Chassis design and analysis

Chassis Load 1. Bending case

Weight of the components distributed along the vehicle frame

2. Torsion case

Upward and downward loads at each axle

3. Combined bending and torsion

Torsion cannot exist without bending as gravitational forces are always present

4. Lateral loading

Vehicle is driven around corner

5. Fore and aft loading

During acceleration and braking

Bending case

Unsprung mass

Dynamic Factors of 2.5 to 3.0 for road going vehicles.Off-road vehicles 4.

Torsion case

• dynamic factors in this case are typically 1.3 for road vehicles. For trucks which often go off road 1.5 and for cross-country vehicles a factor of 1.8 may be used.

Combined bending and torsion

assuming the front track tf = 1450 mm and rear track tr = 1400 mm.The load on the right wheel Re =6184 N, The torque on the body 4328 N-m and RF is 5971 N.′

Lateral loading

Longitudinal loading

Longitudinal loading (Braking)

Asymmetric loading

Allowable stress

Stress due to static load × Dynamic Factor ≤ 2/3 × yield stress

• This means that under the worst dynamic load condition the stress should not exceed 67% of the yield stress

• Bending stiffness

• Torsional stiffness

Chassis types, introductionLadder frames

Cruciform frames

Torque tube backbone frames• main backbone is a closed box section through which runs

the drive shaft between the gearbox and the final drive unit.

• transverse members - resisting lateral loads.

Space frames• Adding depth to a frame considerably increases its

bending strength and stiffness.• All planes are fully triangulated so that the beam

elements are essentially loaded in tension or compression.

Integral structures• This is a structure where the component parts

provide both structural and other functions.• integral structure the whole side frame with its depth

and the roof are made to contribute to the vehicle bending and torsional stiffness.

• relative stiffnesses.

‘Redundant Structure’

Structural analysis by simple structural surfaces method

There are many ways of modelling a vehicle structure.Equivalent Beam Modelsimple structural surfacesComplex models (Computational Model)

• One most useful method was Simple Structural Surfaces. It is possible with this method to determine the loads on the main structural members of an integral structure.

Definition of a Simple Structural Surface (SSS)• A Simple Structural Surface is ‘rigid’ in its own plane but ‘flexible’

out of plane. That is, it can carry loads in its plane (tension, compression, shear, bending) but loads normal to the plane and bending out of the plane are not possible.

Definition of a simple structural surface

Vehicle structures represented by SSS

Examples of simple structural surfaces.

Simple Structural Surfaces representing a box van in torsion

• Consider SSS-2 and SSS-3

The equilibrium of the SSS-2 and SSS-3

SSS-2 (Front cross-beam)

SSS-3 (Rear cross-beam)

Now consider the loads from the cross-beams acting on the left hand sideframe (SSS-6).

Consider the equilibrium of SSS-4, 5, 8, 9, 10.

Examples of integral car bodies with typical SSSidealizations

Simple Structural Surfaces representing a saloon car in bending

Simple Structural Surfaces representing a saloon car in torsion

Computational methods

• Structural analysis is now centred around the Finite Element Analysis method where the vehicle structure is divided into small elements.

• The equations of statics (and/or dynamics) plus the equations of stress analysis and elasticity for each element are solved simultaneously using matrix methods.

• Early models

Complete body Finite Element Model