Shock Absorber

Shock absorber (also known as damper) is a hydraulic device used commonly in automobiles to absorb and dampen the vibrations which a vehicle experiences while traveling on irregular surfaces. Kinetic energy is converted into heat energy and the heat is dissipated.

A shock absorber is usually used in conjunction with coil springs. It actually absorbs the natural frequency of the spring and dampens the unwanted spring motion. Imagine a car with only springs. The drive will be a lot bouncy without shock absorbers.

Design of a Shock Absorber:

Twin tube shock absorber is the most commonly used type of shock absorber. So let’s discuss the working of a twin tube shock absorber. As the name suggests, it consists of two tubes. The outer tube, also known as reserve tube mount is connected to the frame of the vehicle, whereas the inner tube (also known as pressure tube) is connected to the wheel axle.

The reserve tube is internally connected to a piston via a piston rod. The reserve tube with a piston sits on the pressure tube filled with hydraulic fluid. Orifices of very small diameters are pocketed out in the piston.

Working of a Shock Absorber:

When a vehicle hits a bump, all the kinetic energy from the spring is transferred to the reserve tube. The kinetic energy is then transferred to the piston. To explain in a simple way, the reserve tube exerts a lot of force on the pressure tube. The hydraulic fluid in the pressure tube is compressed and a small amount of fluid escapes through the tiny orifices in the piston under a great pressure. Since only a small amount of fluid escapes to the reserve tube, the piston moves down slowly and hence the spring movement is dampened.   

Air Suspension

Air suspension or air spring is a type of vehicle suspension that is powered by an air pump or a compressor. They are commonly used in heavy duty vehicles, but over the last decade it has also made its mark in the smaller commercial vehicles.

Air suspension is made of tough rubber and plastics inflated at a certain pressure to withstand the shocks. Air springs have various advantages over a shock absorber and coil spring arrangement. Several automobile manufacturers use air suspension today such as Rolls Royce, Mercedes-Benz, Maybach, Land Rover, Jeep, Lexus, Volkswagen, etc.

Air Suspension System Design:

As already mentioned, that air suspension is made of tough rubber and plastic (polyurethane) bags that are inflated to a correct pressure with the help of a compressor.  

The bags come in 3 different shapes:

·         Double-Convoluted Bag: This type of bag is in the shape of an hour glass or a double cheese burger. It provides better lateral flexibility than other types and has more load capacity. It has a short stroke and a progressive spring rate. It is best suited for front suspension.

·         Rolling Sleeve and Tapered Sleeve Bags:  These have a smaller diameter and also the stroke length is longer. The linear spring rate is higher and more suited for rear end applications.

Rolling Sleeve

Taper Sleeve

Compressor Motor:

Most of the air suspension systems are provided with an electric compressor motor. It draws the air from the atmosphere, compresses it and then sends the compressed air to inflate the bags through a series of compressed air lines. Apart from compressing the air, it also maintains the pressure to be supplied to the bags. The pressure has to be varied for different driving conditions. Therefore, the compressor provides a transition in pressure.

In vehicles fitted with advanced air suspension system, a separate air tank is provided to maintain the compressed air pressure and also to provide a smoother change in pressure. The pressure of the air might vary from 5 bar to 12 bar and the tank capacity might vary from 7 liters to 20 liters depending on the load that a vehicle carries.

Desiccant Air dryer:

The air drawn from the atmosphere might have moisture which can damage the system. To remove the moisture, air dryer is installed to absorb the moisture before it is sent to the system.  

Air Lines:

High pressure lines made of rubber and polyurethane composition are used to carry the compressed air to the bags. Advanced systems often use steel lines.

Valves (Two way or Four way):

Valves play a crucial role in controlling the amount of air to be supplied to the bags. A two way valve system uses one valve per axle. During cornering, let’s assume a vehicle taking a left turn; the left air bag will be more compressed than the right air bag. Two way valve systems allow the air to transfer from left to right, thus increasing the tendency of body rolls. Body rolls bring a huge disadvantage to a air suspension system.

To overcome the problem, four way valve systems were introduced. It offered separate valve for each bag. It also requires a separate air line for each bag, hence canceling the possibility of body rolls due to air suspension.   

Electronic Control System (Solenoids):

Solenoids are used to electronically control the amount of air supplied to an air bag. The electronic control module receives inputs such as ride height, and pressure through sensors and switches the compressor ON and OFF as required to vary the pressure.

There is also a completely manual setup to regulate the rife height and pressure. This is done with the help of pneumatic valves.

