QUESTION
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 with some tire manufacturers, and what is reason for weight carrying capacity reduction of 12% for Duals, and 18% for Triples?
AG TIRE TALK KEY TAKEWAYS
BKT: Torque is the applied force around an axis, in this case the axle, and produced by an engine.
MAXAM: The AG industry has termed three different air pressure possibilities that a tire may face when operating in dynamic work conditions: Free Rolling, Low Torque, and High Torque Applications.
MICHELIN AG: As free rolling tire positions have no torque, air pressures can be reduced when compared to drive tires.
YOKOHAMA TWS: Increased air pressure reduces the rim slip during extreme torque applications.
PRECISION INFLATION: A tire requires enough air to transmit the torque from the rim to the tire sidewall, without buckling the sidewall.
Michelin Ag
David Graden: Global Account Manager – Agriculture
There are many variables tire manufacturers consider when making tire pressure recommendations, all taking into consideration tire application.
Free Rolling Tire: Lower PSI
Free rolling tires only need to be able to carry the weight from above, while at the same time handling the undulating ground beneath it as it rolls and the movement of rubber and heat build-up, depending upon the speed. As free rolling tire positions have no torque, air pressures can be reduced when compared to drive tires.
Drive Tires: Higher PSI
Pressures for drive tires under torque take these same variables into consideration, while also considering the linear forces put upon them; consequently, higher air pressures are required for drive tires versus free rolling position tires.
Increasing Torque = + PSI Required
Air pressure can also be reduced in Low Torque Applications vs High Torque Applications:
For example, if tire is required to carry 6500 lbs.
- 6 psi can be used in 6 mph low torque “planter application.”
- 7 psi must be used in 6 mph high torque “plowing application.”
Finally, when recommending air pressures for duals or triples, we calculate in a 12% or 18% reduction in weight capacity as a standard safety and performance adjustment. The easiest way to explain this is due to the weight shift of a machine as it moves through a field.
Each tire is mounted to the tire before it. Since there is no pivoting joint between each tire, as they extend out further, weight shift will compound and grow, thus the potential for overloading one tire becomes a concern. Tire manufacturers take this into account when developing their air pressure tables.
Please don’t hesitate to reach out to your local Michelin authorized tire dealers for help in setting up your machine air pressures for maximum efficiency and safety.
Yokohama TWS
Chris Neidert: AG Marketing, Training & Development Manager for Trelleborg & Mitas Tires – North America
WHY ARE AIR PRESSURE REQUIREMENTS DIFFERENT FOR FREE ROLLING, LOW TORQUE, AND HIGH TORQUE APPLICATIONS?
Let’s talk about those two applications. Free rolling tires are not trying to apply any torque to the ground. Their main mission is to carry the load.
Considering a drive tire application, it is trying to grab the soil and the sidewall flexes. Think of what happens in a tractor pull. When the tractor engine rpms start increasing and the engine starts to apply torque to the drive train and then to the tires, the sidewall flexes.
That sidewall flexing needs to have the higher air pressure to help the tire keep its shape. In our below picture, check out the lower sidewall how it is flexing. This is an exaggeration, but the concept is the same. Increased pressure reduces the rim slip during extreme torque applications.
What is reason for weight carrying capacity reduction of 12% for Duals, and 18% for Triples?
Forces are always in action when it comes to tires and tire weight.
I am going to address this comment not as a capacity reduction but as having the tires needing to carry more weight even though the axle weight has not increased. Check out my two graphics.
Single Load/Force
The left picture is showing the load/force situation when a single tire application is in use. I am depicting the axle with the green arrow. I am depicting the center of gravity of the right half of the axle with the blue arrow. There is a load/force being exerted on the axle and the air pressure of the tire is carrying that load, depicted by the red arrow.
Dual Load/Force
When we change to a dual configuration, things start to happen. Many would think because I have more tires on the axle, I can reduce the air pressure. This is true but, we now need to consider the center of gravity has shifted away from tractor compared to the single tire fitment. Considering that, this is putting more strain on the axle, depicted by the larger/bolder red arrow. Going back to my comment at the beginning, even though the axle weight has not changed, the additional strain because the center of gravity has changed, is not good for the axle. Increasing the tire air pressure will help acting to support that changed center of gravity.
