After examining damping in roll last month, we will now look at springs and damping in pitch of the car. As explained last time with roll damping acting in a polar coordinate system, pitch is very similar- except the sprung mass is rotating about the pitch axis instead of the roll axis. Roll damping is a usually overlooked aspect in a suspension system, and pitch damping is overlooked even more so.
Roll Damping: Often an overlooked aspect on racecars is the damping in roll. Most of the time, cars have a damper for each single-wheel spring, but not a damper for the anti-roll bars. The goal of this tech tip is to explain the benefits of using a roll damper. Where this setup is not allowed in the rules, the knowledge will help in understanding the setup and compromises in the lack of a roll damper.
From driving street cars, we all know that if you hit a speed bump going very slow the body of the car (sprung mass) moves vertically almost as much as the wheels. Hitting the same bump going fast (you know you have done it, especially in a rental car) the body of the car does not move nearly as much. The size of the bump was the same, but the body motions were different depending on the speed at which you hit it. The cause of this is that response of the system (the car sitting on the suspension) is dictated by the frequency and amplitude of the input. Hitting the speed bump faster increases the frequency of the disturbance, producing a
different response. To quantify this reality we use the concept of transmissibility.
After understanding the first two tech tips in the Spring & Damper series, you know how to choose ride frequencies for your racecar, calculate the spring rate needed for the chosen frequencies, choose a roll gradient, and calculate the stiffness required from the anti-roll bars to produce your desired roll gradient. Now, what is the deal with these “third” springs people and using, and how in the world do I know where to start when it comes to damping on the racecar? Pitch springs will be skipped for now, as damping baselines will be much more useful.
After understanding last month’s tech tip, you know how to pick ride frequencies for your racecar, and calculate the spring rate needed for the chose frequency. Now, what do you do about the anti-roll bars.
When out on the track, you can spend an entire weekend tuning spring rates and shock settings, and shave a second from lap times. But, how do you know if you are even in the right ballpark, and maybe a completely different spring and shock setup could cut two seconds per lap, especially when working with an unfamiliar car?
Data acquisition is one of the best tools to increase the understanding of vehicle behavior. One can get tons of information just from a couple of potentiometers and accelerometers connected to a simple data logger. However as one adds more and more sensors a number of problems arise. First, the number of cables and connectors increase to where it becomes both a logistical and economical issue.