Leverage and the use of levers occurs in the use of tools such as spanners, pry bars, pliers, and in many vehicle mechanisms such as clutch and brake pedals, throttle linkages and suspension units. The basic principles of leverage are covered by a rule that is known as the principle of moments. The bell crank lever is used in many vehicle systems and the principle of moments may be applied to the solution of numerical problems associated with it. Torque is a turning effect; a clear demonstration of the meaning of torque may be seen in the widely used torque wrench that is used for tightening nuts and bolts to a preset value. Torque is the product of force and perpendicular distance from the centre of rotation to the line in which the force is acting. The torque required at the driving road wheels of a vehicle is larger than the torque available at the engine flywheel.