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. INTRODUCTION The torsional stiffness of a NASCAR Winston Cup Series chassis can have a significant effect on its handling [1]. In order for the suspension to control the vehicle’s motion, chassis flexibility must be minimized. Winston Cup chassis are primarily constructed of mild steel tubular and box beam members. The floor-pan and firewall are constructed of thin gauge, steel sheet metal. An illustration of a chassis is shown in Figure 1. Although much of the chassis’ geometry is dictated by NASCAR rules [2], there are several modifications and additions that can be made to significantly alter the torsional stiffness. Much effort has been made to predict the torsional stiffness of alternative Winston Cup chassis designs using finite element analysis (FEA) [3,4,5,6]. In order to validate these finite element models an experimental method is needed to directly measure torsional stiffness. The purpose of this project is to design and build a twist fixture for measuring the torsional stiffness of alternative Winston Cup chassis designs. The rapid measurement of torsional stiffness allows different chassis designs to be evaluated and compared. The measured data may also be used to validate finite element models. The following design constraints were applied to the twist fixture design: 1. ability to twist the chassis in both a clockwise and counter-clockwise direction about the chassis’ longitudinal axis, 2. ability to measure torsional stiffness with a total error (from uncertainty and regression analyses) of less than 5%, 3. adjustable to different chassis geometries including lateral and longitudinal widths of attachment points, 4. repeatable measured data. The following design criteria were applied to the twist fixture design: