Loads on Vehicle Structures

Author

Professor. Hisham Ibrahim

Loads on Vehicle Structures

Learning Objectives

  • Explain the primary purpose and requirements of vehicle structures.

  • Classify different types of loads acting on vehicle structures.

  • Describe the origin of vehicle loads and their associated dynamic factors.

  • Differentiate between global and local load cases in vehicle design.

  • Calculate combined loading scenarios using appropriate load factors and principles.

Purpose of Vehicle Structures

The primary purpose of a vehicle structure is to:

  1. Support all applied loads without failure

  2. Withstand fatigue loading throughout the designed service life

  3. Maintain structural integrity under various operating conditions

  4. Provide durability and reliable operation

  5. Limit deformations to acceptable levels (both static and dynamic)

The structure must accomplish these objectives while minimizing weight and cost.

Origin of Vehicle Loads

Loads originate from vehicle weight, creating constant static loads. These are transmitted through suspension components. The distribution depends on weight distribution, loading, and suspension geometry.

When the vehicle moves, loads become dynamic. Static loads are multiplied by dynamic factors:

Summary of Dynamic Load Factors Load Case Dynamic Factor Notes
Vertical Symmetric (Bending) 2–3 Higher for off-road vehicles
Vertical Asymmetric (Torsion) 1.3–1.8 1.8 for rough conditions
Braking 1.1–1.84g Depends on vehicle type
Longitudinal Bump 4.5 Wheel size dependent
Lateral (Cornering) 1g max Limited by tire friction
Lateral (Kerb nudge) 1.4–1.75 Dynamic safety factor

Normal Loads

Vehicle loads are classified as global or local.

Global Load Cases

s #### Vertical Symmetric (Bending)

Occurs when both wheels on one axle hit a bump. Bending moment: \[\[\begin{equation} M = \text{Static Load} \times \text{Dynamic Factor} \end{equation}\]\]

Vertical Asymmetric (Torsion)

Occurs when one wheel hits a bump. Torque: \[\[\begin{equation} T = K_{\text{TOTAL}} \frac{H}{B} \end{equation}\]\] Maximum torque: \[\[\begin{equation} T_{\text{MAX}} = P_{\text{AXLE}} \frac{B}{2} \end{equation}\]\]

Longitudinal Loads

  • Braking : Weight transfer to front

  • Acceleration : Opposite effect

  • Bump impact : Large horizontal forces

Force from bump: \[\[\begin{equation} P_H = K_{\text{DYN}} \left( \frac{P_v}{\tan \theta} \right), \quad \theta = \sin^{-1}(1 - H/R) \end{equation}\]\]

Lateral Loads

  • Cornering : Limited by \(Mg\)

  • Kerb nudge : May cause wheel lift-off

Max lateral force: \[\[\begin{equation} F_{\text{LAT}} = \frac{MgB}{2h} \times K \end{equation}\]\]

Local Load Cases

  • Door slam loads

  • Hood and trunk lid loads

  • Seat belt anchor loads

  • Jacking points

  • Towing attachments

Load Combinations

Multiple load cases occur simultaneously. Superposition principle applies: \[\[\begin{equation} Q_{\text{TOTAL}} = Q_{\text{BENDING}} + Q_{\text{TORSION}} \end{equation}\]\]

Impact Loads

Impact loads arise from crash scenarios, causing plastic deformation. Nonlinear FEA is used to simulate structural response, considering strain-rate sensitivity and failure criteria.

Crashworthiness is critical and will be discussed in a dedicated chapter.

Summary

  • Vehicle structures must withstand static and dynamic loads

  • Loads originate from weight and are amplified by dynamic factors

  • Global load cases: bending, torsion, longitudinal, lateral

  • Local load cases affect specific components

  • Load cases often combine and must be analyzed together

  • Accurate load estimation ensures structural integrity