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2007 Honda 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.