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Commando Forks are known to be inadequate by modern standards. There
are many modifications ranging from using different grades or amounts
of oil to buying reworked Showa internals from Kenny Dreer. The following
is a telling of what has come to be known as the Covenant Conversion.
Most of the British Motorcycle manufacturers of the classic era deserved
their reputations for producing well designed and solidly-built machines.
Yet even the proudest names occasionally produced designs which became
famous for their faults rather than their finer points. Triumph fans
prefer not to talk about the sprung hub and Norton devotees change the
subject when you mention Commando Combat engines. Royal Enfield buffs
admit that the Crusader five-speed gearbox was "Made like a gun"
only in that it regularly exploded with a bang. BSA, Velocette and others
all made the occasional blunder. As the saying goes: "A man who
never made a mistake never made anything." In fact it is this fallibility
that attracts many people to these older machines today. There is often
the real chance of actually improving in old design with today's materials
and technology. If such modifications are done sensitively, only the
most fussy of concours types can object. For those of us who actually
ride our machines regularly, modern paints, electrics, tyres and so
on are welcome developments. This article describes a couple of simple
modifications to the later of the two Norton Roadholder fork designs.
These mods improve the forks' damping characteristics, without affecting
the appearance of the machine.
The original Roadholder design, itself a development
of a pre-war non hydraulic version, was launched In September 1946 for
the following season. It used a double-taper damper rod which passed
through a restrictor inside the bottom fork bush. The design worked
well enough at the extremes of travel as the taper took effect, but
the damping provided around mid-travel was limited to moving oil in
or out of the space between the upper and lower fork bushes. This was
the so-called Long Roadholder.
The later Short Roadholder fork was fitted to Featherbed-framed models
from 1953 onwards. It was to continue largely unchanged until Norton
ceased manufacturing in the mid-Seventies. It also appeared on some
of the hybrid AJS and Matchless machines made in the final years of
the Associated Motorcycles (AMC) empire.
For normal road use, this later design was a distinct improvement and
closely followed the pattern of the Matchless Teledraulic fork. The
fork now used a separate internal damper tube and shuttle valve arrangement
which gave more precise control over the middle portion of fork travel.
It is easily distinguished from the earlier design by the lack of an
external spring and the presence of a damper rod screwed to the underside
of the fork top nut. Unfortunately, it was at this stage that two design
flaws, not present in the original Matchless fork, crept in. As the
faults only affect damping near the extremes of fork travel, it is perhaps
understandable that no mention of any deterioration was made in the
road test reports of the day. Doubtless, if riders of the brand new
Featherbed machine, introduced in 1953, had gone for a quick blast round
the local slag heap, some of them would have noticed that all was not
well with their forks. More specifically" the new design lacked
proper hydraulic bump stops to cushion the final metal-to-metal contact
at the two extremes of front suspension movement. The other Roadholder
features which made it such a good example of its type in the Fifties,
were still there: the large-diameter stiff fork stanchions, the solid
fork yokes and the light alloy sliders. The bumpstops, however, had
effectively disappeared.
Most, if not all, motorcycle forks with hydraulic damping incorporate
some arrangement for minimizing noise and uncomfortable metal-to-metal
clashing at the extremes of travel. Car designers, with their concealed
suspension systems, usually resort to crude but effective rubber bump
stops. These are normally tapered, to give a progressively firm control
as the suspension nears the limit of its movement. In motorcycle design,
where appearances have to be taken into account, this simple solution
is not often employed except on spring/damper units, where a rubber
collar is often fitted round the damper rod as a bump stop for the rear
suspension. As an aside, many enterprising riders in the heyday of the
cafe racer seemed to use this system on their front suspension (or a
fibreglass-in compression variant), judging by the number of badly-fitted
and tyre-marked fairings around in those days!
The more conventional methods used by telescopic fork makers almost
always involve either a tapered, moving restrictor arrangement or a
progressively blanked-off hole system. By such methods the damping over
the last inch or so of travel is rapidly increased, in order to avoid
metallic contact between sprung and unsprung components. Those of us
reared on old British field bikes will remember the sickening crash
of slider on stanchion, audible at a hundred yards, as we launched our
tired steeds off yet another death-defying molehill. In our youthful
exuberance, we took these jolts as a matter of course and marveled all
the more at Messrs Smith, Eastwood, Bickers et al. Little did we know
that two-bob's worth of oil and new oil seals would have made the old
banger float like a butterfly!
Getting back to Roadholders, the Norton designers tried to include both
of the above methods, viz a blanked-off pair of holes (one large then
one small, in sequence) at full extension, and a tapered restrictor
on full compression. Sadly, unlike the Matchless version, neither arrangement
works properly.
A quick glance at the illustration shows that as the slider moves down
the stanchion, the oil trapped in the space between the top and bottom
bushes is squeezed out through the large upper holes and the smaller
lower holes. This provides some damping to supplement the effect of
the separate damper assembly. In theory, as the slider nears the end
of its travel, first the large holes and then the small holes are blanked
off as they disappear inside the top fork bush. This leaves a small
cushion of oil trapped between the two bushes to stop them hitting each
other and putting excess strain on the materials (and your eardrums).
