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Home \ How To \ Forks \ Fork Springs and Fluid

The following pictures and descriptions will help guide you through the installation of new fork springs on your motorcycle.  Please note these apply only to conventional (not inverted) forks.  The forks shown below are cartridge style but a similar process can be used on damping-rod forks.

 


Fluid Height
Ensuring the forks have the proper amount of fluid is essential for proper operation.  For this you must measure the actual height of the fluid inside  the fork.  This is a much more accurate means to fill the forks than using fluid volume.  The idea here is that there is enough fluid in the forks to satisfy the damping and lubrication needs of the forks while at the same time creating a specific air gap above the fluid.  This air gap can be used as a tuning device to prevent bottoming.

Start with the fork sitting vertical with the cap off and the spring out.  For cartridge forks you will need some means to pull up and down on the rebound (damping) rod.  I prefer to have the fork off the bike and in a vise as shown.

Before the level can be set the fork lower portion must be fully bled of any trapped air.  Using the appropriate weight, add fluid to the fork.  Begin with about 1/4 quart.  You may hear a bubbling or gurgling sound while adding the fluid.

Next you need to bleed the cartridge and fork body.  For that you first take the stanchion tube (the chrome outside tube) and cycle it up and down fully through it's stroke.  Make sure extends through the ends of travel, this will get the air out of the chamber between the stanchion and lower slider.  If you continue to hear gurgling sounds after a few cycles add another 100cc of fluid and continue bleeding.

Next you will need to cycle the damping rod in a similar fashion.  At first you may not feel an resistance.  To get the cartridge primed you may need to cycle the damping rod fully through it's entire stroke several times.  As the fluid starts to fill from the bottom you will begin to feel a noticeable resistance.  Cycle the damping rod fully until you feel a smooth action with no skipping.  During this time be careful as fluid may try to squirt out of the top of the damping rod.

 

Fork Fluid Height 1

Once the fork is fully bled you will begin to set the fluid level.  I choose to use a metal ruler for this step, but any clean object can work.  The idea here is that we are creating a dipstick of sorts.  For the forks shown the desired fluid height is 120mm.  This measurement is taken from the top of the stanchion tube.  Make sure the fork is completely compressed.  There is often an oil lock device at full compression that will provide some resistance, make sure you are fully pushed through it.

I use a number larger than the fluid height I am shooting for.  In this case I pick 200mm.  Since I want 120mm, I am looking to see the fluid mark on the ruler at 80mm (200-80=120).  Fill slowly and check often as it is easy to overfill past your mark.

You are done with setting your fluid height!  See, that was easy.

 

Fork Fluid Height 2

Fork Fluid Height 3


 

Spring Preload Setting

 

When setting up forks it is often required to replace the springs with the correct rate for that particular rider.  Aftermarket springs are often a different length than the stock springs in the forks.  Manufacturers will also typically use a soft spring combined with an excessive amount of preload in order to make the setup applicable for a wide range of riders (it doesn't work, unfortunately).  because of these factors, it is almost always necessary to use a new spacer tube inside the forks.  It can be a mystery how to know what the length should be, the steps below will outline to determine the correct spring spacer tube length.

 

Once you have set the fluid it is now time to set the spring preload.

Start by sliding the new spring over the damping rod making sure that it falls to the stop inside the fork.  Unless you have a special tool (like shown in the pictures above) you will need to pull the damping rod all of the up before dropping the spring down.

Use a spare piece of PVC pipe (or the original spacer tube) as a starting point for the spacer length.  It's important that this spacer be longer than the final required length but not so long that it compresses the spring during this step.  In this case I am using a tube that is 80mm long.  It might take a couple of iterations to get a good starting point for the tube length.

Install one of the original washers, the spacer, another washer, and the retaining plate (the c-shaped washer).  At this point check that you can still pull up on the damping rod to ensure you aren't compressing the spring. 

 

Fork Spring Preload 1

Install the fork cap to the damping rod.  At this stage it is not critical how far you thread the cap on.  Once threaded to the damping rod, turn the locking up sp that is gently resting against the fork cap.

If equipped, set the spring preload adjustment in the fork cap to approximately it's mid position of travel.  Your assembly should look like the picture to the right.

