Exploring Leaf Springs: The Backbone of Automobile Suspension（一）

一. Overview of Leaf Springs

Leaf springs are crucial elastic elements in automobile suspensions, with unique structures and wide applications. They are composed of several alloy spring leaves, shaped like an elastic beam of approximately equal strength. This structural design enables them to play a variety of important roles in the driving process of the car.

 

From the perspective of structural characteristics, leaf springs are usually placed longitudinally and are composed of many spring steel leaves of unequal lengths. The first leaf is the main leaf, with ears at both ends. The front ear is connected to the front bracket with a pin to form a fixed hinge point. In addition to bearing gravity loads, it can also transmit longitudinal and lateral loads; the rear ear is connected to the ear that can swing freely on the rear bracket, ensuring that the distance between the two ears can be changed when the spring is deformed during operation. At the same time, the lengths of the leaves of the leaf spring are different, mainly based on the requirements of structural strength, so that it can form an elastic beam of approximately equal strength as a whole, which saves materials, reduces weight, and maintains good elasticity.

Leaf springs are divided into two forms: multi-leaf springs and few-leaf springs. The multi-leaf spring is composed of multiple steel plates of different lengths and the same width. Each steel plate is superimposed into an inverted triangle shape. The topmost steel plate is the longest and the bottommost steel plate is the shortest. The number of steel plates is related to the weight of the supported bus. The more, thicker and shorter the steel plates are, the greater the spring rigidity. However, after a long period of use, the plates will slide and rub against each other to produce noise, and the relative friction will also cause the spring to deform, resulting in uneven driving. The less-leaf spring is composed of steel plates with thin ends and thick middle, equal width and length. The cross-section of the steel plates varies greatly, the rolling process is relatively complicated, and the price is more expensive than the multi-leaf spring. However, under the same stiffness, the less-leaf spring is about 50% lighter than the multi-leaf spring, which reduces fuel consumption and increases driving smoothness. The single leaves are in point contact, which reduces relative friction and vibration and increases ride comfort.

 

When the leaf spring is installed in the automobile suspension and the vertical load it bears is positive, each spring leaf is deformed by force and tends to bend upward. At this time, the axle and the frame are close to each other. When the axle and the frame move away from each other, the positive vertical load and deformation of the leaf spring gradually decrease, and sometimes even reverse. The main leaf ear is seriously stressed and is a weak point. In order to improve the stress of the main leaf ear, the end of the second leaf is often bent into an ear and wrapped outside the main leaf ear, which is called a wrap ear. In order to make it possible for each leaf to slide relative to each other during elastic deformation, a large gap is left between the main leaf ear and the second leaf ear. The leaf springs in some suspensions are not made into ears at both ends, but other support connection methods are used, such as rubber support pads.

 

Automobile leaf springs are very important. It is composed of many elastic, uniformly thick and long steel sheets. Its function is to connect the frame and the axle in the form of suspension, exposed between the frame and the axle, bear the load impact of the wheel on the frame, reduce the violent vibration of the body, and maintain the stability of the vehicle and its adaptability to different road conditions. When the leaf spring is subjected to load impact, it forms an extension movement, and strong friction is generated between the steel sheets, that is, an extrusion and stretching phenomenon is generated. The two friction surfaces generate two different directions of movement friction, which causes the temperature of the leaf spring to rise, surface strain, and fine cracks. As the load impact frequency increases, the friction movement force generated by the extension movement also increases. When the fatigue limit is reached at the stress concentration point, it will cause a single sheet or a whole stack to break.

 

The leaf spring has the advantages of simple structure, reliable operation, low cost, and convenient maintenance. It is both an elastic element of the suspension and a guide device of the suspension. One end of it is hinged to the frame, which can transmit various forces and torques and determine the beating trajectory of the wheel. At the same time, it itself has a certain friction and shock absorption effect. But it also has disadvantages. It can only be used for non-independent suspension, heavy weight, high rigidity, poor comfort, long longitudinal size, which is not conducive to shortening the front and rear overhangs of the car, and the leaf spring pins at the connection with the frame are easy to wear. Despite many disadvantages, leaf springs are still widely used on various cars. In order to improve the performance of leaf springs, reduce weight and increase life, variable-section leaf springs and single-leaf springs have emerged.

 

二. The role of leaf springs

1. Shock absorption and buffering

Leaf springs play a vital role in shock absorption and buffering in automobiles. It connects the axle with the body. When the wheel is subjected to impact force, the friction between the spring leaves of the leaf spring can transfer the impact force to the frame. This transmission process is not a simple direct transmission, but through the deformation and friction of the spring leaf, the impact force is gradually dispersed and absorbed, thereby playing a buffering effect.