There are 3 types of Electronic Control Systems:

·         Pressure Based System: The pressure based electronic control system relies solely on the air pressure inputs. Based on the air pressure inputs, the control module has to calculate the ride height of the vehicle. It is more of an assumption than calculating the exact ride height value. The system might not perfectly vary the ride height required based on the pressure inputs alone.

·         Ride Height Based System: With a ride height control system, the issue of assuming the ride height is overcome. Ride height sensors accurately measure the ride height of individual wheel. But without a pressure based system, the vehicle might face cross loading. Cross loading is the difference in ride height between the tires in one diagonal and the tires in the other diagonal. Two diagonal corner tires might be over inflated while the other two diagonal corner tires might be under inflated. To overcome the issue, air suspensions are fitted with a combo of both pressure based system and ride height based system.

·         Combo System: Both pressure based system and ride height system are installed together to overcome both cross loading and ride height calculation issues.

Advantages of Air Suspension:

·         Superior ride quality. Passengers don’t feel like their car is being driven over bumps.

·         It can improve towing capabilities

·         It can maintain the firmness of a vehicle when carrying heavy loads.

Disadvantages:

The only major disadvantage is the cost of buying an air suspension unit. It is mostly used in luxury cars. People who can spend extra money would generally go for this variant. Air suspension is standard in Mercedes S class and Range Rover.

Leaf Springs

Leaf springs are commonly used for suspension in vehicles. It is one of the oldest forms of suspension and is used commonly in trucks and other heavy duty vehicles. Leaf springs are also available in most of the pickup trucks at the rear axle.

Design:

They are long and thin steel metal plates attached either above or below the axle. The metal plates are slightly curved and looks like a bow. The ends of the leaf springs are provided with eye-holes so that they can be attached to the vehicle body. Depending on the number of leaf and the number of eye holes, leaf springs can be divided into different types.

Based on the number of leaf:

·         Monoleaf Springs: These springs consist of only one steel plate in a curved shape. Such plates are thick at the center and get thinner on the edges. The shape is in the form of a semi-elliptical curve. They don’t offer good strength and suspension. Heavy duty vehicles require multileaf springs to provide the strength.

·         Multileaf Springs: It consists of various steel plates of different lengths stacked upon each other. The shortest leaf is kept at the bottom and the longest is kept at the top to give the same semi-elliptical shape that single leaf springs provide. Multiplate springs are provided with rebound clips to hold the plates together.

Based on the number of eye hole:

·         Double eye leaf springs: The longest steel plate in a leaf spring is provided with eye holes on both the sides so that it can be easily bolted to the body of the vehicle.

·         Open eye leaf spring: Only one end is provided with an eye hole. The other end is either flat or provided with a hook.

Purpose of U-bolts in a leaf spring:

Just by securing the eye holes of the spring to the hanger of the frame won’t be enough to steady the leaf springs. The leaf springs might be suspended or attached loosely to the axle of the vehicle. Such conditions lead to overslung and underslung. Overslung occurs when leaf spring is placed over the axle. Underslung occurs when the leaf spring is attached under the axle.

U-bolts are horse shoe shaped metal rods that avoid the overslung and underslung condition. A pair of U-bolts are fit around the axle and bolted to a metal plate that supports the leaf springs and also keeps the metal plates in the leaf spring intact without moving too much when carrying heavy load.

What makes leaf springs so strong?

The number and thickness of these leaves are what decide the spring rate and load capacity. The length of the leaves will decide the ride quality and the width decides the stability. Leaf springs are also known as progressive rate springs because of their tapered design. They grow stiffer when they are compressed which gives them the strength for carrying heavy loads.

Advantages of leaf spring:

·         It doesn’t require an anti-roll bar as leaf springs are twice as stiff in roll as it is in bump. Hence we can reduce weight and cost of extra material.

·         It provides great stability at high loads.

·         It is simple in design and easy to install.

·         The load distribution is even on both the sides of the axle. If a leaf spring is fitted with a shock absorber, then the load on the top of the shock absorber will be drastically reduced.

·         Leaf spring can be mounted in the centre, above or below the drive shaft without affecting the wheel movements. The advantage of this setup is that there will be no off-centre loads like we see in a coil spring.

Disadvantages:

·         Leaf springs are much heavier than coil springs. One leaf will weigh more than two coil springs combined together.

·         Takes up a lot of space.

·         Leaf springs cannot be tuned to adjust with the conditions. Coilovers have a modular design which makes it easy to adjust the ride height and spring rate.

·         Leaf springs are susceptible to noise.

·         Friction occurs between the contacting surfaces of the plates which wears out the leaves and can ultimately lead to a crack.

·         Life of leaf springs is lower than coilovers.