Car design: BIW – resultant of regulations, standards and requirements
Crash energy absorption can be terminal for BIW and for a complete car. Energy dispersion and absorption during high speed crash means unrebuildable total damage for a monocoque structure. However, owing to this, mission of saving occupants life can be accomplished.
BIW static and dynamic stiffness
Preliminary condition to get crash Energy under control is high BIW static stiffness. It provides a good base for a good driving dynamic (car handling) and dynamic stiffness (noticeable sense of comfort) too. At the design stage, global torsional and bending stiffness of the BIW are monitored by Computer Aided Engineering (CAE) called Final Element Analysis (FEA).
During car development process in Europe BIW bending stiffness is considered less important than torsional one. In some sense we could say, acceptable bending stiffness level in Europe is almost side effect of struggle to rich BIW global torsion target. Due to specific conditions (dilatations on concrete made highways and usually much bigger car wheelbase figures), in USA BIW bending stiffness importance seems to be higher than in Europe. All in all, BIW global torsion targets tells us much more about car refinement than bending stiffness figures.
In last 30 years, so called BIW dynamic stiffness analysis (counted in Hz unit) became to be increasingly more important during car development process
Dynamic Stiffness unlike static one is effected by vehicle weight input. Dynamic stiffness analysis is simply FEA simulation run with the aim to define first and second BIW natural frequency (BIW first and second self-excitation frequency modes). It’s also an excellent tool to control local NVH (Noise, Vibration, Harshness) issues effecting occupants comfort, like resonance in a car body generated by vibrating engine or by high speed airflow around the car.
There are actually many driving forces implicating in BIW local or global self-excitation frequency modes. Any driving force being able to excite any component of complete car directly effects driving comfort by vibration or by noise. Local or global increase of static stiffness may or may not stop the car or car component from reaching natural frequency what eventually decrease a driving comfort. Thanks to consideration of the weight factor dynamic stiffness analysis, is simply holistic, getting to the point tool. That’s why, in last 30 years dynamic stiffness analysis became to be prioritized over static one. There is another, down to Earth reason for it. About 30 years ago FEA became to be reliable enough and affordable tool for car manufactures.
Primary task during dynamic load cases analysis is to apply structural modifications to the car body, bringing all type of resonance modes to frequency level higher than frequency of forcing vibrations generated by the road (in general – applicable for torsional or bending modes) or by the airflow around the car (applicable for NVH). It will prevent car from excitation during regular road condition. It is assumed, that target level of BIW natural frequency (with bonded Windshield and Quarter Glass) should be min. 45Hz for major dynamic loadcases. Target figures can actually variate from one car manufacture to another (due to presumed car quality, local road condition or just car producer various experience). Trimmed Body (complete car without wheels and suspension) modal frequency is 1.6-2 times lower.
Reaching or exceeding minimum target value of BIW natural frequency, results in elimination of natural frequency caused by input forces from the road. Additional condition to fulfill is 3Hz separation between achieved results of key dynamic load cases. Requirement is valid both for first and second natural modes. Key dynamic load cases are: first and second mode of global dynamic torsion, global dynamic bending, front lateral stiffness, match-boxing and some more. After static and dynamic stiffness are preliminary on target project can move to next task which is fulfilment of crash safety targets.