1. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Body torsion strength requirement
The body has to recover its shape with little to no permanent deformation
during twist ditch maneuver
The twist ditch torque can be obtained by multiplying axle load (W) by half of
the wheel track (t).
The angle of twist can be determined by 2 x deflection divided by width of the
loaded points (w)
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

2. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Torsion stiffness requirement:
To ensure good handling properties
To ensure a solid structural feel and minimize relative deformations –
squeaks & rattles
As a vehicle turns a corner, it will roll and causes a weight transfer. It
then can affect steering characteristics
High body torsional stiffness is required to ensure good vehicle handling
Typical roll stiffness is 1000 Nm/deg while ride spring rate = 23.4 N/mm
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

3. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Let’s view the stiffness system as a series connection of springs
Keff/Kroll = 1.0
Kbody = 10 Kroll
Kbody = Nm/deg for good
handling
For good solid structure feel:
Vehicle torsional frequency from Hz
Torsional stiffness = Nm/deg
Torsion strength = 6250 Nm
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

4. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Load Path Analysis
To determine loads on individual structure elements
With these loads those elements can be designed
Let’s begin with a simple structure i.e.
a closed box.
The box is loaded by a twisting couple
at the front and rear corners
All panels are loaded
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

5. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
- Edge loads & shear flows can be
calculated
AQ = T
A is a coefficient matrix
Q is an edge load matrix
T is an applied torque matrix
Shear flow, q = Q/L (N/m)
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

6. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 1
Determine the edge loads for the torsion case
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

7. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 2
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

8. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 2
Determine the edge loads for the given torsion load case
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

9. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Analysis of body torsional stiffness: Closed box
Energy method will be used to predict
torsional stiffness by taking into account
panel dimensions, thicknesses and
material properties
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

10. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Effective shear rigidity
to predict realistic torsional stiffness where
in reality the body panels differ considerably
from an ideal flat plate
Typically, the body panels are crown shape,
have holes, cut-outs and framework with
flexible joints
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

11. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 3
Determine torsional stiffness of a box van based on:
Given shear rigidity
Effective shear rigidity: rear hatch opening
Data: w = 1400mm, h = 1250mm, L = 2000mm, G = 80000N/mm^2, t = 1mm
Solution:
K = (2x1400x1250)^2 x (1/(2x( ))
= 6.95E+10 Nmm/rad = 1.22E+6 Nm/degree
b) Work done = Energy in the joints
½ x F x delta = 4 x ½ x Kj x theta^2
theta = delta/b, S = 4Kj/b^2, Gt = 4Kj /ab
Given Kj = 0.1E+8Nmm/rad
K = (2x1400x1250)^2 x (1/( ))
= 1.6E+8 Nmm/rad = 2807Nm/degree
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

12. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Analysis of body torsional stiffness: Sedan
Gt = (Q/delta) x (H/L)
Delta is obtained from
FEA
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.

13. DESIGN FOR BODY TORSION
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 4
From Example 2, determine the cabin torsional stiffness with side-frame.
q = 2678/1250 = N/mm
q/T = 2.77E-7 mm^-2
Let Q/delta = N/mm, Gt7-8 = 374.5x1250/2000 =234N/mm (side frame)
A1=A5= mm^2, A2= mm^2, A3= mm^2, A4=872067mm^2
A6= mm^2, A7=A8= mm^2
Gt 1-6 = N/mm
Thus, K = 6.55E+ 8 Nmm/rad = Nm/degree
All materials in this slide are taken from Donald E Malen Fundamentals of Automobile Body Structure Design, SAE International.