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2007
CRF250R Development 2007
Honda CRF250R: Can it get any better than this? There's
an old saw in motorcycle racing that says motorcycles don't win races,
riders do. What's often unsaid is that a well-designed and well-executed
machine allows the rider to do what they do best: apply their well-honed
skills to winning. It's
no wonder then that Honda's four-stroke CRF250R motocrosser is
fast-becoming the winning platform for numerous competitors in multiple
disciplines. And with the introduction of the 2007 CRF250R, Honda has
seemingly done the impossible: taken a proven design and made it even
better, so that more riders—of all skill levels—can step into victory
circle with regularity. It
doesn't hurt that the existing CRF250R with its innovative dual-muffler
design is already dominating the race track. Before the 2006 AMA
Supercross season started, Honda Racing's Andrew Short reminded the world
of the bike's capabilities, using his skills to sweep all three races in
the Bercy Supercross in Paris, November, 2006—the second consecutive
year that Short achieved this feat. In the AMA Supercross Lites series,
Short, teammate Davi Millsaps and SoBe/Samsung Mobile/Honda Racing Team's
Josh Grant combined for nine wins in the 15-race East-West series. With
help from Grant's teammates Billy Laninovich and Tommy Hahn, the CRF250R
also took second place in 10 out of the 15 races. Even more impressive,
Honda collected the top two positions in all but one competition in the
seven-race East series (13 out of 14 possible first and second place
finishes), earning Millsaps the East Lites title and Grant a third
overall. Short garnered second overall in the West series and won the
East-West shootout at the season-ending race in Las Vegas. In the outdoor
AMA Motocross Lites series, Short has earned one victory (Southwick) and
is second in the championship after three races while Grant is tied for
third. To
fully understand the soundness of the CRF250R platform, consider that Troy
Lee Designs Honda's Cassidy Anderson has won the first three rounds of the
10-race 2006 AMA Supermoto Lites Championship. After taking his third win,
Anderson said, “If you ask me, nothing is more important than the start.
My bike runs really good and I've gotten every holeshot so far. That's the
key. It's really hard to pass and if you're in front that makes it a lot
easier.” Like the larger CRF450R, it appears the CRF250R is destined to
dominate multiple racing disciplines. Changes
to the 2007 CRF250R are evolutionary yet significant. The powerful 249cc
Unicam® engine, already renowned for its superb power delivery, gets even
better. Honda's engineers applied their extensive tuning knowledge to
broaden the powerband for even more usable power. Right out of the truck,
riders will notice that the new CRF250R mill is more tractable, yet gives
away nothing on the top end. It's an exciting feel that equates to lower
lap times on the motocross track. In the chassis department, changes to
the fork valving, front brake and a new-generation Dunlop D742 front tire
work in concert to assure that when it comes time to slow or change
direction, the CRF250R gets the job done faster and with less effort.
Here's
a summary of the changes on the 2007 CRF250R: -
New camshaft design, cylinder-head porting and exhaust header improve
low-end and mid-range power delivery while maintaining output above 8000
rpm and over-rev capability. -
New intake valve springs match revised camshaft profile. -
New ignition map with 13,370-rpm rev limit. -
New crankcase venting for improved engine power. -
Stronger clutch basket and clutch center for improved wear resistance. -
New front suspension valving for improved bump absorption and cornering
traction. -
New link-type front brake master cylinder improves front braking power by
15 percent. -
New Dunlop 742FA front tire provides improved turning feel and traction. -
New one-piece cover helps better protect the throttle cable from dirt and
water. -
Lighter rear chain guide. -
Carburetor features a new accelerator pump and linkage for improved
throttle response. Tuning
the Unicam for more power How
is it possible to increase low-end power without sacrificing top-end? The
flexibility inherent in the Unicam design allowed the engineering team to
test a number of camshaft profiles, porting configurations and exhaust
pipe lengths. One by one, the changes were evaluated through hours of dyno
and seat-of-the-pants testing. When all was said and done, the resulting
powerband was more than broadened. It was enhanced throughout the rev
range, providing a stronger power feeling at low rpm and also more usable
power in the low-, mid- and upper-rpm range. Managing
the CRF's newfound power is even easier thanks to a redesigned accelerator
pump linkage on the Keihin 40mm flat-slide carburetor. First introduced on
the TRX450R and then the 2007 CRF450R, the new direct-acting linkage
system eliminates the previous spring-loaded arm and results in more rapid
and direct throttle response
for more accurate power control. Like
its big brother the CRF450R, the four-stroke 250R utilizes vertically
split crankcases and a twin-sump lubrication system with a built-in oil
pump. Because the total volume of oil circulating to the crankshaft,
piston and valve train is reduced, the oil pump supplying these vital
parts is smaller and lighter. The design not only reduces weight compared
to a conventional design, the dual-supply system also ensures a cooler
environment for the clutch and gearbox while isolating the engine from any
potential clutch material contamination.
