Interpreting Grade Resistance and Rimpull Charts in the Caterpillar Performance Handbook

[MikePhua]

The Handbook’s Role in Equipment Planning
The Caterpillar Performance Handbook has long served as a technical reference for contractors, engineers, and fleet managers. First published in the 1960s, it compiles operational data, machine specifications, and performance curves across Caterpillar’s product line. By Edition 36, the handbook had evolved into a dense, data-driven guide used to estimate cycle times, fuel consumption, and machine behavior under varying terrain and load conditions.
Its charts—rimpull-speed-gradeability curves, retarder curves, and effective grade calculations—are essential for understanding how machines respond to slope, rolling resistance, and payload. However, interpreting these charts requires a solid grasp of mechanical principles and hydraulic behavior.
Terminology Annotation:

• Rimpull: The horizontal force available at the wheel rim to move the machine forward.
  
• Gradeability: The steepest incline a machine can climb while maintaining traction and speed.
  
• Retarder Curve: A graph showing the machine’s ability to slow down using engine or hydraulic retarders under load.
  
• Rolling Resistance: The force resisting motion due to surface friction and tire deformation.
  

Understanding Effective Grade and Resistance
One of the most misunderstood concepts is “Effective Grade,” which combines slope and rolling resistance into a single value. The formula is:

• Effective Grade (%) = Grade Assistance (%) − Rolling Resistance (%)
  

If a machine is descending a 2% slope (grade assistance) and encounters 4% rolling resistance, the result is −2%. This negative value indicates net resistance, meaning the machine must overcome more friction than gravity provides assistance.
In practice:

• Positive values mean gravity aids movement (downhill)
  
• Negative values mean the machine must work harder (uphill or high friction)
  

Operators often misinterpret the minus sign. In engineering terms, resistance is always a retarding force, so the sign is often omitted in charts—resistance is assumed to be negative.
Recommendations:

• Treat rolling resistance as a constant negative input
  
• Use average values for mixed terrain
  
• Adjust for surface type: gravel (2–4%), soft clay (5–10%), sand (up to 15%)
  

In one training session for a mining crew in Nevada, engineers used a simulator to demonstrate how a 3% downhill grade with 6% rolling resistance still required full throttle due to net resistance. The lesson helped reduce fuel consumption by 12% over the next quarter.
Rimpull-Speed-Gradeability Curve Interpretation
Another area of confusion involves rimpull-speed-gradeability charts. These graphs show how much force is available at the wheels across different speeds and grades. Surprisingly, some users report slower speeds for empty machines than loaded ones—counterintuitive at first glance.
This phenomenon occurs because:

• Loaded machines have more weight over drive axles, increasing traction
  
• Empty machines may spin or lose grip on steep grades
  
• Rimpull is a function of torque and traction, not just weight
  

In articulated dump trucks, for example, the drive wheels sit under the bed. When loaded, the added mass improves traction, allowing better grade climbing. On soft terrain, however, the same weight may cause sinking and increased rolling resistance.
Recommendations:

• Use rimpull charts to determine optimal gear selection
  
• Factor in tire type and inflation pressure
  
• Adjust for payload distribution and axle load
  

In one quarry operation in Chile, recalibrating gear selection based on rimpull charts improved haul cycle times by 8% and reduced transmission wear.
Retarder Curve Application and Misinterpretation
Retarder curves show how machines slow down under load using engine braking or hydraulic retarders. Misreading these charts can lead to unsafe descent speeds or excessive brake wear.
Key points:

• Retarder capacity increases with engine RPM
  
• Payload affects retarder effectiveness due to inertia
  
• Steeper grades require higher retarder force
  

Operators should never assume retarder performance is linear. On long downhill hauls, heat buildup can reduce effectiveness. Using transmission retarder in combination with engine braking is recommended.
Preventative tips:

• Monitor retarder temperature gauges
  
• Use staged braking on long descents
  
• Avoid relying solely on service brakes
  

In one case from Arizona, a haul truck exceeded safe descent speed due to misinterpreted retarder data. After retraining and chart review, the fleet reduced brake failures by 40% over six months.
Conclusion
The Caterpillar Performance Handbook is a powerful tool—but only when its charts and formulas are correctly understood. Effective grade, rimpull, and retarder curves require careful interpretation, factoring in terrain, payload, and machine configuration. With proper training and real-world calibration, these tools can optimize performance, reduce wear, and improve safety. In heavy equipment operation, numbers tell the story—but only if you know how to read them.