Spring compressing due to bump response

(Fig. 10.69(a)) When the tyre of the wheel hits a hump in the road, the whole wheel assembly attached to the suspension rises rapidly. This causes the displacer piston to move further into the displacer chamber. Consequently fluid in the displacer chamber will be displaced and pushed into the spherical spring chamber via the transfer port and bump valve. The rapid transfer of fluid into the spring chamber compresses the separator diaphragm against the nitrogen gas and the resilience of the gas therefore absorbs the impact shock. If there was no elastic media between the body structure and the deflecting suspension, any sudden upward movement would be transmitted directly to the body structure and passengers thus producing a very uncomfortable ride.

In actual fact movement of the fluid from the displacer chamber into the spring chamber takes place in three stages:

1 If the road bumps are very small and the vehicle is moving slowly, sufficient fluid flows through the permanently open transfer hole to equalize the pressure on both sides of this restriction.

2 If the road bumps are more severe the increased pressure build-up in the displacer chamber will be sufficient to lift the flaps on the rubber bump valve off a second pair of bleed holes. Additional fluid can now flow into the spring chamber in a shorter time span.

3 If the roughness of the road surface worsens or the speed at which the vehicle travels increases even more, then there will be a continuous rise in pressure of the fluid trapped in the displacer chamber. As a result of the extreme pressure build-up, the rubber bump valve itself will be progressively lifted from its seat to permit more fluid to enter the spring chamber. Thus in total more fluid is transferred from the displacer chamber to the spring chamber in a given time, but the built-in opposing resistance to the flow of fluid produces a measure of damping which slows down the violent uplifts caused by the impact of the tyre with obstacles in the road.

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