Cal Poly Pomona Formula SAE Team
The chassis is the third iteration of the 2010 car. The design features a 4130 chromoly space frame with removable rear sub-frame and integrated rocker posts. To begin, a target stiffness value was conceived using a simplified mathematical model of the chassis acting as a torsional spring in series with the wheel rates. A target stiffness of 95% of infinitely rigid was used after examining the 2012 chassis stiffness and its sensitivity to suspension changes. This target value was found to be 1900 ft-lb/degree of deflection. With this in mind, a finite element model was made using MSC NASTRAN. Component contacts and constraints were applied to replicate actual loading cases seen by the chassis. The model meshed with FEMAP and mesh sensitivity plots were created to check for mesh independence.
In rigidity tests performed on the 2012 car, it was observed that the seat provided stiffness to the chassis. The 2012 seat was not designed to be stressed so compliance was found to be in the mounting locations; subsequently, the new seat was designed to be a fully stressed member of the chassis utilizing multiple fasteners and attaching to tubes that were not previously used for mounting. The multiple mounting locations were also used to adjust chassis stiffness by omitting some fasteners. The seat was fully integrated into the finite element model using an anisotropic laminate material which assisted in reaching the target stiffness. The seat is constructed from multiple layers of carbon fiber and Kevlar, and weighs less than 4 pounds.
Chassis weight loss was constrained by templates and tubing size mandates therefore the weight goal was adjusted to focus more on weight location. The location of the driver's center of gravity was lowered two inches which amounted to an overall drop in cg of 0.5 inches.
This year's aerodynamics package consists of two wings, each composed of two elements which were analyzed. All CFD simulations were computed using the k-ω turbulence model with low Reynold's number corrections, moving floor and rotating wheels. Average mesh cell size was 3.2 million cells with a polyhedral mesh configuration. The k-ω turbulence model was selected for its near-wall treatment, which aided in capturing the boundary layer interaction between the slot gaps in the wing elements.
This wing package was selected because it is capable of generating more downforce than a single element wing package, while keeping drag values reasonable. At average FSAE speeds, the Reynold's number averages 6e5 and it never exceeds 1e6 so a high lift, low Reynold's number airfoil was selected, Selig S1223. The current configuration is capable of generating 115 lbs. of downforce while the single element generates a maximum of 102 lb. The wings are made of a 0.25" composite sandwich mounted using adjustable mounts capable of adjusting the angle of attack by 4°. This adjustability is capable of a low downforce configuration and a high downforce configuration which increases downforce by 16.2% and drag by 7.1%. This can greatly affect the handling of the car since the center of pressure will change location as angle of attack and velocity changes. This migration of center of pressure can influence the lateral acceleration during skidpad events, and autocross.
The wing incorporates a unique top-surface style airfoil to reduce the weight of the wings and increase aerodynamic efficiency at low Reynold's numbers. CFD results showed this unique style of airfoil increased lift by 8.4% while keeping drag values approximately the same. Endplate design was influenced by the spanwise pressure distribution along the wing. In order to further increase downforce and reduce drag, the top surface of the airfoil is smooth in texture while the bottom surface is bumpy; this trips the boundary layer into turbulent flow which delays flow separation at high angles of attack.
The nosecone design was influenced by the addition of the front wing. The nosecone frontal area was decreased to reduce parasitic drag and increase the planform area of the front wing. This reduction of nosecone frontal area along with reduction of the area as a whole reduced the total car frontal area from 18.1 sq. ft. to 14.73 sq. ft. This reduction also yielded a change in CD from 1.34 to 1.24 and CL from 2.13 to 2.15.