tech44

Vintage Front Suspension
by Lynn Bennett

Do you know how 1950 through 1980's front suspension elements, commonly
called front forks, work? Well, I will attempt to walk through
suspension developments and attempt to explain how each style works.

Introduction-The Problem:

As suspension components, front forks have springs to allow absorption
of a bump and hydraulic dampening to dissipate that bump induced spring
energy. Ideally, the system should be critically damped where one
compression cycle of the spring is dissipated in a controlled return of
the spring to its' nominal height at a speed similar to the input rate
and with no over or under shoot. Too much dampening and the fork is not
ready for the next bump and may run out of travel with successive
bumps. Too little dampening and the fork will oscillate up and down
through several cycles. The fork should also guard against end of
travel limits, such as bottoming and topping. The basic concept is
minimal restriction of motion on collapse (compression) and some
retardation of motion on expansion (extension).

Terminology:

The British and us Americans each have our own terms for suspension
bits and pieces and honestly I no longer know which is which, but lets
agree up front what I call these items. The suspension element at the
front of the motorcycle is called the fork. Each complete side is
called a fork leg. The fork is held to the motorcycle using two triple
clamps (triple trees), one above and one below the frame's neck. The
fork leg is telescopic in nature meaning that an outer tube moves over
an inner tube, like a telescope. The inner tubes, the fork tubes, are
usually clamped to the two triple clamps while the lower tubes, the
fork sliders, provides for clamping of the front wheel axle at its'
bottom. Most sliders have seal holders that screw or attach to the top
of the sliders. The seal holder obviously holds the seal that slides on
the fork tube external surface, sealing the internal oil supply. The
upper part of the seal holder provides the lower spring seat for those
bikes with external springs. Conversely, some fork legs have internal
springs, residing inside the fork tube. The fork tubes are topped off
with a thread in top cap, sometimes vented with a spring backed ball
blow-off valve, or perhaps not vented at all. Internally, the hydraulic
dampening can be done in one of several ways: dampening rod, cartridge,
or shuttle valve. The bottoming is handled by the bottoming cone while
the topping can be done by an anti-topping spring. The bushing are the
bearing surfaces for the telescoping action between the tube and the
slider. Usually there are two bushes per leg, one fixed to the top of
the slider and the other to the bottom of the fork tube, both retained
by large nuts or threaded pieces, like the seal holder. They are the
replaceable wear items for the tube and the slider.

Development:

In the 1950's the first fork I had apart was on a 1950 Triumph 650. The
fork used telescopic legs, a set of two bushes per leg with the slider
bush retained by the seal holder, external springs and a crude
dampening cartridge. The entire fork leg was filled with enough oil so
that no mater where the leg was in its' travel the damper was always
submerged in oil. This style was used for many years, even on my 1966
BSA Hornet.

The dampening in this style fork leg is actually done in two locations.
The main damper is the cartridge or damper tube. It is a piece of thin
wall tubing, capped at both ends, about 10 to 12 inches long, small
enough in diameter to fit inside the fork tube, threaded on the bottom
in order to be retained by an allen bolt from the bottom of the slider,
and with a hole in the other end for the control rod. The control rod,
a long slender rod, is attached on the top end to the leg top cap via
threads and a lock nut, and passes through the top end of the damper.
The rod is terminated in a pair of washers so machined and attached to
the control rod that one acts a piston inside the damper and the other
floats on top of the piston washer. The piston washer actually has a
series of small holes that allow oil to flow from above to below the
piston and vice versa. The floating washer, when the fork is
collapsing, is pushed out of the way and doesn't impair oil flow. The
floating washer, when the fork is expanding, is pushed into the way and
partially impairs the oil flow. For still more dampening function, oil
is pumped through holes in the walls of the fork tube near the bottom
of the fork tube into the area between the two bushes. As the leg
collapses the bushes move apart and oil flows into the space between
the bushes and the clearance space of the tube to the slider. As the
leg extends the bushes move towards each other forcing the captured oil
back out the holes it came in. This added dampening tends to be equal
in both compression and extension. The main damper (the cartridge) can
have a short cone machined into its' bottom external surface such that
the bushing retaining nut on the fork tube, with a similar internal
cone shaped surface, captures a small volume of oil at the bottom of
the stroke. Since oil is not compressible it acts as a hydraulic stop
keeping the fork tube from physically banging into the bottom of the
slider's internal space.

During a period of time in the 1960's a simplified approach for
dampening did away with the cartridge and only used the space between
the fork tube and the slider bushes. This style uses an annular shuttle
valve at the bottom of the fork tube that allows oil into the clearance
space on collapse but impairs the oil flow out on extension of the leg.
A cone is attached to the internal bottom of the slider. Under
bottoming conditions a small quantity of oil is captured between the
cone shaped under surface of the shuttle valve and the cone in the
bottom of the slider, creating bottoming protection. Note that the
dampening is unequal, with less compression than extension dampening.
also note that there is no topping protection