 

As introduced by Pacific Automotive Network, the leaf spring is used as an elastic element on trucks because it can effectively connect the axle with the body. When the wheel is subjected to impact force, the relative sliding between the spring leaves generates friction, which transfers the impact force to the frame, playing a buffering and shock absorption effect. At the same time, the leaf spring can also control the wheel to travel according to the prescribed trajectory relative to the body, ensuring good operability and stability.

 

During the driving process, the car will inevitably encounter various road conditions, such as bumpy roads and uneven roads. At this time, the leaf spring can play its unique shock-absorbing and buffering role, relieve the vibration of the vehicle, reduce the violent shaking of the vehicle body, and provide a more comfortable riding experience for the driver and passengers.

 

For example, when the vehicle is driving on a rough road, the impact force on the wheel will be transmitted to the leaf spring through the axle. The spring steel leaves of the leaf spring are deformed by force, and the impact force is gradually absorbed and dispersed through their own elasticity and friction, thereby reducing the impact on the vehicle body. At the same time, the deformation of the leaf spring will also change the distance between the axle and the vehicle body, further alleviating the impact force.

 

In addition, the auxiliary spring steel plate also has a shock-absorbing effect. Pacific Automotive Network pointed out that the role of the auxiliary spring steel plate shock absorption is to buffer and absorb shock. It works with the main spring steel plate to provide a more stable shock-absorbing effect during vehicle driving. On the basis of the main spring steel plate, the auxiliary spring steel plate further enhances the shock-absorbing ability of the leaf spring and improves the driving stability of the vehicle.

 

2. Auxiliary functions

In addition to the shock-absorbing effect, the leaf spring also has a variety of auxiliary functions.

(1) Controlling mechanical movement: In a car, a leaf spring can control the wheels to move along a prescribed trajectory relative to the vehicle body, ensuring the vehicle's driving stability and operability. It is like an invisible commander, guiding the direction of the wheels' movement so that the vehicle can maintain stable driving under various road conditions.

 

(2) Absorbing vibration and impact energy: When a vehicle is driving on an uneven road, a leaf spring can absorb the vibration and impact energy from the road surface, reducing the impact of this energy on the vehicle body and passengers. This function is similar to a car's shock absorber, which converts vibration and impact energy into heat energy through the deformation and friction of the spring leaf, thereby reducing the vibration amplitude of the vehicle.

 

(3) Storing and outputting energy: In some cases, a leaf spring can also store energy and output energy when needed. For example, when a vehicle accelerates or brakes, the leaf spring will be subjected to a certain amount of pressure and tension, at which time it can store part of the energy. When the vehicle is driving on a bumpy road, the leaf spring can release the stored energy and play a role in buffering and shock absorption.

(4) Used as a force measuring element: Although the leaf spring is not a special force measuring element, its deformation can reflect the load borne by the vehicle to a certain extent. For example, when the vehicle is fully loaded, the deformation of the leaf spring will increase. By measuring the deformation of the leaf spring, the load of the vehicle can be roughly estimated.

 

(5) Reset function: The leaf spring has a certain reset ability. When the vehicle is deformed by external force, once the external force disappears, the leaf spring can rely on its own elasticity to restore to its original shape and position. This function is very important for maintaining the stability and operability of the vehicle.

 

(6) Driving function: In some special automotive applications, the leaf spring can also play a driving role. For example, in some mechanical clocks and clockwork toys, the wound spring will produce bending deformation and store a certain amount of elastic potential energy. After release, the elastic potential energy is converted into kinetic energy and driven to rotate through the transmission device. Although the leaf spring in the car is different from these applications, it can also drive the movement of some related parts to a certain extent through its elastic deformation.

 

(7) Buffering function: Leaf springs are installed between the frames and wheels of locomotives and cars, and the elasticity of the springs is used to reduce the bumps of the vehicle. This function is similar to that of shock absorbers, but the buffering effect of leaf springs is more extensive and can be used under different driving conditions.

 

(8) Sound-generating function: When air flows through the reed holes in harmonicas and accordions, it impacts the reeds, and the reeds vibrate and make sounds. Although the leaf springs in cars are not special sound-generating elements, in some cases, the vibration of the leaf springs may also produce some weak sounds.

 

(9) Compression function: In various electrical switches in cars, one of the two contacts of the switch is often equipped with a spring to ensure that the two contacts are in close contact and conduct well. In other parts of the car, such as bayonet lamp holders, screw-on lamp holders, cassette tapes, etc., the compression function of the spring is also used. Although the springs used in these places are different from leaf springs, their compression function principles are similar.

 

In short, leaf springs in automobiles not only have the main function of shock absorption and buffering, but also have a variety of auxiliary functions. These functions work together to provide important guarantees for the car's driving safety, comfort and stability.