Why
For the same reason the CRF250X and many other bikes
come standard with air valves in their fork caps...
to release accumulated pressure inside the forks.
Plus, fork cap air valves provide a quick and easy
way to make temporary spring rate changes while out
on the trail or track.
Note:
Never, ever, EVER use an air pump to ADD pressure
to your forks. You could very
easily damage the seals and/or internal parts
by doing so.
All
popular motorcycle forks contain liquid and air. The
liquid is in the form of fork oil (aka, fork fluid
or suspension fluid), and is used for two reasons:
First,
as the forks compress and rebound while riding, the
fork oil is forced through tiny holes called “orifices”.
Although the actual process is rather complicated,
let’s just say that we can control the rate
of fork compression and rebound by controlling how
quickly the oil can be forced through these tiny holes.
Using larger holes or thinner oil allows the oil to
flow faster through the holes. Smaller holes or thicker
oil slows the flow through the holes. And since oil,
like all liquids, cannot be compressed, the forks
will not collapse (compress) or extend (rebound) faster
than the oil will flow through the holes. It’s
actually a lot more complicated than this, but let’s
just say that most forks have one set of holes for
the compression stroke, and a different set of holes
for the rebound stroke, so that the two different
actions can be controlled separately (i.e., different
rates).
Secondly,
the amount of non-compressible fork oil inside a sealed
fork tube also determines the amount of compressible
air inside the tube. Since fork tubes are not vented
to allow air to move in and out as the forks compress
and rebound, air just naturally fills whatever space
is not occupied by the oil. For example, if the fork
tube volume is 1000 units (it could be fluid ounces,
cubic inches, cubic centimeters, or whatever), and
we pour in 500 units of oil, then air would naturally
occupy the remaining 500 units of volume. Now, what
happens when the unvented fork tube is compressed?
Oil is a non-compressible liquid, but air is an easily
compressible gas, so the air trapped inside is compressed,
and it builds up pressure much like the fork spring
builds pressure when it’s collapsed.
Unlike
a fork spring, however, air pressure doesn’t
build in a linear fashion. A 50 lb. rate spring will
basically collapse 1" for every 50 lbs. applied,
so its response is called “linear”. Compressed
air, however, acts differently. Compress 12 cubic
inches of air into 6 cubic inches of space, and you’ve
doubled its pressure. Compress it again into half
the remaining space, and now it’s four times
the original pressure. Our 50 lb. spring example has
linear response, collapsing 1" for every 50 lbs.
of pressure. Compressed air, however, builds up greater
and greater resistance as it’s compressed, and
the smaller its space, the faster it builds up in
pressure.
Fork
designers know all about this, and they use know compressed
air technology as a secondary spring inside the fork
tube. Since air compression characteristics are known
and predictable, engineers know how much air pressure
accumulates as the fork is compressed. The amount
of compression is thus tunable (adjustable) by varying
the quantity of air inside the fork tube. How do we
vary the amount of air inside a sealed fork tube?
One
way is to change the amount of oil in the fork tube,
which leaves more or less room for air. If we poured
700 units of oil into our 1000 unit fork tube, that
would leave room for only 300 units of air, so it
would build up in pressure more quickly when compressed.
Another
way is to change the volume of the tube. If the fork
tube contained only 800 units, and we added 500 units
of oil, that too would leave room for only 300 units
of air, so it would build up in pressure more rapidly
when compressed.
The bottom line, and the whole point of all this talk,
is that the amount of air inside the fork tube plays
an important role in determining the total resistence
(linear spring pressure plus non-linear air pressure)
of the fork to being compressed. By adding more oil
to each fork leg, the amount of remaining air is reduced,
so as the fork is compressed, the air pressure builds
more quickly, and will hopefully build up high enough
and quickly enough to prevent the fork from harshly
bottoming. By reducing the amount of oil in each leg,
we allow more room for air to exist in the tube, which
slows the compression buildup, making the fork action
softer, but more liable to bottom out.
Our
goal, of course, is to find that elusive ideal combination
of oil and air amounts that allow full use of the
total fork travel, but without allowing harsh bottoming.
Luckily, experimentation in this area is relatively
easy. Just reduce the amount of oil in each fork leg
(but keep both legs the same) until bottoming becomes
troublesome. Then go back to the least amount of oil
that did not allow troublesome bottoming.
So
how do fork cap air valves fit it with all this?
With
air valves in our fork caps, we can quickly and easily
adjust the amount of air inside the fork legs by changing
the volume of the fork tube before it’s sealed.
Here’s how.
If
the air valves are opened while the fork tubes are
fully extended, we admit the maximum amount of air.
This means the total pressure inside each fork leg
will build more quickly during compression, resulting
in somewhat harsher fork action, but more resistance
to bottoming (similar to stronger fork springs, but
without affecting damping rates).
If
the air valves are opened while the fork tubes are
partially collapsed, we reduce the amount of air inside.
This means the total pressure inside each fork leg
will build more slowly during compression, resulting
in somewhat softer and more supple fork action, but
less resistance to bottoming (similar to softer fork
springs).
Air
valve fork caps are also useful to eliminate normal
pressure build-up inside the forks, which eventually
leads to harsh response and weakened fork seals. Fork
seals have oil on only one side (inside the fork tube),
so they are more effective at keeping the pressure
in than in keeping it out. Over time, forks gradually
build up pressure inside. It doesn’t build up
to the point of being harmful, but it can affect tuning
and overall fork action. This is why the owners’
manuals for bikes with vented fork caps usually advise
venting the forks regularly.
