The resistive force opposing vehicle motion through air, increasing with the square of velocity and directly proportional to frontal area and shape. Aerodynamic drag consumes 10-25% of fuel in refrigerated delivery vehicles—R10,000-70,000 annually depending on vehicle size—yet bodybuilders design without wind tunnel testing or computational analysis.
The Physics
Drag force: F = 0.5 × ρ × v² × A × Cd
Where:
- F = drag force (N)
- ρ = air density (kg/m³)
- v = velocity (m/s)
- A = frontal area (m²)
- Cd = drag coefficient (dimensionless)
Power to overcome drag: P = F × v = 0.5 × ρ × v³ × A × Cd
Note the velocity cubed relationship for power. Doubling speed increases drag power requirement by 8×.
Refrigerated Vehicle Drag Problem
Standard refrigerated bodies create massive drag:
- Flat loadbox front wall: Cd ≈ 1.0-1.2
- Cab-to-body gap: turbulent air entrainment
- Roof-mounted equipment: additional frontal area
- Square corners: flow separation and wake turbulence
A 4-ton courier truck with 4m² frontal area and Cd 0.9 at 80 km/h:
- Drag force: 0.5 × 0.95 × 22.2² × 4 × 0.9 = 841N
- Drag power: 841 × 22.2 = 18.7 kW
- Fuel consumption (drag only): 18.7 / (0.30 × 10 kWh/L) = 6.2 L/hr
At R18/L over 2,500 annual highway hours: R279,000/year in fuel fighting drag
Reducing Cd from 0.9 to 0.7 (proper aerodynamic design):
- New drag power: 14.5 kW
- Fuel consumption: 4.8 L/hr
- Annual cost: R216,000
- Savings: R63,000/year
The Condenser Paradox
Everyone assumes larger condensers increase drag. Counter-intuitively, large horizontal condensers mounted on the loadbox wall can REDUCE total drag by acting as fairings that streamline the otherwise flat wall.
The loadbox wall (1.4-2.0 m²) has Cd ≈ 1.15 as a flat plate. A large condenser covering 70-90% of this wall has Cd ≈ 0.65 as a streamlined body. Net effect: reduced drag despite increased frontal projection.
This “fairing effect” saves R2,500-3,500/year in fuel while providing 50-100% more cooling capacity than small roof-mounted units.
Why Bodybuilders Don’t Optimize
Aerodynamic optimization requires:
- CFD analysis or wind tunnel testing
- Custom tooling for shaped components
- Engineering time to integrate improvements
- Higher material and labour costs
Quoting a rectangular box with square corners takes 10 minutes. Optimizing aerodynamics takes 10 hours. Customers don’t demand aerodynamic bodies because they don’t understand the lifecycle cost of drag.
Until operators calculate fuel cost and demand aerodynamic specifications, bodybuilders will continue building expensive bricks.
Related Terms: Energy Efficiency (Cold Chain), Transport Refrigeration Unit (TRU), Total Cost of Ownership (TCO)
Related Articles: The Aerodynamics Tax: R10,000 to R70,000 Your Customers Pay Every Year, Your Truck is a 4-Ton Brick Fighting Wind at 100 km/h