The forks are air sprung and oil damped. Air springing has the advantage of allowing considerable deflection for normal surface irregularities, whilst maintaining the ability to absorb considerable shocks without excess fork movement.
The movement of the synthetic rubber cushions in oil provides approximately equal and constant damping in both directions. These cushions also absorb the shock should the forks extend fully, whilst the oil cushion between the pistons and internal top fittings prevents too rapid closing on compression.
A red dot is marked on the front of each lower sliding tube. When correctly inflated to the load, the bottom edges of the shrouds should coincide with the red dots with rider or riders in position.
To obtain the correct adjustment, over-inflate the fork slightly by removing the inflation valve dust cap and coupling an ordinary tyre pump to the valve. The rider should then sit on the machine, keeping his feet on the footrests and maintaining balance from some convenient support. Air should then be released in small quantities, by depressing the stem of the inflation valve, until the bottom of the shrouds line up with the red dots. Replace the dust cap on the inflation valve.
It will be seen from the above that the forks can be correctly adjusted for solo, sidecar or pillion riding.
The inflation valve is fitted with a special core designed to open at low pressure and fitted with oil resisting rubber seatings. In no circumstances must a normal tyre valve core be used, as the action of the oil would rapidly destroy the natural rubber seatings. Dowty valve cores can be obtained from your Dealer or direct from us.
Topping up becomes necessary only if “bottoming" occurs in spite of correct inflation. Scrupulous cleanliness is essential.
Remove inflation valve dust cap, depress valve stem and allow all air to escape. The forks will close. Rest the crankcase on a block so that the forks are approximately1-in from the fully closed position. Unscrew the filler plugs and fill each leg with one of the recommended oils. (See " Filling.") Replace and tighten filler plugs.
Remove the block from beneath the crank case and depress the inflation valve, thus allowing surplus oil to drain off and the forks to close completely.
Carry out air inflation procedure, adjust to the load and replace valve dust cap.
Forks are supplied correctly filled and inflated. When filling, it is important that the recommended grade of oil be used, as its viscosity does not change appreciably over a wide range of temperatures. Consequently there is little or no alteration in its damping characteristics.
The recommended oils are listed on page 12.
The procedure for filling is exactly the same as described under "Topping-up," except that more oil will be required.
Unless dirt has been allowed to enter with the oil during filling or topping up, the oil need never be changed during the life of the machine.
The bottom bearings in each leg should be greased weekly. Six shots with the grease gun should be given to each greaser, situated at the back of the outer tubes, at the lower bearings. Only clean, high-grade grease should be used. Vent holes are provided in the sides of the outer tubes below the fork crown ; these allow surplus grease to escape.
Nuts and Screws.
Periodically check the tightness of all nuts and screws to ensure completely efficient working. It is particularly important that the steering tube pad bolt is kept really tight, otherwise the fork may become mal-aligned.