Setting your fork sag
First you need to measure the sag in the same way I
described in my last blog post. Your
forks should sag between 1/4 and 1/3 the distance of your full suspension
travel, which you will need to look at your bike’s specification sheet to find
out that number. As an example, if you
have 10” of total fork travel, your total loaded sag should be between 2.5” and
3.3”.
As a guideline, you can use these numbers :*
Road
Race Street Dirt (full size)
Sag 25-35mm 30-35mm 60-75mm
Preload 5-25mm 10-35mm 3-15mm
Stiction Zone 5-15mm 5-15mm 10-25mm
*Race Tech’s Motorcycle Suspension Bible, Paul Thede
and Lee Parks
In order to adjust the sag, you will need to remove the
fork caps to gain access to the springs.
You adjust the sag by adding to or removing length from the spacers
between the springs and the stops. Each
fork configuration is different, so consult your manual.
If you need to add more than the preload spacer lengths
mentioned above to achieve the correct sag number, your springs are too soft
and need to be replaced with the proper spring rate. And in the same way, if you have to remove
more than the recommended preload amount to gain the correct sag, your springs
are too stiff.
It is important to note that these spacer lengths are
important. Springs, in order to function
correctly need to have some preload on them even when your suspension is fully
extended. And in the same way, you
cannot correct for a spring that is too soft by adding a bunch of preload. A .48kg/mm spring is still a .48kg/mm spring
no matter how much preload you cram on top of it. Springs are relatively cheap and the reward
you get for a properly handling motorcycle will be worth it each time you let
the clutch lever out to start on a ride.
Spring selection
Now that you know how to measure and set your sags, in
most cases on bikes where the rider is over 180 lbs or under 160 lbs, you will
need to swap springs. Most bikes are set
up from the factory for a 165-170 lb rider.
(I wish I fit into that demographic)
There are some online spring calculators but please take
those numbers with a grain of salt. I
have found some of the more popular calculators provided by my competitors to
be very far off, especially for dual sport and adventure bikes. Contact me and I can get you the right number
of the spring rate you need. You can
either purchase the springs from me, which is of course my hopes, or take that
information and source your own springs from your favorite source.
Sasquatch Suspensions has a large rage of springs
available to you as well as being able to have custom springs made for your
project. Email me with your specific
needs and I will get a quote to you.
Stiction Zone
What the heck is a stiction
zone? It was actually coined by Paul
Thede, an engineer and the owner of Race Tech Inc. It is a way of measuring how much friction is
within your suspension. This is how to
do it. When you measure your unloaded
sag, have your helper lift up on the suspension and just let it settle. Write down this measurement. Then have your helper press down on your
forks, compressing them, then gently releasing them and measuring where it
stops. The difference between your
higher length and your shorter length is your stiction zone.
Friction is all around us, and I am not just talking
about your boss. In simple terms, it is
the resistance to movement. Friction can
be a good thing, like how a tire uses friction to gain traction on the road
surface. But in other areas great
lengths are traveled to minimize friction, like inside your engine, or within
your suspension. There are points that
generate friction all throughout your suspension. Seals, bushings, bearings, pinch points, etc.,
each adding a small amount of resistance to the smooth movement of your suspension. When you add those small resistances up, it
can actually become quite noticeable and affect how your bike handles.
Here is a simple test to show you what I am talking
about. Stand beside your bike with it
off of the side stand. Apply the front
brake. Slowly press down on your handle
bars to compress the forks. You will
notice that at first the forks do not move, but as you add force, at some point
they will break free and start compressing.
This is friction keeping you from compressing the forks until the force
of friction resisting movement is overcome by the force you are applying on
them.
Friction in forks comes from the bushings and the fork
seals/wipers and you can never completely eliminate it. Friction can also come
from damage to the forks like from a bent fork tube. As a suspension tuner, I
have spent a lot of time looking for ways to minimize this friction. One process that I have been using with great
success is to actually hone the fork sliders putting a fine cross hatching pattern
in the chrome coating. This microscopic
texture traps oil in the grooves which lubricate the bushings and seals better
as well as breaking the hydraulic seal between surfaces. That seal can be described by taking two
pieces of glass and pressing them together, they come right back apart. But if you place some oil, or water on the
glass surface, then pressing them together, they will lock together, sometimes
requiring a great deal of force to dislodge them. Cross hatching the surfaces helps to minimize
this seal. It is the same reasons that
cylinder bores in your engine are honed.
As a side benefit, I have seen a great reduction in seal failures,
lengthening their service life. Each
pair of forks and shocks serviced by us gets this treatment.
Stick around and the next installment on my blog will be
to talk about the difference between high and low speed damping. Just a hint; it has nothing to do with your
speedometer reading……