Word By Paul Berryman.
Director at Visor vision Ltd.
If your motorcycle is old enough
to buy a round of drinks in the USA (that’s 21 by the way) then chances are,
it’s got damper rod forks. A damper rod, in the simplest terms, uses the
resistance created by squeezing fluid through a hole to create a damping force
that will smooth the operation of the suspension. The fluid in this case is
fork oil whilst the hole is drilled into the damper rod.
To help imagine the limitations of
this, and without using a single scrap of the hard to understand physics behind
why, let’s go back to childhood and create a little story in our heads.
In our imaginary story, you have
in your hand a bog standard 1960’s/70’s issue water pistol. We’re going to pull
the trigger 3 times, in 3 different ways, and we’re going to gauge what happens.
Pull 1) Very slowly pulling the
trigger, water dribbles out of the end for as long as you continue to pull.
You’ll notice the trigger doesn’t seem to have any resistance against your
finger. Although there’s water flowing, there’s not enough pressure to slow (or
damp) the triggers progress.
Pull 2) Pull the trigger faster,
it’s now clear there’s some damping as the water is squeezed at pressure
through the exit hole – this is it’s sweet spot. It feels firm against your
finger but yields smoothly to the pressure you’re applying and provides a good
strong squirt. Your target is getting soaked.
Pull 3) Try and pull the trigger
as fast as you possibly can, much faster than before. Water again squirts out
strongly, BUT - not really any harder than before. You notice the
pressure at the trigger is so high, it stops you pulling it as fast as you want
to. At the trigger it now feels solid, unresponsive. The damping has stopped
simply “cushioning” and is approaching “lock up”.
Got all that? Ok, 3,2,1 – back to
adulthood. How boring.
Damper rod forks function by
forcing oil through open holes. That’s the same as in principle as our
imaginary water pistol.
The limit of their design is that
you have a fork that can be too soft to deal with “slow” suspension movements
(see Pull 1) and too stiff to deal with “fast” ones (see Pull 3). A slow
suspension movement may simply be an undulation in the road, gentle braking or
tipping into a corner. A fast suspension movement is akin to hitting one of the
UK’s many unfixed meteorite impact sized potholes.
Worth noting that slow and fast in
this context have nothing to do with the speed of the bike, only the speed that
the suspension is asked to move by the forces acting on the bike – such as
hitting bumps, cornering or braking.
With no damping at the slow end,
the spring alone is there to absorb the forces. The un-damped energy passing up
(and down) the spring will result in a fork that feels “boingy” or “mushy”.
At the fast end, if the damping
“locks up” the suspension won’t travel as far or as fast as needed to absorb
the bump – the forks feel ridiculously solid and very likely you’ll feel your
wrists have taken the majority of the impact. We’re all getting older aren’t
we? We deserve better than sore wrists. Not nice.
It’s not all bad news with the old
forks, because there is a sweet spot (see Pull 2), and it IS possible to make a
damper rod fork with a range of useful operation wide enough to make functional
motorcycle suspension. BUT…the system isn’t anywhere near as good as the
technology that replaced it. For around 20 years now, our motorcycles have been
shipped to us with significantly better ‘cartridge’ forks.
In a cartridge fork the oil
doesn’t just flow through an open hole as in a damper rod. There are multiple
holes in a piston that pressurises oil inside a cartridge as it moves up and
down (hence the name). These holes sit under a stack of deformable shims, also
attached to the piston. These rubber shims, which are really just like floppy
washers, deflect under pressure from the oil flow, thus creating the damping we
need.
The shims are cleverer than they
sound. Although you may imagine they simply increase resistance at all speeds
of the fluid flow, by making a “stack” of them in varying sizes and
thicknesses, they can be tuned to increase damping at low speeds, and decrease
damping “lock up” at very high speeds. The result? Suspension that remains plush
when dealing with gentle undulations at the lower end and bigger bumps at the
top end: the very weakest points of the old damper rod system.
The good news doesn’t end there –
in the cartridge there’s a separate piston for both compression and rebound, so
you have control of the fork’s damping on both the up and down stroke.
So – if you have damper rod forks,
and love the sound of all this new fangled “functional” suspension, how can you
get some of the action without changing your original forks for newer items?
Well, the clue is in the name and they’re called “cartridge emulators”
A cartridge emulator is a simple
yet clever piece of kit that fits into a damper rod fork after some small (and
shed-friendly) modifications to your original fork innards have been made.
They’re also relatively cheap.
We fitted a set made by YSS into
our FZ750 forks, again helped ably by my go-to man for bouncy bits Jon Slenzak
of S1 Suspension Technologies near Silverstone. As well as saving me from
getting my hands dirty, having Jon do this meant that we also get to chart the
forks “before and after” performance by using his shock dyno to measure their
damping rates. Cool!
Who is capable of making this
modification? Well, simply put - if you can change forks seals and can drill
holes in things, then you can do this. The hardest part of the whole job is to
get the forks disassembled and assembled again.
Here’s our step by step guide
Undo the fork cap. Be careful! There’s a big tense spring
inside just waiting to fire the unscrewed cap off in your direction!
