Thursday, November 6, 2014

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……

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