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Motorcycle suspension has two important parts: springs, which absorb energy from bumps, and damping circuits that
convert that energy to heat instead of to uncontrolled rocking.
Your damping circuits have two separate parts, a compression damping circuit and a rebound damping circuit.
Generally, these systems work by having a piston move through oil and a small hole for the oil to pass through. The oil
passing through the hole makes the resistance that absorbs and dissipates the energy from the spring.
Oil passing through a hole has resistance that is proportional to velocity squared. You know another system that has
resistance proportional to velocity squared: a parachute. More or less, no matter how much you weigh, you're going to
fall with a parachute at about 10-12 mph. Square law devices act as speed limiters.
There are a lot of solutions to this problem these days, but they all more or less boil down to one thing: use a
spring to make the hole larger or smaller depending on oil pressure, thereby making a system where the resistance is
proportional to velocity instead of velocity squared. Cartridge forks do this with thin washers that deflect to open up
the oil hole; Works Performance shocks have ball bearings that are spring loaded to open up oil passages; RaceTech Gold
Valves have spring loaded pistons that open up oil passages.
Some years ago it was found that in Supercross (motocross with ginormous jumps) that springs that were strong enough
to handle the jumps were so strong that on normal bumps, the suspension basically didn't move. Springs that worked well
on the normal stuff bottomed out immediately after a big jump and didn't absorb enough energy. So the racing guys came
up with the idea of using the square law to limit suspension speed. Supercross suspension uses springs for the small
stuff, and square law damping for the really big stuff. However, if you never get more than six feet in the air, you
have no need for this stuff. The same square law damping that makes Supercross bikes work is the stuff that makes
normal suspension harsh over square edged bumps. For reasons which I find incomprehensible, nearly all suspension made
in Japan now has this square law damping built in to do speed limiting, even on bikes like large Harleys that will
never get two inches into the air, much less twenty feet.
This is the procedure to get the square law damping under control in most conventional forks, and thereby allow you
to set the rebound damping for better performance without having to put up with a harsh ride. We'll illustrate this
procedure on a set of Suzuki VStrom DL650 forks. Note: Cartridge forks are enormously more complicated inside than
conventional forks, and should not be taken apart unless you have been trained. There's lots of little tiny pieces, and
special tools are required. Most all upside-down forks are cartridge forks. If you have these and have a problem, I
recommend you contact the guys at RaceTech.com.
Fork Disassembly
- From your dealer, buy a new pair of fork seals and a quart of 10w fork oil. You'll also need some large wrenches,
like 20-24mm, and a large allen wrench, like 5-8mm. You'll need a lot of paper towels, get an entire roll. Finally, we
need an electric drill and some large bits, about 3/8" (9mm).
- Prop up the bike so that the front wheel is in the air and the bike is completely stable. We're going to be
shoving pretty hard on some parts, so a wobbly bike is a bad thing.
- Get a large piece of cardboard to work on. You also need an empty water bottle or some such to hold the old
fork oil. We're going to be making a real mess, we don't want to make the mess on the garage floor. If you hope that
your hands will be clean again sometime this month, I recommend you get some shaving cream, put a blob onto your palm,
and rub it all over both of your hands. This will fill up all the pores in your skin and make it very easy to clean up
your hands later.
- Remove both brake calipers and the front fender from the forks. You can just let the brakes hang, they
don't have to come off the bike and you don't have to open any hydraulic lines.
- Remove the front wheel
- Unbolt the handle bars. Put a rag on the gas tank, and move them back an inch or two so that they're out of
the way of the fork caps.
- Remove the upper fork caps. Careful - the fork springs may be pre-loaded, in which case the caps are going
to want to shoot across the garage when you get them to the last 1/2 thread.
