Norton Commando Workshop Manual

## Front Forks/Steering

# G

### SECTION G1

### FRONT FORK DESCRIPTION

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.

### SECTION G2

### HOW FORK WORKS

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.

### FORK AT REST (A)

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.

### FORK COMPRESSES (B)

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.

### FORK EXTENDING (C1 and C2)

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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