Dampers | Suspension Dampers

Dampers

In a vehicle suspension system, springs alone are not satisfactory and sufficient. It must be remembered that the spring characteristics must be compromise between flexibility and stiffness. In must be flexible so that if can absorb road shocks. But if it is too flexible, it will flex and rebound excessively and repeatedly, giving a rough ride. A stiff spring will not flex and rebound so much after a bump has been passed. But, it will give a hard ride because it will transmit too much of road shock to the vehicle. In practice, by using a relatively flexible or soft spring, and a damper, a satisfactory smooth ride can be achieved.

01 - DAMPERS - DAMPER PRINCIPLE

In a bumpy road and especially on a curve, the oscillations of the vehicle frame might become serious enough to cause the driver to loose control of the vehicle. The damper dampens out the spring oscillations quickly once the wheel has passed the hole or bump in the road. The damper is also called shock absorbed by many.

The spring energy is converted to heat which is dissipated partly by the friction of the system, but mostly by the dampers.

Dampers are nothing but a piston in a cylinder filled with oil or gas. The damper’s function is to restrain undesirable bounce of the sprung vehicle mass and to restrain the wheel assembly from loosing ground contact by being excited at its natural frequency.

If a compressed spring is released, it will continue to oscillate back and forth through several gradually diminishing cycles until all the energy is dissipated. This process could provide a very disturbing and dumpy ride. Properly tuned damper will allow a compressed spring, when released, to extend slightly beyond its natural length or shape, then return to rest. This is called critical damping.

In most applications, critical damping is far too severe and the best damping for comfort and adequate wheel control falls between 15 and 50 percent of the critical value, depending on the inherent system friction and the required ride qualities.

The typical function of the damper in a modern car is to reduce the amplitude of the deflection over a bump to about a fifth of its initial value, within two cycles of oscillations, at the natural frequency of the system.

The independent front suspension assembly with a damper in position can be seen with arrangements can be seen in this picture.

01 - DAMPERS - DAMPER ASSEMBLY

There are many types of shock absorber, operating on friction, on compressed air, and hydraulically. The hydraulic shock absorber is the only type in common use at present. The hydraulic shock absorber contains a fluid that is forced through restricting orifices as the shock absorber is operated by spring flexure. The resistance to the movement of the fluid through the restricting orifices from one compartment to the other compartment in the shock absorber imposes a drag on spring movement. This effect quickly dampens out spring oscillations.

The Hotchkiss Drive | Torque Tube Drive

Hotchkiss drive

The Hotchkiss drive is the simplest of the drive systems and is the most widely used. The arrangement of the parts can be seen in the picture.

01 - DRIVE SYSTEMS - HOTCHKISS DRIVE

The suspension spring are bolted rigidly to the rear axle casing. The front ends of the springs are pivoted on pins. These pins are carried in brackets bolted to the vehicle frame. The rear ends of the springs are connected to the frame by swinging links or shackles. This arrangement permits the deflection of the spring when the vehicle is accelerated or braked. The propeller shaft is provided with two universal joints one at each end and a sliding joint at one end. This arrangement permits the rear axle assembly to move up and down due to projections and depression on the road surface.

01 - DRIVE SYSTEM - HOTCHKISS DRIVE

Engine power is always transmitted from the gear box to the final drive in the differential, through the propeller shaft. From the differential the driving torque is transmitted to the road wheels through the axle shafts. In this transmission system, the suspension springs act as torque and thrust members.

Torque tube drive

The torque tube drive, which is still fairly widely used is shown in picture. There is a tubular member called torque surrounds the propeller shaft and is bolted to the rear axle casing. The front end of this member is spherical in shape. The spherical end fits in a cup bolted to a cross member of the vehicle frame. The torque tube incorporates bearings which supports the propeller shaft. The propeller shaft itself is usually made of hollow steel tubing which construction gives it a light weight and torsional strength. The suspension leaf springs are bolted to the spring seats that are provided on the axle casing. Each end of the springs are shackled to the frame. The tubular member will transmit the thrust from the axle to the frame and will also take the torque reaction. Often radius rods are used to assist the torque tube to take the twist and thrust of the vehicle drive.

01 - DRIVE SYSTEM - TORQUE  TUBE DRIVE

With this construction, the centre line of the final drive bevel pinion shaft will always pass through the centre of the spherical cup. Now, if the propeller shaft is connected to the gear box shaft by a universal joint situated exactly at the centre of that cup, no other universal joint will be needed and no sliding joint will be necessary. This is because both pinion shaft and propeller shaft will move about the same centre, namely that of the spherical cup, when the axle moves up and down.

In this system, the spring seats are sometime pivoted to the axle casing by means of spherical pivots. This relieves the springs of twisting stresses when the axle assumes angular positions relatively to the frame.

In one design, however, two extra heavy radius rods are used in place of the torque tube to take the thrust. They are connected between the rear axle housing and the X section of the car frame. An open propeller shaft and two universal joints are used with this design.