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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Torsion stiffness requirement:
1. To ensure good handling properties
2. 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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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 = 10000 Nm/deg for good
handling
For good solid structure feel:
- Vehicle torsional frequency from 22-25 Hz
- Torsional stiffness = 12000 Nm/deg
- Torsion strength = 6250 Nm
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 2
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 3
Determine torsional stiffness of a box van based on:
a) Given shear rigidity
b) Effective shear rigidity: rear hatch opening
Data: w = 1400mm, h = 1250mm, L = 2000mm, G = 80000N/mm^2, t = 1mm
Solution:
a) K = (2x1400x1250)^2 x (1/(2x(21.9+35+31.3))
= 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
a) K = (2x1400x1250)^2 x (1/(21.9+35+35+31.3+31.3+76553))
= 1.6E+8 Nmm/rad = 2807Nm/degree
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
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. 2011. Fundamentals of Automobile Body Structure Design, SAE International.
SMC 4133 AUTOMOTIVE STRUCTURES
DESIGN FOR BODY TORSION
Example 4
From Example 2, determine the cabin torsional stiffness with side-frame.
q = 2678/1250 = 2.1414N/mm
q/T = 2.77E-7 mm^-2
Let Q/delta = 374.5 N/mm, Gt7-8 = 374.5x1250/2000 =234N/mm (side frame)
A1=A5=1170000mm^2, A2=1103087mm^2, A3=1950000mm^2, A4=872067mm^2
A6=3120000mm^2, A7=A8=2312500mm^2
Gt 1-6 = 80000 N/mm
Thus, K = 6.55E+ 8 Nmm/rad = 11491 Nm/degree
All materials in this slide are taken from Donald E Malen. 2011. Fundamentals of Automobile Body Structure Design, SAE International.