Steer Degree of Freedom Example

The effects on vehicle behavior of responses of the undamaged steering system to external forces can be significant during spinout motions. For example, in an angled rear-ender, the struck vehicle releases the steering wheel, the undamaged steering system responds to aligning torques at the front wheels. While the original form of inputs for steering as a function of time in smac, the steer tables, can be used in such an application, the creation of an appropriate table requires a number of iterative adjustments to insure that the amplitude and timing of the steering system response is properly related to the vehicle side-slip behavior.

In msmac, a simple approximation of a steering system degree of freedom, similar to that in the HVOSM simulation program (see HVOSM Curb Impact Simulations (4 megs) for information on HVOSM Steer degree-of-freedom), has been incorporated. A “pneumatic trail” dimension is used to simulate the aligning torque of the front tires. A friction torque is applied to the steering wheel to approximate the effects of driver restraint of the steering system and/or actional friction in the system.

 

There are two main smac input datasets in the project:

•    Steer_degree_of_freedom.si5

•    No_steer_degree_of_freedom.si5

The inputs for the steer degree of freedom are entered on msmac input cards 25 (veh1) and 26(veh2)

The example demonstrates the input format for the steer degree-of-freedom (SDOF) option. In this example, it is assumed that the impact, which occurs at 0.07 seconds, does not produce any steering system limiting damage on vehicle 1 and that the driver does not apply any steering control.

 

An echo of the SDOF inputs used is as follows:

          S T E E R I N G   S Y S T E M

                                      VEHICLE#1         VEHICLE#2

     MOMENT OF INERTIA       XIPS  =     500.000            0.000 LB-SEC**2-In

     COULOMB FRICTION TORQUE CPSP  =    1000.000            0.000 LB-IN

     ANGULAR STOP POSITION   OMGPS =        25.0              0.0 DEGREES

     ANGULAR STOP RATE       AKPS  =  100000.000            0.000 LB-IN/RAD

     FRICTION LAG            EPSP  =       5.000            0.000 DEG/SEC

     PNEUMATIC TRAIL         XPS   =       1.500            0.000 INCHES

 

          STEER DEGREE-OF-FREEDOM OPTION CONTROL INPUTS:

     INITIAL STEER ANGLE     PSIF  =       0.000            0.000 DEG

     INITIAL STEER VELOCITY  PSIFD =       0.000            0.000 DEG/SEC

      (NOTE:PSIF,PSIFD NOT USED IF TSDOF > 0.0 SEC)

     START TIME OF SDOF      TSDOF =       0.200            0.000 SEC

 

     END TIME OF SDOF      TENDSDOF=    9999.000            0.000 SEC

     FINAL STEER ANGLE,      PSIEND=       0.000            0.000 DEG

 

The inputs for the steering system properties were derived from published measurements made for the HVOSM steer degree-of-freedom model (see HVOSM Curb Impact Simulations (4 megs)) as part of a follow up study of the HVOSM curb impact option (see McHenry 1981 (2 megs)). The m-smac SDOF model is based on the HVOSM SDOF model.

Please note that the suggested values and ranges should be used as starting values. Some applications which use the SDOF may require values that are outside the suggested ranges.

 

 

Figure 77 Comparison of rear impact without (left) and with (right) steer degree of freedom option

 

Figure 78 heading and angular velocity responses of rear impact without steer degree of freedom option

 

Figure 79 rear impact with steer degree of freedom option

 

 

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