AG Tire Talk: Proper Tractor Ballast for Improved Traction and Reduced Slippage
Modern Tire Dealer has partnered with AG Tire Talk to provide answers to the insightful questions farm tire dealers have about farm tire technology. This is the sixth in a series, which is designed to help agricultural tire dealers better connect with their customers. A trending question followed by an abridged version of the answers will appear in our Commercial Tire Dealer section every other month; for the complete answers, check out www.agtiretalk.com!
QUESTION: In the 100-plus horsepower tractor segment, how does the grower properly ballast a tractor? In addition, what is the impact on tire traction and soil compaction?
Dave Paulk: manager of field technical services, BKT USA Inc.: There are several important reasons for using ballast on tractors. Adding weight where required will improve traction and reduce slippage. Managing the ballast and tire inflation pressures can maximize traction, minimize compaction, increase the life of the tractor drivetrain, and increase productivity.
There are general guidelines for ballasting a tractor based on the type of equipment being used. A Mechanical Front Wheel Drive (MFWD) tractor should weigh 120-145 pounds per PTO horsepower. 130 pounds is a common weight used per PTO horsepower. The weight split should be 35-40% of the weight in the front, and 60-65% of the weight to the rear. Some manufacturers recommend a 35-65% weight split as it makes it easier to control power hop. Implement dealers have calculators to best determine weights and weight splits based on the type of equipment being used and recommended by the manufacturer.
Cast weights and liquid can be used to ballast tractors. Both have pros and cons. Cast weights on MFWDs and 4WDs are easier to work with if power hop becomes an issue. Cast weights are the most convenient and can be changed according to ballasting needs.
Once MFWD tractors are set up with the proper weights and distributions, they can basically use any type of implement without having to change weight distributions. The exceptions could be heavy implements on the three-point hitch or implements that place large loads on the drawbar such as rippers. These lighten the front end and may require more front weight.
There are potential problems that can be discarded by the correct tractor ballast. If tractors are under ballasted, there is too much slippage; 5-15% slippage is considered in the normal range with 8% slippage considered optimum. Too much slippage effects fuel consumption, field operating time, and tire wear. Over ballasting also has a potential for problems. This can cause tire failures by tires slipping on the wheel, and lug cracking. It can also cause drivetrain problems, increase soil compaction, and reduce efficiency.
James Crouch, national agriculture product manager, Alliance Tire Americas Inc.: It takes a certain amount of weight for a tractor to transfer one horsepower of energy from the engine to the ground — figure 130 to 140 pounds per PTO horsepower for front-wheel assist (MFWD) tractors and 95 to 110 pounds per PTO horsepower for 4WD tractors. That often requires a little extra weight, which is called ballast.
If you have too little weight, your tractor isn’t working as efficiently as it should. Your slip increases and you just aren’t getting all the power you paid for when you bought that machine. If you have too much weight, your soil compaction increases and your fuel efficiency can be reduced. And if the weight is poorly balanced, you can lose efficiency and experience power hop.
Liquid ballast in the tire is a popular way to add weight to the tractor. However, we design tires to be filled with air, not liquid. So when you add liquid to a tire, you’re reducing the tire’s flexibility and performance, and deforming the shape of the tire. That can lead to irregular wear in the center of the tread, a harder ride, and significantly reduced sidewall flex. From a performance perspective, you’re turning a radial tire back into a bias.
The bottom line is that if you’ve paid good money for horsepower, proper ballast is essential to make sure you’re getting your money’s worth. And if you’ve invested in radial tires, that ballast should really be achieved with cast iron weights so those tires can perform the way they were designed to. That way, you get the most out of your tractor and your tires.
Harm-Hendrik Lange, agriculture tires field engineer in North America, Continental Tire the Americas LLC: The right ballasting for tractors depends on many factors. Generally speaking though, the tractor should be as least ballasted as possible, if the ballast is not really needed. Each pound that needs to be moved forward needs energy during acceleration, creates more energy during braking, creates a longer stopping distance on average, and creates more soil compaction during operation on wet soil conditions.
However, there are certain applications and operations where ballasting is needed and reasonable. Those situations can be clustered in two main topics:
1. to prevent an unbalanced situation on the tractor, e.g., when working with a heavy front loader and adding ballast in the rear hitch or on the rear axle, or when using a front ballast as a counter weight for a heavy rear hitch. With higher loads on one end, the maximum speed may be reduced for both the axle and the tires.
2. to make the tractor heavier to transmit more force to the soil, e.g., for implements with lower working speed like a plow where the operational speed is less than 6 mph. Here, all the engine power is transformed into a high torque on the axles, which means high forces in the contact patch between tire and soil. In this instance more weight means better grip for force transmission to the ground – so basically the weight supports the tire.
David Graden: operational agriculture market manager, Michelin North America Inc.: Depending on the machine’s use, improper ballasting will cost more time and money than most would expect.
