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Reinventing the Radial

Michelin Ag says it has taken radial technology to the next level.

Last month the company rolled out its new IF800/70R38 Axiobib radial tire with Ultraflex technology, first previewed last year at the National Farm Machinery Show (see April 2006, page 38). The tire represents a new category of radials with “IF,” which stands for “increased flexion.” IF provides large-volume tires (those wider than 520 mm) with the additional flex or “bulge” in the sidewall they need to support the weight of new 4-wd tractors — without requiring more air pressure.

The technology, which Michelin has branded “Ultraflex,” allows its new Axiobib tire to run at 20% lower inflation pressures, or carry up to 20% more load at the same inflation pressures, as standard radials. “We think that going from standard radials to Ultraflex could be as big a step as going from bias to radial,” says Kevin Lutz, technical manager for Michelin North America Agricultural Tires.

To illustrate what the new technology can mean for tire buyers, Michelin invited journalists to its headquarters in Greenville, SC, for a two-day training and test drive. The new Axiobib tires with Ultraflex were put on a prototype of AGCO Challenger's behemoth 570-hp MT975B tractor, which will be the largest 4-wd tractor in the world when it goes into production later this year.

Tire anatomy

“We sometimes assume people know a lot more about tires than they do,” Lutz says. “And one of the things we assume they know is what a radial tire is.”

He explains that the radial tire, which Michelin patented in 1946, is made from layers of rubber-coated plies of fabric. The plies are laid side by side at a “radius,” or 90°, to the center line of the tire. Additional material is then wrapped around the top of the tire to reinforce the crown or tread area.

Bias tires, in contrast, are made with plies that are laid crisscross over each other. These plies form both the tread and sidewall. Because all the layers in a bias tire are interconnected, the sidewall cannot flex or “bulge” without shifting the whole tire. In radial tires, because the crown is built separate from the sidewall, the sidewall can flex and the tread can stay planted on the ground.

Sidewall flex is important in tires because it allows for better performance, Lutz says. He says radial tires, known for their sidewall bulge, have been proven to provide superior traction, better fuel economy, better ride comfort, improved handling, and longer life than bias tires.

Market penetration

Because of these advantages, 99% of passenger car tires today and 95% of over-the-road truck tires are radial, according to Michelin figures. Ag tires, in comparison, are only 30% radial because not all farm vehicles require the same level of performance. For example, tires on chore tractors and small combines are still predominately bias.

However, the percentage of ag radials jumps to 90% for tractors over 120 hp. “If you look at the 20% of farmers doing 80% of the production, 90 to 95% of the tires on their primary production tractors are radial,” Lutz says. In those applications, radial tires are critical because of their ability to minimize soil compaction.

Soil compaction is caused from downward pressure. Radial tires create less ground pressure because their sidewall flexibility allows for lower inflation pressures. When air pressure is reduced, the tread of the tire lengthens out, resulting in a bigger footprint. A larger footprint distributes the weight of the tractor over a larger area, thereby lowering ground pressure.

Lower ground pressure reduces compaction, which in turn can lead to higher yields. According to a four-year continuous corn study conducted by Iowa State, corn grown on soil farmed with equipment exerting 6-psi surface pressure yielded 9 bu./acre more than corn grown with equipment exerting 16-psi surface pressure. On a 1,000-acre farm, a 6 bu./acre yield advantage results in $18,000 in additional revenue if corn is priced at $3.00/bu.

Story Continued on Next Page >
Horsepower jumps

The standard ag radial sold today was designed for a 400-hp tractor. “The 800/70R38 was reverse-engineered,” Lutz explains. “[That is], if I have a 400-hp tractor, what size tire do I need to be at 8 psi in front and 6 psi in the rear, which are the optimal numbers to minimize soil compaction? But what's happened?”

He says that, in the last five years, horsepower ratings have taken a leap in the category of 4-wd tractors. The most recent examples are a 530-hp tractor released in 2005 and the new 570-hp Challenger MT975B.