HOW
TO USE FORK CAP AIR VALVES
Although
we have a somewhat limited range of effectiveness,
the adjustments are so easy to make, it invites experimentation.
If you can extend and compress the forks by yourself,
fine. If you need a helper, that’s okay too.
Begin by pulling up on the bars or leaning the bike
over on its side stand until the forks are fully extended.
Hold the bike there while depressing the air valve
to release any air pressure inside (positive or negative).
Then release the valve, replace the valve cap, and
go riding. Determine for yourself how you like the
fork response and current resistance to bottoming.
Then
stop, hold the bike straight up and down with your
weight on the seat, and release the valves again.
This time you should hear a little air escape, since
the forks are no longer fully extended. Replace the
valve caps and go riding again. Now your fork action
is softer and more supple, but less resistant to bottoming.
Determine for yourself how you like the new fork action.
If
you’re still not having any problems with bottoming,
then try yet another step toward being even softer.
Stop, hold the bike erect, remove the valve caps,
and while depressing the air valves to open them,
press down hard on the front of the bike to collapse
the forks as far as you can. Do the best you can to
equalize the pressure in both forks, then replace
the valve caps, and go riding again to determine how
you like the new fork action.
One
extreme is to fully extend the forks while the valves
are open. This allows the maximum amount of air, which
makes the fork action harsher but more resistant to
bottoming.
The
other extreme is to compress the forks as far as possible,
using several people to help, or tie-downs on a trailer,
to have as little air as possible inside the forks.
This makes the fork action soft and supple, but less
resistant to bottoming.
Adjusting the air pressure this way is, of course,
secondary to having the proper amounts of oil and
air in the forks to begin with. Adjusting fork action
this way should be considered a temporary adjustment
only, for a particular day or ride, and not a permanent
adjustment. This is simple to do, of course, since
the air adjustments are so easy to make.
HOW
TO INSTALL FORK CAP AIR VALVES
There
are, I’m sure, nearly as many different ways
to do this as there are people who want to do it.
I chose an easy and simple way that costs very little
(about $2.00 each for two SHORT (don’t get the
long ones), automotive tubeless tire valve stems),
but does require an electric drill, some spray solvent
(carburetor cleaner or brake cleaner), and some drill
bits ranging in size from 1/8" up to the same
size as the outside diameter of the valve stem you
bought. The final hole should be just a hair smaller
than the diameter of the air valve stem.
Note
that the tubeless tire valve stems have tapered shafts.
This is important to our installation. When you measure
the outside diameter, measure the small end, near
the threaded portion. If you don’t have calipers,
use an adjustable wrench to match the size of the
valve stem (at its narrow end) to the size of your
final drill bit. The drill bit should be the same
size or slightly smaller than the diameter of the
valve stem. As you can see in the photo, I chose to
cut off the fat end of the valve stem rubber, so as
not to further reduce the air volume of the fork tube,
but it’s not really necessary.
 1.
Place the bike on a box or stand that allows the front
wheel to hang free, off the ground.
2.
Using a 17mm deep-well socket or a box end wrench,
remove the fork caps. The fork caps are very soft
aluminum, so don’t use an open end wrench or
you may round off the corners of the fork caps, making
them ugly and difficult to remove and tighten.
3.
Determine the outside diameter of the tapered valve
stem near the cap. Mine measured 13/32", so my
final hole was made using a 25/64" drill bit
(slightly smaller than the valve stem).
4.
Starting with a 1/8" drill bit, drill a hole
“dead center” (do your very best to get
it dead-center!) through the top of the fork cap.
Then use drill bits 1/16" larger at a time, to
enlarge the hole to your final size. Never attempt
to drill or enlarge a hole in soft  aluminum
more than 1/16" to 1/8" at a time. Again,
my final hole was made using a 25/64" bit, but
yours may differ, based upon the size of your valve
stem..
5.
As you near the size of your final hole, try pushing
the valve stem up through the hole (remove the valve
stem cap first, of course). Once you reach a hole
size that will allow the threaded portion of the valve
cap though, but not the rubber portion, your hole
may be just right.
Again,
your final hole size should allow the threaded portion
of the valve stem to protrude through the hole, but
not the rubber part. If more than 1/16" to 1/8"
of the rubber part will go through the hole easily,
you may have drilled your hole too large. If that
happens, you’ll need to fill the hole with aluminum
epoxy, let it thoroughly harden, and then start all
over.
 6.
Once you have the right size hole, spray some solvent
(carburetor cleaner or brake cleaner) inside the fork
cap and all over the valve stem. (The solvent not
only lubricates the rubber to make it easier to squeeze
it, but the softened rubber will actually be glued
to the aluminum cap when the solvent dries, helping
to make an air-tight seal.) Then insert the valve
stem into the fork cap, and FORCE it in as deep as
it will go. Push it deep into the fork cap using a
Phillips screwdriver, or tap it lightly with a hammer
if necessary. Just get it in there tightly!
Once
the solvent dries, you’re done. Replace the
fork caps on the forks, and you now have air valve
fork caps. Go riding and try ‘em out!
CRF's
Only "How-To" By Gordon Banks, February
2005
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