Dual
mufflers - it's all about balance Introduced
in 2006, the dual-muffler system on the CRF250R consists of two silencers
that tuck in much closer to the bike's center of mass. The two exhaust
canisters of equal weight are arranged on the left and right sides of the
frame. Placed in this manner, the balance of the machine is less upset
when turning, unlike the imbalance that results from the asymmetric weight
of a single, heavier muffler located on one side of the machine. In
addition, the trailing edges of the dual mufflers are nearly four inches
farther forward than the end of a single muffler design—again, moving
weight closer to the bike's center of mass. These improvements in mass
centralization, in turn, significantly improve handling. How
so? Consider that a motorcycle is subject to powerful inertial and
gyroscopic forces. Its handling and feel are judged by the rider's ability
to make course alterations rapidly while the machine is in motion.
Directional changes occur in three axes; these movements are named roll,
pitch and yaw. In simple terms, roll is rotation about an axis parallel to
the direction of travel, pitch is the movement that occurs when the front
and rear of the bike alternately assume an uppermost position and yaw is
the turning motion around the vertical axis. The
dual muffler system on the CRF250R yields a two percent improvement in
roll characteristics, a significant 18 percent reduction in pitch response
and an astounding 28 percent improvement in yaw action. For the rider, the
dual muffler system produces a lighter overall feeling and handling that
is both more responsive and neutral when turning and jumping. Fine-tuning
the suspension and braking systems The
front suspension of the CRF250R features new valving for 2007, including
changes to the secondary compression stack for less compression damping
mid-stroke. This alteration affects the character of the fork, producing
smoother, more linear and more controlled response from mid-stroke through
full compression for better control and feel. Primary compression valving
settings are also changed and a different shim stack changes the main
compression settings as well. Honda's
engineers outfitted the 2007 CRF250R with a new linkage-type front brake
lever, the same type used by the Honda Racing Team. The patented design
features a straight-line actuating system that yields more mechanical
leverage to increase stopping power 15 percent without increasing rotor
diameter or caliper size, or the penalty of additional weight. As a
result, the front brake now provides more positive feedback and feel when
braking. It is also much more
powerful, producing quicker stops. An added benefit is a dramatic
reduction in lever effort; the CRF250R now boasts a true, one-finger brake
system. Rounding
out the upgrades is a new Dunlop D742FA front tire jointly developed by
Honda and Dunlop. The 80/100-21 front skin has a distinctive appearance
and features an innovative knob pattern and profile. Riders will notice
that the new tire initiates turns more easily and rolls into turns with a
more positive feel. In mid-corner the 742FA stays planted and stable and
yet remains easy to control. Bump impact is excellent and the tire
provides great front-end feel and improved traction in hard pack, muddy
conditions, sand washes and more. Fourth
generation aluminum frame: Amazingly better At
the heart of the CRF250R is Honda's fourth-generation aluminum frame.
