Yamaha WR250F(Y) 2009 Owner'S Service Manual Download Page 92

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4-5

CHASSIS

SELECTION OF THE SECONDARY
REDUCTION RATIO (SPROCKET)

<Requirement for selection of sec-
ondary gear reduction ratio>
• It is generally said that the second-

ary gear ratio should be reduced for
a longer straight portion of a speed
course and should be increased for
a course with many corners. Actual-
ly, however, as the speed depends
on the ground condition of the day
of the ride, be sure to run through
the circuit to set the machine suit-
able for the entire course.

• In actuality, it is very difficult to

achieve settings suitable for the en-
tire course and some settings may
be sacrificed. Thus, the settings
should be matched to the portion of
the course that has the greatest ef-
fect on the ride result. In such a
case, run through the entire course
while making notes of lap times to
find the best balance; then, deter-
mine the secondary reduction ratio.

• If a course has a long straight por-

tion where a machine can run at
maximum speed, the machine is
generally set such that it can devel-
op its maximum revolutions toward
the end of the straight line, with care
taken to avoid the engine over-rev-
ving.

Riding technique varies from rider to
rider and the performance of a ma-
chine also vary from machine to ma-
chine. Therefore, do not imitate other
rider's settings from the beginning but
choose your own setting according to
the level of your riding technique.

DRIVE AND REAR WHEEL
SPROCKETS SETTING PARTS

TIRE PRESSURE
Tire pressure should be adjust to suit
the road surface condition of the cir-
cuit.

• Under a rainy, muddy, sandy, or

slippery condition, the tire pressure
should be lower for a larger area of
contact with the road surface.

• Under a stony or hard road condi-

tion, the tire pressure should be
higher to prevent a flat tire.

FRONT FORK SETTING
The front fork setting should be made
depending on the rider's feeling of an
actual run and the circuit conditions.
The front fork setting includes the fol-
lowing three factors:
1. Setting of air spring characteris-

tics

• Change the fork oil level.

2. Setting of spring preload

• Change the spring.
• Install the adjustment washer.

3. Setting of damping force

• Change the compression damp-

ing.

• Change the rebound damping.

The spring acts on the load and
the damping force acts on the
cushion travel speed.

CHANGE IN LEVEL AND
CHARACTERISTICS OF FORK OIL
Damping characteristic near the final
stroke can be changed by changing
the fork oil amount.

Adjust the oil level in 5 mm (0.2 in)
increments or decrements. Too
low oil level causes the front fork
to produce a noise at full rebound
or the rider to feel some pressure
on his hands or body. Alternative-
ly, too high oil level will develop
unexpectedly early oil lock with
the consequent shorter front fork
travel and deteriorated perfor-
mance and characteristics. There-
fore, adjust the front fork within
the specified range.

Secondary reduction ratio =

Number of rear wheel sprocket
teeth/Number of drive sprocket
teeth

Standard secondary
reduction ratio

50/13
(3.846)
* 47/14
(3.357)

* For EUROPE

Part

name

Size

Part number

Drive
sprocket
"1"

(STD)

13T