Increased horsepower requires a corresponding increase in tractor weight to transfer power to the ground. As a rule of thumb, 100 lbs./hp of tractor weight is required, according to Lutz. “So if you have a 570-hp tractor, it should weigh about 57,000 lbs.,” he says.

The inflation pressures required to support that weight are 14 psi front and 10 psi rear (see chart). “The question we asked is, Are farmers who are used to running at 8 and 6 psi going to like 14 and 10 when they go to a bigger tractor?” Lutz says. “Not if they are concerned about soil compaction.”

Lutz says a new type of radial tire had to be designed that could support the weight of bigger tractors without requiring higher air pressures. The result is the IF category radial tire, which Michelin is the first to market.

The IF800/70R38 Axiobib tire with Ultraflex technology is dimensionally the same as the standard 800 radial. But it has several major differences that allow it to carry up to 20% more load at the same or lower inflation pressures as those recommended for standard radials. These differences include:

  • A flatter crown profile and a special rubber compound that provide longer service life and soil protection;

  • Reinforced shoulders that reduce casing distortion;

  • A larger deflection zone — the area on the sidewall that allows the tire to flex.

Is it for you?

The Axiobib can be used for any application that calls for large-volume tires. Michelin expects the biggest market to be 4-wd tractors because of their recent growth in size, horsepower and weight. It expects demand to be less for manual front wheel assist (MFWA) tractors, where load weights are typically less than those of 4-wds.

Large combines are a potential market, but the current IF sizes are not typically used on combines. “We have advanced radial technology that meets the need right now,” Lutz says. “We have the only steel belted radial combine tire on the market.”

The Axiobib is being sold through the original equipment market. Michelin plans to make it available through the replacement market later this year. Michelin tires will be standard on the Challenger MT900B series 4-wd with the Axiobib available as optional equipment. The tires are available in five sizes at a suggested list price of about 20% more than the price of standard radials.

Michelin will continue to make its standard radial, the MachXbib, for tractors whose weight puts them at the recommended pressures of 8 and 6 psi. “Or if soil compaction is not a concern, the MachXbib is a good option,” Lutz says. “But if you own a big tractor and your tire air pressures have gone up, we now have this new tire option.”

For more information, contact Michelin North America Inc., Dept. FIN, Box 19001, Greenville, SC 29602, 888/552-1213, visit or, or circle 102

IF vs. standard radial

I tested the new Axiobib tires with Ultraflex on the 570-hp Challenger MT975B. The tractor was set up with two different sets of tires: the Axiobibs on the left and standard 800 radials by Firestone on the right. Both brands were inflated to their recommended pressures, which means the Axiobibs had 20% lower pressure.

Bob Reese, Michelin's manager of survey tires, guided me through the two-part course. On the first leg, railroad ties laid across half of the road to simulate rough field conditions. I drove over the ties in 10th gear with the Axiobib tires first, then doubled back to test the standard radials. With the Axiobib tires, I could feel the tread spread out over each tie, which dramatically eased my ride. I swayed back and forth as we drove over the ties but remained in full control of the wheel.

When I turned the tractor around to make the same run with the standard radials, the difference in ride was dramatic. The higher air pressures and shorter footprint magnified the uneven road, resulting in an extremely bumpy ride. My body jerked with each bump, and there were a few times when I almost slid off the seat. All the jerking resulted in instant back pain.

On the second part, Reese instructed me to shift up to 16th gear to simulate road conditions. One side of the road was deeply rippled so I could assess the tires' handling ability. Even at top speeds, the Michelin tires were able to hug the road despite the ruts and gravel. I kept the wheels straight and stayed on the road without a problem. When I doubled back to test the standard radials, I experienced noticeable wheel slip, which made driving more challenging. The ride was rougher, too, causing my compromised back to hurt even more.

