Automobile Suspension Spring

2.5.1. Automobile Suspension Spring

Spring absorb shock forces while it maintaining correct ride height. Automotive springs are generally classified by the spring rate (deflection / load).

Suspension springs are the link between wheels and car body. Their primary task is to compensate uneven road surfaces and thus provide an assurance of high levels of ride comfort. Secondly, they must ensure that the wheels always have safe contact with the road regardless of its condition. Reliable transmission of drive, braking and transverse forces relies on these requirements being met. As such, suspension springs are one of the most safety-critical components of modern vehicles. They affect handling, road-holding and braking performance.

Most conventional suspensions use passive springs to absorb impacts. The majority of land vehicles are suspended by steel springs, of these types:

1. Leaf spring

 a. Full – elliptic type

 b. Semi – elliptic type

 c. Three Quarter – elliptic type

 d. Transverse Spring type

 e. Helper Spring type

2. Coil spring / Helical springs

3. Torsion bar / Torque rod

Automakers are aware of the inherent limitations of steel springs, which they tend to produce undesirable oscillations, and have developed other types of suspension materials and mechanisms in attempts to improve performance:

4. Rubber springs: 

 a.      Compression Springs

 b.     Progressive Springs

5. Plastic springs

6. Air springs: 

 a.      Bellow Type

 b.     Pestoon Type

7. Hydraulic springs

1. Leaf Spring:

Leaf springs are multi-layered steel plates clamped together. Leaf springs are formed by bending. They are made of long strips of steel. Each strip is named as Leaf. The long leaf is called master leaf / main leaf, and it consists of eyes at its both ends. 

Fig : Leaf spring

One end is fixed to the chassis frame, the other end is fixed to the shackle spring. The spring will get elongated during expansion and shortened during compression.

This change in length of spring is compensated by the shackle. The U-bolt and clamps are located at the intermediate position of the spring. The bronze or rubber bushes are provided on both eyes on the master leaf.

Fig: Leaf spring

a. Full elliptic:

The advantage of this type is the elimination of shackle and spring. The lubrication and wear frequently, which are one of the main drawback of that type of springs.

Fig: Full elliptic

b. Semi – elliptic:

This type is more popular for rear suspensions are used in 75% of older cars.

Fig: Semi-elliptical spring

c. Three – Quarter – elliptic type:

This type is rarely used in now-a-days. It gives resistance, but occupies more space than other types.

Fig: Three-quarter-elliptic spring

Fig: Quarter-elliptic spring

d. Transverse Type:

This type of spring is arranged transversely across the car instead of longitudinal direction. The transverse spring for front axle as shown in figure, this is bolted rigidly to the frame at the centre and attached to the axle by means of shackle at both ends.

Fig: Transverse spring

e. Helper Leaf Springs:

The helper springs are used in heavy vehicles for rear suspension. When vehicle fully loaded the main spring as well as helper spring to come in action and absorb the road shocks. When the load of the vehicle is less the helper spring will not act and the main spring only absorb the road shocks.

Fig: Helper spring

2. Coil springs:

Coil spring is made of thick steel wires a length of special spring steel, usually round in section which is wound in the shape of coil. The ends of coil spring are kept flat so that could seat properly. They can store twice energy per unit volume in comparison to leaf spring. To seat the coil springs pan shaped brackets or spring seats are attached to the axles. This suspension is also used in combination with torque tube or torque rod.

Fig: Coil spring

3. Torsion bar / Torque rod:

Torsion bars are long steel rods of either circular or square cross section. The springing action is generated by the torsional forces when the torsion bar is twisted.

A torsion bar suspension, also known as a torsion spring suspension, is any vehicle suspension that uses a torsion bar as its main weight-bearing spring.

The effective spring rate of the bar is determined by its length, cross section, shape, material, and manufacturing process.

Fig : Torsion bar

4. Rubber spring:

As rubber can store more energy per unit mass than any other type of spring material, considerable weight can be saved with rubber suspension. Rubber springs, if works on compres­sion or shear, can be used as the main suspension spring, otherwise can be fitted along with metal springs to improve the suspension characteristics. Large rubber ‘bump’ stops used in many suspension layouts stiffens the suspension spring against maximum deflection.

Fig: Rubber spring

Figure represents a rubber suspension system in a simplified form, that is similar to the one used on a popular small car. The spring is installed between the frame and the top link of the suspension system. When the spring is connected to a point near the link pivot, deflection of the spring reduces to a minimum, without affecting the total wheel movement. This arrangement of spring provides a rising-rate characteristic, which is ‘soft’ for small wheel movements but becomes harder as the spring deflects.

