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Mechanics Corner: Under pressure

Michael Aguon Published on 20 July 2011
Proper maintenance and management of your tires can have far-reaching effects on your operation – from the performance of your equipment to the productivity of your fields. A few pounds of air pressure quickly find their way to your bottom line.

Proper pressure
Proper inflation is not a single number – it’s a factor of tire construction, load, speed and pressure, which is why manufacturers create extensive inflation pressure tables. Radials are designed to operate at significantly lower air pressure than bias tires. As load or speed increase, air pressure should be increased, too. It requires a bit more effort to adjust the air pressure to the load-speed conditions, but it’s worth it.

A study by Frank Zoz of John Deere and Reed Turner of the Alberta Farm Machinery Center found that overinflating radial tractor tires by 8 to 10 psi led to a decrease in peak tractive efficiency and power delivery of 4 to 7 percent – pretty much eating up the benefit of having switched from bias-ply tires to radials in the first place. With diesel around $4 a gallon, every percentage point of efficiency adds up in a hurry.

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When tire engineers design a tire, they factor in tire construction, materials, design, load and air pressure to create an optimally sized and shaped area of tread rubber on the ground, which is called the contact patch. Sidewalls are constructed to deflect just the right amount, and the tread is engineered to create the proper footprint for optimal traction.

The result is a contact patch that supports the load in the best possible way and allows the tire to operate efficiently, minimizing rolling resistance while providing maximum traction force for pulling.

When a tire is underinflated, more of the load is concentrated near the edges of the tire. If the tire is overinflated, more of the load is concentrated toward the center of the contact patch, and the tire footprint becomes narrower and shorter.

At that point of overinflation, you are prematurely wearing your tread down in the center of the tire … and there goes some of your tractive efficiency.

Also, if you’re traveling across a field with overinflated tires – especially on compaction-prone soils – you can also be setting yourself up for compaction-related crop problems for months or years to come. There are many studies relating soil compaction with reduced crop yields.

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Change with the load
As your load changes, your tires’ air pressure should, too. There’s a huge difference between a full manure tank or feed grinder and an empty one. If you’re going to haul that equipment any sort of distance – or across a field – take the time to adjust your air pressure.

Remember that load isn’t just isolated to the implement or trailer. A heavy load on your three-point hitch or drawbar may require you to increase pressure on your tractor tires, too. Also, don’t forget to factor in the weight of a full fuel or spray tank.

Operating on slopes
If life were simple, we could just take the weight of a piece of equipment and divide by four wheel positions to get an evenly distributed load. If you farm flat or gently sloped ground, life may indeed be that simple (at least when it comes to tire inflation).

But if you operate along slopes or tight curves, you have to consider that some of the equipment’s weight shifts to the tires on the downhill side.

If you do a lot of hillside work – or navigate sharp turns or curves – talk to your tire or equipment dealer about proper pressures and ballasting to help you do your job safely. Several resources are available for determining proper tire inflation pressure on sloped ground, including your tire dealer, the Tire and Rim Association Yearbook and the load-inflation tables published by your tires’ manufacturer.

Pay attention to field conditions
Just about everything you do in farming depends on your paying attention to what’s happening on your operation. Tires are no exception.

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Pay attention to your tires. Do daily visible checks and watch for signs of excessive stalk damage, foreign objects stuck in the tread or anything out of the ordinary that could indicate a potential problem.

Tires are like elephants: They don’t forget. Damage from prior operating conditions doesn’t go away – it just accumulates and eventually you will have to remove the tire prematurely from service.

Pay close attention to soil conditions before you drive out onto the field. Do you need extra flotation because of wet soil? If you do, lean toward the lower end of the range of recommended air pressures for your load and speed.

Just be sure not to underinflate your tires and risk causing premature wear and structural damage. Maximizing field efficiency and maximum tire performance is a balancing act.

Monitor your slip
Some wheel slip – in the range of 6 to 15 percent – is inevitable in the operation of your tractor. It depends upon the type of tire – bias or radial – as well as loads, air pressure and ground conditions. Excessive air pressure for the conditions will reduce the tire contact patch and increase slippage.

If you have too much wheel slip, your fuel efficiency is reduced and your tires may wear down prematurely. Check with your tire dealer, equipment dealer or local Extension agent for instructions on how to measure wheel slip by counting tire revolutions, or, if you have one of the newer generation of tractors that monitors wheel slip, consult the monitor.

Proper maintenance of your tires contributes greatly to the safety and efficiency of your equipment and the productivity of your operation. With just a few minutes of attention to your tires every day, you can help ensure operations roll right along. PD

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Michael Aguon

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