We have been discussing different alignment setting of a car like its negative/positive toe settings and negative/positive camber setting. Another very important part of wheel alignment is the castor angle of the wheel.
Castor angle defines how much forward or backward a tire is in relation to the mounting point of the strut. Castor angle is slightly confusing to understand, without a diagram. Take a look at the photo below.
See how the tire is placed forward compared to the vertical axis. If the tire is moved forward, it’s called positive axis, and if the tire is pushed behind the vertical axis, it’s called negative castor.
Wheels of shopping trolleys you find at super markets are best way to explain them. When you push the trolley, the wheels fold backwards, creating positive castor angle. It makes it easier for you to push and turn the trolley to where you want it to go. If you turn the tires to 180 degree, it will create negative castor. It will make it not only hard to steer the trolley, but also it will be unpredictable. You might end up hitting some unsuspecting fellow shopper.
Usually, for normal cars, castor is kept positive. Castor angle is required to give straight line stability to the cars. It gives directional steering ability to car.