Vibration- Suspension Technology

Question 1

What are the most commonly used springs in suspension technology, however what is their downside?

Answer 1

Steel coil springs, as they are cheap and reliable. However they have lightly damped modes of vibration leading to structurally transmitted noise

Question 2

What can be done to reduce the amplitude in spring coil resonances?

Answer 2

An elastomer isolating pad can be placed between the spring and the car body to dissipate energy

Question 3

What are the 2 main solutions for springs in suspension?

Answer 3

Steel coil springs and air springs

Question 4

What is the benefit of air springs?

Answer 4

The static displacement and stiffness can be adjusted by adjusting the air volume or pressure respectively, and the sprung mass mode natural frequency can be made independent of the sprung mass.

Question 5

What are the 2 main solutions for dampers in suspension?

Answer 5

Monotube and twin tube dampers

Question 6

Describe the structure of a monotube damper

Answer 6

There is just one tube, sealed at the top where the piston rod (which has a top mounting attached to the rest of the vehicle, using an elastomer isolating pad) comes into the working chamber that is filled with oil. The end of the piston rod is attached to a piston head/valve which can move up and down in the working chamber. Below the working chamber there is a floating piston that seals the oil above it from a chamber of compressed high pressure gas below it which is at the end of the tube.

Question 7

What is the purpose of the high pressure gas chamber below the floating piston in a monotube damper?

Answer 7

It accommodates the oil that is displaced by the piston in the working chamber above it. It also prevents cavitation and causes a static force.

Question 8

Describe the structure of a twin tube damper

Answer 8

The twin tubes are placed coaxially with each other. The piston rod comes in the top of the assembly, into the working chamber that is filled with oil. Unlike a monotube damper, there is no floating piston at the bottom of the working chamber, but a compression valve with orifices that allow the oil to be transferred between the inner tube working chamber, and the outer tube. The outer tube is part filled with oil at the bottom and above it is gas at low pressure. Therefore the cavity between the concentric tubes provides space for the oil to be displaced by the piston rod.

Question 9

What are the 5 differences of a twin tube damper compared to a monotube damper?

Answer 9

• Larger diameter and shorter
• heavier
• lower static force (no high pressure gas chamber to provide the high static force)
• must be mounted upright (gas from cavity could get into the inner tube/working chamber
• cavitation is a greater risk

Question 10

How does the valve arrangement change in a monotube damper and how it is achieved?

Answer 10

The valve consists of a fixed orifice and several spring controlled orifices. At low speeds, flow is jut through the fixed orifice, and at high speeds the flow is through both the fixed and spring controlled orifices.

Question 11

Is damping usually set greater in compression or tension?

Answer 11

Damping in extension is greater (damping is represented by the gradient of a force-velocity diagram)

Question 12

How can the damping performance be tuned in the case of a monotube damper?

Answer 12

The curves can be optimised by changing the springs that controlled the spring controlled orifices in the piston valve. Non-linear springs can be used.

Question 13

Name 4 types of advanced ‘active’ suspension configurations

Answer 13

• Inerter
• switchable damper
• continuously variable damper
• fully active

Question 14

What is an inerter, and how is it used in advanced suspension?

Answer 14

A device which generates a force proportional to the relative acceleration across the device
It is used in parallel with a spring and damper

Question 15

What is switchable damping?

Answer 15

Where you can switch the damper valve between several fixed settings. It allows the trade off between performance criteria to be varied. Switching can be performed manually or automatically.

Question 16

What is a continuously variable damper?

Answer 16

The piston contains electromagnetic fluid which properties can be varied by a magnetic field.
It uses Magneto-rheological (MR) damper where the yield stress can be varied by the presence of a magnetic field.

Question 17

What is fully active suspension?

Answer 17

The damper is replaced by an electronic motor, the motor force can be controlled independently of displacement and velocity across the motor. This can improve ride and handling.

Greater mass than conventional damper
Possibility for energy recovery

Question 18

What is the purpose of linked suspension

Answer 18

Can be used to increase roll stiffness by linking the left and right wheels hydraulically or by using an anti-roll bar. Increasing the pressure in the hydraulic system increases the roll stiffness.

Question 19

What modes of vibration are considered in secondary ride?

Answer 19

Wheel hop 
Wheel and tyre vibrations 
Engine vibrations 
Driveline vibrations 
Flexural modes of body

Question 20

What modes of vibration are considered in primary ride?

Answer 20

The main body bounce mode (sprung mass mode)

Question 21

What are the frequency ranges of primary and secondary ride vibrations?

Answer 21

Primary ride: 0 Hz - 3 Hz

Secondary ride: >3 Hz

Question 22

What method is used to address wheel vibration from longitudinal force inputs, eg from bumps in the road?

Answer 22

Longitudinal compliance is provided by an elastomer mounting bush with slots

Question 23

What method is used to address engine vibration?

Answer 23

The engine is mounted using hydraulic and elastomeric elements (recent innovation is electro-rheological engine mounts)