Some people believe that castor wheels are named after one of the three major wheel alignment parameters used in cars and other 4-wheeled vehicles. These parameters are: toe, camber and caster.
What is Caster?
Technically speaking, a wheel alignment that tilts the axis of the steering pivot from vertical to either forward or rearward is called caster. When the axis is tilted such the top pivot of the wheel is farther from the rear than from the bottom of the pivot then the caster is said to be in the “negative position”. But when the axis is tilted in a way that the wheel is allowed to freely move forward in a straight course, then the caster is in the “positive position.”
This is the wheel movement mechanism that is used in shopping carts. If you have observed how these shopping carts move, it will be easier for you to visualize how caster wheel alignment works. If you have noticed, the wheels of the shopping cart moves forward when it makes contact with the ground. As you push the shopping cart forward, the wheels are also pulled along by the steering axis and your cart moves in an unrestricted straight path. Furthermore, the more force you exert on the cart, the move distance that it covers. The distance covered is called the “trail” of the caster.
In cars, the steering axis is placed at the wheel hub in order to avoid making the “trail” equal to zero. The trail is zero when the steering axis is coincident with the tires that make contact with the ground. To avoid this, the steering axis is put someplace where the wheel can freely spin about the tire patch. By aligning the steering axis in the “positive position”, cars are able to make the most from the effects of castering.
In short, castering happens when the steering axis is tilted in the right position. This condition also has significant effects to the suspension geometry of the car. When done properly, the car will also experience the effect of camber (another wheel alignment parameter) which useful for cornering and for turning directions.
Going Back to Castor Wheels
Although it might be too difficult to imagine just how castor wheels and car wheels are alike, it is still good to know that the castor wheels that we use in the office and at home has long been an inspiration in the automotive industry. So, the next time that you see a castor wheel, take the time to appreciate it for what it’s worth.