Studies were conducted to evaluate the quasi-static and dynamic performance of an energy absorbing (EA) and a standard (STD) steering wheel at the lower spoke and rim (LSR) and at the center part of the unsupported rim (USR) locations. In addition, tests were conducted at the USR location of two steering wheels using intact fresh human cadaver heads. Using a custom designed impact drop test system, either zygoma was impacted at the USR location at velocities ranging from 1.73 to 6.71 m/s. Generalized force histories were recorded using a six-axis load cell under the hub of the steering wheel placed at 30 degrees to the horizontal to simulate restrained driver impact in collisions. Steering wheel deformations were recorded using a potentiometer. Accelerometers were placed on the specimen and under the wheel rim at the impact site, opposite to the impact site, as well as at locations 90 degrees from the USR. High speed photography documented wheel kinematics. Our previously published data at the LSR location on the EA wheel (32nd Stapp Conference) was combined with additional human cadaver tests (from this study) to obtain the facial fracture probability distribution at this location of the EA wheel. Facial trauma was assessed in all specimens using gross dissection, plain radiography, computed tomography, and defleshing techniques. Fracture severity was graded according to accepted techniques.
The interface force-time response at the wheel rim typically exhibits bimodal behavior. The first force peak appears to be a function of wheel rim inertia: it is coincident with maximum rim acceleration at the impact site, at a time when rim deflection is small. The second peak appears to be largely driven by the force-deflection behavior of the wheel since it occurs when rim tion is approaching its maximum value and when rim velocity and acceleration have fallen to low values. However, the magnitude of the second force peak often exceeds the value which would be expected on the basis of quasi-static force-deflection behavior alone.
At low impact velocities, the first (inertial) peak is smaller than the second. However, at the higher impact velocities in this experimental series, the magnitude of the inertial peak can exceed the second peak value. Therefore, it is concluded that an understanding of both inertial and stiffness characteristics is necessary for optimal design of steering assemblies protective of the facial skeleton.
Data has been gathered and presented in four discrete sets: EA and STD wheels at LSR and USR locations. Additional biomechanical data was acquired for the EA wheel at the LSR location and combined with the previously reported information to permit a Weibull probability analysis. Results indicated that a force level of 1525 N corresponds to a 50% probability of facial fracture at the LSR location on the EA wheel. At the USR location on the EA wheel, no fractures were detected in seven tests conducted at velocities up to 6.7 m/s with peak forces up to I335 N. Additional tests are necessary on the STD wheel to deduce probability distributions at both the USR and LSR locations.
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