Are You Getting the Most Out of Your Tractor?

April 6, 2020 10:04 AM
Blog Post

Are You Getting the Most Out of Your Tractor?

Determining the proper ballast and tire pressure for your tractor should be a key part of spring equipment maintenance. Proper ballast and tire pressure will maximize traction and tire life while minimizing fuel consumption and compaction.

Proper Ballasting

Ballasting a tractor is the act of adjusting the weight its frame to allow for optimal fuel efficiency and pulling power in the field based on the field operation. Most planters from the last decade are attached to the tractor’s three-point hitch or the rear drawbar. The rear axle often carries more of the planter’s weight in the field. This results in the tractor needing less rear ballast and transferring more ballast over the steer axle to pull the planter. In a tillage application, the tillage tool often has little to no tongue weight when the tool is engaged in the soil, requiring more weight on the rear of tractor to compensate for high draft load. Before altering the ballast or the front-rear weight ratio of the tractor, consider the following questions.

  • Does the tractor have a mechanical front wheel (MFW) assist?
    • The front-rear weight ratio will be higher on an MFW tractor than a 2-wheel drive model.
  • What is the gross weight of the towed implement?
    • The tractor needs enough ballast to properly stop the implement while being towed at transportation speeds.
  • What is the operational speed of the implement, both in the field and on the road?
    • Higher field speed implements often require more ballast to limit wheel slip during operations. The same implement might require 2000 pounds less if towed 3 miles per hour slower.
  • Does additional weight transfer to the tractor when the implement is in the transportation position?
    • A centerfold planter shows how additional weight can be added to the rear axle when in transport.
  • Is the implement balanced in a way that the tongue will put additional weight on the rear axle?
    • When a manure tank is loaded, a significant amount of tongue weight is added to the tractor from the tank.

A guideline to the correct ratio between the front and rear axle weights is determined by the Original Equipment Manufacturer (OEM) and can typically be found in the back of the owner’s manual. A worksheet may be found in the owner’s manual to help calculate ballast changes, like this example from John Deere.

If the manual is not available, the PM2089G factsheet available in the ISU Extension Store is a good reference for general recommendations by tractor type (Table 1).  This factsheet also provides information on required weight per horsepower based on tractor type and operating speed.

Front-to-rear axle weight ratio as percentage of total weight

Tractor Type

Towed / drawbar

% Front / % Rear

Semi-mounted

% Front / % Rear

Fully-mounted

% Front / % Rear

2WD

25/75

30/70

35/65

MFW

35/65

35/65

40/60

4WD

55/45

55/45

60/40

Table 1. Provides general recommendations on tractor ballasting by tractor type. The ballast distribution can change front to back up to 10% depending on the type of implement used.

Setting Tire Pressures

Tractor tire pressure should be set according to the tire manufacturer’s specification. Proper tire inflation significantly improves the traction of the machine and reduces soil compaction and operator fatigue. Several factors should be considered when setting tire pressures. If the pressure is too high, the tire’s contact with the soil is reduced. The tractor can experience a higher wheel-to-ground slip and machine bouncing can occur, reducing the pulling power of the tractor. Over-inflation can cause decreased tractor productivity and fuel-efficiency, as well as increased soil compaction. A higher wheel slip can equal higher fuel consumption. Over-inflation also reduces the tire’s longevity.

Over-inflation can be detected in the center of the tire where it has the most contact with the ground. The center lugs are often worn and shorter than the lugs near the outer edge of the tire. If the tire has experiencing a sustained duration of wheel slippage, the tire will often show signs of wear in the lugs as thin lines or groves where the tire was scraped past hard objects like rocks and hard packed soils.

Underinflated tires can also cause uneven tire wear and can ultimately end in tire failure. Underinflated tires show wear in the shoulder and sidewalls of the tire. Extremely low tire inflation can damage the tractor rim. Underinflated tires are extremely unstable and can lead to tractor rollovers.

LSW (Low Sidewall) tires can also be used. Compared to traditional tires, these have a larger rim and slimmer sidewall that allows them to operate at lower pressures while maintaining weight carrying capacity. This reduces soil compaction and minimizes issues with hopping under high load conditions. 

To set the tire pressure, know what speed you tend to operate and the load on each axle. Axle weights can typically be found in the back of the owner’s manual or by weighing the tractor. Consider aftermarket additions to the tractor, such as saddle tanks for carrying liquid fertilizer, when making the axle weight determination. Most elevators or grain cooperatives have large enough scales to weigh a tractor and will often let you use them free of charge if you are a customer.  OEM tire manufacturers offer load and inflation tables used to set tire pressure based on the size of the tire, the tractor’s intended speed, and weight applied to the axle (Figure 1).


Figure 1: It is important to inflate tires to the proper pressure based on the machine and axle load. As the axle load increases for a specific tire, it will require more pressure to safely handle the load. (Table courtesy of Titan International).

Example: Based on the table above, we will calculate the needed tire pressures for a given machine setup. Assume we are operating a MFW tractor with dual tires on all 4 wheels (8 tires total). The rear tires are a 480/80R50. The rear axle of the tractor has 16,000 pounds of weight on it, measured before the implement was attached. When an implement is added to the 3-point hitch, the rear axle weight increases to 23,000 pounds. We can determine the per tire load by dividing 23,000 by 4 since we have 4 rear tires. We need to be able to support 5,750 pounds per rear tire. Looking at the chart above for our tire size with duals, we would need a minimum pressure of 17 PSI to support this machine setup.

In the age of smartphones and mobile technology, some tire manufacturers have created web-based resources to calculate the correct inflation pressure for many agricultural applications (Titan/Goodyear App , Firestone App). Select the correct application for the tire (spraying, harvesting, tillage, etc.) and input the configuration of the machine—single, dual or triple tires on each side of the axle. A drop-down list of different tires sizes and a box to select the weight of the machine will appear. The calculator will produce the recommended inflation pressure for each tire.  

Authors: 

Ryan Bergman Program Coordinator in Ag Technology

Ryan Bergman is a Program Coordinator in Ag Technology at Iowa State University where he is part of a 20+ person research team focusing on precision agriculture, big data, telematics, data analytics, aerial imagery, and ag machinery automation. Ryan has received both his bachelor’s and mas...