When driving, turning the steering wheel causes your vehicle to change direction. Have you ever wondered why that is? Let’s explore the principles behind the steering system in cars to help you drive more safely.
Here is some basic knowledge about the steering system of a vehicle. We will examine how it works, some basic steering systems, and their impact on the vehicle’s fuel economy. Let’s consider what makes a car turn. It’s definitely not as simple as you might think!
Figure 1: Steering System Layout in Cars
First, you will be surprised to learn that when turning, the front wheels do not move in the same direction. Why is that? For a car to turn smoothly, each wheel must follow a different circular path. The inner wheel moves along a circle with a smaller radius, making it more challenging to turn compared to the outer wheel. If you draw a perpendicular line from each wheel, these lines will intersect at the center of the turn. The geometric diagram below illustrates that the inner wheel must rotate more than the outer wheel.
Figure 2: Turning Radius Diagram
Historically, there have been two different types of steering mechanisms. The most general summary includes the rack-and-pinion and the screw-and-ball mechanism. Let’s first examine the principle of the rack-and-pinion system.
Figure 3: Click on the steering wheel to see the principle
The rack-and-pinion steering mechanism has emerged and quickly become popular in passenger cars, small trucks, and SUVs. It is a relatively simple mechanical system. A gear is connected to a metal tube, while a rack is mounted on another metal tube. A tie rod connects the two ends of the rack.
The circular gear is connected to the steering shaft. When you turn the steering wheel, the gear rotates, moving the rack. The tie rods at both ends of the rack are connected to an arm on a pivot (see Figure 4).
Figure 4: Steering System with Directional Gear in Independent Suspension
The rack-and-pinion mechanism serves two main functions:
- It converts the rotational movement of the steering wheel into the linear movement necessary to turn the wheels.
- It provides a reduction in speed, increasing force to facilitate easier and more accurate wheel turning.
In most modern cars, the driver usually needs to turn the steering wheel three to four times to turn the wheels from full left to full right and vice versa. The steering ratio is the relationship between the angle of the steering wheel’s rotation and the angle that the wheels turn. For example, if the steering wheel makes one full rotation (360 degrees) while the car turns 20 degrees, the steering ratio is 360 divided by 20, which equals 18:1. A higher ratio means you need to turn the steering wheel more for the wheels to change direction by a given distance. However, a high ratio may not be as effective as a low ratio. Generally, lightweight and sports cars have lower steering ratios compared to larger vehicles and heavy trucks. A lower ratio allows for quicker steering response, meaning drivers don’t need to turn the wheel as much when making sharp turns, which is advantageous for racing cars. These smaller cars are quite light, so they only require a steering type with a low ratio, while larger vehicles typically use a higher steering ratio to reduce the driver’s effort when turning.
Some vehicles with variable steering ratios still use the rack-and-pinion system but have different tooth pitches in the middle and the outer part (the tooth pitch is the number of teeth per unit length). This design enables the car to respond quicker when the driver begins to steer but reduces effort as the wheels approach their limit.
Power-Assisted Rack-and-Pinion Steering System
Figure 5: Power-Assisted Steering System Diagram
In this steering system, the rack is designed slightly differently than the standard type. A part of the rack contains a cylinder and a piston that is always in the center position. The piston is connected to the rack. There are two fluid lines on either side of the piston. A stream of fluid (usually hydraulic oil) under high pressure is pumped into one end of the line to push the piston, assisting the rack’s movement. Thus, when you steer in one direction, there is hydraulic assistance in that direction.
Screw-and-Ball Steering Mechanism
This mechanism is commonly used in most trucks and SUVs. The connection of the components in this mechanism is slightly different from the rack-and-pinion steering system.
Figure 6: Diagram of Components in the Steering System
You can imagine that the mechanism has two parts. The first part is a metal block with a hollow screw thread inside. Outside this metal block are several teeth that mesh with a gear (which can move an arm). The steering wheel is connected to a threaded shaft (like a large nut) that meshes with the thread grooves on the metal block using spherical balls (see Figure 7). When the steering wheel is turned, the nut rotates as well. Normally, turning this nut would cause it to screw deeper into the metal block according to the screw principle, but it is held back, causing the metal block to move in the opposite direction. This action makes the gear connected to this metal block rotate, leading to the movement of the arms that steer the wheels.
Figure 7: Screw-and-Ball Steering Mechanism
As illustrated in the diagram, the nut engages with the metal block through spherical balls. These balls serve two purposes: they reduce friction between the components and minimize play in the mechanism. Play occurs when changing the steering direction; without the balls, the teeth would separate momentarily, causing play in the steering.
