Royal Enfield CONSTELLATION 1958, Workshop Maintenance Manual

Page: 85 / 106

SECTION J1
Front Fork
With Casquette and Aluminium Alloy Bottom Tubes
1. Description
The telescopic fork consists of two legs each
of which comprises a main tube of chrome
molybdenum alloy steel tubing which is
screwed into the Casquette fork head at the
upper end and securely clamped to the fork
crown. Fitted over the lower end of the main
tube is the bottom tube made of high strength
aluminium alloy with an integral lug which
carries the wheel spindle. Fitted on the lower
end of the main tube is a steel bush which is a
close fit in the bore of the bottom tube. The
upper end of the bottom tube carries a bronze
bush which is a close fit over the outside
diameter of the main tube. The bush is secured
to the bottom tube by means of a threaded
housing which contains an oil seal. A stud
known as the "spring stud" is fitted in the lower
end of the bottom tube and a valve port is
secured to the lower end of the main tube. As
the fork operates oil is forced between the
spring stud and the bore of the valve port
forming a hydraulic damping system. A
compression spring is fitted inside the main tube
between the upper end of the spring stud and the
upper end of the main tube. The lower end of
the main tube and upper end of the bottom tube
are protected by a cover secured to the fork
crown.
A special fork is available for sidecar
machines. This has bottom tubes with extended
wheel lugs giving less trail and is fitted with
stronger springs and a steering damper.
2. Operation of the Fork
The fork provides a range of movement of 6
in. from the fully extended to the fully
compressed
position. The movement is
controlled by the compression spring and by the
hydraulic damping system. The hydraulic
damping is light on the bump stroke and heavier
on the rebound stroke, thus damping out any
tendency to pitching or oscillation without
interfering unduly with the free movement of
the fork when the wheel encounters an obstacle.
The fork is filled with S.A.E. 20 to a point
above the lower end of the fork spring so that
the damper chamber "B" is always kept
full of oil. Upward movement of the wheel
spindle forces oil from the lower chamber "A"
through the annular space between the spring
stud (38067) and the bore of the main tube valve
port (38138) into the damper chamber "B."
During this stroke the pressure on the underside
of the valve plate (38073) causes this to lift so
that oil can also pass from "A" to "B" through
the eight holes in the valve body. Since,
however, the diameter of chamber "B" is less
than that of chamber "A" there is not room in
"B" to receive all the oil which must be displaced
from "A" as the fork operates. The surplus oil
passes through the cross hole in the spring stud
and up the centre hole in the stud, spilling out
through the nut (38076) which secures the upper
end of the spring stud to the bronze guide at the
lower end of the fork spring.
On the rebound stroke the oil in the damper
chamber "B" is forced through the annular space
between the spring stud and the bore of the main
tube valve port. During this stroke pressure in
chamber "B" closes the two disc valves at the
upper and lower ends of the chamber so that the
only path through which the oil can escape is the
annular space between the spring stud and the
port. Damping on the rebound stroke is therefore
heavier than on the bump stroke. At the extreme
end of either bump or rebound stroke a small
taper portion on the spring stud enters the bore
of the valve port, thus restricting the annular
space and increasing the amount of damping. At
the extreme end of the bump stroke the larger
diameter taper on the oil control collar (38075)
enters the main counterbore of the valve port
thus forming a hydraulic cushion to prevent
metal to metal contact.
3. Dismantling the Fork to Replace Spring,
Oil Seal or Bearing Bushes
Place the machine on the centre stand,
disconnect the front brake control and remove the
front wheel and mudguard complete with stays.
Unscrew the bottom spring stud nut (38080)
which will allow oil to run out of the fork down
to the level of the cross-hole in the spring stud.
Now knock the spring stud upward into the fork
with a soft mallet, thus allowing the remainder
of the oil to escape. Pull the fork bottom tube
down as far as possible, thus exposing the oil
seal housing (38157). Unscrew this housing
either by means of a spanner on the flats with
which it is provided or by using the gland nut
handgrips (E.5417). The bottom tube can now
be withdrawn completely from the main tube,
leaving the bottom tube bush, oil seal housing
and oil seal in position on the main tube.
Now unscrew the main tube valve port using
"C" spanner (E5418). The spring stud and
spring can now be withdrawn from the lower
end of the main tube.
The steel main tube bush (38156) can now
be tapped off the lower end of the tube, if
necessary using the bottom tube bush for this
purpose. Before doing this, however, it is
advisable to mark the position of the bush with
a pencil line so as to ensure reassembling it in
the same position on the main tube. The reason
for this is that these steel bushes are finish
ground to size after fitting on to the tubes so as
to ensure concentricity to the main tube. After
Section J1 Page 2
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