Variable-assist power steering -- or speed-sensitive steering -- is kind of like continuously variable transmissions; a hypothetically perfect idea that has yet to reach approval or acceptance among driving purists. But like the continuously variable transmissions, there's a better than even chance that systems like this will eventually reach a point of proliferation that enthusiasts will have to either get used to them or go out of their way to avoid them. Still, for 99 percent of drivers on the road, speed-sensitive steering has become a boon to convenience and driving enjoyment.
The idea behind variable-assist steering goes all the way back to the first variable-rate steering boxes. In a variable-rate box like those found in most cars, the gear ratio between the steering column and steering rack is high, with narrowly spaced gear teeth. At the extremes of the rack gear, the gear tooth spacing widens out. This causes the wheel to move very little just off center, when you're cruising down the road and only need to make minor adjustments in direction. But the wider-spaced teeth on the extremities move the wheels more quickly as you turn them, as when parking or navigating tight corners.
While the variable-assist mechanism works well under most circumstances, it's an inherently flawed mechanism. Your steering system does two things. It's primary job is to move the wheels back and forth, but its secondary job is to relay information back to the driver about traction at the steering wheels. The faster your wheels spin, the more quickly and easily they're able to turn. This causes a linear drop in both steering effort and driver feedback at higher speeds. A mechanically perfect system would compensate for this drop in feedback by increasing steering effort as road speeds increase. That way, the driver would never have to wonder if steering effort just dropped because of an increase in speed or because the front wheels lost traction.
A mechanical speed-sensitive steering system like Fords first-generation variable-assist power steering uses a valve in the steering rack to vary hydraulic assist with speed. At speeds below 5 mph, the hydraulic pressure valve in your rack is completely open, allowing full hydraulic pressure from the pump to reach the rack and assist with steering. The valve slowly closes in incremental steps; in Ford's case, at 5 mph, 20 mph, 40 mph and 60 mph. At 80 mph, the valve is fully closed and the steering receives no power assist at all. Second-generation variable-assist power steering systems and those used by many other manufacturers use a variable-orifice valve, which closes progressively with speed and removes the "notchiness" from stepped designs.
Many steering boxes use a torsion bar, or twisting spring bar, between the end of the steering column and a weight to sense steering effort. The faster you turn the wheel, the more the torsion bar bends and the farther the drag weight at its end lags behind. The lagging weight or friction disc, opens a valve that routes more fluid pressure into the steering rack, thus increasing steering effort on demand. A magnetic speed-sensitive steering system like GM's Magnasteer uses a pair of electromagnets on the drag element and the case, or the drag element and the column. Increasing electrical power to the magnet or magnets will increase drag -- and thus assist -- if the second magnet is on the case and decrease drag if the second magnet is on the column.
The variable-assist and speed-sensitive steering mechanisms are clever solutions to a mechanical problem. They do work as advertised, and the newest generation systems are for all intents transparent in operation. If you notice the system at all, you'll only notice that the wheel feels solid and stable at high speed and compliant at lower speeds. But speed-sensitive steering is perceived -- perhaps rightly -- as something of a bane among performance drivers and purists. A performance driver relies on consistent and predictable performance to extract the greatest-possible traction from each tire. If given a choice, many performance enthusiasts would prefer a bicep workout while parking to a system that artificially changes steering feel and effort according to speed.
