Body

The body protects occupants, shapes airflow and defines the car's look. Learn body-in-white (BIW), closures and trim, crashworthiness (crumple zones, safety cage), aerodynamics (drag, lift), materials and lightweighting, and corrosion and NVH.

The body is where engineering meets design: it must look good, keep out water and noise, carry loads, slip through the air and — most importantly — protect people in a crash.

Crash structures are designed to crumple progressively at the front and rear while a strong safety cage keeps the cabin intact.

An analogy

A crumple zone is like catching a ball with soft hands: extending the stopping distance lowers the force on the occupants.

Body-in-white and closures

BIW: the welded structural shell before paint and trim (≈ 250–400 kg for a car). Closures: doors, bonnet, boot lid, tailgate. Trim: interior panels, seats, glass, sealing.

Crashworthiness

Front and rear rails fold progressively to absorb energy; the passenger cell (pillars, sills, roof rails in ultra-high-strength steel) resists intrusion; restraint systems (belts, pretensioners, airbags) control occupant deceleration. Tests: Euro NCAP, IIHS, NHTSA — frontal offset, side impact, pole, roof strength, pedestrian protection.

Aerodynamics

Drag power grows with v³, so at motorway speeds aerodynamics dominates energy use. Typical C_d: SUVs 0.33–0.40, sedans 0.25–0.30, efficient EVs 0.20–0.23. Techniques: smooth underbody, tapered tail, wheel air curtains, active grille shutters, flush handles. Lift affects high-speed stability.

Materials, corrosion and NVH

Mixed materials: mild/HSS/AHSS/press-hardened steels, aluminium, magnesium, CFRP, plastics. Corrosion protection by galvanising, e-coat and sealing. NVH: body panel damping, acoustic glass, cavity foams; aerodynamic noise from mirrors and seals becomes dominant in quiet EVs.

Formulas

• Crash energy: E = ½ m v²
• Average crush force: F_avg = E / s, deceleration = F_avg / m
• Aerodynamic drag: F_d = ½ ρ C_d A v², P = F_d v
• Mass–fuel rule of thumb: −10 % mass ≈ −6 to −8 % fuel/energy use

Worked example: Crash energy and crumple zone

Given: 1500 kg car at 56 km/h (15.6 m/s) into a rigid wall; Crush 0.6 m

Find: Energy, average force, average deceleration.

• E = ½ × 1500 × 15.56² = 181.5 kJ
• F = 181 500/0.6 = 302 kN
• a = 302 000/1500 = 202 m/s² ≈ 20.6 g

Answer: 181 kJ, 302 kN, ≈ 21 g average

Worked example: Drag power

Given: C_d = 0.30, A = 2.2 m², 120 km/h

Find: Drag force and power.

• F = ½ × 1.2 × 0.66 × 33.3² = 440 N
• P = 440 × 33.3 = 14.7 kW

Answer: 440 N, 14.7 kW

Common mistakes

• Wrong: Making the whole body as stiff as possible for crashes. — Right: Crumple zones must deform; only the cabin must stay rigid.
• Wrong: Ignoring underbody and wheels in aero work. — Right: Underbody and wheels cause a large share of drag.

Applications

• Safety ratings: Body structures engineered for 5-star NCAP results.
• EV efficiency: Low-drag shapes add tens of kilometres of range.
• Lightweighting: Aluminium closures and AHSS cut mass without losing strength.

Key takeaways

• Crumple zones absorb E = ½mv² over crush distance s.
• Rigid safety cage protects occupants.
• Drag power ∝ v³ — aero matters at speed.
• Mixed materials balance weight, cost and safety.

Going further

• Crash simulation: Explicit FE (LS-DYNA, PAM-CRASH) predicts crush, intrusion and dummy loads.
• Active aerodynamics: Moving spoilers, shutters and ride-height control trade drag and downforce.
• Pedestrian protection: Deformable bonnets and bumper energy absorbers reduce injuries.