Transcript 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. 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.