Tips For Picking The Right Tractor Tire – for Jul. 25, 2011

A few years ago, all you needed to know was a tractor’s rim size when you went to a tire shop to order replacement rubber. Today, things are considerably different. Tire technology has progressed far beyond the days when you only had three ag tire choices: bias ply, bias ply or bias ply.

Now, radials offer a superior alternative, and even the options available within radial tire lines are increasing. Picking the right tire for the job is a little more complicated than it used to be. “[It begins with] an overall assessment of your needs, requirements and the machinery you’re using,” says Michael Burroughs, marketing manager for Michelin tires.

Because tractor ratings have now hit the 600 horsepower mark, getting all that power to the ground without excess slippage or creating serious compaction represents a challenge for tire manufacturers. On top of that, tractors and self-propelled machines are now travelling faster, adding high-speed capability to the list of things on farmers’ wish lists for tire performance. All of those factors continue to drive new tire technology development across the industry.

When it comes to selecting the best tire for field work on high-horsepower tractors, Michael Vandel, farm segment marketing manager for Michelin, says many of those workhorses can be retrofitted with updated tire options to increase traction and reduce compaction without having to change rims.

“As an example, when you look at the 350 horsepower range, there are numerous tractors on the market that are on a standard dual configuration starting with a 520 (metric) or 20.8 X 42 (bias) wheel,” he says. “Let’s say he (a farmer) is worried about traction, but it’s not feasible to spec new wheels. In that case he can look at the Multibib in a 650-65 X 42 or an Omnibib or a MachXbib in a 620- 70 X 42, and those tires will fit on the same rims he has. The only extra expense would be an extension to go in between the wheels for dual spacing. He’ll see a reduction in air pressure of about 20 per cent and a dramatic increase in footprint which will reduce the compaction rate and it will also help with traction.”


Because the technology used to produce bias-ply tires is 40 or 50 years old, they simply can’t compete with the performance offered by any radial. “You’ll see a 20 to 25 per cent increase in footprint by going to radials,” adds Vandel. “That’s mostly due to how the tire casing flexes. A bias tire under load moves as a single unit; as the sidewall flexes, so does the tread area. With a radial, the sidewall is independent of the tread. As it flexes the tread will simply lengthen. With a bias tire as it comes under load, the sidewall will flex but the centre (of the tread) will actually pick up, which is going to reduce your footprint under the same load and conditions.” That is one of the key factors that gives radials their advantage in the field.

But what about equipping a front-wheel assist tractor that may have to do double duty, working in the field and handling front-end loader work? That increases the demands on tire performance beyond just maximizing traction and minimizing compaction.

“My advice to anybody who is using a loader on a front-wheel assist machine is to look for the widest and highest volume air chamber (the biggest tire) you can to put on it,” says Vandel. “When you put your loader on, you have an increase in load carrying capacity to support that weight, but if you remove that loader you have a large air volume allowing you to decrease pressure and increase traction in the field.”


“It’s the volume of air inside the tire that carries the load,” continues Burroughs. “Fundamentally, a tire is a pressure container. As a rule of thumb, the air carries 90 to 95 per cent of the load. The tire, itself, only carries five per cent. The more you can increase the volume of air inside the tire, the more you’re able to reduce the psi (pounds per square inch of pressure) required. PSI has a linear relationship with ground contact pressure. If you have a tire that has 35 psi, you’re putting 35 pounds per square inch of compaction onto the ground. But if you have a tire with 12 or 14 psi, you’re cutting compaction in half.”

And lower tire pressures also make things more comfortable for the operator. “Your tire is your shock absorber,” notes Vandel. So lower psi settings in radial tires allow them to cushion the ride much more than bias tires can.

But while making the jump from bias to radial tires represents a giant leap in performance, new technology in radial design offers further gains. Tires like Michelin’s UltraFlex VF (very high flexion) designs, which were introduced to North America in 2006, allow for air pressure reductions of up to 40 per cent over bias tires, which further reduces soil compaction. “It’s still an emerging technology,” notes Burroughs. But tires like Michelin’s Xeobib model use this design (see the profifield test article in this issue that looks at their performance).

Michelin’s website,, offers an interactive tire selection tool as well as useful information on how to minimize wheel hop, calculate tire pressures or minimize compaction.


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About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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