Honda has been crafting aluminum CR and CRF frames for 11 years, starting
with the 1997 CR250R. The CRF's fourth-generation design incorporates
Honda's years of experience and current state-of-the-art engineering to
produce amazing handling. The
CRF250R frame is purpose-built and tuned specifically for the bike. This
is one of aluminum's great advantages. Material can be added or taken away
in precise increments to gain strength or flexibility in specific areas.
These changes alter the stiffness of the frame members, making them
resistant to up-and-down flex, while still allowing side-to-side
resilience. For example, the main spars of Honda's aluminum frames are
rectangular in cross-section, allowing engineers to work with variables
unavailable to round-tube-steel fabricators. The top and bottom sections
of the box can be made thicker or thinner than the sidewalls by specifying
a change in the extrusion dies. The most visible features of the
fourth-generation frame are the crucial chassis elements connected to the
main spars: the steering head and the swingarm pivot. Here engineers can
take advantage of aluminum's amazing tunability. A forged gusset at the
rear of the forged aluminum steering head adds strength to the entire
assembly. The aluminum swingarm pivot plates feature a web of internal
gussets for increased strength, and are shaped to create a narrow overall
cross-section to optimize rider comfort and enhance maneuverability. In
the end, the CRF250R frame provides the optimum balance between chassis
rigidity and lightweight agility. The flex characteristics of the fourth
generation design create a chassis that feels compliant on bump impact and
improves cornering characteristics, while still delivering the solid feel
and exemplary durability which Honda's aluminum frames are known for. It
all adds up to this The
2007 CRF250R has more power, accelerates better, turns and holds cornering
lines better, maintains higher cornering speeds—and all the while
requires less effort from the rider. Which means that everyone who throws
a leg over the 2007 CRF250R can become a better rider—not to mention a
race winner. Honda's
Innovative Unicam Design It's
official. With more than 100,000 Honda Unicam-powered CRF250R, CRF450R,
CRF250X and 450X models setting the pace in Supercross, motocross,
off-road and Supermoto races across the country, Honda's vanguard
four-stroke engine design has developed a cult-like following. It's
not surprising—the Unicam engine has earned a performance reputation
second to none. In the CRF250R rendition, the high-tech, liquid-cooled
four-valve engine boasts a bore and stroke of 78mm x 52.2mm, yielding a
displacement of 249cc. The heart and soul of the CRF250R engine is the
innovative Unicam single-overhead-camshaft design, which incorporates a
carburized single camshaft that directly actuates two 31mm titanium intake
valves. The camshaft's single exhaust lobe actuates two steel 26mm exhaust
valves via a unique forked, low-friction, roller rocker arm. The
lightweight titanium intake valves produce a high-revving engine, and the
high-temperature steel exhaust vavles utilize heat-resistant Inconel®
alloy in the neck area just before the flare of the valve head. (Inconel
alloy contains high levels of nickel and can be thought of as a
super-stainless steel that has exceptional anti-corrosion and
heat-resistance properties.) Valve adjustment is accomplished with shims;
the intake valves use a direct shim-under-bucket system, the exhausts
incorporate shims between the rocker arms and keepers. The
most significant benefit of the Unicam design is a shorter engine. The
lighter titanium valves permits the use of smaller valve springs and steel
exhaust poppets handle the stratospheric 13,370-rpm rev ceiling. A roller
bearing on the rocker arm reduces friction and therefore wear, allowing
the cam lobe to be narrower—and lighter—than conventional designs. The
compact SOHC configuration and separate cam holder contribute to a notably
compact design that saves weight over a comparable DOHC motor and also
permits a narrow included valve angle. This, in turn, flattens the
combustion chamber to facilitate free flame propagation, allowing a high
compression ratio of 12.9:1. Since less space is taken up in the cylinder
head, the camshaft sits lower in the head for a more compact engine and a
lower center of gravity (Cg) is achieved. With a lower Cg, the CRF250R
feels light and handles nimbly and predictably, reacting swiftly to
steering inputs and directional changes.
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