 

Fork Spring Preload 1

Pull the stanchion tube until it stops.  You will now make a measurement of the distance between the top of the stanchion tube and the flange where it seats on the fork cap.  If you assembled the fork now this number would be the spring preload - it's the distance the spring compresses to install the cap.  For most bikes the target spring preload is 16mm.  As you can see in the picture I'm at 36mm, so that is too much.  Since my preload right now is 36mm and I want 16mm, my spacer tube is 20mm too long (36-16=20).

From above, I started with a tube that 80mm long.  At this point I need to cut the tube down to 60mm (80-20=60) which should give me the correct length for this application.

 

Fork Spring Preload 1

The next steps will finish the assembly of the forks.  This step is very often done incorrectly, so it's important to take care (this only applies to forks that have external rebound damping adjustments).  Take the adjuster screw and turn it out, counterclockwise, until it stops.  From this point start turning it back in while counting the revolutions.  Most bikes will have 2.5 to 3 turns of adjustment range.

 

Fork Spring Preload 1

With the adjustment screw turned in to the desired position, thread the cap back on the damping rod until the needle gently seats at the bottom.  Do not tighten the cap or you can damage the needle or valving inside the fork. It is best to have the locking nut turned down as far as it will go on the damping rod.  You should see a gap between the locking nut and the adjuster as shown to the right.

Gently tighten the locking nut against the adjuster.  Before tightening the nut, turn the damping screw out about 1/2 turn.  Tighten the locking nut.

Next is to thread the cap into the stanchion tube and the forks are fully assembled.  Congratulations! 

Fork Spring Preload 1


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  This page was last updated Sunday July 21, 2013.

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Honda Motorcycle VF1000R VF1000F vf1000 vf500 shock fork shocks forks engine top end CBR600 racing roadracing performance products street footpeg relocator suspension cam oil modification information Interceptor interceptor interceptor interseptor VF500F VF700 VF750 VFR750 F2 F3 vfr1000 vfr1000f vfr500f vfr500r shock wheel conversion rebuild repair brakes VFR1000 VFR1000R VFR1000F VF1000 VF500 vf500f horsepower power fast quick light turning riding race stainless steel rearsets footpegs pegs peg foot exhaust 4-1 aftermarket Honda Motorcycle VF1000R VF1000F vf1000 vf500 shock fork shocks forks engine top end CBR600 racing roadracing performance products street footpeg relocator suspension cam oil modification information Interceptor interceptor interceptor interseptor VF500F VF700 VF750 VFR750 F2 F3 vfr1000 vfr1000f vfr500f vfr500r shock wheel conversion rebuild repair brakes VFR1000 VFR1000R VFR1000F VF1000 VF500 vf500f horsepower power fast quick light turning riding race stainless steel rearsets footpegs pegs peg foot exhaust 4-1 aftermarket Honda Motorcycle VF1000R VF1000F vf1000 vf500 shock fork shocks forks engine top end CBR600 racing roadracing performance products street footpeg relocator suspension cam oil modification information Interceptor interceptor interceptor interseptor VF500F VF700 VF750 VFR750 F2 F3 vfr1000 vfr1000f vfr500f vfr500r shock wheel conversion rebuild repair brakes VFR1000 VFR1000R VFR1000F VF1000 VF500 vf500f horsepower power fast quick light turning riding race stainless steel rearsets footpegs pegs peg foot exhaust 4-1 aftermarket Honda Motorcycle VF1000R VF1000F vf1000 vf500 shock fork shocks forks engine top end CBR600 racing roadracing performance products street footpeg relocator suspension cam oil modification information Interceptor interceptor interceptor interseptor VF500F VF700 VF750 VFR750 F2 F3 vfr1000 vfr1000f vfr500f vfr500r shock wheel conversion rebuild repair brakes VFR1000 VFR1000R VFR1000F VF1000 VF500 vf500f horsepower power fast quick light turning riding race stainless steel rearsets footpegs pegs peg foot exhaust 4-1 aftermarket Honda Motorcycle VF1000R VF1000F vf1000 vf500 shock fork shocks forks engine top end CBR600 racing roadracing performance products street footpeg relocator suspension cam oil modification information Interceptor interceptor interceptor interseptor VF500F VF700 VF750 VFR750 F2 F3 vfr1000 vfr1000f vfr500f vfr500r shock wheel conversion rebuild repair brakes VFR1000 VFR1000R VFR1000F VF1000 VF500 vf500f horsepower power fast quick light turning riding race stainless steel rearsets footpegs pegs peg foot exhaust 4-1 aftermarket aluminum titanium