The same will be true for triples, shifting the center of gravity even more and the need for even more increased pressure to support this change.
WHY ARE VF SINGLE TIRES ALLOWED MINIMUM INFLATION OF 12 PSI, WHEREAS DUALS/TRIPLES ARE ALLOWED MINIMUM INFLATION OF 6 PSI?
First of all, Trelleborg & Mitas Tires never advocate going below 12 psi on any application or configuration.
For several reasons:
First – many air pressure gauges being used may not have enough increments to show that preciseness of the air pressure reading.
Second – Ourselves. We tend to ignore checking air pressure. Ignoring the pressure when you have only 6 psi in the tire does not leave a lot of room for the normal fluctuation that occurs with air pressure. When was the last time you checked your passenger vehicle air pressure. The answer is probably embarrassing.
Shown below are some pictures of some testing we performed. These pictures are showing a tire running 6 psi, which according to the data book, is sufficient to carry the load. This is a recipe for disaster as this tire has a good chance of coming off the rim resulting in some unwanted downtime.
Here are some images showing tire footprints at different air pressures.
My point is, the 6 psi image is showing the footprint is now starting to lift up in the middle. That is not the footprint we want. Traction will be sacrificed. Irregular wear will start and shoulder wear will start.
25 PSI – 18 PSI – 12 PSI – 6 PSI
However, Trelleborg & Mitas in some cases can be run at lower air pressures, but we still advocate the 12 psi rule.
BKT USA, Inc.
Dave Paulk: Manager Field Technical Services
Radial vs Bias
There are differences between bias tire and radial tires when running in a free rolling application versus a drive wheel application (high or low torque). In most cases, radial tires are used in mixed service (drive wheel and free rolling) without a noticeable difference in air pressures and load carrying capacities. With radial tires, the load capacities don’t change with the application change.
This is not the case with bias tires because of the construction of the tire and the effects of load, speed and torque when used in a drive wheel (DW) application. When used in a free rolling (FR) application, the tire is pulled by a tractor on a wagon or cart. It is pulled across the ground and has no slippage and no torque applied to the tire. Without the effects of torque, the tire can carry more load.
Torque Explanation
When used in a DW application, the engine, bearings, and gears all create a bit of friction. Some slippage from torque occurs from the power of the machine to the tire and to the ground. Used as a drive tire, this limits the amount of weight the tire can carry. Torque is the applied force around an axis, in this case the axle, and produced by an engine. The amount of torque generated is predicated by gear ratios and the rpm’s used to generate power. When you think of a torque wrench, the person is generating power. The longer the handle, the more force that can be applied. This is the same concept as torque on bias tires.
Load Index Impact
Air pressure requirements are dependent on the Load Indexes (LI) for radials, and the ply ratings (PR) for bias. The higher the ply rating or the load index, the more air pressure the tire can use. Air pressure carries the weight of the equipment. The more air pressure that can be used, the more weight carrying capacity of the tire. Radials can run lower air pressure if there is enough air to carry the weight of the equipment. Because of the construction of bias tires, they should be run at near maximum air pressure all the time.
Single vs Dual vs Triple PSI
VF technology tires are becoming more prominent and more widely used because the weight carrying capacity is greater than a standard tire and they can be ran at lower air pressures to minimize compaction. Air pressure carries weight, so the right amount of air pressure must be used to minimize damage to the tires. Lower air pressure can be run in duals and triples versus running singles. When using duals/triples, the tires provide more floatation and share the load of the tractor/implements. When using singles, there is only one tire per side to spread the weight of the tractor. When running duals and triples, air pressures can be dropped below the required pressures. Duals/Triples can carry the same weight at lower air pressures vs a single tire.
When using duals/triples, the weight carry capacity of the tire is diminished by 12% on duals and 18% on triples. Although low air pressures can be used in this application, thought must be given to ensure that enough air pressure is being used to carry the weight based on these percentages. When using duals and triples, the deflection of the tires when weighted must be considered. Minimum spacing must be maintained between tires so that they clean out and don’t pick up sticks, rocks, and other trash between the tires and damage the sidewalls.
