SECTION G1
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FRONT FORK DESCRIPTION
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The Commando Fork is a development of the
"Roadholder" Fork which is world famous as an
extremely strong and precise steering unit under
all conditions. The forks comprise high quality
seamless chrome steel main tubes with light
aluminium sliding members for reduced unsprung
weight. Fork movement is controlled by long single
rate springs and two-way oil damping The sliding
members are supported on oil impregnated sintered
bronze bushes at the top and steel at the bottom to
give maximum support and to minimise wear. The fork
yokes are substantial cast components which provide
great rigidity. The steering head pivot comprises
two pre-packed and sealed ball journal bearings The
steering head arrangement illustrated is that usad
on 1971 and later models with the stem captive in
the upper yoke and incorporating non-adjustable
bearings.
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SECTION G2
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HOW FORK WORKS
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Consider that as the front wheel meets a bump in
the road surface the front forks are compressed
against the main springs. As the wheel passes the
bump the forks are permitted to return to the
static position. Should the motor-cycle become
"front end light" such as negotiating a hump-backed
bridge, the fork extends fully but once again on
level road, returns to the static position. The
natural tendency for a spring controlled suspension
system is to over react and oscillate before
returning to static position. Such a system wouid
result in a pitching motion on rough roads and a
subsequent loss of stability. To counter this
condition, oil damping is provided which in the
case of the Commando fork slows down the spring
return both on compression and extension. Each fork
slider contains 150 cc of oil which is metered by a
rod and valve damper assembly. The damping sequence
is detailed in the following text and the complete
fork assembly is shown in Fig. G5. In the latter
diagram "A" illustrates the fork at rest with parts
descriptions, "B" the form compressing and "C1" and
"C2" the fork extending.
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FORK AT REST (A)
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The oil lies in the bottom of the slider and
occupies the spaces between top and bottom bushes,
damper tube and main tube and occupies the lower
end of the damper tube.
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FORK COMPRESSES (B)
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Consider that the main tube and damper rod
remains stationary thus as the fork compresses, the
slider and damper tube rise. As the slider rises,
oil passes into the lower end of the damper tube.
There is no passage of oil between the damper rod
and damper tube cap but oil passes from the slider
through the main tube bleed holes between the
bushes. The vacuum caused between the fork bushes
during compression causes oil to be sucked from the
space between the fork bushes. As the fork
compresses further, the damper tube passes further
into the main tube until the cone shaped bottom
area enters and the oil passage into the main tube
is progressively cut off until only the bleed hole
into the damper tube remains. Oil is then able to
pass through the bleed hole slowly, thus slowing or
"damping" the fork action, finally providing a
hydraulic bump stop on full compression.
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FORK EXTENDING (C1 and C2)
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The main tube remains stationary whilst the
slider and damper tube descend. Oil remains trapped
between the top and bottom bushes. The damper valve
within the damper tube remains closed thus as the
damper tube descends, oil passes between the damper
rod and damper tube cap into the space between the
damper tube and main fork tube. As the fork
continues to extend, oil trapped between the fork
bushes is forced back into the slider through the
large hole then the small hole to slow down the
fork action and prevent the fork "topping" heavily.
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