Yamaha YZ250FX 2021, Owner'S Service Manual

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CHASSIS
10-1
EAM20119
CHASSIS
EAM30168
SELECTION OF THE SECONDARY
REDUCTION RATIO (SPROCKET)
<Requirement for selection of secondary gear
reduction ratio>
•
It is generally said that the secondary gear ratio
should be reduced for a longer straight portion
of a speed course and should be increased for
a course with many corners. Actually, howev-
er, as the speed depends on the ground condi-
tion of the day of the race, be sure to run
through the circuit to set the machine suitable
for the entire course.
•
In actuality, it is very difficult to achieve settings
suitable for the entire course and some set-
tings may be sacrificed. Thus, the settings
should be matched to the portion of the course
that has the greatest effect on the race result.
In such a case, run through the entire course
while making notes of lap times to find the best
balance; then, determine the secondary reduc-
tion ratio.
• If a course has a long straight portion where a
machine can run at maximum speed, the ma-
chine is generally set such that it can develop
its maximum revolutions toward the end of the
straight line, with care taken to avoid the en-
gine over-revving.
TIP
Riding technique varies from rider to rider and
the performance of a machine also vary from
machine to machine. Therefore, do not imitate
other rider’s settings from the beginning but
choose your own setting according to the level of
your riding technique.
EAM30169
DRIVE AND REAR WHEEL SPROCKETS
SETTING PARTS
EAM30170
TIRE PRESSURE
Tire pressure should be adjusted to suit the road
surface condition of the circuit.
• Under a rainy, a muddy, a sandy, or a slippery
condition, the tire pressure should be lower for
a larger area of contact with the road surface.
•
Under a stony or a hard road condition, the tire
pressure should be higher to prevent a flat tire.
Secondary reduction ratio = Number of
rear wheel sprocket teeth/Number of
drive sprocket teeth
Secondary reduction ratio
3.923 (51/13)
Part name Type Part number
Drive
sprocket “1”
(STD) 13T 9383B-13218
Rear wheel
sprocket “2”
47T 17D-25447-50
48T 17D-25448-50
49T 17D-25449-50
50T 17D-25450-50
(STD) 51T 17D-25451-50
52T 17D-25452-50
Standard tire pressure
100 kPa (1.00 kgf/cm², 15 psi)
Extent of adjustment
60–80 kPa (0.60–0.80 kgf/cm², 9–
12 psi)
Extent of adjustment
100–120 kPa (1.00–1.20 kgf/cm²,
15–18 psi)
Part name Type Part number