tech37

Off-Road Japanese Forks for British Bikes
by Lynn Bennett

I have been an advocate and a do’er in adapting Japanese motocross
suspension units to our British bikes. My successes have been 1981
Yamaha IT465 forks to a BSA B44 Shooting Star, Yamaha YZ forks to a
Mark 4 Rickman, Yamaha YZ forks to a B50 frame, and Yamaha YZ forks to
a Triumph Trophy Trail (basically a modified B50 frame). All used an
extended travel late 70’s, early 1980’s Japanese motocross fork set.
The years selected actually were just ones of opportunity that showed
up at the right time and happened to be Yamaha, by coincidence. These
forks all had 10 to 12 inches of travel, were dampening controlled by
dampening rods (not cartridges, like the much later variety), had
internal springs, were leading axle type (the axle mount is on the
front of the slider rather than directly under the leg), and came with
a wheel and brake assembly. One set was a take from one of my earlier
bikes, another came off a whole bike whose remains were sold off to a
guy building an AHRMA EVO racer, another came off a bike purchased at
the El Camino swap meet for $100, I believe, with the remnants going
into the trash, another off a later disk braked bike, whose remains
were sold off. In each case where they were purchased, the cost of the
forks, wheel, and triple clamps was never more than $150 and usually
less than that. Remember each included a wheel with brakes and an axle.
The B44 had a CZ wheel fitted with adapters machined by me to allow the
CZ axle to fit the Yamaha forks.

It is important that the triple clamps for the Japanese forks stay with
the fork tubes in order to not upset the trail dimension of the bikes
geometry. Leading axle fork legs fitted into standard British triple
clamps will reduce the trail by about an inch or so, with the
possibility of instabilities in the handling under certain adverse
condition. In my experience the easiest way to fit the triple clamps to
the British frame is to use the British upper and lower head bearings
and the stem from the original British bike modified to fit the
Japanese triple clamps. As it is difficult to generalize on fitting the
Brit’ stem to the Jap’ triple clamps I will explain what I had to do
for the ones that I did. Some required I turn down the bottom of the
stem to fit into the Jap’ triple clamp. Some required that I make a
pressed in adapter for the triple clamps and then press the stem into
the adapter. The fit must be hydraulic press tight. On some I had to
make spacers to place under and /or on top of the upper triple clamp to
allow the upper stem nut to be able to tighten onto the upper triple
clamp. The bottom line is that machining of parts and the making of
parts and pieces is necessary, always considering what the ultimate
strength will be so that its’ strength is not compromised: a little
engineering is in order.

After the triple clamps are fitted it is necessary to figure out how to
stop the fork tubes from hitting the gas tank. All leading axle forks
have the tubes set back closer to the gas tank so stop modifications
are most certainly going to be required. On some I was able to grind
down the original stops so that the back of the triple clamp hit up
against them. On some I had to make a bolt on stop bolt, drill and
thread the lower triple clamp at the correct place so that the new stop
bolt hit the frame’s stop. Again a little engineering might be in order
to make sure the stop will not be over ridden in a fall down. In any
case, some wheel lock (how far the forks will move side to side) will
be lost in this conversion but in the ones I have done it has never
impaired actual usage.

