The three general vehicle motions that I used for kinematic design were used once again. While the rear wheels do not steer, the complex nature of suspension geometry can cause the rear wheels to experience a steering-like effect in heave or pitch, a phenomenon called bump steer. This can cause instability in braking or acceleration, harming vehicle balance and reducing driver confidence. In the front, the same principle applies, with an additional goal of creating geometry that enables ideal ackermann, as discussed in my tire analysis. 

Shock absorbers are responsible for the ride quality of the vehicle and maintaining a stable platform while driving. 

In a similar manner to kinematic design, collaboration with the aerodynamics subsystem was paramount in this phase of suspension design. Using aerodynamic efficiency plots, I was able to determine the maximum allowable pitch and roll angles before aerodynamic performance experienced severe drop off. In tandem with G-force data from the previous year's car, I developed target pitch and roll gradients. These two numbers represent the amount that pitch and roll angle change with lateral and longitudinal G-force. This number would allow me to develop relationships between spring stiffness and pitch/roll gradient.

Steering goals were developed from previous steps. Target ackermann angle was established in my aforementioned tire studies, and target toe variation in pitch and heave was set to zero. This is often a rule of thumb in automotive design, aimed at maintaining vehicle stability and performance. Additionally, in prior testing, toe compliance resulted in braking behavior that drivers strongly disliked, validating the choice to neutralize bump steer.

When considering how to design the car's actuation package, I had to consider two main factors. The first, and most important, was chassis compatibility. Ensuring the shocks were mounted in a location that would support suspension loads was key, as was making sure the package was rules compliant. The other important consideration was motion ratio. This is the ratio between wheel travel and shock travel, which can be altered based on geometry. A motion ratio of 1:1 was selected based on testing data, which showed that a 1:1 ratio would result not only in ideal suspension and shock travel, but also would eliminate additional conversion factors in future calculations.

In OptimumK, I set up the targets and configured a set of points based on recommendations from the chassis designers. The overall layout we ran optimizations on was a cantilevered suspension design with shocks mounted in-line with the wheel centers. This configuration resulted in easier accessibility to the shocks, minimal interference with the cockpit, and also allowed for easier manufacturing. After some adjustments to mounting location based on feedback from the chassis subteam, I reran the optimizations to finalize a set of points that would serve as shock mounting locations, bellcrank dimensions and pushrod mounting locations.

In the rear, the main purpose of the toe rod is to prevent the rear tires from experiencing bump steer. To eliminate bump steer, I needed to ensure that kinematic alignment is retained throughout the range of wheel travel. This was achieved by aligning the toe rod with the rear instant center, and also ensuring alignment with the ball joint axis on the inboard and outboard side. This ensures that the relative lengths of all the suspension linkages are retained, eliminating the possibility for the toe rod to push the wheel outwards or inwards. For this reason, I aligned the rear toe rod points on the inboard and outboard side with the upper and lower control arm points established previously. Verification in OptimumK's simulation module showed effectively zero toe deviation in both heave and pitch.

In the front, the tie rod geometry is responsible for ackermann steering, as well as bump steer control. Bump steer, as mentioned, can be controlled with tie rod placement, and ackermann can be controlled with relative upright and steering rack placement. To achieve neutral steering, as prescribed in my tire analysis, I aimed to align the outboard pickup point for the tie rod to be directly in line with the steering axis intersection point. To iterate through many combinations of these two variables, I used OptimumK's optimization module to find an arrangement that achieved both goals. Observing ackermann in steering and toe in heave and pitch validated the geometry.

Determining the material and sizing of the pushrods and tie rods was done in a similar fashion to suspension sizing. Using unit vector representations of the linkages and decomposing vertical and horizontal loads allowed me to calculate the force components in each part, and find a combination of material and size that provided sufficient factor of safety. The tubes were manufactured in the same way as the suspension arms, with welded hex inserts to attach threaded ball joint rod ends. 

Another part worth noting here is adjustability. Compared to control arms, pushrods and tie rods require more adjustment and attention during vehicle testing, and therefore need method to adjust their lengths. The method chosen was a turnbuckle, which was threaded into the linkage and the ball joint. When loosened, the turning the turnbuckle would result in a lengthening or shortening of the linkage.

The bellcrank, also known as a rocker, is the component responsible for transmitting the mostly vertical suspension load to the mostly horizontal shock. This required me to design a triangular piece that attached three points: the pushrod, the shock, and a pivot point that would allow this motion transfer to take place. One difficulty is making a part that could accommodate three mounting points in double shear, and for this reason, I elected to use a two-piece design. This could be made easily using a waterjet, and would allow for a more simple assembly process. Once a general outline capturing the three points was developed, I performed several FEAs with calculated suspension loads. To do this, I calculated the maximum reaction force exerted by the shocks, and applied this in combination with the maximum pushrod load. I gradually removed material to save rotational weight while maintaining appropriate factor of safety. I addressed areas of stress concentration with larger radius fillets, reducing the likelihood of developing stress cracks over time. The bellcranks were then waterjetted, post processed using a reamer and bearing press before being fitted to the car.

Another high load component in the suspension assembly are the tabs that connect the shocks to the chassis. Once suspension loads are transmitted through the bellcrank, they end up being reacted by the shocks. However, the shocks require a rigid connection so that they can behave in the way they are designed. I designed tabs that could be welded to the chassis, and ran FEA simulations to ensure they did not fail under load. Another challenge is spacing the tabs on the car so that the shock is installed at the correct angle. I created a series of 3D printed jigs based on existing references, and used them to mock up the tab placement on the car. This allowed for more precise welding, reducing the chance of part failure due to manufacturing errors.