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...Now, my green car, 123k mi, everything 4WS related was replaced or refurb'd by a previous owner at about 110k. That car handles like none other. ...Here's what I really want to know, what goes wrong in the 4WS system that causes the cars not to handle well? My black car? Scary above 80.
When you turn the rear wheels in the direction of the turn, you will increase understeer...simple geometry.
Perhaps your green car's superior handling can be attributed to good overall maintenance including shocks, sway bar end links, ball joints, bushings, etc. And tires can make a HUGE difference in handling so you might consider what tires are on the various GVR4s you have driven as well as whether or not they were properly inflated (which can also make a big difference).