Fork legs from this era actually have too much travel for our uses. If
the travel is not reduced the frame may drag on the ground in the full
collapse mode or the tire hit the bottom of the triple clamp (an
excellent braking action occurs but is definitely not desirable). I
have found that the Brit geometry is not grossly altered if we limit
front fork travel to about 8 1/2 inches. To keep the bike from looking
like a chopper with the front 2 to 3 inches higher than the rear we
will have to raise the back at least one inch by replacing the original
shocks with longer travel new shocks. The most readily available I have
found are the standard Progressive Suspension units with eye to eye
lengths available from 12.5 to over 15 inches. We’ll get to shock
selection later. The fork legs must be disassembled (take this
opportunity to replace the seals) and a machined spacer placed between
the anti-topping spring and the top end of the dampener rod. Before
disassembly, after spring removal, and with all the oil removed the
stock travel must be determined. With that number in hand, say 11.5
inches for example, you subtract the desired travel, 8.5 inches, to get
the length of the spacer to be made. It’s outside diameter must not
exceed the anti-topping spring outside diameter and its’ inside
diameter must give an easy sliding fit over the dampening rod where the
anti-topping spring will use it as a spring seat. Our example would be
a 3 inch long spacer (11.5-8.5=3.0 inches). Here is a little
disassembly trick, by the way, for the allen bolt on the bottom of the
fork slider that holds in the dampener rod: use an allen wrench 3/8
inch socket driver fitted to an air or electric impact wrench. Leave
the spring inside the leg, compress the leg by pushing the air impact
wrench down, and rattle away. It will come out instantly, usually. When
the travel limiting spacer is fitted reassemble the legs. The main fork
springs will either have to be cut to shorten them or the shorter of
the two piece ones will have to be discarded. Most of these adaptations
worked fine with just the main spring. Additional spacers will have to
be made up to pre load the main springs about 1/2 to 1 inch when the
caps are screwed in and seated. To assure that the main springs will
not coil bind count the number of coils and measure the wire thickness.
Multiply the number of coils times the wire diameter (this is the
springs collapsed length) and subtract that number from the springs
total free length and subtract the amount of pre load intended to be
used. This result is the amount of total spring travel available and
this number must be 1/2 to 1 inches more than the designed 8.5 inch of
fork travel we will use. On all of the adaptations I have done, this
was absolutely not a problem but you should make the calculation to be
sure. The springs won’t last long if you exceed their travel and will
come out of the fork leg looking like a pretzel.

Since all the long travel forks I have seen from this era are made to
slide (adjust) up and down in the triple clamps we will use this to
level the bike. First we must determine how high the tire is going to
go on full compression of the forks. Remove the springs and raise the
tire into the fender (don’t forget to support the frame with a box)
until the forks are hard bottomed internally. While holding the forks
in full compression move the legs in the triple clamps until the tire
is clear of the fender with about a 1/4 to 1/2 inch of clearance.
Tighten down the triple clamps onto the fork legs. Add oil into the
fully collapsed fork leg until it is within six inches from the top of
the tube. Allow time for the oil to drain down into the internals of
the fork leg and stroke it to insure there is no air inside and
remeasure for the 6 inch figure. I always suggest using 20 wt fork oil
for vintage forks as that seems to work out best. But the oil’s weight
can be adjusted both up or down to make the ride more comfortable but
at the increase or loss of dampening action. The oil height can also be
adjusted to effect the anti bottoming characteristics of the forks but
I find the best action when the dampen rod is fully covered by oil with
the fork legs fully extended. Put the springs and pre load spacers back
in, and tighten the caps onto the tube tops.

Now for some numbers. With the bike unladden, standing on its’ wheels
the front should settle 1/2 inch to no more than 1 inch. This initial
sag is adjusted with the pre load spacers but the need for much more
than 1 inch of pre load means you really need a bigger spring rate
spring. The forks have 8.5 inches of travel available and the sag with
the rider on board the bike (full weight, tippy toes on the ground)
should use up about 25% of that travel or about 2 inches. Stroke the
forks to get them to over come stiction and settle in the right place.
Faster riders might like the forks to ride a bit higher in their
stroke.

With the rider on the bike with his full weight measure how close the
bike is to level. Usually the rear shocks only have to be about an inch
longer, eye to eye, to get the bike back to level but I have 2 inch
longer shock on one of my bikes. If the new rear shock turns out to be
too long you can drop the fork legs in their triple clamps, but don’t
ever raise the fork tubes in the triple clamps more than the position
determined at the fully collapsed test. After the bike is level and
with the rider’s weight on the bike the swing arm should be almost
level but on the jacked up side (frame swing arm pivot slightly higher
than the wheel axle).

Addendum:

While not absolutely critical I decided that the upper dampening holes
in the dampening rods needed to be plugged and duplicated down the
dampening rod so as to emulate the piston/dampening rod holes
relationship in the original non-shorten condition. Just plug the
originals with brazement and move them down the dampening rod to
correspond to however much of the fork travel was removed. I did this
on my Triumph Trophy Trail TR5T and frankly I could feel no difference.
(Maybe it takes a huge bump or hole to appreciate that there is a
difference as the theory says there should be)