Powersports / Motorcycles / Motocross / CRF150R / Development
The Revolution Continues: Introducing the Honda 2007 CRF150R and CRF150R-Expert
Anybody with even a passing interest in motocross and Supercross racing recognizes that a revolution has recently taken place. Within a few short years four-stroke machines supplanted two-stroke bikes almost completely throughout the ranks of professional racing, and Honda has played a major role in this transformation. Since the debut of the CRF450R during the 2002 season, the buying public has purchased well over 100,000 motorcycles spread between the CRF450R, CRF450X, CRF250R and CRF250X, which makes Honda's four-stroke Unicam® engine format a runaway best-seller.
In 2007 the revolution continues with the introduction of the CRF150R, a machine destined to transform another segment of the motocross market in a shift from two-stroke to four-stroke-with all the stellar attributes that have been quickly proven in Honda's big-bike ranks. In keeping with the trend begun by the CRF450R and continued with the CRF250R, Honda's new CRF150R and big-wheel CRF150R-Expert are destined to shake up the small-displacement classes of two-stroke MX machines. And if the new little CRF150R looks strangely familiar, it's for good reason: Check out the engine-it's a scaled-down and updated miniature version of the tried and proven big CRF Unicam engines.
Think of it as an exercise in modern-day miniaturization, analogous to the ongoing trend in computer design where evermore-powerful units are shoehorned into ever-shrinking packages.
As such, the 149cc CRF150R follows almost every key design element common to both of the larger Unicam four-valve, single-overhead-cam powerplants. A single chain-driven camshaft directly actuates a pair of intake valves, in this case measuring 26.0mm. However, instead of employing a forked roller rocker arm to actuate the two 22.5mm exhaust valves, the CRF150R incorporates two separate low-friction roller rocker arms. This arrangement makes it easier to change valve-lash shims when adjusting the valves, and it also allowed the spark plug tube to be situated between the two rocker arms to create a very compact cylinder head layout with the spark plug centrally located in the combustion chamber for optimum flame propagation. Like its larger CRF siblings, the CRF150R's Unicam layout produces a very flat 21.5-degree included valve angle that also opens up the combustion chamber for efficient burning. Because they are lighter and smaller, all four valves can be made of tough, durable steel, and single valve springs are used all around. Also in keeping with established Unicam practice, the CRF150R incorporates a small, lightweight automatic decompression mechanism that opens the left-side exhaust valve slightly to make starting a snap.
In keeping with the Unicam practice and race-engine design in general, the bore and stroke are radically oversquare: 66.0mm x 43.7mm, standing in stark contrast to the dimensions of the recreationally oriented two-valve CRF150F, which displaces its 149cc by way of a nearly square bore and stroke of 57.3mm x 57.8mm. Likewise, the CRF150R's slipper-type racing-style two-ring piston slides through a Nikasil-lined cylinder to generate a compression ratio of 11.7:1, which stands much higher compared to the CRF150F's 9.5:1 ratio. The crankshaft is made of high-strength, low-carbon steel with carburized main journals for optimal durability at sustained high-rpm operation. In addition, this engine follows Unicam engine design in its use of two separate oil chambers, one for the engine and one for the gearbox. This design is instrumental in reducing weight and creating less oil drag around the crankshaft, as well as lowering the heat load in the clutch.
If that little carburetor looks familiar, albeit smaller, you've got a good eye; this FCR type, flat-slide 32mm unit with a throttle position sensor is a mini-sized version developed specifically for this machine; the CR85R, in contrast, uses a 28mm flat-slide. And because this is a larger-displacement engine compared to the CR85R, the CRF150R features dual radiators in place of the single cooler mounted aboard the two-stroke.
Speaking of the CR85R, that race-proven platform formed the basis for the CRF150R rolling chassis, from the tough but light high-tensile steel frame to the trick aluminum swingarm, fully adjustable 37mm inverted Showa cartridge fork and Pro-Link rear suspension system with fully adjustable Showa shock. As would be expected, the CRF150R incorporates its own specific spring and damping rates. Also, the CRF150R features a smaller and lighter rear brake system like those found on the big CRF racers, and new, stronger wheels also sport lighter hubs for less unsprung weight.
In keeping with prior Honda small-displacement motocrosser practice, the CRF150R is available in small-wheel or big-wheel versions to tailor-fit growing racers. The wheel configurations are identical to those found with the CR85R and CR85R-Expert: a pairing of 17-inch front and 14-inch rear wheels, or the larger duo of 19-inch front and 16-inch rear. As with the CR85R, the larger wheels found on the CRF150R-Expert model raise seat height and ground clearance about 1.5-inches, while steering rake is also increased minimally. The front axle is a stout piece measuring 15mm in diameter, while the rear axle measures a full 17mm across.
As this new motorcycle progressed from the conceptual to the development stages, Honda R&D called in some teenaged experts in the field. This sharp-eyed crew was perhaps even more critical than any in-house engineers; these youngsters have spent years in the crucible of racing, and their focus is on winning. Period.
From the get-go, no one had any problems starting this 149cc four-stoke engine, even the less experienced riders and those who had ridden two-strokes extensively. More to the point, every young tester simply loved the CRF150R; they found it amazingly fun and fast to ride, and remarkably confidence inspiring regardless of racing rank. Many comments concerned the bike's light and maneuverable feel. With a low CG the CRF150R is highly maneuverable, and turn-in is remarkable light. Others opined that because it's so rideable, with so much more torque compared to a two-stroke, it doesn't really feel any heavier than the CR85R.
Regardless of the physics involved, the net outcome is that the CRF150R makes the rider more confident; like its bigger four-stroke brothers, this bike offers a decidedly broader powerband while also pumping out lots of muscle down low and on the top end. This broad-shouldered power delivery makes the CRF150R notably easier to ride than a two-stroke, especially while coming out of turns, and coming off jumps. It's simply a better power delivery, one that is not as explosive as a two-stroke when the two are tuned to high states for racing use. Speaking of high states of tune, the CRF150R generates its peak power at 12,500 rpm, and its rev limit is higher yet at a remarkable 14,300 rpm, which is 1000 rpm higher than the limit of the CRF250R, making it a real thoroughbred, through and through.
Still and all, the CRF150R doesn't give up anything for all its gains; this bike is slightly heavier than the CR85R but that was never an issue with the toughest critics of all, the assembled group of young racers. The proof? Every single one wanted to take the bike to the track that weekend in place of their current race bike, including many that were highly modified models.
With the introduction of the CRF150R, the transformation of the sport of motocross continues. This is one triple-distilled, pure racing motocross bike through and through, in keeping with the winning heritage of Honda's CRF450R and CRF250R. It's not much of a stretch to predict that this new pocket-rocket four-stroke will immediately become the favorite of the closed-course crowd. But there may be one hitch.
As the assembled group of young testers was putting the prototype CRF150R through its paces, more than one parent was looking on with envious eyes. With the popularity of adult mini racing, it may be necessary to stock each garage with two CRF150Rs to satisfy dad as well.