Return of active suspension - 2017

[DaveW]

Dave, in this example you said inerter fitted to the front axle I assume you mean to the front third, yes? So this is a heave mode only test of just the front suspension, I'm not sure what you can talk about freely here, but what was the rear doing in this test?

The data was produced by a "bicycle" model driven by a typical vertical input. A front inerter was added (corner or 3rd, as you wish) to a typical suspension set-up of a GP2 vehicle. The rear axle had no inerter. Other suspension parameters were unchanged. Here is a an equivalent plot of the rear axle contact patch load time histories (same notation).

For the sake of completeness, here is an estimate of the "straight line" energy dissipated by the tyres for the four cases (red front, green rear).

[DaveW]

It seems fairly obvious that the "spinning top" will carry inertia and result in a secondary unload after it has "damped" the initial load but that should be in equal force value to the "damping" of the initial load. In the graph you posted the secondary unload event, even with the low inerter setting is larger magnitude than the forces absorbed in initial compression. This is curious to me, why? Before I read your post I assumed that the car will now be on the "gravity spring" if you will and that is totally undamped but you reference to the hub mode, has me curious if I had that backwards?

I wasn't quite sure how to respond, so I put together an animated GIF of the suspension frequency response functions for the four cases. Here (hopefully) it is (apologies, I had to sanitize the legends). That for the front axle is shown in red, the rear axle in green.

Concentrating on the front axle, at low frequencies (say 4 Hz.) the dynamic spring rate is reduced with each increase in inerter mass. At high frequencies the suspension tends to towards a pure spring with a rate equal to the installation stiffness (the inerter mass ultimately causes the damper to lock up). What happens at intermediate frequencies is complicated, particularly when the phase angle exceeds 90 degrees (implying a negative dynamic spring rate).....