In the 1970's the fork designs more closely matched what was being done
with off-road racer suspensions. In this style the fork is still
telescopic but the seal holder and external springs are discarded. The
sliders are now aluminum with minimal clearances between the slider and
the fork tube, the springs internal, and the main dampening
functionality is handled by the damper rod. Although the rod is
physically similar in appearance to the earlier cartridge, it works
entirely differently. It is attached from the bottom of the slider with
an allen bolt and is about 12 inches long. The piston is an annular
ring that is held into a recess in the bottom of the fork leg. The
piston has oil flow control holes which can be masked off by a loose
washer. The damper rod fits inside the fork leg and slides inside the
annual piston. The top end of the rod is slightly larger so as to act
as piston on the inside surface of the fork tube, using a ring or
O-ring as a seal. As the forks collapse and extend the two pistons, one
in the bottom of the fork tube and the other sliding inside the fork
tube as part of the rod, they move oil through the oil control holes of
the piston affixed to the fork tube. Or the damper rod "pumps" oil
through the fork leg piston. The resistance to motion is greater in the
extension motion than in the collapse motion because the floating
washer can float to block or not to block the oil flow control holes of
the fork leg piston. Holes in the dampening rod body, including one top
to bottom, act as addition control holes for controlling the rate of
dampening and as oil supply passages for the main fork tube piston
cavity. A short spring is placed coaxially around the top end of rod,
caught between the underside of the rod piston and the top side of the
fork tube piston when the fork tries to extend fully. This is
anti-topping protection. A conical shape on the bottom of the rod
matches with a similar shape on the bottom of the fork tube piston
retaining nut to provide bottoming resistance from captured oil. The
fork springs are internal and seat into a machined cavity on the top of
the damper rod and onto the underside of the top cap. The fork tube
seals are retained in the slider inside a cavity in the slider's top by
e-clips or spring clips. Small rubber boots usually protect the seals.

Other manufactures use variations on the last style. One key one is
done by Ceriani. Instead of a straight walled rod Ceriani uses a hour
glass shape which eliminates virtually all dampening in either
direction in the mid stoke of the fork leg. This gives a plush ride
most of the time with no harshness caused by compression or extension
dampening. Another is the elimination of bushes of any kind where the
slider is a slip fit to the fork tube making the bush the entire length
of engagement between the slider and the tube.

Modern Suspension Elements:

Well, we've come full circle as the modern fork legs use a cartridge
for dampening. The dampening is a lot more sophisticated using spring
washers of various sizes to control the flow of oil through metering
holes in both compression and extension. Some part of the dampening for
both compression and extension is adjustable externally. But the
variations of washer stacks and sizes makes them extremely adjustable
during disassembly. To reduce the un-sprung weight the sliders are on
top and the fork tubes are on the bottom (the internals stay the same),
hence the term "upside down" forks.

Servicing Vintage Style Forks:

As stated before the vintage fork contains an oil supply for the
dampening feature. The oil gets really abused and most often is not
changed regularly, typically waiting until the forks blow a seal.
Fluids used in forks over the years has gone from motor oil to:
Automatic Transmission Fluid (ATF), hydraulic jack oil, and most
recently to suspension fluid. Modern suspension fluids (fork oil)
include additives for anti frothing and seal softeners. Both these
issues impact fork performance, so these should be our first choice.
But do change the fork oil at least every year or two. What weight? I
found that my early BSA cartridge forks like 30 wt to keep from topping
while my Vintage race bikes and my BSA Victor dual sport liked 20 wt.
It doesn't hurt to try various weights of suspension fluids until you
find the one for you but 20 wt is a good starting point. How much?
Follow the book but I always start mine level such that with the
springs removed and the fork tube full collapsed into the slider, the
oil level is 6 inches below the top of the fork tube. You, of course,
need to stroke the sliders full stroke many times to get the oil into
all the nooks and crannies and to get the air out.

Seals need only be changed when oil leakage is detected or at the time
of restoration after years of storage. Be sure to check all of the
surface that the seal rides on for damage. Seals don't work over rust
or dings or metal surface damage or rust erosion. New tubes are readily
available for most vintage bikes. The Vintage motocross scene has
started doing re-chrome and grinding to size for dirt bike forks. It's
being done regularly in England.

Fork leg removal and replacement on Vintage bikes can be frustrating!
The problem is that the top of the fork leg is machined as a taper and
the top triple clamp has the mate. Years of service seat these tapers
very well. Add to this the clamps on the lower triple clamp and some
rust and it gets tough. The secrets for easy removal are as below.
First, force a screwdriver or better yet a punch into the slit of the
lower triple clamp. This relieves the pressure and drag when the tube
starts to slide out of the top triple clamp. Sacrifice a top cap for
hammering. Place the top cap into the threads of the fork tube top
until it is unseated off the triple clamp by about 1/8 inch. Now hit
the top cap with a hammer. It should move down , stopped by the
shoulder of the top cap. Loosen the top cap again, hammer, loosen,
hammer, etc. The punch in the lower triple clamp is key. For
installation make up a tool using another sacrificed top cap that has
been drilled and threaded for a 12 inch piece of threaded rod. The
shoulder of the top cap must be sawn off so that the modified top cap
will pass through the tapered hole of the top triple clamp unimpeded.
The threaded rod is threaded into the modified top cap and locked in
place with a lock nut. Now thread the tool into the positioned fork
leg, follow with a sturdy washer and a nut. Tighten the nut to pull the
leg up into the taper of the top triple clamp until the fork tube seats
on its' shoulder in the triple clamp. Remove the tool, the punch from
the lower triple clamp, and put on the top cap. If you find that the
second leg does not align with the its' hole in the triple clamp then
you've got bent triple clamps. Throw them away and get new if you can.
Any straightening will hurt the metal and how would you like for your
fork legs to fall off while out riding?

As an aside modern cartridge upside down forks have no drain holes. To
be serviced they must be removed from the bike, dumped upside down,
stroked, and drained for oil removal. Replacing the oil requires
special techniques and careful removal of all air from the system. Yea
for easy to maintain Vintage forks!

Conclusion:

As time went on forks got better. I suspect that dirt bike development
was the real reason not street riding. From the simple cartridge fork,
to the shuttle fork, to the damper rod fork, to the new cartridge
up-side down fork, they just got better and better. The Vintage fork,
unlike the modern cartridge fork, are easy to maintain and fix.

That's about it for Vintage style forks. Hope this article helped you
understand how they work.