Design of a Twist Fixture to Measure the Torsional Stiffness of a Winston Cup Chassis

The torsional stiffness of a vehicle's chassis significantly affects its handling characteristics and is therefore an important parameter to measure. In this work a new twist fixture apparatus designed to measure the torsional stiffness of a Winston Cup series race car chassis is described. The twist fixture is relatively light weight, adjustable, and easily transportable by one person for quick set-up on different chassis. Measured values of torsional stiffness are reported for several different chassis. The fixture applies vertical displacements (using linear, jack-screw actuators) at the front spring perches of the chassis while holding the rear perches fixed. Conventional race car scales located under the front assembly measure the resulting reaction forces due to the displacements. Dial indicators are placed at selected locations along the chassis to measure deflections. Using the dial indicator readings, the measured reaction forces and the chassis geometry, the torsional stiffness of the chassis can be calculated. Ball-joint connections between the twist fixture and chassis have been carefully designed to minimize unwanted rotational restraints. A typical test involves twisting the chassis in increments up to a set point and then untwisting it back to the starting point. The average torsional stiffness value is determined from a least-squares fit. An uncertainty and repeatability analysis of typical data is presented to determine the sensitivity of the stiffness measurement as a function of uncertainty in scale readings, dial indicators and geometry measurements. To help validate the twist fixture, the torsional stiffness of a standard frame structure with a known stiffness value based on an analytical mechanics solution is measured and compared. Tests conducted using the standard resulted in measured values of torsional stiffness slightly higher (about 6%) than the analytical prediction. The difference between measured and predicted values is within the expected uncertainty of material constants, geometry, dial indicator measurements and the assumptions inherent in the analytical solution.