In terms of evaluating damping and how well it controls body motion, the seven poster operates in a heave mode. For this type of testing, only the four posts exciting the wheels are in motion and the chassis is not constrained. The four posts will heave each corner up and down in unison to compress and extend each suspension and do this at increasing speeds from 1 to 25 hertz. (A video of this test is available on www.SportCompactCarWeb.com).
Six accelerometers are placed on the car, one on each wheel and two on the chassis on the front and rear axle lines between the wheels. These accelerometers measure the acceleration forces on each wheel and the front and rear of the chassis as the machine sweeps through each frequency. Since most cars are not perfectly balanced in front to rear weight distribution, the idea is to figure out the delay between chassis motion and wheel motion. If improperly tuned (like in the video example), the chassis will show signs of unnecessary chassis pitching at the natural frequency of the front or rear suspension. All of us have felt this while driving on concrete paved freeways with constant undulations that cause the car to dive and squat unnecessarily because the chassis is moving more than the wheels were moved.
This is due to the spring mass natural harmonic frequency of each axle, which is stipulated by the spring rate and the masses attached to it. Every system has it, including you. Just jump up and down on your bed, you'll notice that the frequency of your bounces is pretty consistent and difficult to change even if you tried. That's the harmonic frequency of your weight and the spring rate of the bed.
Without proper damping control, this pitching will occur as the suspension is excited at the natural frequency of the front and rear suspension. This is the whole point of the heave test, to find the natural frequencies where the car isn't happy and adjusting damping of each axle to eliminate unnecessary motions.
Mathematically speaking, this goes back to my rant about spring mass systems, critical damping forces, and damping ratios (August 2007). By looking at the relative acceleration of the chassis to the wheels, you can figure out the delay in motion. Perform a little FFT (Fast Fourier Transform) and covert the data from the time domain into frequency domain, and you can look at the phase delay in terms of degrees and frequency. Without going into this in detail, ultimately, a delay phase angle of 50 degrees correlates to an ideal (according to some schools of thought) damping ratio of 0.6, or slightly underdamped. A critically damped suspension will have a phase angle of 90 degrees. Try explaining this to your drunk car friends next time you're up late wrenching.
The seven poster also has several other modes of operation. Twisting or warp tests can be used to establish the influence of anti-roll bars. A full track simulation, which is what F1 and NASCAR teams typically use this machine for, can run the car through a known track using previously collected data to ensure that the suspension is near perfect from a mechanical perspective.
While few people will ever see or understand the immense possibilities that a seven post machine offers, it's good to see one that's trickled down to the aftermarket and being used for products we can afford. It might be overkill for a street suspension, but in some cases, even suspension gurus with highly calibrated butts have to resort to the hard data the seven post machine spits out.