At Michelin, we strongly encourage producers to work towards proper weight distribution of their machine. This should be an all-encompassing solution to maximize traction (which reduces slippage and soil compaction), torque, and fuel efficiency, minimize power hop and road loping, and overall productivity.
Further, it is very important to keep in mind each task and corresponding implement can require different ballasts and tire pressure recommendations. For instance, a front-wheel assist machine requires about 120 lbs. per PTO horsepower at 6 mph to fully transfer the torque. Additionally, of the total machine weight, 35% should be on the front axle and 65% should be on the rear axle. Let’s say you add a hitch mounted strip till on the back. Now, you have to take into account the weight of the implement, while working in the field, and adjust your ballast weights and air pressures accordingly.
Finally, this change only occurs a handful of times per year. Investing in ballast weights that allow you to change quickly and easily will give you the ability to properly distribute your machines weight for every season.
Mark Turner, senior product marketing manager, agricultural tires, Maxam International Inc.: Ensuring you are using the correct ballasting weight is not always an easy exercise, but ultimately it is one that is worthwhile. Unnecessary extra weight or not enough weight can both lead to higher fuel consumption as well as possible uneven tire wear and thereby increase input costs.
Correct ballasting for field work is entirely about ensuring that slip rates are kept in the best window that allows optimum tractive efficiency and drawbar pull. Typical recommended rates of slip are 10-15% for 2-wheel drive tractors (2WD) and 8-12% for 4-wheel drive (4WD) or mechanically assisted 4WD tractors (MFWD).
The best type and weight of the ballast should be assessed for each implement used and should be added to the front and/or rear to ensure optimum weight distribution between the two axles.
The general recommendation for field work is to make sure that at least 20% of the total weight is on the front axle; this ensures good steering response and vehicle control as well as limiting the possibility of an adverse reaction between suspension systems and automatic draught controllers.
For prolonged road work applications, a weight split of around 40% front and 60% rear should ensure good vehicle stability and control as well as ensuring that dynamic overload is not excessive when the vehicle is braking.
It is also important to remember to alter your tire inflation pressures to suit the new load on the tires when weights are changed.
Norberto Herbener, OE applications engineer, Trelleborg Wheel Systems Americas Inc.: For a tractor to perform at its peak capabilities and efficiency, consider the following points.
1. The first point is the correct weight and weight distribution that the tractor must have to transfer power with efficiency. As a rule of thumb, a correct weight ratio begins considering that between 100 to 120 lbs. per horsepower of the tractor is needed. Therefore, a 300 hp tractor should weigh approximately 30,000 to 36,000 lbs. This weight will allow the tire lugs to penetrate the soil deeply enough to create optimum grip and acceptable tire-to-soil slippage. The best way to increase a tractor’s weight is by adding solid ballast to the axles or on the tractor chassis.
2. The second point is weight distribution. Once the correct tractor weight is defined, the distribution should be 50% on the front axle and 50% on the rear axle for 4WD. For a front wheel assist tractor, the distribution should be between 40-45% on the front axle and 55-60% on the rear axle. If the design of the implement creates a high load on the rear axle (reducing the load in the front axle), the ratio on front-wheel assist tractor should be closer to 45-50% in the front and 50-55% in the rear. For 2WD tractors the front axle should have between 25-35% of the total load. It is very important to measure this weight per axle with calibrated scales --individual per tire or per axle, like on a grain elevator scale. An incorrect weight distribution can create issues like “power hop,” where the front axle is too “light” (losing traction) and the rear axle is overloaded, creating excessive “grip.”
3. The third point is tire inflation pressure. Each tire’s size load capacity changes with inflation pressure changes -- the higher the inflation pressure (more air volume in the tire chamber) the higher the load capacity. This load capacity is defined by the type of tire (bias, radial, IF, VF, IMP, etc.) and design (width, rim diameter and ratio), and is regulated by the TRA (Tire and Rim Association) in the U.S. or the ETRTO (European Tire and Rim Technical Organization) in Europe.
A note related to minimum pressure: Even if the load is low enough to allow for very low inflation pressure (below 10 psi), the minimum inflation pressure should not be below the 10-12 psi mark. Below this pressure value is risk of rim-to-tire slippage… as there is less pressure from the tire bead against the rim flange, reducing the efficiency of power transmission from the tractor to the tire.
Scott Sloan: ag product manager/global LSW, Titan International Inc.: Whether running in heavy tillage or moving heavy hay bales, the correct ballast is important.
Fact is, too much weight can cause a tractor to feel sluggish, increase soil compaction, reduce fuel efficiency and result in premature drivetrain wear. Too little weight can result in tire slippage, reduce fuel efficiency and cause premature tire tread wear.
The end game is trying to minimize tire slippage when you head to the field. Most manufacturers shoot for slip in the 8% to 15% range. Anything over that is extremely noticeable and not very efficient. Growers feel they are sending a lot of money with little return through the exhaust stack. Most growers I deal with do prefer to be running in the 3% to 9% range.
