QUESTION: Why does the same size tire, in a different brand, often have a slightly Larger/Smaller Overall Diameter & Section Width?
AG TIRE TALK KEY TAKEAWAYS
YOKOHAMA TWS: “That’s the situation AG Tire Dealers have to deal with on a daily basis- if we follow the ETRTO suggestion and go ±3% the tire section width could vary from 494 mm (19.4 inches) to 465 mm (18.3 inches).”
ASCENSO: “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.”
BKT: “The MFWD is a 4-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.”
MAXAM: “Inflated tire dimensions (tire geometry) can vary with tire construction based on the material positioning of layers after curing despite meeting the standards requirement by size and tire type: Standard, IF, or VF.”
MICHELIN: “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.”
Yokohama TWS
Chris Neidert: AG Marketing, Training & Development Manager for Trelleborg & Mitas Tires – North America
Why does the same size tire, in a different brand, often have a slightly Larger/Smaller Overall Diameter & Section Width?
That’s the situation ag tire dealers have to deal with on a daily basis. The ETRTO which stands for European Tire and Rim Technical Organization and the TRA which stands for Tire & Rim Association, do have suggestions on the maximum and minimum variance between tire brands as it relates to diameter and section width. Notice I wrote SUGGESTIONS. Some measurements are not as critical as others.
Section Width
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 ±3% the tire section width could vary from 494 mm (19.4 inches) to 465 mm (18.3 inches).
ETRTO Max Tire Spec Variance = +-3%
Now if you check some real live data, I’m sure you’re going to notice that there are even larger variances between brands. Reiterating that comment, using our 480/80R50 size example, some popular brands can vary by almost 7% in section width. Remember, I wrote the 3% variance is a suggestion. The section width measurement may not be as critical because as long as you have good clearance around the tire, the width differences can be acceptable.
Overall Diameter
Let’s talk about Tire Diameter- now, 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 +- 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 & 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. To read more on the subject of tractor Lead/Lag, see AG Tire Talk January 2023 article: https://agtiretalk.com/lead-lag-tractor-tire-calculation/
If the equipment is a self-propelled sprayer or 4WD (4 Wheel Drive) tractor, this difference is not critical since all the tires are the same size. Again, tire clearance with the vehicle needs to be considered when replacing with a different brand.
Trelleborg and Mitas build tires to the nominal value as the objective.
RC (Rolling Circumference)
It is the distance covered in one complete revolution of the wheel on an asphalt road at reference load and pressure. Value is in inches measured according to ISO11795. This measurement is very important when it comes to tire replacement. You want to stay within 1% of the RC of the tire you are replacing. Extremely important on a MFWD tractor as it relates to Lead/Lag.
Replacement Recommendations
- If possible, we do not suggest mixing brands on the vehicle and for sure the axle. Our above discussions about variances could be a factor.
- Suggest replacing the OE (Original Equipment) tire with the same brand. Sometimes, the particular brand of tire has been engineered to operate better on that particular piece of equipment.
- Look for an ECE number on the tire. ECE stands for Economic Commission of Europe. This commission verifies that the tire specifications that are being published by the manufacturer are accurate and true.
- If ever in doubt, contact your ag tire dealer and/or tire manufacturer to get the correct specs or have a discussion about brand substitutions.
Ascenso Tires North America
Nick Phillippi: Product Manager / Technical Support North America
TRA & ETRTO Standards
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.
Raw Materials
This process all starts with the purchase of raw materials. With varying qualities of raw materials, there are also levels of variance, specifically in elongation characteristics. The higher the quality of the raw materials, the higher the cost but assures less variances throughout the manufacturing process. Rigorous testing of raw materials is required to ensure the final product is of the highest quality. Ascenso Tire performs testing on all raw materials to ensure they meet the Ascenso standards, we also test during the manufacturing process and perform rigorous testing on final products.
In Service Variance
Next is the tire dimensions while “in service”, this is about how the dimensions change on a tire while under load, at higher speeds, and as tire temperatures increase. Ascenso’s unique CTT build technology minimizes the dimensional changes in working conditions to maintain tolerances. The use of steel belts or all steel construction will also help to maintain OD at high speeds and help with heat dissipation. Ascenso cuts no corners and builds to the highest standard for each product and the application it was intended for, during the design stage. All radial rear tires also undergo runout testing to be sure they will perform at the highest level in use.
So, as you can see the OD and SW of tires are highly regulated to standard. Load index, speed index, and structural safety specifications also have standards that all tire manufacturers adhere to.
Variances Matter in Fit
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 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 OD’s 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.
AG Tire Dealer Importance
As always, the AG Tire Professional at the Tire Dealership is a great resource for information and they know their market and the specific applications. Dealers need to stay on top of new trends and new products and know the details of tire design and tire construction to be the best asset to their customers. Farmers and dealers should be in consultation as the investment is large and the performance is important.
