load distribution per axle by platform for example:
- 2WD Tractors: 35% of the total weight is on the front axle and 65% of the Weigh is on the rear axle
- MFWD/MFWA Tractors: 40% of the total weight is on the front axle and 60% of the Weigh is on the rear axle.
- 4WD Tractors: 55% of the total weight is on the front axle and 45% of the Weigh is on the rear axle
The advent of “increased flexion” technology tires such as IF or VF tires does not change the above working rules for dual or triple fitments nor the inherent weight distribution from one axle to another experienced by each type of platform. These new technology tires are providing improved solutions to increase either the load carrying capacity at a given speed or increase the tire footprint at a lower speed through lower air pressures to improve productivity through reduced soil compaction and greater crop yields. Radial tires can operate as low as 6 PSI under low torque conditions, however, any greater speed required to move the machinery faster or necessitate greater torque will require increased air pressure to meet the axle load need for that given speed. IF and VF tires due to their unique construction and materials can be more forgiving in terms of air pressure adjustments, but they will still require some adjustments to their working air pressure to meet the dual or triples load limits and front or rear axle load distribution for each platform.
MAXAM recommends the following best practices or working recommendations:
- Inflate their tires to the correct cold inflation pressure based on the axle position, number of tires, axle load and required work speed.
- Installing a Central Inflation System lets you adjust tire pressure on the go, optimizing performance for both fieldwork and transport – saving time and improving efficiency.
- The right air pressure will reduce soil compaction, increasing the probability of healthier crops and increased yields.
- Greater crop yields will be achieved if soil compaction can be minimized by using the best tire footprint based on the right inflation pressure by axle configuration, position, axle weight distribution, and speed required.
- The right air pressure will improve tire wear and reduced sidewall deflection giving the optimum tire gross flat plate or footprint allowing improved fuel efficiency with reduced tire slip.
- The right air pressure will ensure greater productivity in the field as the tire’s footprint and traction will be fully optimized.
MAXAM’s agricultural product range has been engineered and designed to ensure the optimal contact patch without compromising flotation or traction regardless of wheel position, axle configuration, or platform weight distribution. MAXAM is developing both VF and standard tire sizes to not only complement our existing offer, but to meet the evolving market demand for improved tires that will deliver greater productivity, longer wear, and a better value to both farmers and growers.
Need help determining the right air pressure for your operation? Contact MAXAM Tire and discover how our Standard or VF Agricultural products can help you improve productivity and reduce your operating costs.
