[arthurus]

. Even when driving down a perfectly straight road, it is necessary to turn the steering wheel vigorously from left to right every few moments.

That comes from old movies because old cars had to be steered much more positively than do newer models. You couldn't take your hands off the wheel for a few seconds.

[Old Professer]

Steering geometry hasn't changed much in 75 years; suspensions have. We still have the toe-in relationship (parallel tracking of the tires in an axial line), camber angle (tilt toward the center or outside of the sideline of the vehicle of the front tires), and caster angle (the shopping-cart wheel set-back is positive and used on most cars) and finally the kingpin inclination angle (an imaginary line drawn from the top center of a wheel to the outside bottom of the tire at the contact point). When all of these angles and relationships are correct to design specifications, a car should track without correction at the wheel (assuming a flat, level road surface).