[MichaelDay]

The PT understeers. Virtually all production cars are designed to understeer. The logic used by the manufacturers is that it is easier to explain why you drove too fast and plowed off the road headfirst than to defend why the rear let go and you spun out or went off the road tailend first. The pre 1965 Corvairs were criticized for having oversteer as a result of the rear independent suspension lift causing the tires to lose contact with the pavement on the inner part of the tread. With the rear end losing traction first, the rear end of the car would start to spin. Since the engine was in the rear and thus the weight distribution was significantly biased toward the rear the rear of the car would lead.

For most drivers (read unskilled at reading and controlling a car pushed to its limit of traction) understeer is "safer" and more predictable. Front wheel drive cars are even more difficult to get to oversteer due to the weight bias toward the front on the car. Cars that provide the best handling strive for a neutral balance so that they may oversteer or understeer depending on the conditions. The performance driver has a feel and practices to determine the conditions that will create each situation. The ordinary driver doesn't want to know and doesn't care, they want the car to behave the same all the time. I suspect that the true reason DC dropped the rear bar on the GT was to save cost but they were able to do it since the stiffer springs on the GT tended to reduce the understeer somewhat making the rear bar less "necessary" for the casual driver. I suspect that the reduced torsional rigidity of the convertible again increased the understeer and caused DC to reinstate the rear bar.

P.S. In 1965, Chevrolet redesigned the rear suspension on the Corvair with lower links to keep the tires flat on the ground when the rear end lifted thus decreasing the likelihood that the rear end would lose traction as quickly. Unfortunately, that resulted in the vehicle having a tendency to plow when the lightly loaded front tires lost traction. Hello corn field, goodbye road.