Every year students from California State University Northridge design and build a race car to compete in the Formula SAE competition. The design philosophy of the 2013 CSUN Formula SAE car emphasized light weight, simplicity and low cost. In keeping with this philosophy a single cylinder engine was used, a spool was used in place of a differential, and 10 inch wheels were used in place of the usual 13 inch wheels. The combined effect of these design decisions was the lightest car in CSUN's history at approximately 340 lbs, a decrease of approximately 80 lbs from the previous lightest car.
The choice of using small wheels and a spool to reduce weight resulted in new suspension design challenges. The choice of 10 inch wheels made the development of suspension geometry more difficult and the use of a spool required special attention to weight transfer in the rear of the car in order to ensure good turning performance. Adams/Car was used in order to meet the unique challenges posed by this design.
Adams/Car helped us meet our kinematic design challenges by allowing for the rapid iteration of suspension design, and providing us with detailed results as to the functionality of the suspension. The suspension kinematics were designed with the primary goals of obtaining a camber gain of one degree per inch of wheel travel, and keeping the front and rear roll centers as low as possible. Many iterations were required to meet these goals within the constraints posed by the small wheels.
In addition to helping us meet the previously mentioned kinematic goals Adams/Car was used to check the steering geometry for bump steer, and to make sure that we met our goal of 100% Akerman. Finally, the camber change throughout the steering travel was analyzed, and many iterations were performed in order to set the kingpin and caster angles.
Understeer was expected due to the lack of a differential. To check this assumption a skidpad simulation with constant acceleration was performed. A bar was added between the two half shafts and joined in order to rotationally lock them. Analysis indicated that the degree of understeer present was lower than expected, and should not preclude the use of a spool.
So far the team has successfully complete multiple drive days with the new car, and understeer has not been found to be a problem. Additionally, the drivers all agree that the new car handles better than our older testing car which does have a differential, justifying the decision to save weight by using a spool.