Within the last couple years, Honda rocked the motocross world with a pair of stunningly brilliant four-stroke machines, the CRF450R and CRF250R. Although the two race bikes quickly proved their fundamental excellence, that really came as no surprise; since the company's inception, four-strokes have been the mainstay at Honda, and innovation a way of life.
However, it may come as a surprise to many that the newly released CRF250R motocrosser would be followed so quickly by a high-performance off-road machine--the CRF250X. Typically, other companies would release such a machine much later. But the rapid succession of the two quarter-liter bikes simply reflects how much technology the two Hondas share.
Two machines, one design
Like the CRF250R, the CRF250X incorporates a new-generation four-stroke Unicam® single-cylinder engine, one designed to run at speeds in excess of 12,000 rpm. That engine speed, for the record, is faster than the majority of high-tech street machines. Like the motocrosser, the 250X engine is also cradled within a new, fourth-generation aluminum frame equipped with top-drawer suspension and chassis components. No corners have been cut here; in fact, extra R&D effort was spent in retuning the entire chassis to suit off-road needs virtually to perfection. In like manner, the engine faithfully follows CRF-R architecture, but the 250X incorporates modifications that make it even better suited to serious off-road work.
Establishing the state of the art in off-road engines
The CRF250X engine features forked rocker arms that actuate two lightweight and very tough 31mm titanium intake valves, which feature a special surface processing. The two 26mm exhaust valves are made of high-temperature steel with heat-resistant Inconel® alloy used in the neck area just before the flare of the valve head. Inconel alloys contain high levels of nickel and can be thought of as super-stainless steels that have exceptional anti-corrosion and heat-resistance properties.
Like the CRF250R, the 250X incorporates a compression ratio set at an extremely high 12.5:1. This creates a piston with a relatively tall dome, but overall the 250 piston height is very low and its skirt is ultra-short, so instead of looking like a piston used in run-of-the mill four-stroke engines, it looks more like a piston crown with wrist-pin bosses. The amazingly short and narrow piston skirts come from the factory impregnated with low-friction molybdenum to ensure a smooth piston-to-cylinder fit beginning with first use.
The forged piston runs in a cylinder made with a tough, low-friction Nikasil® liner, a setup that also enhances cooling and reduces weight because it eliminates the need for a separate cylinder liner. In addition, the 250's connecting rod is double-carburized for extra toughness, and it utilizes a needle bearing in the big end to maximize high-rpm performance and increase durability. Likewise, the high-strength, low-carbon steel crankshaft features carburized main journals for maximum durability under high engine speeds.
In keeping with CRF-R engine design, the 250X incorporates a twin-sump lubrication system that separates the engine oil from the oil bathing the clutch and the wide-ratio five-speed transmission. This dual-supply system ensures a cooler environment for the clutch, while also isolating the engine from any potential contamination caused by clutch and transmission material. Because the total volume of oil circulating to the crankshaft, piston and valve train has been reduced, the oil pump supplying these vital parts can be made commensurately smaller and lighter. The new engine also features redesigned internal baffles to manage oil level and circulation.
To make life on the trail easier, the CRF250X features an electric starter system, complete with a sealed, maintenance-free battery and charging system. A kickstart mechanism is retained for backup purposes, and a lightweight, compact internal automatic decompressor system makes that starting drill easy as well. A gear-driven counterbalancer system does double duty by quelling vibration and driving the water pump-again--another testimony to efficiency in design. Fuel/air mixing chores are handled in thoroughly modern fashion by a 37mm Keihin FCR flat-slide carburetor featuring a throttle position sensor that helps maintain linear throttle response throughout the rev range. The flat slide features four rollers to ensure smooth operation while keeping throttle effort commendably light; older-generation flat-slide carbs were subject to sluggish slide movement due to high engine vacuum pressures. To better cope with changing conditions and elevations, a longer air-screw on the CRF250X carburetor allows for easy access and adjustment.
A world-class motocross chassis--with improvements
The CRF250R boasts a fourth-generation Honda aluminum frame tuned to take on the toughest challenges thrown up by world-class Supercross and motocross tracks. But off-road events take place in an entirely different world, which called for entirely new performance parameters for the CRF250X chassis. So Honda's development engineers took to the roughest and tightest off-road areas in America to learn new lessons that would be applied to the basic fourth-generation motocross frame and suspension components, thereby creating a new, off-road-specific chassis.
Blessed with an overarching inherent soundness, the CRF250R's aluminum frame made a great starting point, as changes were incorporated into the frame that was to become the X-model's skeletal structure. And Honda's engineers enjoyed a huge advantage: Aluminum has a lower specific gravity compared to steel, so adjustments can be made easily compared to working with steel frames. That made it easy to add a bit of thickness here and trim a little bit off there, in order to arrive at an ideal balance between lightweight agility and frame rigidity in an off-road setting.
Also, because the main spars of Honda's aluminum frames are rectangular in cross-section, engineers can work with two more variables unavailable to round-tube-steel fabricators. With aluminum, Honda can make the top and bottom sections of the box thicker or thinner than the sidewalls, simply by specifying a change in the extrusion dies. This can alter the stiffness of the frame members in huge ways, making them resistant to up-and-down flex, while still allowing side-to-side resilience, both in precise increments.
By altering frame downtubes, engine cradle, engine mounts and side pivot plates, the R&D team defined a new frame that yields greater bump absorption over the many small obstacles typically encountered off-road, a change that also provides greater steering precision and enhanced rider comfort. These subtle but significant changes, however, do nothing to lessen the 250's light and nimble feel--a result of the machine's relatively low inertial mass and forward-located center of gravity. Perhaps the one element most astute observers will note is the newly strengthened oval-section subframe, which is beefier to handle the weight of the CRF250X's spark arrestor and battery.
All of the first-rate chassis components found on the CRF250R still remain aboard the CRF250X, but again these elements have been retuned specifically for off-road applications. The lightweight 47mm inverted Showa® twin-chamber cartridge fork is fully adjustable with 12.4 inches of travel, and it features a new taper in the fork tubes and lighter springs and compression damping that make it X-bike specific. Honda's renowned Pro-Link® suspension system graces the rear end, and while it too delivers 12.4 inches of fully adjustable travel like the CRF250R, a new linkage system with a less-progressive curve and lighter springs and compression damping make it more off-road-friendly.
Large, 240mm disc brakes mount front and rear, with a dual-piston caliper up front. The aluminum swingarm features a dual-axis, double-taper design, with a stout cast aluminum crossmember for rigidity. HRC works-type lightweight aluminum spoke nipples help reduce unsprung weight.
Other Team Honda motocross-inspired touches add a special aura to the CRF250X, such as the durable Renthal handlebar, quick-adjust clutch perch for easy on-the-fly adjustments, and newly designed racer-style plastic bodywork for a more aggressive look plus lighter weight. Then there's the trick non-slip seat cover, plus the new quick-access airbox, svelte headlight and slimline LED taillight and so much more.
It all makes sense. To make the best four-stroke 250 off-road bike in the world, all you have to do is take the best 250 four-stroke motocross bike in the world--and make it better yet.