As far as it goes, this design is perfectly sound. Unfortunately, it
literally doesn't go far enough. Unlike the Matchless, Triumph and countless,
other designs, the large and small oil passages are not actually blanked
off at all; this is because the slider never moves far enough down the
stanchion for the top bush to cover them. What happens is that the delicate
damper valve assembly strikes the underside of the damper tube top at
a point where the stanchion oil-holes are still about an inch below
the top bush. In practice, therefore, instead of a nice oil cushion
providing a proper bump stop, the damper valve has the job of limiting
fork extension when it suddenly hits solid metal - a job it should never
really be expected to do.
If those of you with Roadholders don't believe me, remove the fork top
nut from one side of your forks, (having first taken off your front
wheel and mudguard) and fully extend the fork leg. You will notice that
the damper rod disappears an inch or so below the end of the stanchion.
In other words, when the top nut is in place, it and the attached damper
rod stop the forks from extending as far as they otherwise would. For
those doubting Thomases who are still unconvinced, have a look at the
underside of the damper tube top and you'll see the tell-tale marks
in the alloy where it has been struck by the damper valve.
You may have noticed during the earlier procedure that the last bit
of fork extension happened only slowly, no matter how hard you tried
pulling the slider down (assuming you have some oil in the fork leg).
This was because the hydraulic 'lock" - ie the bump stop - was
working for a change, since the damper tube no longer limits fork travel
when you take the top nut off. In a moment you will learn, if you haven't
already guessed, how to get that bump stop working properly with the
fork fully assembled.
Meanwhile, what about the other bump stop, the one supposed to work
at the point of full compression? Well again the Norton designers missed
the chance to design a proper system. In fact if, as seems likely, they
more or less copied the Matchless design, their copy was not as good
as the original. From the illustration you can see that as the slider
rises on the fork, the base of the stanchion and its bottom bush act
like a piston, forcing the oil up between the stanchion and the damper
tube. As the forks near full compression, however, the base of the damper
tube (which is tapered) progressively restricts this escape of oil.
The widest part of the tapered section almost completely blocks the
passage of oil up the inside of the fork stanchion. The oil is therefore
trapped and forms an hydraulic cushion that prevents bottoming out under
normal circumstances. Or rather it should do, but again the Roadholder
idol turns, out to have feet of clay.
In this case, the design is flawed because there are holes drilled on
or under the taper section of the damper tube, rendering the desired
hydraulic lock impossible. Naturally, some sort of hole is required
in the damper tube to allow oil to flow in behind the damper valve,
as it travels up the tube during fork extension. The Matchless design,
amongst others, puts this hole (or holes) near the base of the damper
tube but just above the taper. In this way the oil can enter the damper
tube freely during extension, but when the fork nears full compression
the oil has nowhere to escape from below the taper, so the proper cushioning
effect is available.
At first, the Norton people put four 1/4 in. holes in the plain section
at the very bottom of the damper tube below the tapered part. With four
such holes the oil is never really compressed below the taper on full
bump, it simply flows away up the damper tube past the open shuttle
valve. Since no hydraulic lock can possibly occur, the forks may bottom
out on rough surfaces.
By the time the Commando appeared, the Norton designers
had hashed the hole (sic) idea, so that now there were two holes instead
of four and they were drilled through the flanks of the taper itself
rather than below it. In as much as this modification allows the very
last part of the fork travel to form an hydraulic lock, it represents
an improvement over their first attempt. However, in so far as it represents
a repeat of the earlier mistake, albeit in a less spectacular fashion,
it really is rather daft.
It does not take a degree in mechanical engineering to work out that
there may as well be no taper at all above the lowest edge of the holes.
This is because, as before, there is no real restriction of oil movement
during fork compression, since it just squirts up inside the damper
tube through the holes. Only when the bottom of the stanchion passes
beyond the lower edge of the holes, does the taper cause any restriction
and thereby provide a belated cushioning effect. Now you know why Norton
never used an offset spindle like Velocette and Royal Enfield did -
they had to keep the spindle under the bottom of the fork leg to stop
the stanchions poking through when twin leading-shoe brakes were invented
(don't laugh, it might be true!).
Still, despite everything I’ve said so far, most people seem more
or less content with their Short Roadholders as they are. Certainly
they give a reasonable ride - commendably free of fork flex - over average
road surfaces. Nevertheless, by means of a couple of simple modifications,
their behavior on poor surfaces can be transformed. As some of our roads
begin to resemble the rolled and graded scrambles tracks of yesteryear,
the modifications will be of benefit to almost any machine fitted with
these forks.
To provide a proper bump stop on full extension, it is only necessary
to arrange for the holes already in the stanchion near the bottom bush,
to be blanked off in sequence. At first sight, the easiest way might
seem to be to make up a longer damper rod to allow the forks to extend
fully. However, even if the fork springs were packed up to provide the
extra extension required such an arrangement would not be desirable
because of the limited overlap, or engagement, of slider over stanchion.
