Modern Tire Dealer has partnered with AG Tire Talk to provide answers to insightful questions that farm tire dealers have about farm tire technology. This is the next installment in our ongoing series, which is designed to help farm tire dealers better connect with their customers. A trending question, followed by answers, appears in our Commercial Tire Dealer section every other month. For complete answers, click on www.agtiretalk.com.
QUESTION: Why does the same size ag tire, in a different brand, often have a slightly larger or smaller overall diameter and section width? How much variance can there be and what is the impact on equipment? Do you build product in line with a specific standard or benchmark?
NICK PHILLIPPI, product manager/technical support, North America, Ascenso Tires North America: Ascenso builds all tires to very specific standards. These standards are TRA (Tire and Rim Association), predominantly a standard for North America, and ETRTO (European Tire and Rim Technical Organization) — currently the most widely used standards for off-highway use.
There is an acceptable variance of 3% for outside inflated no load diameter and for section width inflated no load dimensions. Ascenso R&D will normally design tire specifications to the center of the allowable tolerance.
Where tolerances might come into play would be if the tire fits in the space available on the equipment. Many tolerances on equipment today are very tight. Also, if one manufacturer’s tire is within specifications but at the low end and another is at the high end it could be enough to cause lead or gear issues on MFWD or 4WD equipment. It would be important to calculate RCI (rolling circumference index) of both fronts and rear when replacing either. Use the RCI chart specific to the exact tire you are considering. We always suggest replacing an entire axle to be sure the load is being evenly distributed across all tires and the ODs (overall diameters) are the same. If you are only replacing the front or rear axle it might be a good final check after fitment to manually check the lead-lag to be sure you are still in tolerance.
DAVE PAULK, manager, field technical services, BKT USA Inc.: ETRTO (European Union) and TRA (USA Tire and Rim Association) are governing organizations that recommend and furnish engineering information for overall diameters (OD), section widths (SW), section heights (aspect ratio) and tread depths for specific tires. They also provide data on new tire dimensions and allowable grown tire dimensions. Both organizations are made up of representatives from various tire manufacturers working for a common cause. Not all manufacturers are members and may have different specifications for tires than these organizations dictate. BKT is a member of both organizations and designs and builds tires according to the specifications laid out for the industry.
Bias ply and radial tires differ in body construction. Both are used on tractors depending on the application and what the tractor is used for. They can be interchangeable, but they react differently to ride, traction and handling. Where a radial tire can be run at lower air pressures, a bias ply tire cannot without possibly damaging the casing of the tire. Air pressures can dictate the growth of the tire and change rolling circumferences. Both are used on MFWD (Mechanical Front Wheel Drive) tractors where the rolling circumferences are critical.
When looking at the size of a bias ply tire (ex. 18.4-38), it tells you the tire should have a section width of 18.4 inches (measurement of sidewall to sidewall when inflated) and is mounted on a 38-inch wheel. The size doesn’t tell anything about the height of the tire. Because the process of building a bias tire often differs between companies, there are oftentimes differences in heights. Because of the construction, a bias tire will grow more than a radial when aired up.
When looking at the size of a radial (ex. 480/80R38), it tells you the tire should have a section width of 480 millimeters (18.897 inches), the sidewall is 80% of the width (aspect ratio), and it’s mounted on a 38-inch wheel. This tire should be approximately 68.2 inches in height. In a perfect world, all radial tires this size should be approximately the same height and width. If they have the same height and width, they should have close to the same rolling circumference. Unfortunately, this is not always true. Once the tire is aired up, it will grow some as the fabric in the tire will stretch. TRA regulations allow for a growth in the overall tire width of 8% and the maximum overall diameter (height) of 4%. This excludes radial high flotation tires.
It is always wise to check the specifications of the tires replaced to make sure they are within the same size ranges, as sizes between manufacturers can be different. Where the difference can be noticeable is on MFWD tractors. The MFWD is a four-wheel drive tractor with different sized tires on the front and rear. The transmission must be geared to allow for the differences between tires. The rolling circumference is extremely important to maintain the correct lead/lag ratio between the front and the back. (The rolling circumference of a tire is the distance in inches it travels in one revolution.) While a 2-3% lead is optimum, 1-5% is in the allowable range. Air pressures can change the rolling circumference and flat plate measurements but should still be in the range of the allowable.
Source: MTD, Published: Jan 15.2025