Keeping Your Customers’ Vehicles on The Straight & Narrow
Modern automotive steering gears evolved from the quaint tiller-controlled steering systems used in the first automobiles to the hydraulically controlled systems now used in most import vehicles. Unfortunately, steering tillers that attached to axles with little or no steering geometry didn’t provide much steering feedback from the road surface. When vehicle speeds approached 20 mph, auto manufacturers began to incorporate more sophisticated steering systems to help the driver keep the vehicle traveling in a straight line.
As vehicles became heavier and more difficult to steer, auto manufacturers developed different types of steering gears designed to change the rotating motion of the steering wheel and shaft into a lateral motion transmitted through the steering linkage to the steering knuckles and front wheels. This system has prevailed to this day.
As steering gear technology progressed, innovations in steering geometry also aided steering gear functions. The popular introduction of kingpin inclination (KPI) during the early 1920s, for example, eased steering effort by tilting the kingpin inward at the top, so that a line drawn through the kingpin angle intersects with the center of the tire tread. KPI is also present in ball joint or MacPherson strut systems, with the angle of the upper and lower ball joints or MacPherson strut intersecting with the center of the tire tread.
KPI actually accomplishes three separate, but related, steering functions. First, KPI reduces steering effort by allowing the tire to pivot on the center of its tread rather than be swung around the axis of a vertical kingpin. Second, KPI greatly reduces the probability that a wheel striking a pothole may suddenly spin the steering wheel out of the driver’s hands. Third, KPI actually lifts one side of the vehicle and lowers the other as the wheels are turned. This lifting effect, combined with caster angle, forces the front wheels to return to a straight-ahead position. For all of the above reasons, steering effort increases and the steering wheel becomes more sensitive to road shock whenever offset custom wheels are installed on a vehicle.
THE ROLE OF CASTER ANGLE
Caster angle also plays a function in all steering gear systems. Positive caster angle can be easily visualized in the backward tilt of a bicycle’s front fork. Positive caster angle, of course, allows the bicycle to steer itself when the rider takes his hands off the handlebars. Although some auto manufacturers may employ negative caster angle to accomplish the same purpose, caster helps force the front wheels to return to center when the driver releases the steering wheel.
CONVENTIONAL STEERING GEARS
Although conventional parallelogram steering linkage systems are being replaced by the more modern rack and pinion steering gears, many heavier vehicles such as luxury sedans and pickup trucks still use conventional steering gears. Early steering gears were crude, bronze-bushed units that required a great amount of steering effort to turn the wheels. As vehicles became heavier, ball and needle-roller bearings were used to reduce friction in the steering gear assembly. In addition, a recirculating ball-bearing worm gear assembly was introduced that greatly reduced friction between the worm gear mounted on the steering gear input shaft and the sector gear mounted on the steering output shaft.
As vehicle speeds increased, it became very important for the steering gear to transmit a sense of “road feel” to the driver. Without a fine-tuned sense of steering wheel center, it became difficult for a driver to drive in a straight line at high speed. To accomplish this, engineers designed a worm and sector gear that would develop zero clearance or lash when the steering gear operated in the centered or straight-ahead position. The fact that the steering gear ‘tightens” up when the front wheels are in the straight-ahead position gives the driver a much finer sense of when the vehicle is tracking in a straight line.
As the wheels are turned, the steering gear rolls off the “high spot” and develops clearance between the worm and sector gears. This added clearance reduces friction and helps the front wheels return to the straight-ahead position when the driver releases the steering wheel. In addition, the high spot in the steering gear tends to stop or hold the steering gear in the straight-ahead position by increasing friction between the worm and sector gears. This particular feature helps prevent the driver from over-steering the vehicle.
RACK-AND-PINION STEERING GEARS
Rack and pinion steering gears were popularly introduced into the import market during the 1950s because they provided fast steering wheel response and had fewer parts. In this system, a rack gear is attached to the right and left steering knuckles by tie rods that swivel to allow vertical movement in the suspension system. The pinion gear, of course, is attached to the steering shaft and moves the rack in a lateral direction in response to steering wheel input. Like the conventional steering gear, the rack and pinion is machined to decrease gear lash as the steering wheel returns to the center position.
Steering assist systems incorporate a hydraulic pump driven by an engine accessory drive belt to provide pressure to a steering assist cylinder attached to the sector gear in conventional systems or the rack gear in rack and pinion systems.
To provide full steering assist, the power steering pump must produce at least 1,000 psi pressure upon demand. Because a power steering pump is used only a small percentage of time, the pump is equipped with a hydraulic boost or computer-controlled, pulse-modulated valve assembly that allows it to free-wheel during highway driving. To fine-tune steering wheel response, the steering gear shaft is equipped with a torsion bar that opens a metering valve as torque is applied to the steering wheel. Due to the action of the torsion bar, the amount of oil pressure metered to the steering assist cylinder is directly proportional to the torque being applied to the steering wheel.
The metering valve assembly is very sensitive because it’s often equipped with compressible reaction discs that further increase the steering gear’s sensitivity to steering wheel torque. Some manufacturers, such as Honda, have used axle-driven mechanical systems that limit power steering assist to low-speed driving situations. In other applications, steering pump pressure may be controlled by using a pulse-width modulation system that varies pressure in response to steering wheel demand and highway speed.
On high-mileage vehicles with a steering wander complaint, the sector gear lash might require adjusting to eliminate excessive lash between the worm and sector gears. Early manual rack and pinion gears also have a similar adjustment that reduces lash between the pinion and rack gears.
A steering gear may also transmit steering feedback to the driver only when it’s installed in the centered position. In most cases, a four-wheel alignment may be required to recenter the steering gear. The first step is to always make sure that indexing marks on the steering wheel and steering shaft are aligned. The last step is to adjust front and rear toe angles to correspond with the vehicle’s centerline or thrust angle. When removing the steering wheel, always follow the manufacturers’ instructions regarding the disarming of the air bag and preventing damage to the air bag clock spring.
Any power steering gear can develop a “lack of assist” complaint. Generally speaking, a worn power steering pump is usually indicated if steering assist deteriorates as the steering oil begins to warm up and lose viscosity. In contrast, some rack and pinion steering gears may develop an intermittent condition called “morning sickness.” This particular lack of steering assist is caused by the metering valve oil seals wearing grooves into the soft aluminum steering gear housing. As the steering gear oil warms up, the valve assembly reseals itself to the housing and restores steering assist. Because special tools and expertise might be required to rebuild a steering gear, it’s more cost-effective to install a new or remanufactured gear than to repair or rebuild an old one.
Last, it’s always important to use application-specific power steering fluid in import applications. In the case of Honda, using non-OE specification fluid can ruin the steering gear seals. In other applications, using the wrong fluid or mixing fluids can cause the steering fluid to foam, which may cause a howling noise and temporary reduction of steering assist.