1975 CZ 250 Motocross GP - 9-Page Vintage Motorcycle Road Test Article

Description

1975 CZ 250 Motocross GP - 9-Page Vintage Motorcycle Road Test Article
Original, vintage magazine article
Page Size: Approx. 8" x 11" (21 cm x 28 cm) each page
Condition: Good
CZ250
MOTOCROSS GP
The new production
racer is a replica
of Jaroslav Falta’s
works racer
• The pattern of development for CZ’s
production motocrossers has been one
of gradual, disciplined progress since the
first ones showed up in this country in
the early Sixties. The works bike that
powered Joel Robert to a World Champi-
onship in 1964 looked very much like
those winning scrambles races in New
England in 1962. From the onset, the CZs
have been tough, fast, good handling
machines for the serious privateer.
We saw the finishing stages of devel-
opment being hammered into the new GP
motocross models last season. It was then
that Jaroslav Falta came within a silly
millimeter of winning the 250cc World
Championship on his factory team racer.
Riding this same bike, the young Czech
absolutely dominated the Superbowl of
Motocross in the Los Angeles Coliseum
last summer. Recalling our examination
of Falta's bike at the Superbowl, we see
that several experimental parts that
proved reliable on the works bikes are
now incorporated in the new production
models; things that gave trouble are not.
The most obvious new pieces are the
CZ air-oil rear shock absorbers. These
shocks have no central damper rods or
springs in the conventional form. They are
similar to a very short version of the front
forks. Instead of the familiar coil springs,
compressed air is used as the elastic
medium. And in place of the skinny (up
to 13mm diameter) damper rod on most
shocks, a 48mm tube slides directly inside
the finned aluminum outer housing. The
absence of a spring inside the huge tube
leaves plenty of room for damping fluid.
Up to 190cc of SAE 10 hydraulic oil is
held in each shock body. A 42mm dia.
piston is attached to the bottom shock
mount with a hollow aluminum damper
tube. This big piston pumps 13.9cc of oil
for each centimeter of shock absorber
travel. The average piston diameter for
conventional type shocks is 22mm, which
moves 3.8cc of oil per centimeter of
travel. In order to get equivalent damping
force from the smaller piston, its smaller
quantity of oil must be forced through
much smaller metering valves at corre-
spondingly higher pressures. Often the
total amount of oil held by the smaller
shocks is only 60cc. It is this over-working
of the oil through the small valves, to-
gether with very small reservoir capacity
to dissipate the heat, that causes quick
damper failure in severe motocross races.
Apart from the obvious reduction in
weight, the main advantage of using
compressed air as a spring is that the
pressure is quickly and fairly easily regu-
lated. With steel springs the ride height
must be changed with spacers or cam-
rings. Pumping higher pressure into the
new CZ shocks from a storage tank ac-
complishes the same thing.
Changing the air spring’s compression
rate is accomplished by using more or
less oil in the shock bodies. Using more
oil decreases the volume of air in the body
(at any given pressure) and makes any
movement of the suspension create a
higher pressure than it would with less
oil. It is the same principle as increasing
the compression ratio in an engine. And
whatever the proportions of air and oil,
the air acts as a progressive-rate
spring—stiffening as the shock moves to-
ward full compression.
The rate of rebound damping can be
changed by using higher viscosity oil, but
we never experimented because the rear
suspension never oscillated excessively
due to under-damping. After we took the
shocks apart for examination and pho-
tography, they were reassembled with the
full 190cc of Torco 10-weight fork oil. It
was the wrong thing to do: the ride was
harsh and stiff during testing at Valley
Cycle Park. Meanwhile, American Jawa’s
technicians had cross-referenced the CZ...
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