I didn't know there were small shaker rigs like that for shock/suspension diagnostic testing. This is good to know.
The Hunter model appears to be out of production.
Here is a Muller model (European) that is currently available: http://www.actia.co.uk/muller/pdfs/Broc ... ochure.pdf
These testers use fixed amplitude and known frequency ranges, so it's simple to determine peak velocity vs frequency. These testers are generally not interested in primary frequencies. The Muller only goes down to 3 Hz, and the amplitude at the lower freqs may be smaller than ideal. The Hunter goes down to lower freqs, but again the fixed amplitude may not be optimized for true primary ride motion at these low freqs.
The technical paper on the Hunter unit also helped me realize that the phase angle between input and response roughly corresponds to overall system damping. I knew this in theory, but it was a good reminder of how much info can be had from a simple test if you think about what to look for.
DaveW commented about missing the perspective on overall suspension setup compromises if I only consider the primary frequencies. I agree that development people are generally good at balancing all these compromises, and the tradeoffs are rarely understood later on by pure test/eval people.
I would try to avoid this dilemma by using the primary shaker rig as a pure research device as opposed to doing low-frequency development on cars where I didn't understand the other tradeoffs. I don't think we have a good understanding of why different cars have good or bad primary ride. I'd try to improve this knowledge so development people can use it in the future. Our current primary ride stiffnesses are set by identifying competitor vehicles with good primary ride and copying their static deflections. This is a reasonable pragmatic approach, but there is room for improvement.
A primary shaker rig would need to be 4-post so I could get into some of the "action" issues that Olley identified. In particular I would be interesting in finding the pitch center location for pitch and bounce modes (bounce does have a bit of pitch that is apparently important to how it feels). The pitch center will probably have a clear location at the resonant frequency of each mode, but I'm also interested in the pitch center location vs the entire primary frequency range.
Overall, I'm trying to decide if the primary shaker rig would be worth the time or if I'm simply enamored with the idea of making a shaker rig. It may come down to what is my most useful alternative activity. More feedback always appreciated.