To locate the wheel relative to the chassis in every direction except the one the suspension is meant to move in, and to carry the resulting loads back into the frame.
Front and rear upper and lower A-arms. Each arm is two AISI 4130 steel tubes with threaded inserts, running from two chassis pickup points to a single spherical bearing holder at the outboard end. The upper arms locate the top mount and, on the front, carry the actuation tab that feeds the pullrod. Because the arms are effectively two-force members, tube sizing is a buckling problem before it is a yield problem. Upper arms run on FK WSSX4TV wide-series alloy grooved spherical bearings, chosen for a shorter height than CR25's and a documented thrust load rating. Lower arms run on Aurora HAB-5TG high-misalignment Grumman-groove bearings, which give the large misalignment angle the lower arm needs for actuation clearance.
Size for buckling with a target factor of safety above 1.8, then check yield. The sizing calculator sweeps outside and inside diameter against critical buckling force, critical buckling stress and actual stress, and returns both a buckling and a yielding factor of safety plus an estimated weight, so the trade is visible in one table. Machine the bearing holder complete rather than welding tabs to it. CR26I moved the upper bearing holders to a three-setup mill part using soft jaws as part of the assembly, instead of welding actuation rod tabs in a jig. That took A-arm build time from over 100 hours on CR25 to about 50 hours on CR26I. 1. Take the geometry points from kinematics and the wheel loads from the load cases: CR26I front outside cornering 648.691 lbf vertical and 786.094 lbf lateral, braking 585.523 lbf vertical and 763.26 lbf longitudinal, combined 768.095 / 785.550 / 444.124 lbf. 2. Resolve loads into each member with Altair HyperWorks MotionView. 3. Size the tubes in the calculator against buckling and yield. CR26I front upper 0.375 in OD / 0.305 in ID at minimum FoS 1.946; rear upper same tube at 2.581; front lower 0.625 / 0.527 at 2.49; rear lower 0.5 / 0.402 at 2.62. 4. Analyse the spherical bearing holder with two FEA setups so the boundary conditions are cross-checked: bearing fixed with loads at the pullrod bolt and arms, then arms and pushrod bolt fixed with a remote load at bearing centre. Validate the tab stress against a hand calculation for a hole centred in a wall (calculated 18.95 ksi against 22 ksi FEA). 5. Weld in the jigging assembly, then measure the finished arm.
- Maintain overall weight within 1 lb. CR26I saved 0.28 lb per rear upper and 0.01 lb per front upper, 0.58 lb across the system. - Increase serviceability. - Decrease compliance. - Improve design and validation tools. - Reduce stiction in the moving assembly.
- Spherical bearing retention. CR25 had a retention problem caused by an improperly designed chamfer in the bearing holder. - Misalignment-induced bending in the rear. CR26I slightly altered the rear geometry to reduce it. - Bearing holder thickness against bearing choice. CR26I used a variable-thickness bearing holder design to accommodate a thicker bearing without gaining weight. - Manufacturing hours. A-arms were consuming over 100 hours a set before the move to fully machined bearing holders.
Altair HyperWorks MotionView for load paths. The in-house tube sizing calculator for buckling and yield. SolidWorks FEA with two independent setups per part plus a curvature-based mesh convergence study (element size growth ratio 1.4). Hand calculation for stress concentration at tabs. Welding jigging assemblies for both upper and lower arms. Staking anvils manufactured to Timken-Aurora 3072G-x dimensions for bearing retention.