Spring extending due to rebound response

(Fig. 10.69(b)) After the wheel has passed over a hump in the road the bounce action of the nitrogen gas pushes some of the fluid from the spring chamber back to the displacer chamber causing the displacer piston to extend from the displacer chamber.

The return of fluid from the spring chamber to the displacer chamber takes place in two stages:

1 If the bumps in the road are small or the vehicle is moving very slowly, then only a small amount of fluid needs to be transferred back to the displacer chamber in a given time. The movement of this fluid out of the spring chamber can be coped with adequately by the permanently open transfer hole. This means the damping action takes place as fluid is bypassed through the permanent bleed hole for low speed conditions.

2 If the bumps in the road are larger and the speed of the vehicle is higher, then the highly pressurized fluid in the spring chamber will lift the rubber rebound valve progressively from its seat, thus permitting a greater rate of flow of fluid back into the displacer chamber.

Because the progressive opening of the rubber valve is pressure sensitive, the flow of fluid is restricted and it is this tendency to slow down the fluid movement that produces the retarding effect on the rebound expanding gas.

Comparison of bump and rebound fluid damping control The extension (rebound) of the displacer piston is slightly slower than on contraction (bump) because there is not an intermediate flap valve second stage opening as there is on bump. Thus for small deflections of the displacer piston the permanent bleed transfer hole controls the movement of fluid in both bump and rebound directions. For more rapid displacement of fluid on rebound there is only the rebound compressive rubber block valves which regulate the flow of fluid in the extending direction, this being equivalent to both the flap valve and compressive rubber block valve opening on the contracting (bump) stroke.

Bump and pitch mode (Fig. 10.69(c)) When the front or rear wheel passes over a bump, the contraction of the displacer piston inside the displacer chamber at that wheel causes fluid displacement through the interconnecting pipe to the other wheel spring unit on the same side of the vehicle.

This movement of fluid into the other spring unit's displacer chamber extends its displacer piston within the chamber and thereby lifts the suspension and body up to the same level as that at the car's opposite end. Fluid movement from one suspension spring unit to the other therefore prevents pitch and enables the car to ride at a level attitude.

At moderate speeds the fluid is simply displaced from front to rear spring unit and vice versa, the fluid pressure remaining constant so that the coupled nitrogen gas springs are not further deflected.

Roll or bounce mode (Fig. 10.69(c)) If the body rolls due to cornering or the car bounces as a whole, then both front and rear suspensions are deflected together. The simultaneous fluid displacements increase the fluid pressure and dynamically compress and contract both of the nitrogen springs. Thus with the inward movement of the pistons the projected effective piston areas increase so that a larger fluid area has to be lifted. Consequently both the front and rear spring stiffness on the side of the body furthest away from the turn considerably increase the suspension's resistance to roll.

Similarly, if the body bounces at both ends together, then the spring stiffness rates increase as the displacer pistons approach their inner dead centres so that a much greater resistance against the downward movement of the body occurs if the bounce becomes violent.

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