tech39

How Do Springs & Dampening Work?
by Lynn Bennett

In the years of most of our bikes, meaning the 1960's, there was not
much to know about springs and dampening since there was not much you
could do about it anyway. Oh yes, you could opt for sidecars springs
and heavier weight oils, but that was about it. But how do springs and
dampening work and how would we change from the factory parts?

SPRINGS:
Springs support the weight of the bike plus rider and any extra
equipment carried (could be a passenger). Springs can be thought of as
a long slender beam that deflects downward under a load (the above list
of stuff on the bike plus the bike). The slender beam is just bent and
wound so that it is in the form of a cylinder with no two parts of the
beam touching each other. Consequently, when a weight is placed on one
end, the beam deflects downward (the spring compresses). As the load is
removed the beam moves upward back to its' original natural unloaded
position (the spring recoils to its' natural length). Springs are
measured in how much of a load they can carry for a measured amount of
movement or deflection; i.e., 75 lbs per inch or 110 lbs per inch or
0.44 kilograms (kgs) per millimeter(mm). This measure is called the
spring constant. Springs can be dual rate or progressive. That is done
by the coil's spacing made such that part of the spring's coils touch
each other under load, reducing the number of active coils. In the beam
analogy, that is, the beam is shortened making it deflect less for a
given load (higher spring constant). If the spring is designed so that
more and more coils touch as the load progressively increases, the
spring constant progressively increase with the shortening of the
overall length of the spring; hence a progressive spring. Another way
to modify the system's spring constant is to stack two spring end to
end then load them. This is seen in motorcycles as two piece springs,
with one substantially shorter than the other. The resultant spring
constant is less than either spring taken individually until the weaker
spring coil binds (all the coils of the spring touch one another and
the spring effectively acts like a solid spacer of steel). The spring
constant for the system then becomes the constant of the still active
spring from that point on as the load is increased. This gives a softer
initial travel then a substantially harder travel until the second
spring totally collapses and it too becomes a steel spacer. We never
let springs in motorcycle applications completely coil bind to limit
the total travel.

The spring constant of a spring is determined by the uncoiled length of
the analogist "beam" and its' strength or thickness. Manufactures can
change the length of wire used, the coil spacing, the coil diameter,
and the diameter of the spring's wire to handle almost any situation.
We can increase the spring constant by simply removing coils but at
some point there might not be enough compression capability left in the
modified spring so that it will bottom (coil bind) before the
suspension runs out of travel. Not good! We can add buffer springs to
make the initial travel softer without impacting the ultimate spring
constant at full travel.

But we also must be able to support the static bike as well, with its'
load. For this the spring is said to be preloaded. We do that by
forcing the spring to be shorter than its' natural length, statically.
We'll talk about selections of springs and adjustment of preload later.

Dynamically, as the bike moves across the terrain the suspension
travels to absorb the bumps. This action is equivalent of increasing
the static load. The bike's inertia wants the bike to travel in a level
path while the ground wants it to go up. The suspension defects upward
and the spring compresses, pushing back with a force dependent on the
spring constant. Too soft and the suspension easily runs out of travel.
Too hard and the suspension doesn't move at all and the bike is force
to travel up with a resultant jar to the rider.

So it can be seen that the softness or hardness of a motorcycle's
suspension can be changed by exchanging springs or modifying original
ones. Several manufactures offer springs designed for our application,
Progressive Suspension being one. Their technical representatives when
given bike/rider specifics can usually suggest several alternatives
dependent mostly on our usage.

Once we have after market springs we need to tailor them to our
specific use. The only real control we have after selecting the spring
is the amount of preload we give the springs. This is done with spacers
above the springs or with the preload adjuster on the rear shocks. For
the front forks the spring preload should not be much more that about
an inch. Various spacer should be tried until the suspension settles
about 25% to 33% of its total travel (25% for 6 inch travel vintage
bikes and 33% for modern 12 inch travel bikes) from the rider and the
extra stuff expected to be carried. You have probably selected the
right spring constant if after adjusting the preload the bike settles a
small amount when sitting unladden. For the rear the same rule of thumb
applies (25% and 33% but the nominal travel is 4 inch travel for
vintage bikes and 12 inch travel for modern dirt bikes). The rear
should settle 1/4 inch to 1/2 inch when sitting unladden in any case.

DAMPENING:
When springs compress they absorb energy which they must give back. It
left undampened they will oscillate in length, loosing a little of the
energy internally to heat with each cycle, eventually stopping at their
natural length for the load they carry after many, many cycles. This is
not ideal for a motorcycle as sea sickness would surely limit the
length of the rides. Ideally, we would like the spring to travel from
its loaded position to the natural rest position in one cycle. The
spring's absorbed energy has to dissipated and a dampener (shock
absorber is a misnomer) is the device that does it. The dampener does
very little as the springs collapses but resists the springs rebounding
action. It does it by forcing oil through small orifices internally.
The orifices have one way valves in series with them giving virtually
no resistance in compression and lots of resistance in rebound. Moving
the oil causes the oil to heat up. If the design is not done right the
dampener changes its' action as it heats up, which is obviously bad. In
recent years suspension designers have found that they can use softer
springs than dynamic loads suggest if they add a small amount of
compression dampening. In fact modern dampeners, both front and rear,
allow adjustments for both rebound and compression, and some form of
design element to handle the heat. That can be moving the dampening oil
supply to an external reservoir or making the supply of oil so big as
to not be substantially effected by the heat. For our vintage bikes
companies like Progressive Suspension, Works Performance and Olins make
replacement dampeners that use more modern designs than the original
Girlings. But note that cold Girlings are the performance measure that
others attempt to attain. They were good but wore out early and were
effected by heat build up.

It should be mentioned that dampening forces generated by moving oil
through orifices don't increase in a linear manner as desired but go up
exponentially. This doesn't match the dampening requirement so
manufactures have added extra circuits that are speed related. Speed in
this sense is the speed of the dampener action, not the speed of the
bike. These circuit blow off at high oil movement speeds, reducing the
increased dampening action, in an attempt to make the dampening action
more linear. The adjustments to all these circuits is now mind
boggling; high and low speed for both rebound and compression for front
and rear suspension units. Rear dampeners are gas pressurized to
eliminate cavitation of the oil movement through the dampeners cavities
and valves. Front forks now have bladders separating the air in the
tubes from the oil for the same reasons. The result is a bunch of
adjustment that most riders haven't a clue as to how to set. Add to
that, the adjustments have a limited usage range. A bike setup for
motocross will kill you in the open desert and vice versa. Small
companies have been opened across the US to provide internal changes to
get the dampening and spring "right on" for a price, of course.

So there you have it. Get ahold of a good suspension supplier that can
provide springs and dampeners either selected or designed to your
requirements. Why? Because the technology has progressed to where it
can make a difference to your comfort while riding. And being old, like
our bikes, we need all the help we can get.