Remove the preload spacer
Remove fork spring
Remove dust seal, circlip, oil seal and top bush (if fitted)
Drain out fork oil, turn fork upside down and pump the
stanchion back and forth into the slider – Top tip - don’t bother using the
tiny drain hole fitted to some forks, it takes forever and never drains the
fork completely.
Option - If you’ve got a mid 80’s Yamaha a Variable Damper
may fall out during point 5 – it looks like a cartridge emulator, it isn’t.
Remove the bolt from the underneath of the fork to release
the damper rod. An air tool is useful for this to crack the thread before the
damper rod spins, or you can lock the damper rod in place by inserting
something from the open top of the fork – this could be anything from the
correct hex-head tool to a firmly applied broomstick – anything that stops it
spinning as you undo the thread will work.
Remove damper rod and top out spring if fitted. A good way to get
the damper rod out is to undo the bolt about 5 turns and then tap the head of
the bolt to ease the damper rod away from it’s snug fit in the fork bottom.
Your fork is now empty – give it a clean out with brake
cleaner and get the last vestiges of old oil out of it.
Clamp the damper rod into a vice and drill out the holes as
per instruction – on our forks we had 4 x 5mm holes, the YSS instructions ask
for 6 x 8mm holes, that are no closer than 10mm. Doing this means the damper
rod flows enough oil through it to now let the emulator do the damping. Clean
up any burrs and swarf from the drilling both inside and outside the rod. Be
careful to avoid damaging any threads that may be inside your damper rod if you
clean up the burrs with a file.
Put the modified damper rod and top out spring back into the
slider. The bottom bolt can again be done up using the air/correct
tool/broomstick method from point 7 above.
Refit top bush, seal, circlip and dust seal. You may or may
not have a top bush depending on the fork.
Now it’s time for the Emulator itself. If using the YSS PD
Valves like us, you’ll have the chance to adjust the Compression damping level
within the range of 0 to 7 turns in from the end of the adjustment. 2 turns is
the suggested setting, whilst more turns will give firmer damping. I chose 4 turns
because my FZ will be used on track and could use a little extra support. It’s
also cunningly in the middle of adjustment, meaning I can move it either way.
Cut down the preload spacer. Because the PD valve will add a
little length to the innards of your fork assembly, you may need to adjust the
preload spacer length (or accept that you have preloaded the spring more than
it was) Measure the amount of the PD valve that will separate the damper rod
from the fork spring and remove that same amount from the preload spacer by
neatly cutting it and de-burr it too. (We changed fork springs on this rebuild,
and left the preload spacer out completely as our new forks springs are longer
and used without a spacer)
Dry run first - Drop the PD valve into fork the correct way
up! (it sits between the damper rod top and the fork spring with the threaded
adjuster facing to the top.) Add the fork spring and be happy in yourself that
it all fits together nicely. If it does than it’s ready for the oil…
Add the fork oil. Remove the PD valve and spring from the dry
run and you’re ready to add the fork oil. You’ll need to find your own models
suggested oil height and viscosity (the URL at the end of the article should
give you what you need). On our FZ, 10-weight oil was suggested along with an
oil height of 130mm from the top of the fork tube. This measurement is the
distance of the oil from the top of the fork tube, with no spring fitted, and
the fork tube resting at it’s lowest point in the fork bottom. Worth noting
that rebound damping in the new fork is controlled only by the weight of the
fork oil, the emulator itself doesn’t offer any control or adjustment over
this. Again because of the track use my FZ will have to endure, we chose to use
15W viscosity fork oil to balance the extra compression damping that we’ve
dialled in. Obviously upping the fork oil viscosity will affect the compression
damping as well, but if it’s proven to be too much then we’ll go back and
reduce it using the adjustment on the emulator.
Drop the PD valve, fork springs and preload spacer back in.
Nearly done – the fork tops are now ready to go back on to
finish the job. Do this with the same care with which they were removed. These
do not need to be torqued up insanely high, as they’re held under compression
from the spring inside and often also pinched from the outside by the top yoke
or clip-on.
So, at the end of all that what
did we end up with?
Well, even just a quick squish
against the floor of the workshop proved that the results were very noticeable.
The old fork offered very little
of anything you’d call damping. On the bike they’d felt reasonable until asked
to take fast riding, then they became mushy and unhelpfully vague. The new fork
however feels totally transformed. A smoother, firmer action in both
stroke directions with more support on offer; comparing it’s feel with the old
fork is like comparing a dainty Filo Parcel with a traditional Cornish Pasty!
The shock dyno supported what we’d
felt in the hand (see pic) and shows the extra compression and rebound on offer
– where the red line forms a bigger circle than the green one, that’s where we
have increased damping (Compression above the horizontal centre line, rebound
below it)
So it seems that our FZ forks may
well have been much like the first squeeze in our water pistol analogy – very
little on offer by way of controlled damping!
As we go to press I’ve yet to ride
on them, but having felt the difference in the hand I know it’ll be leaps and
bounds better. Even if it’s not perfect off the bat, I know I can tweak it with
the adjustment on the PD valve and messing around with oil grades.
It may not be 100% as good as a
cartridge fork, but compared to way an old damper rod fork worked, really;
what’s not to like?
