PRIVATE CAR STRUCTURES BASELINE LOAD PATHS to demonstrate that a car structure can be represented by SSS to introduce the load paths in a car structure for different load cases
INTRODUCTION The structures of PCs vary according to: Size Vehicle layout & type Design Assembly methods However, there will be similarities on the integral constructions of the PCs For load paths demonstration purposes: A simplified sedan structure is used A simplification of payload is done for further clarity The suspension input loads is also simplified
THE STANDARD SEDAN Consist of: A closed box passenger compartment Floor Roof Sideframes Front and rear bulkheads Windscreen For simplicity, all surfaces are assumed to be plane
Suspension loads are carried on element 3, 4, 7 and 8 Elements 3, 4, 7 and 8 are the simplified version of the inner wing panels
There are also need to add a supplementary SSS element to carry out-of-plane loads Elements 1, 2, 6 and 10 are the examples
SSSs in Standard Sedan 1. Transverse floor beam (front) carrying the front passengers 2. Transverse floor beam (rear) carrying the rear passengers 3. Inner wing panels carrying the powertrain and supported by the front suspension 5. Dash panel Transverse panel between passengers and engine compartment 6. Front parcel shelf 7. Rear quarter panels Carrying luggage loads and supported by the rear suspension 9. Panel behind the rear seats 10. Rear parcel shelf 11. Floor panel 12. Left-hand and right-hand sideframes 14. Windscreen frame 15. Roof panel 16. Backlight (rear window) frame Structure engineer must make his/her own subjective assessment for the model that best represent a particular structure
BENDING LOAD CASE FOR STANDARD SEDAN
Payload Distribution R F F pt R ( L + l ) + F ( L l ) + F ( L l ) R F pt pf l pf + pf F pr l pr pf L + F L l pr ( L + l ) R + R F + F + F + R F pt pf l pr F F l pr pt l pt F l l l
Free Body Diagrams for the SSSs
For bending analysis, it is essential to start at the central floor area Assumed that the passenger loads are carried by transverse floor beam 1 and 2 These elements are supported at the end by forces P 1 and P 2, which is equal but opposite the loading
The powertrain and front suspension loads, F pt and R FL respectively, are carried by inner front wings 3 and 4 Both loads are held equilibrium by forces P 3, P 4 and P 5. Additional SSS, i.e. element 6 is required to support P 4.
Free Body Diagrams and Equilibrium Equations for Each SSS 1. Transverse floor beam (front) P 1 Fpf 2. Transverse floor beam (rear) P F 2 pr 3. Left and right front inner wing panel P 2 2 R FL F 3 pt 2 { R ( ) } FL l 1 Fpt l 1 + l pt 1 P 4 2 h P 5 P 4
5. Dash panel P 6 P 3 6. Front parcel shelf P 7 P 4 7. Rear quarter panels P P R RL F 8 l 2 { R l F ( l l ) }h 9 RL 2 l 1 + 2 2 P 10 P 9
9. Panel behind the rear seats P 11 P 8 10. Rear parcel shelf P 12 P 9 11. Floor panel 13 2 ( P ) 2P P 10 5
12. Left-hand and right-hand sideframes Pl P 6 P1 P2 + P11 P 7 + P13 P12 0 0 ( h ) 0 1 3 + P2 l4 P11l5 P12 2 h1
Shear Force and Bending Moment Diagrams In Major Components Design Implications
Joint at the end of the beam must be suitable for carrying P 1 Centre section has a constant bending moment Must be designed to provide suitable bending properties Note: positive bending moments sagging
Negative bending moment hogging Bending stresses & deflections will be small However, stiffening at the top & bottom edges will be necessary to prevent buckling Outer sections carrying P 6 will probably require swaging to prevent shear buckling
Positive bending moment deflected towards the rear Negative bending moment deflected forward Both shelves must have good bending properties in the centre and adequate shear connections to the sideframe
TORSION LOAD CASE FOR THE STANDARD SEDAN Car is subjected to torsion when a wheel an one side strikes a bump or pothole Causing different wheel reactions on each side of the axle Asymmetric load case Combination of bending & torsion on vehicle For calculation, torsion is calculated separately pure torsion load case
Equal & opposite reactions is obtained at the rear, so that the vehicle is in pure torsion R FT is applied to both front right and left suspension towers in equal & opposite direction R R T FT RT R FT T R S S FT F F S F R S RT R S R T S R
End Structures Front inner fender R P FT FT L 1 R FT P FT L 1 h 1 h 1 TL 1 ( S h ) F 1
Rear inner fender P ( S ) RT RRT L2 h2 TL2 Rh2
Dash T P FT S F Parcel shelf / upper dash X 1 X 2 B P B P FT RT S S F R
Passenger Compartment
Engine bulkhead 1TOP 2 LEFT T h 1 1 1BOTTOM + 2 RIGHT 2 B 1 2 Front windshield h 1 3 3B 0
L Roof 1 7 4B 0 Backlight frame h 1 4 5B 0 Rear set bulkhead T 1 h2 + 6B
R RT F FT S P S P B L + 7 5 1 )... ( ) ( 2 2 1 1 7 7 6 6 5 5 4 4 3 3 2 2 Z h Z h r r r r r r x x + + + + + Sideframes Floor
Summary Baseline Closed Sedan R T S R T S P FT F RT FT TL1 ( SFh 1 ) P ( ) RT TL2 SRh 2 r 2 x1 X 1 X 2 P P FT RT S S F R R B TL 1 B TL h B 1 1 + 2 T 1 3 + 3B 0 h L h 1 7 + 4B 1 4 + 5B 2 0 0 h B T 1 2 + 6 L B P S + 2 1 5 + r 3 7 3 + r FT ( h1 Z) + x2( h2 4 4 + r 5 F 5 P r Z)... R h FT h B 1 RT 6 2 S 6 B R S + r F 7 S 7 R
Structural problems in the torsion case - if any one of the structural is missing, the shear panel load path breaks down - example: the faux sedan-rear seat bulkhead is missing.
Remedies for the faux sedan - replacement with ring frame/a triangulated bay (fig 5.13 pg. 80) - provision of closed torque box in part of structure - provision of true grillage structure in floor
Lateral loading case - is generated when a vehicle travels on a curved path
Lateral loading case Rear structure Front structure Compartment structure
Braking loading - is generated during braking event
Braking loading sideframe structure Rear structure