- Loosen the bolts on the triple clamps that hold the fork tubes. The tubes should now have nothing connected
to them and they should easily slide down and off the bike. Pull them out one at a time. Holding the tube over the
large cardboard, remove the fork spring (dripping disgustingly dirty oil), then tip the fork over and pour the rest of
the fork oil into your oil container. There's likely to be about a pint (500ml). From now on, every step will involve
spilling some really nasty looking oil, so do all your work over the large piece of cardboard.
Fork tube removed from bike
- Remove the dust cap from the top of the fork leg. This prys off with a flat screwdriver. Careful, don't
damage the dust cap, don't score the metal on the fork leg, and don't scratch the chrome on the fork down tube.
- Under the dust cap there will be some sort of retaining clip for the fork seals. If it's a metal ring,
carefully and gently pry it out with a flat screwdriver. If it's a retaining clip, get a retaining clip pliers and
remove it.
- Holding the fork tube over the cardboard, turn it upside down. Looking from the bottom, there will be a
large allen bolt going up into the fork tube. Remove this allen bolt. When the allen screw is removed, the chrome
down tube will pull out of the fork leg. If it pulls out smoothly, that's all there is to it. If you get it almost but
not quite completely out, then the fork seal is holding it in. In this case get a friend to hold the fork leg. Push
the chrome down tube back into the fork leg about four inches, and pull the chrome fork down tube firmly and quickly
out. You're using the down tube as an inside hammer to remove the fork seal. Four to eight pulls like this and the down
tube will come out, pulling the fork seal and a large washer with it.
fork seal washer, fork seal, retaining wire, fork dust cap
- Turn the down tube upside down over the cardboard. From inside the down tube, out will spill the damping
piston and rod and the top out spring.
top: damping tube and top out spring.
middle: chrome down tube with (from left) fork bushing, seal washer, fork seal, seal retaining clip, and seal dust cap.
bottom: the other fork tube, still in one piece.
- Repeat all this for the other fork tube.
- Clean everything up. Take your time and do a good job. There's lots of little nooks and crannies on the fork
tubes and the damping rods, get them shiny clean. It's ok to use gasoline or kerosene to clean up these parts. It's
also ok to blow everything out with compressed air.
Changing your damping
Your damping is set by three things: the oil viscosity in the forks, the diameter of the compression damping holes,
and the diameter of the rebound damping holes. We would like to increase the rebound damping, which we can do simply
by increasing the oil viscosity. Most forks are shipped from the factory with incredibly cheap 5w oil. However,
if we simply increase the oil viscosity, we'll also increase the compression damping and the forks will become more harsh.
So we'll first increase the diameter of the compression damping holes. In the damper rod shown below, the compression
damping hole is about .2" in diameter and is near the right hand end. The rebound damping hole is about 1/16" diameter
and is close to the piston on the left.
Stock Damper Rod, piston at left, rebound damping hole near piston, compression hole on right
We clamp the damper rod in a vise and drill out the compression holes. There are two holes on this rod, one on each
side. Sometimes there are four holes. Here the top rod has been drilled out to 3/8" diameter.
Two damper rods, top rod drilled out, bottom rod still stock.
Reassembling your forks
- Place the top out spring on the damping rod. Place the damping cone into the bottom of the chrome down
tube. Slide the down tube into the fork leg. Use the allen screw to screw the damping rod and damping cone to the
fork leg. The chrome down tube should now slide in and out of the fork leg by the amount of travel of your forks,
most likely 4" to 6".
- There are two bushings on the chrome down tube. Normally there is a bushing which is attached to the bottom
of the down tube, and a second bushing which is fixed in the fork leg. The down tube lower bushing should already be in the
fork leg. Get the fork leg bushing and place it on the chrome down tube. At this point we have to insert the bushing
into the fork leg. There are several ways to do this:
- Bring your forks to the motorcycle dealer. They will charge you $50 to $200, and give you a massive ration
of crap for starting a project you couldn't finish. Then they'll tell you most likely in the next 10 days your wheels
will fall off, your wife will become pregnant with a red-headed kid, your dog will run away, and your teen age daughter
will start dating a convicted child molester. All because you opened up your forks. Never touch your bike with a
wrench.