The whole drive lasted only about 20 minutes — far short of a typical 12-hour day in the field. But it was long enough to show what a difference the Ultraflex makes in ride comfort, performance and handling.
Jodie Wehrspann

Pressure comparison: Standard tires vs. Ultraflex

Year to market

Tractor hp

Standard tire








8 psi

6 psi

6 psi

6 psi



12 psi

9 psi

9 psi

6 psi



14 psi

10 psi

11 psi

7 psi

Source: Michelin Ag

Goodbye tracks?

Does putting its large track tractors on wheels mean Challenger is moving away from tracks? No, says Paul Sparenberg, product marketing specialist for AGCO Challenger. “Our story for years has been about better traction and lower compaction,” he says. “That story doesn't change with the expanded product range.”

But he says there is a large demand for articulated 4-wd tractors for various reasons. “People are either not familiar with tracks or are in areas not conducive to track advantages,” he says. “So they want tires. And we have answered the call.”

When track tractors first came out in the early 1980s, experts predicted tracks would claim 70 to 90% of the tractor market. “Although track adoption is growing, roughly 60% of the market is still wheels,” Sparenberg says. “So the demand is out there.”

Challenger hopes to claim 12 to 15% of the wheeled market with the new MT900 series. At the same time it believes its wheeled offerings will help sell more tracks by making more farmers aware of both options. “In the past, some farmers saw Caterpillar and Challenger as only track brands,” Sparenberg says. “And if they didn't want tracks, they didn't see the need to talk to us. Now some of the guys will talk with us. And we can show them the benefits of both tracks and tires.”

Challenger MT900B tractors will be released to dealerships this spring. They will come down the same production line as the Challenger track tractors made in Jackson, MN. Suggested list price will be about $320,000, roughly 8 to 10% less than the price of their counterpart track tractors.

For more information, contact AGCO Corp., Dept. FIN, 4205 River Green Pkwy., Duluth, GA 30096, 770/813-9200, visit or, or circle 101.

Tire replacement tip

“If you are only going to buy two new tires, where should you put them?” asks Michelin test driver Mac Demere during a crash course on car tires that the company gave for journalists at its Laurens, SC, Proving Grounds.

Demere says most people, including those who install tires for a living, will say “the front.” But the correct answer is the rear because the rear tires are what stabilize the vehicle. “Without stability, without the rears having grip, steering and braking are useless,” Demere says. “And this is whether you are driving a car, truck, van, SUV, front-wheel-drive or rear-wheel-drive vehicle.”

To prove it, Demere took us to a wet track, where high-pressure sprinklers provided a steady inch of standing water. He told us to put helmets on and take a spin at 50 mph with two different cars: one with the new tires placed in front; the other with new tires on the rear.

“On the car that has the new tires on the rear, the front tires will lose grip first, making it impossible to steer,” Demere says. “Your natural reaction will be to lift off the gas and leave your hands where they are. And most likely it will be a non-event. On the car where the good tires are placed in front, the rear tires will lose grip first and the car will start to fishtail.”

On that note, I signed a liability waiver and started to drive the car with the new tires installed in back. As Demere predicted, the drive was basically uneventful. When I reached my top speed of 50, I could feel the front tires hydroplane as the water from the sprinklers exceeded the tread depth of the tires. But with a quick touch of the brake, and a firm grasp of the wheel, I was able to keep the car on the track.

Then it was time to switch cars. Same drill. Only this time, the car did a 360 despite all my attempts to correct for it.

Demere says the reason is that new tires have a deeper tread depth and will grip the road and channel water more effectively than worn tires. “If all four tires have the same tread depth, the tires will hydroplane at the same time and it will be no big deal,” he says. “But if the front tires are newer, the rear tires will lose their grip first and the average driver will be unable to control the vehicle.”

The message for tire buyers, Demere says, is “insist that the two new tires be installed on the rear of the vehicle.”
Jodie Wehrspann

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