Norton Front Forks
Factory shop manual errors
The fork assembly's comprise of the fork stanchions, lower
legs or bodies, the dampers, and springs.
The factory shop manual pn 065146, I believe, is in error in
it's description on how the commando forks function. Figure G2 and text section
G2...especially in regards to the early commando (and atlas).
potential damper travel- 6.25/7.312
damper body length- 7.75/9.125
fork travel- 6.15/6.15
damper rods- 19.875/19.875
fork staunchions- 21.875/23.187
damper bottom bolt- 26cei/24unf
damper top cap- 26/28 TPI
damper rod top thread 3/8"- 26cei/24unf
Bottom bump stop mod analysis
The Covenant conversion is an interesting bit of craftsmanship.
This is mainly due to the positioning of the bump stop holes on the early
models below the taper apex. This kit is legitimately needed for
these early versions. If the vent holes are below the taper apex, then
the oil flow into the center of the damper body is unimpeded. In this condition
the forks have NO bottom hydraulic bump stop. The conversion kit plugs
these holes to restore oil flow past the apex restriction by relocating
the holes above the taper apex like the later commando forks which do have
some hydraulic bump stop.
A few specifications to allow analysis.
(I assume I have typical components......)
The damper body taper apex is 1.010" in diameter.
The fork tube ID is 1.022" diameter
The vent holes (2) on the late commando are .250" diameter
The 2 vent holes area summed is .098 in/sq.
Some high school math shows that at full restriction (fork and damper meeting
at taper apex) you have .0192358 in/sq.
This represents the minimum area that fluid can pass through and thereby constituting
hydraulic bump stop.
If the math is done to find out the diameter of the taper where the aperture,
between the taper and fork tube ID, is .098 in/sq, it can
be determined that that happens at .959" diameter. It seems like poetic justice
that it just happens to cooincide with the center of the 2 vent holes.
Therefore as the fork tube passes down the ever decreasing size vent hole,
and increasing size of taper on the damper body, the hydraulic bump stop
becomes ever harder as would be expected. If the vent holes are moved even
1/8" higher, there would be NO further modification on hydraulic bump
stop operation possible. It simply has no further effect. Finally, the fork
bottoms out against the fork leg before coil bind happens. The bump stop
taper apex is .55"above final fork bottoming out
The factory manual eludes on the drawing and in the text that as the fork
extends that the large hole becomes blocked off by the upper bush to start
the function of a hydraulic bump stop and finally the small hole becomes
blocked by the upper bush and final hydraulic stop is complete. THIS IS AN ERROR.
this is why
The fork damper rod tops out in the damper, 0.6"before the fork
is extended enough to start to block off the big hole in the fork tube. This
topping out action is also 1.25" short of reaching the smaller bump stop
hole. The damper topping out limits the fork to 1.55" less travel than
the bushes would otherwise allow. The reasons the factory manufactured the
forks this way may hopefully be made known to us all one day.
Top hydraulic bump stop
A longer damper rod 'would' allow for more fork extension, an therefore more
The need for this extra travel may continue to be controvertial. Topping
out is usually cured by renewing the damper cap and rusty damper rod
The pix shows
(left) a late model commando damper body with 2 vent hole above taper apex.
(center) a early commando with 4 vent below taper apex,
(right) a shorter atlas damper body with 4 vent holes below the taper apex.