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.
QUESTIONS: What is rolling resistance and why is it important consideration for fuel usage in transport mode? What are the main contributors to energy loss, how can a producer minimize it and what specific tire attributes can help reduce it?
DAVE PAULK, manager, field technical services, BKT USA Inc.: Rolling resistance, also known as rolling friction or rolling drag, is the resistive force that opposes the motion of a rolling object over a surface. It primarily occurs due to the deformation of the tire and the surface it rolls on.
Smoother surfaces typically offer less rolling resistance compared to rough surfaces. Rolling resistance increases as torque and speed increase. This is why it is important to run proper air pressures in the field to reduce slippage (loss of efficiency) and gain fuel economy. It is equally important to run the correct air pressures on the road to reduce rolling resistance and minimize possible damage to the tires.
Rubber is not a perfectly elastic material. The primary cause of rolling resistance is called hysteresis. This is the deformation of the tire that occurs under a load because it doesn’t decompress as quickly as it compresses. As a tire turns under a load, it is continuously running through these cycles of compression and decompression. This generates heat and is a cause for rolling resistance.
Tread compounding and tire construction can increase or minimize rolling resistance. There are different compounds used for different surfaces and various applications. The body construction of a tire is important depending on the application. Radials tend to run cooler on the road than bias tires. Radials can run with more deflection and traction and less slippage in the field.
The tread’s thickness and shape affect rolling resistance. The thicker and more contoured the tread, the greater the rolling resistance.
Hybrid tires (lower void-to-ratio design) can be used with better results where highway usage is high, but you give up traction in the field.
Chevron- style farm tires give the best traction in the field, but have higher rolling resistance and rate of wear on the road.
It all depends on where the tires are used and the application they are used in.
Rolling resistance is significant in transportation and engineering as it impacts fuel efficiency, energy consumption and overall performance of vehicles and other wheeled equipment.
GREG GILLAND, vice president, global agriculture, Maxam Tire North America Inc.: Rolling resistance is used to explain the physical reaction between two objects in contact acting against each other. In the case of all tires, but specifically in ag tires, it is the force that acts against the forward motion that the tire must transmit against the ground when a forward force or engine power is applied. There are three forces that the ag tire must overcome to move the tractor in the working direction forward or in reverse. This is true in all tire applications:
- X Force is the forward or backward force to transmit the engine torque;
- Y Force is the lateral forces caused by the side-to-side forces as the tire deforms or achieves “plastic deformation” due to the air pressure required to carry the weight, in relation to the Z force or vertical load flexing;
- Z Force is the vertical force caused by the suspended load or vehicle load acting on the tire.
Unfortunately, farmers and growers cannot change the physical nor structural elements of their tires to overcome the “rolling resistance” that is inherent in any tire operation.
However, farmers can implement best practices to achieve improved tire performance and mitigate the impact of rolling resistance.
By inflating tires to the correct cold inflation pressure based on the axle load and required work speed, the tire can absorb the heat and overcome the hysteresis.
The right air pressure will minimize ground compaction and reduce the “plastic deformation” of the soil or ground.
The right air pressure will improve tire wear and reduce sidewall deflection, giving the optimum tire gross flat plate or footprint while minimizing hysteresis or rolling resistance.
The right air pressure will ensure the tire can transmit the forces necessary to overcome rolling resistance, allowing improved fuel efficiency and reduce tire slip.
The right air pressure will ensure greater productivity in the field, as the tire’s footprint and traction will deliver increased traction, overcoming the rolling resistance required to work the field efficiently.
DAVID GRADEN, operational market manager, agriculture, Michelin North America Inc.: Rolling resistance, as it relates to agriculture tires, is far different from rolling resistance related to vehicles primarily working on hard surfaces. In general, rolling resistance is the main contributing factor for fuel consumption.
On both hard and soft surfaces, you’ll want to optimize your air pressures so as to maximize your fuel consumption but preserve the tread life of your tires at the same time.
For ag tires specifically, a higher pressure will produce a smaller footprint on the ground.
Of course, this will generate the best fuel economy — on the road.
When you enter a field, however, that same small footprint won’t roll over the soft soil very well and alternatively, it generates very poor fuel economy.
Historically, farmers and ag tire manufacturers, alike, have had to find a happy medium between the two: set the air pressures for the fastest speed the machine will run down the road and to carry the greatest weight it will operate in the field.
JAMES CROUCH, national segment manager — agriculture and forestry, Yokohama Off-Highway Tires America Inc.: Rolling resistance is the opposing force that a tire encounters as it tries to move forward. The more rolling resistance your tires encounter, the more energy is wasted trying to overcome it — your fuel consumption increases and your tires may build up heat. It is important to note that all tires encounter some amount of rolling resistance, regardless of whether they are on drive axles or free-rolling ones.
Rolling resistance sounds simple at first, but it is actually a complex set of reactions between a tire and its environment — and the rules are different depending on whether you’re driving on soil, mud or pavement.
In the field, a farm tire encounters increased rolling resistance when it creates a trench and has to push soil out of its way to move forward, or when it slips as it tries to gain traction.
That means overinflated tires, narrow tires or tires with poor flotation encounter greater rolling resistance, especially in wet or soft soil. Your best bet for reducing rolling resistance in those conditions is maximizing flotation and traction.
That means operating at the lowest appropriate inflation pressure for your speed and load.
Choosing wider tires or duals is the next step, and mounting VF tires and operating them at the low pressures they’re capable of is the gold standard.
On the road, the situation is almost opposite. You want to reduce friction between the tire and the pavement with a smaller contact patch and less deformation in the tire — basically, more inflation pressure.
Tread pattern can also impact rolling resistance. Deep lugs with lots of space between them — just what you want for traction in soil — can increase rolling resistance on hard surfaces.
Many farmers are put in a bind by rolling resistance because they often have to operate extensively both in the field and on the road.
If you do a lot of both, VF radials and a central tire inflation system (CTIS) unit could be a worthwhile investment that pays off remarkably quickly in better performance, lower fuel bills and longer tire life.
You'll get less soil compaction and be able to run more quickly on the road, too.
CHRIS NEIDERT, ag marketing, training and development manager for Trelleborg and Mitas Tires — North America, Yokohama TWS: The force that resists the motion of a body rolling on a surface is called the rolling resistance or the rolling friction.
As the tire and the ground are distorted, the tire sidewall flexes while the ground compresses, until both opposing forces become equal.
The tire’s rolling resistance therefore results from these two distorting movements, or energy loss.
Railroad cars have steel wheels. On the track, they encounter almost no rolling resistance because neither of the two components — the steel wheels or the steel track — undergoes any distortion, whatever the load in the car.
What tactics can a producer use to minimize energy loss? Running the correct air pressure will be a huge influence on rolling resistance and minimizing energy loss.
We at Trelleborg and Mitas are a huge proponent of running at field pressure and a road pressure.
(Imagine) two tire pictures depicting field pressure and road travel pressure.
One picture (shows) field pressure and you can see the sidewall squatting. This is because it is being operated at slow speed (5 mph) which enables you to run less air pressure to carry the load.
When you get out on the highway you need to increase that air pressure. The higher air pressure at road speed helps the tire to not deform and it reduces heat buildup. The higher pressure reduces rolling resistance and energy loss