BKT USA, Inc.
Dave Paulk: Manager Field Technical Services
Specification Governing Organizations
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 vs Radial
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.
Bias Tires Grow More
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” rim. The size doesn’t tell anything about the height of the tire. Because of 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.
Radial Sizing is Metric
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” rim. This tire should be approximately 68.2” 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.
MFWD Tractor Specs Critical
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 4-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.
2WD & 4WD Less Sensitive to Spec Variance
The rolling circumference on 2-WD and fulltime 4-WD tractors are not so critical. The height and width may be more critical on these to make sure they fit and don’t rub. On any tractor with duals or triples, the minimum spacing between tires needs to be followed.
Varying Tread Depths Alter Specs
There are some differences between R-1’s, R-1W’s, and R-2’s (Rice and Cane) that should be noted. The tread depth on the R-1W is approximately 20% deeper than on the standard R-1. The R-2 tread depth is approximately 2 times the R-1. The section height of the R-1 and R-1W should be about the same. The section height of the R-2 is approximately 6% more than the R-1. This means that the R-2 casing is a little taller.
Summary
Between manufacturers, radials are generally close in size and rolling circumference, but not always. It is always a good idea to do the math and make sure the tires being used match up to minimize any issues that can be avoided. There can be some differences in brands. Most tire dealers that sell agriculture tires are good at determining what will and won’t work.
Maxam Tire International
Greg W. Gilland: Vice President Global Agriculture
The Tire & Rim Association of America, established in 1903, became the governing body that drove to standardization of tires, tubes, valves, flaps, and corresponding rims 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 1960’s to standardize pneumatic tires and rims across the European Union. Today the TRA, as well as the ETRTO and other regional associations, are responsible to standardize and interchange 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 addition to the above design limits, there are other considerations that will affect how the tire design and construction will interplay when compared to the industry tire standards for any given size:
- Inflated tire dimensions (tire geometry) can vary with tire construction based on the material positioning of layers after curing despite meeting the standards requirement by size and tire type: Standard, IF, or VF.
- Radial tires with a steel belt package or belts may have different inflated OD from the same size tire with nylon breakers.
- The tire tread depth based on the lug design affects the overall diameter (OD) of a standard R-1 pattern versus the same sized tire with deeper R-1W, or R-2 tread depth.
- Tire tread depth will also differ from manufacturer to manufacturer which will affect the tire geometry despite tires being of the same size and within the above industry tolerances.
- In addition, tire weight is not mandated in the standard beyond the above dimensional requirements, therefore the type of tire or materials used will affect the deflection and geometry of the tire under load.
- Specifically, in bias ply tires, the different ply rating or load ratings of the same size can result in different inflated overall diameter (OD) based on a given air pressure/tire load.
- Lug design can also influence how the tire under load compares to the industry standard when fully inflated.
- Bead and sidewall optimization by each manufacturer can impact the type and width of the rim recommended that could deviate from the industry standard as an alternative rim fitment.
Below is an MAXAM example of our new VF480/80R50 AgriXtra 85 tire (under development) tire specifications compared to the industry standards from both the ETRTO and the T&RA:
In the agricultural market, it is how the equipment is utilized or the operational requirements that will affect tire design and performance within the industry standards. As agricultural tires are considered “off-road” and not US Department of Transportation (DOT) regulated, there is some flexibility in the design process for each tire manufacturer to enhance features to deliver improved performance for a given size or tread.
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 in the below table are some general considerations related to using tires from different manufacturers on the same machine or axle based on application differences:
Michelin Ag
David Graden: Operational Market Manager – Agriculture
When I was a young Ag Tire Sales Representative, I would often get called out to visit farmers with technical tire issues. I would visit these farms and spend an exhaustive amount of time trouble shooting issues like irregular wear, rapid lug wear, melting tire lugs, etc. One of the rare culprits for these issues was mismatched tire brands and, therefore, mismatched overall diameter tires. At the time, it was strange to me that the sidewalls of these tires stated they were the same size, but the height of one tire was taller than the other. After seeing this a few times, I came to the uneducated conclusion that Michelin was one of a few tire manufacturers that actually sized tires accurately to the actual measurements. As I gained experience, however, I quickly realized my conclusion to this issue was wrong! In fact, tire manufacturers follow strict standards set forth by the ETRTO or TRA.
The European Tire and Rim Technical Organization was founded 1964, but was formerly knows at 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 of these organizations set guidelines to standardize and harmonize rims 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, as described in my earlier scenario, 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.
Due to this standardization threshold, tire manufacturers have to pay close attention to where they are shipping tires, to be sure they follow the proper standards.
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 match 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.
If you need help determining whether a tire match is acceptable or not, please don’t hesitate to reach to your local authorized Michelin Ag tire dealer and/or your local Michelin Ag sales representative.
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.