To produce the largest share of the car's downforce at the lowest drag cost by accelerating air under the vehicle, and to do it consistently across the ride heights the car actually sees.
A venturi-style undertray with outboard tunnels and, from CR26I, a center tunnel. Lift is generated primarily by the pressure drop from continuity and Bernoulli through the tunnel throat, assisted by the Coanda effect and by momentum change through the diffuser. A shelf and half-tube along each side seal the tunnel against atmospheric pressure so the low-pressure region is not bled off from outboard. Gurney flaps at the tunnel exits raise the pressure recovery. Transverse stiffeners (a top-hat center spar spanning the tray, elliptical fore and aft spars between the inboard mounts) provide bending stiffness so the tray holds its designed shape under load.
The undertray is where the points are. CR26I's undertray more than doubled CLA against CR25 (1.84 against 0.91) at lower weight (9.48 lb against 10 lb) and improved component efficiency from 10.94 to 18.90 L/D. When the CR26I scope was written, the new undertray was named as the single largest predicted performance gain of the season. Design to the ride height the car reaches, not the ride height on the stand. CR26I was designed at 0.25 in and built at 0.61 in, which cost CLA (1.84 designed against 1.63 as-built) and is the clearest example of why the transient ride-height work matters. 1. Sweep tunnel geometry parametrically in CFD: expansion ratio, choke height, inlet shape, outlet length. Findings were peak efficiency at expansion ratio 5.14, peak downforce at 5.44, flat inlets greatly reducing drag, greater choke heights helping, and outlet length having little effect. Stall is the limit. 2. Add the center tunnel where packaging allows. CR26I outboard tunnel width fell from 11.63 in to 6.94 in because of the wider EV frame, so the center tunnel recovered the lost area. Center section efficiency 25.2, outboard 12.6 on Iter17. 3. Check the design against the aero no-go surface derived from worst-case heave and roll with Suspension. 4. Package around the exhaust, oil pan, jack bar, radiators and sidepods, with inlets and outlets positioned away from direct tire effects. 5. Validate on track with Flo-Vis on the bottom surface and constant-speed CL measurement.
- Maximise downforce per unit drag; the undertray is the most efficient device on the car. - Make the part in one infusion. CR25 used a multi-step honeycomb sandwich panel needing film adhesive; CR26I moved to a single infusion layup, cutting time, consumables and laminate density from 0.351 to 0.309 lb/ft^2. - Reduce weight. CR26E targeted -3 lb using an Innegra and G-10 phenolic core with reduced surface area, taking the tray from 10 lb to 5.5 lb. - Make ride height and rake adjustable independent of the chassis, so Suspension can change setup without moving the aero platform. - Cut attachment and removal time.
- Mold warping. The CR25 mold warped enough to be unusable for CR26I, which forced cross-compatibility with the EV part. Root cause was fiberglass molds from CNC foam plugs; fixed by machining tooling board directly. - Inboard mounting was inaccessible on CR25, making removal slow. CR26I revised the inboard mounting to improve attachment and removal time and cut mount weight from 0.889 lb to 0.556 lb while raising the load requirement from 400 lb to 486 lb. - Waterjet-finished mounting components. The cut surface leaves stress raisers that can propagate; CR26I moved mounting to milled parts. - Disturbed flow at the tunnel outlet from rear wheel wake, visible in Flo-Vis. Not solved, characterised. - Ride height sensitivity. The as-built tray sat 0.36 in higher than designed and lost CLA accordingly.
ANSYS Fluent parametric tunnel sweeps, half-car iteration and full-car PMAP. Steady-state Excel calculation to generate the ride height surface map. Simulink four-wheel transient quarter-car model for dynamic ride height. SolidWorks FEA for mounting and spars. Flo-Vis and tufting for validation. Laminate weight calculator for layup schedule selection.