Pump: All forks have air springs, but not
all forks have coil springs.
WHERE AIR MEETS THE OIL
MXA wrote something that
bothered me. You said that “all forks
are air forks,” whether they have
steel springs in them or not. This
can’t be true, because air forks don’t
have springs and regular forks do.
What was your point?
It is true that air forks like Kayaba
PSF forks don’t have coil springs, but
it is equally true that coil-spring forks
have air in them that is also used as
a spring. The most obvious spring in
a motocross fork, except in the case
of those bikes equipped with Kayaba
PSF air forks, is the mechanical coil
spring. It is made of steel, and it
holds the bike up. It is a given that
when motocrossers talk about “fork
springs,” the venerable coil spring is
what they are referring to.
Yet, even coil-spring forks have a
secondary spring—an air spring. The
air spring is incredibly important.
When you set the oil height on your
forks, you are actually changing the
volume of the air spring. Each fork
leg has a quantity of trapped air
(approximately 6 inches of air is
contained in the typical motorcycle fork). As the fork moves downward through its stroke, the 6 inches
of air are compressed as the oil is
pushed upward by the fork’s downward motion. The farther the fork
moves down, the more the air
chamber is squeezed—and the more
it is squeezed, the more it resists.
Most racers think that the coil
spring is what stops their forks from
bottoming over a big jump, but in
reality, it is the air spring. The air can
only be compressed so far before
it refuses to compress any further.
Once the air has reached maximum
compressibility, the motorcycle fork
Since the amount of air (and its
compressibility) is determined by
how much fork oil is poured into
the fork, the air spring’s stiffness is
derived from the empty space left in
the fork. The more oil in the fork, the
less air. The less air, the stiffer the
spring rate of the air (and the sooner
it comes into effect). Conversely, if
you remove oil from your fork, the air
volume gets larger—and the larger
the volume of air, the slower it gets
When does all this kick in? At mid-
stroke. The fork’s compressed air
space doesn’t become pressurized
enough to affect the spring rate until
it has reached the halfway point of
Tuning the feel with the fork oil
level enables a rider to find the
perfect compromise between small-bump sensitivity and big-hit bottoming resistance. In many ways,
adjusting your fork’s oil height offers
bigger dividends than having the
fork re-valved. Why? (1) You can do
it yourself. ( 2) It’s nearly free (you do
have to pay for the fork oil). ( 3) You
can do it at the track, between motos,
and in less than 15 minutes.
Just to make this more complex, we
must discuss Boyle’s law. Boyle’s law
states that “for a fixed amount of gas,
pressure and volume are inversely
proportional.” That means that as
the volume of air in your forks is
compressed by the rising oil level,
the air pressure increases. How
much does it increase? When the
volume is cut in half, the pressure
doubles. In simpler terms, if you
have 15 pounds of air in your forks
with the oil height set at 6 inches,
when the oil rises enough to reduce
the oil height to 3 inches, you will
have 30 pounds of air in your forks.
And, the pressure doubles every time
the volume is halved. Thus, when the
air volume in your forks is reduced to
1-1/2 inches, you will have 60 psi
and so on.
An answer to your question—it
would be accurate to say that not
every fork has a coil spring, but
every motorcycle fork is an air fork.
Orphan: It may have won the 450 World Championship, but it doesn’t win our
coveted 450 shootout.
THE MISSING-MAN FORMATION
I loved the 2014 MXA 450 shootout in the January 2014 issue, but I wondered
why you did not include the KTM 350SXF?
The most obvious reason is that, as a 350, it does not meet the criteria for
“MXA’s 450 Shootout”—namely, it isn’t a 450. We included it every year since
it was first introduced in 2011, and it has always finished fifth or sixth overall,
largely because it gives up 10 horsepower in the midrange to a 450. We are
willing to admit that the 350SXF would win MXA’s 350 shootout, but we felt
that riders in the market for a new 450 would benefit from a pure comparison
between the five available 450cc bikes—not five 450s and their little brother.