Steering System Basics
The steering system (power or not), provides the operator with a way of controlling the direction that the vehicle travels. It gives the driver a mechanical advantage over the angle of the front wheels. Mechanical systems also provide the driver with “road feel.” Road feel is something that some drivers like and other drivers do not. It provides the driver with a feel for what the wheels are doing. Steering systems can use a steering rack or a steering gear to steer the wheels. Newer electric systems make the engine more fuel efficient, but they provide almost no road feel. Many drivers say that it feels like they are driving in a video game.
Pieces of a Steering System
The steering wheel is the drivers interface for the steering system. steering wheels with a larger diameter allow the driver to deliver more torque to the steering column with less effort. Many drivers install a smaller “racing steering wheel” which provides less of a mechanical advantage. This makes the steering feel more like a go-cart.
The steering column provides the connection between the steering wheel and the rack or steering gear. They may have one or two universal joints, which allow the rack or steering gear to be somewhere other than right under the steering wheel. Many older steering columns were completely solid. In a front end collision, it was common for steering columns to force the steering wheel up and into the driver. Modern systems have some kind of device that will break or sheer in a front end collision to allow the column to collapse, preventing further injury to the driver. The steering column will most likely have 2 bearings, holding the shaft in place but allowing it to rotate when the driver turns the steering wheel. These bearings, over time, can make noise when turning the steering wheel and will need to be lubricated or replaced.
Steering Knuckle and Spindle
The steering knuckle is the connection between the suspension and steering systems to the wheel. The wheel is bolted to the spindle, which is connected to the knuckle through a bearing. The bearing can be pressed into the knuckle or it may be bolted to the knuckle. This bearing allows the wheel and spindle to rotate while the knuckle remains stationary. The knuckle is allowed to pivot on its ball joint(s) when the steering system pushes or pulls on the tie-rod ends.
The tie-rods are what allow the steering system to control the angle that the steering knuckle and wheels pivot to. On a rack and pinion style of steering system, tie-rods have an inner tie-rod end which attaches to the rack itself. The outer tie-rod end is connected to an arm on the knuckle that will stick out to the front or rear of the vehicle, depending on the steering systems design and location. They are both a ball-in-socket type of connection, which allows the tie-rod ends to pivot on its ball freely when the suspension moves up and down. They are connected to each other by the tie-rod itself.
Steering racks, commonly referred to as rack and pinion steering, use a pinion gear which receives input from the steering wheel and column. The pinion gear meshes with the teeth on the rack, which are connected to the tie-rods and tie-rod ends. When the pinion gear is turned by the steering wheel and column, the pinion gear moves the rack from side to side. This pushes one tie-rod out, and pulls the other tie-rod in, which turns the front wheels. Even without power steering, the pinion gear (and driver) have a mechanical advantage over the rack because of the gear action of the pinion.
Steering gears tend to be used more on heavy duty applications. In this type of system, the steering column is connected to the steering gears worm shaft. This worm shaft has the ball nut assembly riding on it with ball bearings. The ball nut assembly has teeth on one side which mesh with the pitman or sector shaft, which controls the arms and tie-rods of the steering system. As the worm shaft is rotated, the ball nut assembly is moved from side to side depending on which way the worm shaft is rotated. This movement of the ball nut assembly rotates the sector shaft. The worm shaft needs to be rotated several times by the steering column to get just a few degrees of sector shaft rotation. This means that the worm shaft has a mechanical advantage over the sector shaft, even without power steering.
Manual steering systems (rack or gear systems) rely on gear ratios alone to give the driver a mechanical advantage over the steering system. These systems are not that difficult to operate once the vehicle is in motion, but require much more effort to turn the steering wheel when the vehicle is stationary. A power steering system is used in almost every modern vehicle on the road today. Many new models are using electric power steering, which I will discuss later in this article.
Basic Power Steering Operation
The power steering pump, pumps power steering fluid to the control valve on either the rack or the steering gear. This creates fluid pressure of about 1000PSI to 2500PSI depending on the system. The control valve senses torque input from the operator and directs fluid to either side of the rack or steering gear, which allows the hydraulic pressure to assist the operator in controlling the steering system.
Pieces of a Power Steering System
The pump makes the hydraulic pressure that the system uses to assist the driver in controlling the steering system. These pumps are driven by the engine, usually by an accessory belt. These pumps can be a vane-type pump, a slipper pump or roller pump. To get the extra pressure necessary for the power steering system, these pumps act as a double pump. They have an inlet and an outlet on each side, creating two separate pumps out of one, or causing twice as much displacement per revolution. The pump must create enough pressure and fluid flow to allow the wheels to be turned with the engine at idle and the vehicle stationary. This is when the power steering system works the hardest. As engine RPM increases and the vehicle begins to move down the road, if fluid pressure was not regulated, the steering assist would cause the steering be dangerously light. As engine RPM increases the flow control valve, located inside the pump assembly, will limit the amount of fluid which leaves the pump. These can be simply spring and pressure controlled or they may be PCM controlled. PCM controlled systems take vehicle speed into account when deciding how much assist is necessary. The faster the vehicle is travelling, the less assist is necessary to turn them.
The Control Valve
The control valve is located on the rack or steering gear and is in charge of sending/regulating hydraulic pressure to either side of the rack or steering gear. Torque from the steering column must travel through the control valve to get to the rack or steering gear. The control valve senses the torque applied to the steering system and applies hydraulic assist accordingly. The steering column connects to two pieces inside the control valve, the torsion bar and the stub shaft. The torsion bar is the only connection from the steering column to the rack’s pinion gear or steering gear worm shaft. It is a very thin piece of metal that twists when force is applied to the steering wheel and is located inside the stub shaft. The stub shaft is allowed to rotate freely with the steering column but does not transmit torque. When force is applied to the steering wheel, the torsion bar twists and the stub shaft turns with the steering column. When the torsion bar and the stub shaft do not rotate at the same speed, a passage for fluid flow is uncovered. The fluid is directed to one side of the rack or steering gear to assist in the movement of the ball nut or rack. The more the torsion bar twists, the more fluid is allowed to flow. At the same time, fluid that is in the opposing side of the rack or steering gear, is allowed to flow back to the reservoir under the low pressure created by the rack piston or ball nut assembly moving. Once the hydraulic assist has provided enough flow, and the torsion bar has caught up to the stub shaft, the passage is closed and fluid flow stops. This is how the control valve senses system requirements and sends fluid to where it needs to be for proper assist.
The hose between the pump and the control valve, must hold the high pressure created by the pump. These hoses should be noticeably thicker in construction as compared to the suction side hoses between the control valve and the reservoir and the reservoir and the pump.
The reservoir stores excess fluid and makes sure that the pump always has access to fresh fluid if it needs it and provides a place to add or top up fluid. This also ensures that the pump never sucks in air. (similar to a dry sump oil system)
Electric Power Steering
Many modern power steering systems do not use hydraulic pressure for steering assist. This means that they do not have a power steering pump driven by the engine resulting in better fuel economy. Instead the steering column has torque sensors on it which detect how much steering assist is required. This information may be sent directly to a module located right on the steering gear or rack or an external control module. This module controls a powerful electric motor which applies assist to the steering system. These systems can vary assist based on system demands. These systems tend to feel very light and more “like a video game.” They also cause the steering rack to be much heavier than a hydraulic power steering system’s rack.