Prototyping
There are nearly infinite design considerations which influence the real world performance and predictability of a vehicles suspension system. To further complicate design, Ultra 4 cars have to perform in a variety of different racing environments, within a single race. Because of this we spent considerable amounts of time researching and prioritizing the suspension geometry and dynamics.
In order to finish a race, a car has to at bare minimum hold together and stay upright. Considering this we decided that the racing environment, where suspension geometry stood to hurt us the most was high speed cornering. Here, more than anywhere, a suspension-link-end that is even a few inches out of place could send a car unpredictably hurdling out of control, and out of the race.
One of the most important considerations for predictable high speed cornering, is the relationship between a vehicles center of gravity, and what’s known as it’s roll axis. Roll axis can be described as a line drawn on a side view of a vehicle through the points where body pivots around the front and rear suspension systems. Time and experience have proven that by placing the front roll center below the vehicles center of gravity, and higher than the rear roll center, will make the car the most predictable in a near rollover situation. Because the front roll center is higher than the rear roll center, under heavy cornering, or a rollover situation, the front inside tire will lift before the rear inside tire. This does two things; 1) provides feedback to the driver that the car is close to rolling over. 2) A higher roll center in the front than in the rear, will induce under-steer limiting traction, and keeping the vehicle from rolling.
We used this spreadsheet based calculator to design the relationship between our cars center of gravity and roll center. It took in location and magnitude of the center of gravity of the sprung mass, and the locations of all of the suspension mounts, and outputted roll centers, roll axis inclination and also anti squat values, another useful suspension tuning parameter. We would place components where they fit in the model, and then run dimensions through the calculator to check how they would perform. We repeated this process several times on both the front and rear suspension until we had reached what we believed was the best arrangement, given our intent.
Bumpstops prevent the axle from bottoming out on the frame and rear shocks, and provide a soft landing when the suspension is fully compressed.
Fig1. Radflo Bumpstops
The shocks are fully adjustable and offer 5 separate bypasses. At different stroke lengths of compression or extension the shock can be tuned to desired characteristics. A driver may want the shock to be hard when fully compressed and soften as the stroke of the shock extends. It all depends on the type of terrain and driving style of the driver. But having tunability is crucial to be competitive in the sport of Ultra4 racing.
Fig. 2 Radflo Rear Shocks