A bell crank is a device which changes the angle at which a suspension piece or a linkage is operated. For example, a throttle linkage can be attached to a crank of this type, and when the linkage is pulled straight back, the crank can pull the throttle sideways. This is typically used on multiple carburetor and fuel injection set-ups. Another feature of the bell crank is its ability to increase or reduce the ratio of a component's effect on a suspension. By attaching a spring to the crank, the amount of travel — and, therefore, the rate of spring tension — can be altered by the mounting position on the crank.
Resembling a capital letter L, the bell crank is actually two arms with a pivot position at the 90-degree angle. The arms on the angle can differ in length; however, most arms are of the exact same length, forming a perfect 90-degree angle with different mounting positions on each arm. The mounting positions alter the ratio of pull and can be made to move one component a great deal with only minimal movement of a corresponding component. While sometimes used with cable linkages, the unit is typically used with hard line or rod-activated linkage lines.
In a bell crank steering system, the steering arm pushes or pulls one end of the crank forward or to the rear. In response, linkage from the other end of the bell crank pushes right or left, causing the wheels to turn in a particular direction. This type of steering is most commonly found on a Model T Ford or a sprint or midget race car. The steering exits the driver's cockpit and actuates a long steel rod, making its way to the front axle. That rod attaches to one end of the bell crank's arm while the other end of the crank's arm is connected from one front wheel hub assembly to the other.
This type of steering control linkage is also used on boats. The rudder is controlled in the same manner as an auto's steering. This manipulation of the push-and-pull ratio aids the ship's operator with the ability to turn the rudder swiftly with only minimal input from the ship's wheelhouse. By increasing the ratio of the rudder's movement over that of the ship's wheel input, the ship is able to make very quick changes in direction, which helps it to avoid collisions and objects in the water.