The energy released from the rubber spring after deflection is considerably less than that imparted to it. This internal loss of energy is called hysteresis, which is an advantage, because lower-duty dampers may be used. Some rubber suspension systems have a tendency to ‘settle down’ or ‘creep’ during the initial stages of service, therefore allowance for this must be provided.

Fig: Rubber suspension

Fig: Rubber suspension spring

5. Plastic springs:

Plastic spring for motor cars is about to roll out fibreglass-reinforced epoxy road springs which match the function of steel coils in coping with severe loading, but are around 40% lighter.

The engineers work determinedly to avoid every scrap of unnecessary weight – not only in the body but also in every other component and assembly.

Ultra-lightweight design is a particular priority for the car’s chassis and suspension, since any reduction in unsprung mass improves ride comfort and handling.

Plastic spring is capable of absorbing torsional loads extremely well if it is designed with this specific purpose.

Application: Audi R8 e-tron

Innovations in suspension design, such as those demonstrated by Audi and Citroën, show that there are several ways in which development can progress. The use of lighter metals and plastics can provide engineers with greater freedom of design as they develop the next generation of suspension systems.

Lightweighting of vehicles has become one of the key trends in the automotive industry, as OEMs endeavor to meet future emission targets. Lighter cars enable the downsizing of engines, better performance and better fuel efficiency, and every sector within manufacturing is under scrutiny for weight reduction.

6. Air springs:

Air springs offer several advantages over metal springs, one of the most important being the possibility of controlling the spring rate. 

Inherently, the force required to deflect the air unit increases with greater deflection, because the air is compressed into a smaller space and greater pressure is built up, thus progressively resisting further deflection.

Air spring generally made of rubber material with two major types are, Bellow Type and another is Piston Type.

Fig: Air spring

Air spring can be tuned for the required application by adjusting the following parameters:

a) Size of effective area Aw.
b) Size of air spring volume (air volume) and
c) Outer contour of roll piston.
The air spring basically comprises;
a) an upper housing with an outer guide.
b) the air spring gaiter.
c) the roll piston (lower housing).
d) an auxiliary accumulator.
e) the integrated vibration damper.

7. Hydraulic springs:

Hydraulic springs are comparatively small, thick-walled cylinders in which the spring effect is produced by applying a load to the fluid in the cylinder through a small piston entering at the centre of one end of the cylinder.

The piston movement, or deflection, is produced by the compression of the fluid and the deformation (bulging) of the cylinder walls. These springs are particularly useful in applications requiring high load capacities and stiffness's.

Hydrolastic Suspension

This suspension is intended to improve the vehicle’s resistance to pitch, the tendency of the body to oscillate in a fore-and-aft direction when the front springs are compressed and the rear springs are expanded simultaneously.

The continuous forward and backward pitching motion provides a most uncomfortable ride, which may become serious when the frequency of vibration of front and rear springs is the same.

The Hydrolastic suspension layout on a vehicle uses inter-connected rubber displacer units (Fig.) installed between the frame and the independent suspension linkage controlling the wheel. The interconnection is carried out using two pipes. One pipe links the left-hand side units together and the other on the right-hand side. 

The system is pressurized with an anti-freeze liquid after removing air. Each displacer unit contains a rubber spring; metal separating member, which holds two rubber damper valves; rubber diaphragm attached to the suspension linkage, which holds the wheel; and a metal body, which is secured to the frame of the vehicle.

Fig: Hydrolastic spring

A sudden upward movement of the front wheel causes the diaphragm to displace the liquid through the damper. This action is turn forces liquid along the pipe to the rear unit where it moves the diaphragm and raises the rear of the car to the level of the front (Fig.). When the front wheel descends, the liquid returns and the vehicle comes to its normal riding position.

During this sequence the liquid has to pass the damper valve in each unit, and the restriction to liquid flow at the valves and in the pipelines damps out the tendency of pitch oscillation.

Fig: Action of hydrolastic units

When a vehicle is cornering, the body of the vehicle tilts or rolls outwards due to centrifugal force. This tilting action is apparent when ‘soft’ conventional springs are used.

The hydrolastic system is ‘soft’ during movement of a single wheel, but if the two outside suspension units are loaded during cornering, a stiffening of the hydrolastic system occurs.

Under this type of loading displacement of the fluid from one unit to the other does not occur. Instead the increased liquid pressure deflects the rubber springs, which provide a marked resistance to the tilt of the body.

During bouncing of the vehicle four wheels deflect at the same time. To resist this motion all the hydrolastic units perform in the similar way as to react to roll.