- Go to the dealer and purchase the official fork bushing insertion tool. Here, we're working on a Suzuki
VStrom. The official Suzuki Fork Bushing Insertion Tool is part number 09940-52861, about $350. On back order for six
weeks. Really, I don't make this stuff up.
- Make your own. This is known as "Red-Neck Engineering." I made my own. Get about a 3.5" length of 1.5"
diameter PVC pipe. Slice the pipe length wise. I cut the pipe on one side only, I suppose you could cut it on both. Get
a couple of 1.5" hose clamps and some duct tape (duct tape is the mark of a true red-neck solution.) If you're a
liberal, sorry, you're just going to have to check your sensibilities at the garage door for the next half hour. Or
choose A or B.
fork bushing, pvc pipe hose clamped on, duct tape (the mark of a true non-professional)
Place the fork leg bushing on the down tube, and let it slide down to the fork leg. Place the PVC pipe on the fork leg,
and slide it down to the bushing. Compress the forks completely. Now pull the fork leg as far out as it will go. Slide
the PVC pipe down 1" on the fork leg. Hose clamp it in place. The bushing is likely to not really want to enter the
fork leg - this is the first time it's been out since it was installed at the factory, and it hopes to find a
girlfriend. Wrap duct tape around the fork leg immediately above the PVC pipe, about about eight times around. The duct
tape is to make certain the PCV doesn't slide up the leg. Get a bit of fork oil on your finger, and get the bushing wet
with oil all the way around. Now stand the fork leg up, and use the down tube as a slide hammer to hammer the bushing
into place. Make sure you left plenty of room so that the hose clamps don't hit the fork leg.
- Now remove the duct tape and the PVC pipe. Use Acetone (finger nail polish remover) to get the glue off the
fork down tube. Do not put the new fork seal on the down tube until it's sparkly clean.
fork seal washer, fork seal, PVC pipe
Put the fork seal washer, if any, on the down tube, then the fork seal, then slide the PVC pipe on the down tube again.
Bottom out the fork, then pull the down tube as far out as it will go. Slide the PVC downwards 1", and hose clamp it
into place. Get some fork oil on your finger, and get the fork seal wet with fork oil all the way around. Note: Fork
seals have a top and a bottom, make sure you get this right. Normally the bottom is open and the top is closed with
some numbers or something embossed onto it. The fork seal is not so independent-minded as the bushing and you most
likely won't need duct tape this time. Stand the fork leg up, and use the down tube as a slide hammer to press the fork
seal into place. You're done when you can see the machined groove where the retaining clip goes.
Hammering the fork seal into place
- Now put the fork seal retaining clip into place, and put the dust cover back on.
- Although you've no doubt tried to do a good job of cleaning out the forks, I expect there's still a bunch
of grudge in there. I suggest you flush the forks at this time. Stand the legs up, bottom out the down tube, and fill
them to about 6" from the top with some cheap motor oil. You can get 30wt oil at Walmart for $2 / quart. Work the forks
up and down a dozen times or two, then dump the oil. Repeat with fresh oil until your compulsive tendencies are
satisfied.
- Now pour some fork oil into each leg. You want to fill the leg until the oil level is about 5" to 6" below
the top of the down tube. This measurement is made without the fork spring in place, and with the fork tube bottomed out.
You have to have some air in your forks - oil is incompressible, if your forks were filled to the top they would hydraulically lock.
The air space allows for compression. As the air compresses the pressure goes up, so the air acts like an extra spring.
Higher oil level equals less air space equals more air compression and more spring effect. However, more oil also equals higher
pressures, and the pressures can get high enough to blow out the seals. Your bike's service manual will give a factory
spec for oil level, but you can experiment around &plusm;1" if you like.
fork seal washer, fork seal, retaining wire, fork dust cap
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