AG Tire Talk: Why Aren't All Tire Specs the Same?

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.

GREG GILLAND, vice president, global agriculture, Maxam Tire North America Inc.: The Tire & Rim Association of America, established in 1903, became the governing body that drove the standardization of tires, tubes, valves, flaps and corresponding wheels to help both manufacturers as well as consumers solve the problem of sourcing products. The European Tyre and Rim Technical Organization (ETRTO) came into existence in the early 1960s to standardize pneumatic tires and rims across the European Union. Today the TRA, as well as the ETRTO and other regional associations, are responsible for standardizing and interchanging tires, rims, valves and all associated components, from passenger vehicles to bicycle tires, and to include agricultural tires. These organizations provide huge advantages as they eliminate ambiguous interpretations of nominal sizes between tire manufacturers, vehicle makers, and/or by country providing standardization across all platforms.

From a standard perspective, as an ag tire manufacturer, each size that we develop either as a bias ply or as a radial product must conform and be within the following tolerance for any given tire size in millimeters or inches:

• Tire Overall Diameter (O.D.) = + / – 2% versus the industry size standard
• Tire Section Width (S.W.) = + / – 4% versus the industry size standard

In all instances, the type of machinery, soil conditions, application (intensity of use), and air pressure maintenance will have a direct impact on its performance even after improvements. Listed below are some general considerations related to using tires from different manufacturers on the same machine or axle based on application differences:

DAVID GRADEN, operational market manager, agriculture, Michelin North America Inc.: The European Tire and Rim Technical Organization was founded 1964, but was formerly known as the ETWTC (European Tyre and Wheel Technical Conference), founded 1956. The TRA (Tire and Rim Association Inc. for the USA), was founded in 1903, originally to standardize passenger car tires.

Both organizations set guidelines to standardize and harmonize wheels and their corresponding pneumatic tires across all typical types of vehicles, from bicycles to mining equipment. This standardization improves the consumer’s ability to purchase proper tires for our vehicles and, in some cases, match up sizes when a direct replacement doesn’t exist. Now, this does not mean that any two tires of the same size will match up. In fact, within these standards, there is a measurement threshold that a tire of a certain size needs to fit within.

For example, a 480/80R50 should fall between 2,008 mm with a maximum of 2,068 mm. But the TRA standard is slightly different, in that the minimum threshold for the same tire should be 2,014 mm. Either way, matching up two different tire brands can be a pretty big problem if you were to match up one brand of tire with a 2,068 mm overall diameter with another at 2,014 mm overall diameter (about a 2 inch difference). Best case, depending on the machine and position of the tire, you could see rapid wear on one of the tires or, worst case, you could damage your transmission.

Given this information, for whatever reason, if you are one of those people looking to match up a tire and you are considering different brands, there are some specs that you and your tire professional should carefully consider. First, once you have found a matching tire size, make sure the rolling circumferences match between the two brands. This will be the measurement around the outside of the tread, circumferentially, around the tire. Second, make sure the tread depths are similar in comparison. This will also have an effect on the rolling circumference and loaded radius. Third, make sure the loaded radius of each tire is comparable. This is the measurement between the center of the hub and a hard surface. It takes into account the tire sidewall bulge at the bottom, under load, and will also have an effect on the revolutions per mile. If these three specs are comparable between brands, you are probably safe to match them up. However, buyer beware, the tread rubber of each tire could possibly wear faster or slower than the tire it is matched up to. In this case, whereas you were safe to match up from the start, over time these tires may no longer be a good match and you could have major problems down the road.

From a Michelin perspective, I highly recommend dualling up the same brand and tread name of tire (ex: Michelin Agribib 2 matched to a Michelin Agribib 2 and not a Michelin Yieldbib). I would also go so far as to say these should be the same across the axle if it is a drive/powered axle.

CHRIS NEIDERT: ag marketing, training and development manager for Trelleborg and Mitas Tires — North America, Yokohama TWS: Let’s start with the section width. According to the ETRTO, a suggested maximum variance is 3% from the nominal measurement. An example would be the tire size of 480/80R50 — where the 480 is the nominal measurement in millimeters of the section width or the overall width of the tire. In inches, this is 18.9. This is measured from the widest part which includes the sidewall and even raised lettering, if applicable. If we follow the ETRTO suggestion and go within 3%, the tire section width could vary from 494 mm (19.4 inches) to 465 mm (18.3 inches).

Let’s talk about tire diameter. This is where it gets critical! Let’s stay with our size example of 480/80R50. According to the ETRTO, the nominal diameter of that size is 80.2 inches. If we apply the plus- or minus-3% rule, the minimum diameter is 77.8 inches and the maximum is 82.6 inches. If you are considering replacing tires on a MFWD (Mechanical Front Wheel Drive) tractor, you could follow the minimum and maximum measurements. However, at Trelleborg and Mitas, we suggest to stay within 1% of the diameter measurement of the tire we are replacing. This is extremely critical as tire replacement could affect the lead/lag of the MFWD tractor.