To carry pure axial load between the wheel and the actuation system, or between the steering rack and the upright, at minimum weight.
Pullrods at the front and pushrods at the rear connect the upper A-arm or upright to the bellcrank, converting wheel travel into bellcrank rotation. Tie rods connect the steering rack (front) or a chassis tab (rear) to the tie rod mount on the upright, setting toe and, at the rear, providing the fifth link. Each is a tube with a bonded aluminium insert at each end and a rod end threaded in, so length is adjustable and the load path is purely axial. From CR25 the tubes are Rockwest Composites 3k 2x2 twill CFRP rather than AISI 4130 steel. Modulus is roughly 42,000 ksi and compressive yield roughly 60 ksi at 0.0567 lb/in^3, against 4130 at 29,700 ksi and 63.10 ksi at 0.284 lb/in^3, so the composite tube is both lighter and stiffer for the same section.
Buckling is the primary failure mode for a composite tube in compression, so the sizing method is built around it. The calculator uses a universal slenderness ratio to determine the reduction factor in the critical buckling load for composite tubes, following the ECCS curve fit rather than a plain Euler result. Choose purely symmetric layup orientations. That allows useful assumptions about tensile and compressive properties, and Rockwest's part numbers are verified against their documentation to confirm the layup really is symmetric. Validate the calculator, then trust it. Five test specimens designed to fail at 2280 lb were loaded to 2300 lb or failure; all failed within 5% of the expected load, which validated both the adhesive calculator and the manufacturing process at once. CR25's parts had been designed overly conservatively, which is why CR26I's factors of safety sit well above target (front tie rod 7.82, rear tie rod 9.093, rear pushrod 23.056). 1. Take the axial load in each rod from the HyperWorks load path study. CR26I front tie rod 390 lb over 13.67 in, rear tie rod 95 lb over 10.72 in, rear pushrod 1208 lb over 8.1875 in. 2. Run the carbon component calculator for every available Rockwest part number: it returns tensile, compressive and buckling factors of safety, estimated weight and price against inputs of load, length and target buckling FoS. 3. Size the insert: the calculator returns insert diameter, required length and adhesive factor of safety from tensile strength, load, required area and a minimum L/D ratio. Minimum insert length is 2 x diameter, which maintains the ideal slenderness ratio. 4. Select adhesive. DP420 at 4500 psi listed shear strength was chosen over ET5401 (2900 psi) and EA9460 (3500 psi) because it is already on hand and the required quantity is about 60 g. 5. Manufacture: cut to length with a diamond blade Dremel wheel, surface prep with IPA and sanding, bond at a 0.005 in gap using 0.0049 in silica bond gap controller with a 0.010 in clearance fit on insert diameters, cure 24 hours at room temperature under static pressure. 6. Proof test the manufactured parts: tensile to 1.5 FoS and compressive to 1.5 FoS, pass/fail.
- Large weight decrease and stiffness gain against steel. CR26I actuation rods 684.35 g, tie rods 434.5 g. - Refine and develop the structural analysis tools, and build a new sizing calculator on research data validated by a simple test procedure. - Gain access to a high-capability materials test machine for impact loading, fatigue loading and non-invasive CFRP quality tests. - Test prototype carbon arms, including a test rig and an adjustable arm design that accounts for frame tab accuracy.
- Adhesive tear-out is the failure mode seen in testing, which is why insert length and bond gap are controlled so tightly. - One test sample failed low because its insert was shorter than designed, which is a manufacturing control problem rather than a design problem. - Front pullrod tubes could not be made in carbon on CR26I because of packaging constraints, so that one component stays steel. - CFRP has no visible warning before failure and no ductility, so the design has to carry margin rather than rely on inspection.
The in-house carbon component calculator (buckling by ECCS curve fit, adhesive sizing, insert sizing). MTS Criterion 43 for tensile and compressive validation. Altair HyperWorks MotionView for loads. Rockwest Composites published tube properties based on classical lamination theory.