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Honda CRF250R Owner's Manual & Competition Handbook

Honda CRF250R
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Page #141 background image
Adjustments for Competition 135
Gearing
You can “adjust” the power delivery of the
standard engine to suit track conditions by
changing gearing. This allows you to utilize a
different portion of the engine’s power range at a
given throttle setting. New gearing may provide
the change you are looking for without the need
to consider further modifications.
The portion of your engine’s power range you
use can be adjusted by changing the final drive
ratio with different sized rear sprockets. Gearing
changes allow you to more closely match the
type of terrain and the available traction.
Normally, a change of one tooth on the rear
sprocket will be sufficient.
There is a choice of both higher and lower final
drive ratios with two optional aluminum driven
sprockets. Like the optional springs, these
sprockets are listed in the Optional Parts List
section of this manual (page 154).
Unless you have the required mechanical
knowhow, tools, and Honda Shop Manual,
sprocket changing should be done by your Honda
dealer.
Higher Gearing (less driven sprocket teeth)
increases top speed in each gear (provided the
engine will pull the higher gearing)
reduces frequency of shifting (wider gear
ratios)
reduces engine min
-1
(rpm) at a given throttle
setting or ground speed (which may allow
better rear wheel traction on slippery or loose
terrain)
However:
the engine may not pull the higher gearing
the spacing between gears may be too wide
engine min
-1
(rpm) may be too low
Lower Gearing (more driven sprocket teeth)
decrease top speed in each gear
increases frequency of shifting (narrower gear
ratios)
increases engine min
-1
(rpm) at a given
throttle setting or ground speed (which may
provide more power-to-the-ground on good
traction surfaces)
However:
spacing between gears may be too narrow
engine min
-1
(rpm) may be too high
Some tracks may be watered heavily prior to the
first race, then lightly or not at all during the day.
This results in a track surface that is slippery
during the first few races, then changes from
good to great and back to good, and possibly
ends the day with a slick rock-hard consistency.
Ideally, your gearing should be adjusted to suit
all these conditions.
•Wet and slippery or sandy conditions: use a
higher gear (less teeth) to keep engine min
-1
(rpm) down, and avoid unwanted wheelspin.
The engine may bog in certain corners so
you’ll need to slip the clutch to compensate;
downshifting may be too drastic a change in
speed.
•Average conditions: use the standard sprocket.
Hard (but not slippery) track conditions: use
lower gearing (more teeth) to keep the engine
min
-1
(rpm) high where the engine produces
the most power. This may require an extra
upshift on certain sections or perhaps you can
just rev it out a bit longer.
For tight tracks, consider lower gearing to avoid
having to slip the clutch frequently. Repeated
fanning or pulling of the clutch lever in a turn to
raise engine min
-1
(rpm) may eventually damage
the clutch system.
A gearing change may help for riding in sand,
where you want to keep the front end light so it
can float from the peak of one sand whoop to the
next. Generally, with higher gearing, it is easier
to maintain that perfect attitude (maximum rear
wheel traction and a light front end) because you
remain in the powerband longer in each gear.
The higher gearing allows you to steer more
efficiently with throttle control and body English.
If you are riding a track with sections where you
choose to over-rev the engine temporarily rather
than shifting up, higher gearing might help.
Sometimes you have to sacrifice performance on
one section of the track to gain a better overall
time. Your goal is the fastest overall lap time,
even if the trade-off is gearing that feels wrong in
some sections.
If you decide to try a gearing change, have
someone check your times with a stopwatch
(before and after the change) to get an accurate
appraisal of the gearing change. “Seat-of-the-
pants” feelings can’t be trusted. Eliminating
wheelspin with a gearing change can make you
feel like you’re going slower when, in reality,
you’ve decreased your time by increasing your
speed with better traction.
These gearing recommendations should be
evaluated by considering your ability, your riding
style, and the track.
250R_120-139.qxd 2008.07.17 15:43 ページ135

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Honda CRF250R Specifications

General IconGeneral
Engine TypeLiquid-cooled single-cylinder four-stroke
Displacement249cc
Bore x Stroke79.0mm x 50.9mm
Compression Ratio13.9:1
StarterElectric
TransmissionClose-ratio 5-speed
Final DriveChain
Front BrakeSingle 260mm disc with twin-piston caliper
Rear BrakeSingle 240mm disc
Front Tire80/100-21
Rear Tire100/90-19
Wheelbase58.3 inches
Seat Height37.8 inches
Ground Clearance13.1 inches
Fuel Capacity1.7 gallons
Valve TrainUnicam OHC, four-valve
InductionProgrammed Fuel Injection (PGM-FI)
Throttle Bore44mm
IgnitionFull transistorized ignition
Front Suspension49mm inverted Showa coil-spring fork with rebound and compression damping adjustability; 12.2 inches travel
Rear SuspensionPro-Link Showa single shock with adjustable spring preload, rebound and compression damping; 12.4 inches travel
Rake27.2°
Trail115mm (4.5 inches)

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