To translate the motion of the actuation rod into displacement of the spring and damper, giving a more linear motion ratio through wheel travel and a single parameter to tune that ratio with.
A pair of Al 7075-T6 plates pivoting on the frame, with three connection points: the actuation rod (pullrod at the front, pushrod at the rear), the damper, and at the rear the anti-roll bar drop link. The pivot is in double shear where kinematics and chassis have worked together to allow it, which makes sizing straightforward: find the minimum cross-sectional area that withstands the shear load from the vector sum of the actuation rod and damper forces, with bending moments usually negligible but still checked. Where a single-shear bung has to be used instead, bending dominates, and the welded bung brings a softened heat affected zone, potential weld stress concentrations, and a minimum weld radius that has to be specified and followed. The bellcrank also carries the damper position sensor, an Amphenol Piher PST360G2 rotary hall-effect unit mounted at the pivot.
Deflection, not stress, is the governing requirement. Bellcrank deflection shows up directly as a wheel rate change, so the design target is a deflection goal with stress checked afterward. CR26I front bellcranks run Al 7075-T6 at 0.19 in thick with 57 ksi maximum von Mises and FoS 1.3, which would be uncomfortable if stress were the driver; it is acceptable because the deflection goal was met and the load case already carries a shock factor. Material follows the load. The rears moved from 6061 to 7075 specifically because aerodynamic loads rose, not as a general upgrade. 1. Take the wheel rate target from kinematics; the bellcrank is named by it (CR26I front is the 150 lb/in wheel rate crank, rear the 200 lb/in). 2. Set the architecture: connection points, pivot location, double or single shear. 3. Size the pivot for shear from the vector sum of rod and damper loads. 4. Run single-plate FEA for deflection first, then stress. CR26I rear single plate at 0.19 in gave FoS 3.54. 5. Calculate the wheel rate change that the measured deflection produces, and confirm it is inside tolerance. 6. Fit and validate the damper position sensor.
- Improve the reliability of the damper position sensors. - Validate deflection targets. - Maintain weight within 10%. CR26I fronts 82 g both plates against CR25's 87 g; rears 177 g against 182 g. Total change -10 g, -3.9%.
- Damper position sensor reliability is the long-running problem. CR21 and CR22A used linear potentiometers, which gave poor data from excessive noise and wander, cost roughly $1000 to replace, and were difficult to package and isolate. CR25 moved to a rotary hall-effect sensor mounted between the plates, but retention relied on friction from a 3D print and the data was still unreliable. CR26I's mounting fixed it, giving roughly 0.1 mm damper position resolution with reduced noise. - Weight optimisation is limited by geometry. The bellcrank has to reach three fixed points, so there is little material to remove.
SolidWorks FEA for single-plate deflection and stress. Wheel rate change calculation from measured bellcrank deflection. Amphenol Piher PST360G2 sensors validated against known weights and Hooke's law in the sprung region, which also produces the bump stop resolution figure used by Aerodynamics.