Aero/suspension design

As I've mentioned, we don't yet know with enough confidence what specific vehicle dynamics behaviour to target or optimise for when designing aerodynamic components. I won't even claim we are entirely confident in the best approach to designing suspension, but we do have a reasonable idea as we've been making suspension for just as long as we've been making cars (which isn't that long either mind you). Combined with the fact that poor suspension will doom an FSAE car more than poor aero, we decided between the aero and suspension teams that suspension would be designed entirely to suit the vehicle concept while ignoring all aerodynamic forces other than pure steady-state downforce. Setting a low downforce goal allows us to do this confidently; it's is the beauty of aiming small to start with, as I've also mentioned. I'll go over some more complex interactions to consider in a future post; 5 year plan. 

The optimal suspension setup initially offered to the aero team already had a bit of progression to keep roll rates in check while avoiding having to design and manufacture anti roll bars. This was a good start, but it meant the chassis was expected to drop 11mm near Vmax; nearly half its available bump travel. Our test track is pretty bumpy, and a few quick tests in our transient/impulse suspension simulation showed it getting worryingly close to bottom-out when our heaviest driver experienced a moderate bump. The progression was increased at the front and rear (by simply adjusting the rocker design) in proportion to the expected aero balance until the chassis only dropped 8mm (equally front and rear), which gave us a slightly more reassuring clearance in the same simulation case (Figure 1). 

Pitch rates looked reasonable and I had no reason to request a change. Positive longitudinal acceleration wasn't a concern, and under 2g of braking the pitch was expected to be 1.4 degrees. If we had a front wing we may benefit from a stiffer pitch rate, particularly for maintaining aero balance, but with just an undertray I didn't expect any significant issues. Roll rates also looked ok, especially after increasing the progression further. At 2g lateral the prediction was 1.9 degrees of roll. Perhaps more than is ideal for an undertray, but I can't really say having not yet run any CFD.

After going through the initial suspension parameters, there was still an outstanding issue with the roll of the car. As it was, a hypothetical undertray filling the entire regularity box between the front and rear axles and sitting flush with the chassis floor plane would impact the ground at its lateral extremities under 1.9 degrees of roll. One solution was to further increase the roll rate but I was told this wasn't really possible at this point without starting to see some significant sacrifices elsewhere (the drawbacks of a non-decoupled suspension setup). A goal of the suspension team this year was to reduce jacking, (when the roll centre is significantly to the outboard of the chassis, such that the car lifts as it rolls). Instead of increasing roll rate, I requested the jacking goal be lifted slightly and worked to evaluate the trade-off between this and achieving an additional 15% usable plan area for the undertray (without having to deviate upwards from the chassis ground plane). In the end we decided to go down this route; it also meant a smaller change in suspension since the previous year, which seemed safer. The end result was 5mm of ground clearance for the outboard tip of the hypothetical undertray with the heaviest driver pulling a theoretical 2.1g through a corner. Figure 2 shows the effect of the jacking.

The final change we requested for suspension was a slight increase in anti-dive geometry. Our goal was to maintain the ground clearance of the region at and immediately forwards of the expected location of the undertray's "throat". This would significantly reduce downforce pitch sensitivity, and potentially also decrease balance sensitivity. The anti geometry can be changed small amounts without significantly affecting other kinematics, and the chassis up until this point was designed to allow room for suspension points to shift within a reasonable tolerance without putting any tubes outside of rules, so this change was easy to make. The result of increasing the anti-dive more is that the centre of the car now lifts slightly under braking (Figure 3), but without increasing minimum ground clearance. The lifted rear will increase the expansion ratio, but as long as the floor is designed to handle this without separation (hence the importance of a braking design/simulation case), there shouldn't be any issue. 

I'm not going to claim all of these changes were worth it for just an undertray (although they might be...time will tell), but these are ongoing designs and things will only get bigger and better. Making sure these changes can be implemented without damaging potential suspension performance/response will be another learning point come testing time.