To maintain adequate stiffness of the front forks, there should be several
inches of stanchion inside the slider even on full extension.
The best way of providing a bump stop in this design is simply to make
longer top bushes - about one and a half inches longer in fact. The
exact length is adjusted so that even when completely extended, with
top and bottom bushes in contact, the damper valve stays just clear
of the underside of the damper tube top. With such bushes fitted and
the fork topped up with oil, the last fraction of travel is properly
cushioned as the oil holes are blanked off in turn until the hydraulic
lock occurs. The effect is immediately obvious if the engine is out
of the bike, since even under such circumstances (when the rolling chassis
on its own is seriously oversprung) it is impossible to top out the
forks. If making special fork bushes is not possible, then the insertion
of a collar under the original fork bush is a reasonable compromise.
Again, the extra collar should be about one and a half inches long,
with the final adjustment best made by measuring up the various fork
components to see -how far below the top bush the stanchion oil holes
are kept by the damper assembly at full extension. Unlike an extra-long
top bush, the collar will need to be prevented from falling down inside
the slider and the best way of doing this is to make it a snug fit in
the top of the slider and a looser fit over the stanchion. In this way
the collar can be tapped down into place as the top bush is fitted.
A drop of Loctite Bearing Fit could be smeared on the outer diameter
if there is any suspicion of looseness further down inside the slider.
Triumph used a separate bush like this (made of a plastic material)
in the forks they manufactured between 1964 and 1971. With the Roadholders
it is possible to use an old fork top bush if you can't get a spacer
made up. A slightly worn bush is probably best, as there is less chance
of it being pulled down the slider by the stanchion. Trim the brim from
the top hat shape and ensure the bush is a firm fit in the slider don't
forget to check that it is actually long enough to stop the damper valve
touching the underside of the damper tube top. In an ideal world it
would be possible to give the required length, accurate to the nearest
millimetre, but where old British bikes are concerned it is often unwise
to be so precise. Each (possibly mixed-up) set of fork components should
be measured for the best results. In fact, by experimenting with different
damper rod and top bush (or spacer) lengths, it is possible to adjust
the effective length and travel of Roadholders I quite easily. However,
this should only ever be necessary if the forks are fitted to a non-original
type of frame, when the ride height may be incorrect. The average home
mechanic is advised to leave this kind of thing well alone.
Arranging a credible bump stop on full compression is even easier than
providing one of full extension. All that is necessary is to block the
existing holes in the lower section of the damper tube and re-drill
them in a position just above the taper. The only question is, how to
block the holes? Those with brazing or welding gear (and the skill to
use it properly) will even now be reaching for their goggles and spark
igniter. The rest of us will have to indulge in a bit of lateral thinking.
A simple method is to tap the holes (say 8mm) and insert a suitable
bolt or screw.Using Loctite, and with the bolt head cut off and the
ends peened over, this works quite well. It can even look reasonable
if the ends are dressed with a file, but as nobody is going to see the
result it only matters that you preserve the taper and don't leave any
metal protruding. The seal doesn't have to be totally oil tight either,
although a good job will be and it is
as well to aim for this.
I use a simple piece of alloy rod cut to length, with the ends squashed
in a vice after fitting and then filed smooth. Those of you with the
earlier pattern tubes with four holes can obviously not use the bolt
or rod method for the second pair of holes at right angles to the first.
Instead, you can fit a collar over the lowest part of the damper tube
to block off all four holes; this can be held in place by the taper
above and the bottom of the slider below. Of course, it must be no wider
than the taper at its widest point, so that the base of the stanchion
can pass over it. Alternatively, the bottom of the four hole damper
tubes could even be filled with Isopon or an epoxy resin, but I'm sure
you can do better. Having blocked off the original holes, by whatever
method, the two replacement holes (only two are necessary about 1/4
in.) can be drilled just above the taper, as in the original Matchless
design. The position is not critical, as long as they are below the
lowest point reached by the damper valve inside the tube: there is plenty
of leeway, so just above the taper will do.
Having performed these simple modifications, your bike will have a set
of forks the equal of most and better than many. On the neglected roads
of the Eighties, the need for good damping at the extremes of suspension
movement is becoming more pressing and as these alterations are totally
concealed, they should not offend the purist or devalue your machine.
Instead, they should give you that little bit more pleasure as you ride
your classic.
Those of you ready, willing but unable to carry out these changes, will
be glad to know that a kit of parts is available to do the job. The
kit consists of two alloy bushes, four alloy dowels and fully illustrated
instructions (in English, French, German and Dutch).
Editor's Note: This is an old article and Peter Crespin is no longer
making the conversion kits. Peter's contact details are below and if
there is enough demand they may become available again.
Peter Crespin
The Writers Bureau
P.O. Box 57
Buxton
Derbyshire
SK17 7AE
UK
Tel: +44 (0)1298 27112
Fax: +44 (0)1298 71093
Cell: +44 (0)7973 854452
Web: www.thewritersbureau.com
Email: pc@thewritersbureau.com
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