Weight carrying capacities and recommended air inflation for the loads being carried can be found in Load and Inflation Charts on the manufacturers’ websites or in Data Books. If only the load and inflation for singles are given in the Load Chart, multiply the carrying capacity by .88 for duals and .82 for triples at the corresponding air pressure. This will give the carrying capacity in dual/triple applications.
Maxam Tire International
Greg W. Gilland: Vice President Global Agriculture
Tire Forces
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.
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:
- Axle Load or Weight
- Speed
- Desired Tire Contact Patch or Footprint
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.
Application PSI Impact
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 such as planting.
- 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.
Working Requirement PSI Impact
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:
- Machine: MFWD Tractor, 4WD Tractor, Harvester, Floater, Sprayer, Grain Tank, or Implement etc?
- Size & Wheel Position: Front Axle Tire, Rear Axle Tire, Trailer Axle Position?
- Configuration: Single, Dual, or Triple?
- Maximum Axle Load: Weight per Tire?
- Cyclic Load: Work conditions fully laden and then fully unladen?
- Speed: What is highest speed to complete work requirements?
Central Tire Inflation System is BEST
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.
Singles vs Duals vs Triples
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 Total Weight is on the Front Axle and 65% of the Weight is on the Rear Axle.
- MFWD Tractors: 40% of the Total Weight is on the Front Axle and 60% of the Weight is on the Rear Axle.
- 4WD Tractors: 55% of the Total Weight is on the Front Axle and 45% of the Weight is on the Rear Axle.
VF PSI Impact
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 Tires to the correct cold inflation pressure based on the axle position, number of tires, axle load and required work speed.
- Install a Central Inflation System to adjust tire pressure on the go, optimizing performance for both fieldwork and transport – saving time and improving efficiency.
- Correct 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.
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.
Precision Inflation Systems
Ken Brodbeck: VP of Technology
Tire Pressure, How Low is Too Low?
The proverbial question: “WHAT PRESSURE SHOULD I RUN IN MY TIRES?” OR: “HOW LOW CAN I GO?”
Tires require enough air to:
- Seat the Tire Beads to the Rim- Maximum Pressure to Mount and Seat the Beads is 35 PSI!! NO MORE!!
- Carry the Vertical Load.
- Transmit the Torque from the Rim to the Tire Sidewall, without Buckling the Sidewall.
- Provide Lateral Stability to provide Adequate Steering Response for both Field & Transport Conditions.
- Under High Torque and Turning with Too Low PSI, the Tire could Dismount from the Rim.
- In Transport, too Low of PSI could cause Poor Steering Response (squishy feeling).
- With the Correct Air Pressure for EACH application, you will see Maximum Life & Performance from All your Tires.
Let’s look at some examples of VF tires at LOWER psi and various conditions.
Tractor Standing Still, NO TORQUE:
Now under HIGH TORQUE…
12 psi & High Torque ,VF looks good!!
VF Duals Planting work fine at 9 psi with CTIS!!
Remember!
Setting AG Tire Air Pressure Requires:
- Tire Size
- Verify whether the tire is standard, IF or VF designation
- Single, Dual or Triple Setup
- Application
- Tractor: Low Torque = Not Pulling a Ground Engaging Tool: Sprayer
- Tractor: High Torque = Primary Tillage: Ripper
- Cyclic Loading Service: Combines
- Road Transport or Field Use
- Weigh each Axle
- Take the information above and consult the tire manufacturer’s website or data book for the correct tire pressure.
- FINALLY, Central Tire Inflation Systems will allow you to set your valuable tires to their “Sweet SPOT” whether planting or heavy tillage for MAXIMUM PERFORMANCE.
All information is provided in this blog solely to provoke thought. All deductions made from information on this site must be confirmed by Certified Ag Tire Dealer & Tire Manufacturer before use. Ag Tire Talk does not recommend anyone conduct tire service work with exception of Certified Ag Tire Dealer Professionals.