To change lateral load transfer distribution so the mechanical balance of the car can be moved toward understeer or oversteer, and to raise the roll rate of the chassis so the aerodynamic platform stays where it was designed to be.
A torsion bar mounted transversely across the car, with an actuating blade at each end connected by a drop link to the bellcrank. When the two sides of the axle move together the bar does nothing; when they move differently the bar twists and reacts the difference. The effective stiffness is tuned by sliding the drop link along the blade, which changes the blade's effective length and therefore its bending stiffness in series with the bar's torsional stiffness. CR26I runs a rear bar only. The front was deleted because it could not be packaged against the frame, the aerodynamic package, the steering rack and the cockpit template.
For a system that exists mainly for aerodynamic stability rather than mechanical balance tuning, the sizing process is: determine the maximum tolerable roll angle, determine the rough maximum lateral capability, find the roll rate without an ARB, determine the ARB spring rate needed to raise that roll rate enough to keep roll acceptable at maximum lateral, then design adjustable rates centred on that value. Centre the adjustment range on the last car's maximum. CR22 ran its rear ARB at maximum stiffness, so CR26I's nominal middle setting reproduces that stiffness and the adjustment goes both ways from there. 1. Take the roll rate targets and the LLTD range from vehicle dynamics. 2. Size the torsion bar. CR26I: AISI 4130, 0.5 in OD, 0.33 in ID, 68.12 lb-in/deg torsional stiffness, contributing 1160 lb-in/deg (96.7 lb-ft/deg) to roll rate at nominal. 3. Design the blade and set the drop link positions. Five settings give blade lengths from 4.43 in to 5.57 in and roll rate contributions from 113.8 down to 72 lb-ft/deg. 4. FEA the blade at each setting to confirm deflection and the resulting change in roll rate contribution (1.24% to 1.34% across the range). 5. Design retention: a machined square boss and bore to minimise backlash, a threaded stud for blade retention, and a transition-fit needle bearing for bar mounting. 6. Confirm the resulting LLTD range on the car.
- Improved reliability. - An adjustability range of 3% LLTD. - Keep the assembly serviceable in disassembly.
- The front ARB could not be packaged on CR26I. The vehicle consequences are stated rather than glossed: less adjustability in front axle stiffness, less opportunity to reduce understeer, and no ability to test roll stiffness against bump stiffness independently. - Rear adjustment range is limited by packaging against the drivetrain. - Backlash in the bar-to-blade connection, addressed with the machined square connection.
SolidWorks FEA for the blade with mesh control at the fixture. Torsion bar sizing calculation. LLTD calculation across bump stop states (CR26I: 39.55% front with no bump stops, 41.22% partial, 48.20% bump stops only). INA-SCE88 needle roller bearing selection (2000 lb static rating against a 200 lb maximum static load seen).