The power assist system of this steering mechanism is similar to that of the rack-and-pinion system. The assistance is also provided by directing high-pressure fluid into one side of the metal block.
Hydraulic Pump
Figure 8: Location of Power-Assisted Pump in the Steering System
To provide hydraulic assistance for the steering system, a vane type hydraulic pump is used (see Figure 8). This pump is driven by the engine’s torque through a pulley-belt system. It consists of numerous vanes that can move within the grooves of a rotor. As the rotor spins, centrifugal force pushes the vanes outward, pressing them against an oval-shaped closed space. Hydraulic oil is drawn from a low-pressure line (return line) and compressed to a high-pressure output. The amount of oil supplied depends on the engine speed. The pump is always designed to provide enough oil even when the engine is idling, which results in an excess supply when the engine operates at high speeds. To prevent overloading the system at high pressure, a pressure relief valve must be installed (see Figure 9).
Figure 9: Vane Type Power Steering Pump Structure
The power steering system assists the driver when he applies force to the steering wheel (when wanting to change the direction of the vehicle). When the driver does not apply any force (when the vehicle is moving straight), the system provides no assistance. The device used to sense the force applied to the steering wheel is called a rotary valve.
Figure 10: Rotary Valve Structure Diagram
The main component of the rotary valve is a torsion bar. The torsion bar is a thin metal rod that can twist when torque is applied to it. The upper end of the torsion bar connects to the steering wheel, while the lower end connects to a gear or screw, balancing the total torque of the torsion bar with the total torque the driver uses to steer the wheels. The greater the torque the driver exerts, the more the bar twists.
Figure 11: This animation will show you the operation of the rotary valve and the opening and closing of valves when you apply force to the steering wheel (please click the white circle in the center to see how the rotary valve works)
The input of the steering shaft is an internal component of a cylindrical valve block. It also connects to the upper end of the torsion bar. The lower end of the torsion bar connects to the outside of the valve. The torsion bar also rotates the output of the steering mechanism, which connects to a gear or screw depending on the type of steering system.
When the torsion bar twists, it causes the inside of the valve to rotate relative to the outside. Since the inner part of the valve is also connected to the steering shaft (that is, connected to the steering wheel), the total angle of rotation between the inside and outside of the valve depends on how the driver turns the steering wheel.
When the steering wheel is not being turned, both hydraulic lines provide equal pressure to the steering mechanism. However, if the valve is turned to one side, the lines will open to supply high pressure to that side. Nevertheless, the effectiveness of these auxiliary systems is low. Let’s explore some future systems for higher performance.
Electric Power Steering System
Electric Power Steering System
1. Steering wheel (steering force); 2. Electric power assist motor; 3. Reaction force from the road onto the tires; 1+2. Steering assistance when turning.
The principle of operation of the electric power steering system is based on signals from a torque sensor located in the power steering assembly. When the driver applies force to the steering wheel to change direction, the reaction force from the road through the tires causes the steering column to apply torque to the torsion bar in the electric assist assembly. The torque sensor measures the steering torque (the deformation of the torsion bar) and sends signals to the control unit. Based on the signals from the torque sensor, the control unit provides sufficient current to the power assist motor to help turn the steering shaft in the direction controlled by the driver, thus making steering much lighter and easier.
Comparison of Electric Power Steering and Hydraulic Power Steering
When comparing electric power steering systems with hydraulic power steering systems, electric steering has several outstanding advantages over hydraulic steering:
- Electric steering does not require power from the engine, so the engine does not lose power for the steering assist system, thereby saves 2%-3% of fuel during operation.
- Electric steering does not use a medium (hydraulic oil) for power assist, ensuring environmental cleanliness.
- Electric steering has a simpler, more compact structure compared to hydraulic steering (reducing weight -> saving fuel).
- Electric steering provides a more realistic steering feel at high speeds. In hydraulic steering systems, the faster the speed, the lighter the steering feels, making it easy for the driver to oversteer and the vehicle to become unstable, especially during sharp turns. In electric systems, the assistance is light at low speeds and heavier at high speeds, providing a more realistic and secure steering feel.
- Electric power steering is lighter and more responsive than hydraulic power steering.
Despite its numerous advantages, the biggest drawback of the electric power steering system is the repair cost. When components within the assist unit fail, the entire unit must be replaced rather than repaired to ensure safety while driving and to avoid sudden loss of steering control.
Steering Systems in Automobiles
