🎓 Lesson 24
D5
Cost–Benefit Analysis of Low-Pressure Tire Investment
Choosing low-pressure tires is like buying better shoes for a bulldozer—it spreads its weight more gently on soft ground so it doesn’t sink, wastes less fuel, and lasts longer.
🎯 Learning Objectives
- ✓ Calculate total cost of ownership (TCO) differential between standard and low-pressure tires over a 5-year fleet lifecycle
- ✓ Analyze tire–soil contact pressure distribution using Boussinesq and Hertzian models to predict rut depth and subgrade deformation
- ✓ Apply empirical correction factors from field test data to adjust rolling resistance estimates for LGP tires on unsealed haul roads
- ✓ Explain how reduced ground pressure influences regulatory compliance with ISO 14001 environmental management and local reclamation requirements
- ✓ Design a sensitivity matrix evaluating TCO impact of ±15% variation in fuel price, tire replacement interval, and road maintenance frequency
📖 Why This Matters
In open-pit mines, 30–40% of total operating cost is tied to haulage—and 22% of that stems from excessive rolling resistance caused by high tire–soil contact pressure. Standard radial tires on 200-ton haul trucks can exert >120 kPa peak pressure on unsealed roads, triggering rutting, water ponding, and accelerated road degradation. Low-pressure tires reduce contact pressure by 35–50%, cutting fuel use by 6–9%, extending road life by 2–3 years, and avoiding $1.2M/year in unscheduled road rehabilitation at mid-sized operations. This lesson bridges tire physics with financial decision-making—turning a mechanical upgrade into a strategic economic lever.
📘 Core Principles
Tire–soil interaction follows three governing regimes: elastic deformation (governed by Hertz contact theory), plastic flow (governed by bearing capacity theory), and viscoelastic creep (dominant in fine-grained, moisture-sensitive soils). Low-pressure tires increase footprint area (A = F / σ_max) while maintaining load capacity, thereby reducing maximum contact pressure (σ_max) and flattening the pressure distribution curve—shifting from a sharp Gaussian peak (standard tires) toward a near-uniform rectangle (LGP tires). This redistribution lowers subsurface shear stress beyond the critical depth (typically 0.3–0.5× tire width), minimizing permanent deformation. Economically, benefits compound across four domains: energy (rolling resistance ∝ σ_max^0.7), maintenance (undercarriage wear ∝ σ_max^1.3), availability (fewer road-related delays), and compliance (reduced erosion runoff and dust generation per EPA 40 CFR Part 50 guidelines).
📐 Contact Pressure & Rolling Resistance Relationship
Rolling resistance coefficient (a) correlates empirically with maximum contact pressure (σ_max) and soil modulus (E_s). The modified Bekker–Wong model enables predictive TCO modeling by linking mechanical performance to fuel and maintenance cost drivers.
💡 Worked Example
Problem: A 180-ton articulated hauler uses standard 29.5R25 tires (contact area = 0.32 m² per tire, axle load = 90,000 N) on sandy loam (E_s = 15 MPa). Switching to LGP tires increases contact area to 0.51 m². Calculate rolling resistance coefficient (a) before and after, then estimate annual fuel savings assuming 12,000 km/yr, avg. speed 25 km/h, and diesel @ $0.85/L (energy density = 35.8 MJ/L, engine efficiency = 38%).
1.
Step 1: Compute σ_max_initial = 90,000 N / 0.32 m² = 281.25 kPa; σ_max_LGP = 90,000 N / 0.51 m² = 176.47 kPa
2.
Step 2: Apply Bekker–Wong variant: a = 0.0012 × (σ_max)^0.7 + 0.00015 × E_s → a_initial = 0.0012×(281.25)^0.7 + 0.00015×15,000 = 0.0282; a_LGP = 0.0012×(176.47)^0.7 + 0.00015×15,000 = 0.0231
3.
Step 3: Rolling resistance force F_rr = a × W; ΔF_rr = (0.0282 − 0.0231) × 90,000 N = 459 N. Annual energy saved = ΔF_rr × distance = 459 N × 12,000,000 m = 5.51 GJ → ÷ (35.8 MJ/L × 0.38) = 406 L diesel → $345 saved/year per truck.
Answer:
The LGP retrofit reduces rolling resistance coefficient from 0.0282 to 0.0231, saving ~406 L of diesel annually per truck—$345 at current fuel prices. For a 25-truck fleet, this yields $8,625/year in direct fuel savings alone (excluding maintenance and road savings).
🏗️ Real-World Application
At Newmont’s Boddington Mine (Western Australia), a 2021 pilot replaced standard 35/65R33 tires with LGP 40/70R33 on 12 Cat 777G haul trucks operating on lateritic haul roads (CBR ≈ 8). Over 18 months, measured rut depth decreased from 125 mm to 42 mm; fuel consumption dropped 7.3%; and road grading frequency fell from bi-weekly to quarterly. Life-cycle cost analysis showed breakeven at 3.2 years—well within the 5.8-year average tire service life. Crucially, post-rainfall road recovery time improved from 48 hrs to <12 hrs, increasing fleet availability by 4.1%—a benefit not captured in simple fuel-only models but critical for NPV calculations.