Also known as variable-rate or variable-assist depending upon the application, non-linear steering showed up in the latter 20th century as a means to increase parking agility while still providing for decent feedback and performance at speed. While steering systems that provide non-linear feedback or movement -- based upon steering angle or road speed -- do exactly as advertised, they're not completely free of drawbacks.
Linear steering is the simplest type of steering system out there, and it is still preferred for performance applications. A linear steering rack uses a pinion gear -- the one on the end of the steering column -- and a flat rack gear -- the one that moves the steering rack side-to-side -- with the same number of teeth from one end to the other. This means that the wheels will always turn by the same amount for every turn of the steering wheel. So, if you've got a linear gear ratio of 3-to-1 and your wheels can move by 120 degrees side-to-side, they'll always move by 40 degrees regardless of angle or road speed.
Variable-rate steering boxes are quite common today primarily because they help in parking the car. A VR box's bar-like rack gear has teeth spaced farther apart in the center than they are at the ends of the rack. So, with the wheels pointed straight ahead, one full turn of the steering wheel might turn the road wheels by 30 degrees. But another full turn only moves them by 20 degrees, and a third turn moves them by 10 degrees. This configuration gives the driver more leverage when turning the road wheels to their extremes for tight parking maneuvers and minimizes steering wheel movement at cruise speeds when the wheel's pointed relatively straight.
A variable-assist box works on a different principle than a variable-rate box. The VA system doesn't vary gear ratio like a VR arrangement does; it only alters the amount of power steering assist. VA systems come in several varieties, with the simplest being one that uses a springy torsion bar and a "drag weight" in the box to determine how fast you're turning the wheel. Turn the wheel very quickly and the drag weight will lag behind and twist the torsion spring. This opens a hydraulic fluid pressure valve in the steering box, immediately boosting steering assist. When the drag mechanism catches up, the valving goes back to normal. Some systems actually default to zero-assist -- no pressure to the system -- in the absence of any detected steering effort.
The speed-sensitive system is actually something of a cousin to the variable assist system and may or may not incorporate a variable ratio steering rack. An SS system takes information from the car's wheel speed sensors -- also utilized by the antilock braking, traction control and stability control systems -- to vary power assist in accordance to the car's actual road speed. The faster your wheels spin, the easier they'll change direction and the less effort you need to exert to turn the wheel. Speed-sensitive steering compensates for this effect by keeping the required steering effort consistent regardless of road speed.
Steering feedback is more important to some than to others. When the steering tires lose grip on the road, the steering wheel gets easier to turn; performance drivers use this feedback to keep the tires right on the limit of adhesion, and depend on it to extract maximum performance. Speed-sensitive and variable-assist steering systems negate a certain amount of this natural feedback by artificially altering steering effort; fine for those who just want the car to go where they point it, but a serious problem for those who rely on consistent feedback. Variable-rate gearboxes aren't as big a problem since they're at least predictable, but fully linear racks are still preferred for most serious racing applications.
