On extension, that is on recoil after impact, the damper tubes move downwards with the unsprung sliders, to which they are attached, away from the damper piston rods which are fixed to the sprung part of the fork at the top, and the damper pistons move into contact with the piston rod washers closing the damper valves. The upward movement of the pistons relative to the damper tubes draws oil through radial holes in the tubes at the bottom and fills the space below the pistons.
On compression of the fork the damper pistons move downward relative to the damper tubes, but are lifted off the piston rod washers allowing the oil below them to pass them freely through the valves into the upper parts of the damper tubes.
Again on recoil the damper tubes below the pistons are replenished, but the oil above the pistons is trapped and can only get out through restricted openings at the top ends of the damper tubes. This provides the resistance necessary to give the desired degree of damping.
The oil from the upper ends of the damper tubes drains down outside them into the sliders and is available for further use.
To prevent metallic contact and "bottoming" under very severe impact a hydraulic lock is formed by the internally tapered ends of the main fork tubes coming down over the externally tapered or coned lower ends of the damper tubes.
The above cycle of operations is going on all the time the machine is being driven over rough surfaces, and the more irregular the surface the greater is the degree of damping. It has been found in practice that over almost smooth surfaces the fork works almost undamped, and moves freely over slight inequalities, thus overcoming the objection usually found with most damping systems that they prevent movement of the fork altogether under slight impact.
Maintenance. In normal use no attention should be needed for at least 10,000 miles. After this it is optional to change the oil. If it is desired to change the oil remove the ¼-in. drain plugs at the bottom to the rear of the fork ends. Do not confuse them with the larger hexagon nuts in the centre - these hold the damper assemblies. Refit the plugs after draining.
Unscrew the two large plated hexagon head plugs (the damper piston rod adaptors) from the top cross members and fork tubes. These will lift up far enough to allow oil to be poured into the fork tubes. Put 1/8-pint (71 c.c.) of S.A.E.20 Viscosity oil into each one. Refit and tighten the adaptors.
Should one or both of the fork spring dust covers rattle put this right at once, otherwise the rubber seating washers
will be damaged and the fork will have to be fully dismantled to renew them. See next paragraph.
Raise the front of the machine and bounce the front wheel sharply on the ground to shift the split sleeve up the fork tube and compress the rubber washer.
If the clamp bolt is too slack the sleeve will move down again when the fork spring extends. If not loose enough the sleeve will not move.
A hydraulic lock will form if any oil is left in the fork and this also will prevent the sleeves moving, so that both sides must be drained.
Tighten the clamp bolts when the covers are firm. They will never be quite tight and can always be turned by hand, but must be secured against rattle.
Refill fork with oil.