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  Firestone OTR Tires  Solving Motor Grader Road Loping

Factors Influencing Grader Road Loping
Lope (Bounce) is a natural phenomenon in motor graders occurring at resonant speeds that correspond to the natural frequency of the vehicle/tire/wheel unit. Many grader attachments and configurations affect the nature and frequency of the bounce. Other factors include:

  • Weight Distribution of the Motor Grader
  • Inflation Pressures
  • Tire Size
  • Road/Operating Conditions
  • Out of Round Rims and/or Tires

Each of these conditions can act alone or combine to change the natural frequency of the system, thus changing the speeds at which lope occurs and the intensity of the disturbance.

How to Solve Motor Grader Road Loping
1. Evaluate the Basics:

  • The roundness of the tire/rim assembly
  • Uneven Wear
  • Flat Spots

It is normal for Bias ply tires to develop flat spots from sitting for extended periods of time. This can affect ride since the roundness of the assembly is deformed (possibly up to 0.4 inches). Prior to operating the machine in finish grading conditions or measuring tire roundness the machine should be run for an appropriate period of time to warm the tires up.

2. Configure machine to design weight distribution:
Field testing shows that vehicles with an actual weight distribution matching the design distribution were easier to correct than machines that had unevenly distributed weights. Design load distribution for OE Motor Graders tend to be 33% front and 67% rear. Testing has shown that correct weight distribution combined with other factors lead to the best improvement in ride. At worst, the machine should have a weight balance of 30% front, 70% rear. Attachments such as rippers, scarifiers, pushblocks, ROPS, etc. have a pronounced effect on the weight balance of the machine and should be accounted for.

While not always practical for machines in-service due to the added expense of adding counterweights, work at the OE level and the dealer level is important to see that new machines are correctly equipped.

3. Correct Inflation Pressure Settings:
The spring rate of a tire is related to the load and inflation pressure of the tire. The best ride results are achieved by setting the cold inflation pressures to match the actual load per wheel position. It should be noted, however, that these settings are for road transport and light material finish grading only (the common loping field conditions). For slope and ditch work or extreme use, the air pressures need to be increased from standard by 10 to 15 psi.

Grader GVW = 32,000 lbs. with weight distribution of 30% front, 70% rear:

  • Front Tire Load = 5,280 lb./tire
    Typical Air Pressure = 30-35 psi**
    Correct Air Pressure = 24 psi**
  • Rear Tire Load = 5,360 lb./tire
    Typical Air Pressure = 35-40 psi**
    Correct Air Pressure = 25 psi**

**All pressure settings are cold inflation for a 14.00-24 bias ply tire.

4. Additional Solutions:

  • Use Larger Tires
    Larger tires will allow greater loads to be carried with less inflation pressure. The testing revealed that the reduced air pressure requirements helped the ride, while the larger tires provide more traction, flotation and stability.
  • Use Radial Tires When Possible
    Radial Tires, in addition to increased stability, traction, flotation, and cut resistance do not suffer from flat spots as do bias ply tires. This will save the end user the burden and down time of warming up a machine prior to finish grading.

In Summary
Testing has revealed that the combination of weight distribution, inflation pressure, and tire size had the most dramatic effect on reducing road loping. When possible all of these measures should be enacted; however, the use of any one method or combinations of methods will help to improve overall ride. If you have any questions about ride disturbance and how to improve the ride of your Motor Grader, please call (800) 905-2367.