A disadvantage of a fixed-ratio system is that towards the lock positions, more effort is needed by the driver.
This is because the angle of the steering arms reduces their effective length, and that reduces the leverage on the wheels.
To overcome this, many rack-and-pinion systems use variable ratio steering. The ratio is made variable by changing the shape of the teeth on the rack, between the centre and the outer edges of the rack.
Then, as the steering moves away from the straight-ahead position, the ratio, and therefore, the mechanical advantage, increases progressively.
As the pinion turns, and moves on the rack, the gear contact point between the pinion, and the teeth on the rack, changes. This change in tooth contact changes the effective diameter of the pinion.
Then, for the same amount of steering wheel rotation, the rack moves a shorter distance near the ends of the rack than near the centre. Effort needed to turn the wheels stays approximately the same through the whole range of movement.