What factors determine steering ratio?

Question

• What are the factors that can be used to determine the steering ratio of manually driven rack & pinion steering gear?

• What are the relations of steering ratio with other steering factors like turning radius, design of steering gear etc.?

Answer 1

The steering ratio of a car is strictly an input v. output ratio. You input a number of degrees into the steering wheel and you get so much steering movement out of the wheels. This applies if the rack & pinion is manual or assisted. Here is what the Car Bible has to say about how it's figured:

Steering ratio is the ratio of the number of degrees turned at the steering wheel vs. the number of degrees the front wheels are deflected. So for example, if the steering wheel is turned 20° and the front wheels only turn 1°, that gives a steering ratio of 20:1. For most modern cars, the steering ratio is between 12:1 and 20:1. This, coupled with the maximum angle of deflection of the wheels gives the lock-to-lock turns for the steering wheel. For example, if a car has a steering ratio of 18:1 and the front wheels have a maximum deflection of 25°, then at 25°, the steering wheel has turned 25°x18, which is 450°. That's only to one side, so the entire steering goes from -25° to plus 25° giving a lock-to-lock angle at the steering wheel of 900°, or 2.5 turns (900° / 360).

As far as how does it relate to how far you can turn the wheel, there isn't one. The steering angle itself will only go as far as the physical limits of the steering system itself. For instance, if you want to turn a car into a drift car (successfully), you need to modify the steering components so they will travel further. You'd need to make it so there would be no obstruction as the tires turn. You'd need to ensure the rack & pinion has enough travel to push the tires/wheels to these limits. The steering ratio itself will have nothing to do with these limits.