A manual gearbox has a high ratio to reduce the amount of effort needed to turn the steering wheel. Power steering gearboxes have a lower ratio. Constant and Variable Ratio Steering A variable ratio gearbox changes the internal gear- ratio as the worm wheels are turned from the centre position. The variable ratio types recirculating ball gearboxes are used in most modern vehicles.
A variable ratio steering is faster when cornering, requiring fewer turns of steering wheel from full right to full left. It also provides better control and response when maneuvering.
Variable ratio steering is accomplished by changing the length of gear teeth on the Sector Shaft gear.
This changes the effective lever arm action between the gears. Many manual steering gearboxes and most power steering gearboxes are variable ratio. A constant ratio gearbox has the same gear reduction from full left to full right. The sector gear teeth are of the same length.