So a quick, fast approach to get in the ballpark for example on a 4WD depending on the application to have anywhere from 85 lbs. to 125 lbs. per rated horsepower. For instance, a 4WD rated at 500 hp should be in the 50K to 55K total weight with 55%-60% on the front and 40%-45% on the rear. Not only is the total machine weight important, the distribution of that weight is just as critical (see Table 1 below for a quick reference to proper ballasting and weight distributions).
At the end of the day, each grower needs to understand the factors involved and the objective for their individual operation. One end user may be okay with a little more slip and a lighter machine, while the other wants to pin all the power to the ground.
Jim Enyart, technical manager, CEAT Specialty Tires Inc.: Ballasting has been utilized for equipment functionality and stability as well as transferring power to the ground. Many types of ballasting materials have been utilized, including rocks, soil, cast iron, water, calcium chloride, magnesium chloride, beet juice and more. The choice of ballasting materials includes many factors, but the most critical aspect of adding ballasting is how much weight to add and where to place these materials.
The first critical factor is knowing the engine horsepower of the tractor and the axle weights. Ideally, the weight per horsepower ratio should be between 100 and 120 pounds/hp. The lighter weights are more desirable and efficient as long as there are no performance issues such as stability or wheel hop. Increased weight ratios may be utilized to reduce slip, which increases efficiency, especially in demanding applications.
For 2-wheel drive tractors the weight distribution should be 30% on the front axle and 70% on the rear. MFWD tractors should have 35% on the front and 65% on the rear axle. Four-wheel drive tractors should carry 60% on the front and 40% on the rear axle. Allowances for additional weight due to attachments should also be calculated into your weight distribution adjustments.
Air pressures should be set just above the maximum load carrying capacity you are expecting from your tires based on the most demanding application. Changing air pressures between road and field usage is time consuming and not practiced. Air pressures should be set for the road application due to the higher speeds, which is the most demanding from a load carrying aspect.
Bradley Harris, manager, global agricultural field engineering, Firestone Ag Division, Bridgestone Americas Tire Operations LLC: To get the most efficiency from a tractor, it’s critical to have the proper amount of ballast and the weight appropriately split between the front and rear axle. There are some general rules that tractor and tire manufacturers use when helping customers set up tractors used for fieldwork.
Step One: Identify the horsepower rating of the tractor. For 2WD and mechanical front-wheel drive (MFWD) tractors, the rated PTO horsepower is used. For 4WD tractors, the tractor’s rated engine horsepower is used. For the following calculations, we will use a MFWD tractor with 180 PTO horsepower being used to pull drawbar implements as the example.
Step Two: Calculate the target total tractor weight. Example target tractor weight: 180 PTO HP x 130 lbs./PTO HP = 23,400 lbs. (see Chart 1).Step Three: Calculate the recommended front and rear axle weight splits. The weight split is based on the tractor type and how the tractor is being used. Example tractor weight splits: front axle, 23,400 lbs. x 0.35 = 8,190 lbs.; rear axle, 23,400 lbs. x 0.65 = 15,210 lbs. (see Chart 2).
Step Four: Weigh the tractor. The most accurate way to find the weight is to use a scale. When weighing a tractor, make sure all the fluids are topped off, especially the fuel tank.
If for some reason the tractor cannot be weighed, please consult your operator manual or your equipment dealership. Example tractor weighed with portable scales: front axle, 10,150 lbs.; rear axle, 15,500 lbs.Step Five: Add or remove weight. In most cases a tractor cannot be ballasted to the exact target weight outlined in Chart 2. In the example, the target front axle weight is 8,190 lbs., but it weighed 10,150 lbs.
Let’s assume the example tractor has six front suitcase weights. Since the front axle weight is heavier than the target weight, we would remove the suitcase weight and re-weigh the front axle.
Even though the suitcase weights are 100 lbs., they add 130 lbs. to the front axle because the weights are in front of the axle. When the weights are removed, the tractor is weighed and the new front axle weight is 9,370 lbs.
Step Six: Set the inflation pressure of the front and rear tires. Now that the front axle and rear axle weights are known, we can use the tire inflation tables to look up the minimum inflation pressure required to carry the load. In this example, the front tires were 420/90R30 used as singles. The minimum inflation pressure to carry the 9,370 lbs. is 17 psi. The rear tires were 480/80R46 used as duals. The minimum inflation pressure to carry the 15,500 lbs. is 12 psi.
When the tires are inflated to the recommended inflation pressure, the tire is able to develop the proper footprint. ■
James Tuschner has spent 25 years in the tire industry, primarily focused on the agricultural and specialty tire markets. His experience includes time spent at Alliance Tire Americas Inc. (first as director of marketing, then as director of business development) and Denman Tire Corp. He started www.agtiretalk.com in 2016.