Honda Discusses CRF250L Development
Honda CRF 250 L Development Story
Following is an in-depth analysis of Honda’s development of the all-new CRF250L. For a review of the machine, click here.
In 1972 Honda reconfigured the motorcycling landscape by introducing the XL250 Motosport, a four-stroke four-valve overhead-cam single-cylinder street/dirt bike in an era dominated by two-strokes. At the time, highly regarded Cycle magazine said of the XL250, “No more than you would expect from Honda, and no less: An absolutely brilliant, inspired engine, superb styling, careful detailing… ”
Riders responded to the XL250 with a rush, a generation set for fun and affordability scooped up more than 80,000 of these machines over the course of its run. Subsequent generations of models were strong testimony to the popularity of these versatile, fun-to-ride dual-sport machines.
Flash forward 40 years, and Honda introduces our newest dual-sport bike with the CRF250L. The generation that became part of motorcycling in the 70′s and 80′s are still riding and new generation X and Y are looking to enter motorcycling. Thanks to a very affordable price, fuel economy estimated at 73 mpg,* plus the ease of use, convenience and practicality of an electric-start everyday rider, the CRF250L is an incredible value at only $4499. This is a great motorcycle for the existing customer to add to their ownership and for a new rider just getting started in the sport.
CRF250L Engine Design
At the core of the CRF250L beats a heart suited for the global market: like the popular CBR250R, this is a bike manufactured in Honda’s Thailand production facility and the two bikes share the same basic engine architecture. This new-generation four-stroke liquid-cooled 249.6cc single-cylinder powerplant features a dual overhead camshaft layout for improved combustion efficiency, thanks to the reduced weight of the reciprocating valve train, large valves (30mm intake and 24mm exhaust) with thin stems for superior flow, and a centrally located spark plug for efficient flame propagation. This design also permitted great freedom in choosing the narrow included valve angle, port shape, and a pent-roof combustion chamber-all key elements for optimal performance.
A roller rocker arm is used in combination with the DOHC engine configuration, which allows for a low-friction valve train and very compact cylinder head, plus an ultra-compact layout for the roller rocker arm. The application of a shim design for valve tappet adjustment reduces rocker arm weight, while internal engine friction is further reduced by setting the valve spring load to a low level. For ease of maintenance and reduced operating costs, the valve shims can be replaced for valve adjustment maintenance without removing the camshafts.
The oversquare, short-stroke engine with a bore and stroke of 76mm by 55mm promotes engine responsiveness. To reduce reciprocating weight and friction, the piston carries a very short skirt and features a slick molybdenum coating. Friction was further reduced through the application of light striations on the piston to facilitate retention of lubricating engine oil, lowering the tension of the piston rings, and applying a smooth, shot-peen hardened finish to the piston pin.
Another slick bit of design insight further reduces engine friction: the cylinder centerline is offset from the center of the crankshaft, 4mm toward the exhaust side. Doing so reduces the lateral resistance generated between the piston and the cylinder during the power stroke. Also, to reduce the flow of blow-by gasses and minimize oil consumption, a spiny sleeve design was adopted for the cylinder sleeve. With this configuration, small spines have been added to the outer surface of the cylinder sleeve to improve cooling performance and help reduce distortion of the cylinder’s inner shape. In addition, centrifugal casting allowed a thin, uniform wall thickness, which aids weight reduction.
While the CRF250L and CBR250R share engine architecture, the CRF250L incorporates a number of unique elements that enhance its off-road capabilities-changes that place a premium on a broad spread of low-end and midrange torque over high-revving power. To that end, the CRF250L has been tuned for a more tractable powerband with its own ECU to control ignition and EFI mapping; new airbox; new intake manifold with a straighter path from the airbox to the cylinder head; a head pipe that’s smaller in diameter (down from 31.8mm in the CBR250R to 28.6mm) but about 200mm longer; a new muffler; and a smaller throttle body. The smaller 36mm throttle body for the Programmed Fuel Injection (PGM-FI) system (the street-only CBR250R uses a 38mm throttle body) yields higher airflow velocities in the intake tract, especially at lower engine speeds, which is just the ticket for slower-speed maneuvering in off-road applications. Other changes made for the CRF250L’s off-road applications include a new clutch with a judder spring to absorb shock loads through the driveline, plus a dirt-tough gearbox with wider gears and strengthened dogs. The internal gear ratios remain unchanged from those in the CBR250R, but lower final gearing is achieved via a 40-tooth rear-wheel sprocket in lieu of a 38-tooth sprocket.
The new-generation crankshaft incorporates a metal bearing (half-split, press-fit) for the crank journal, and a cast-iron bushing supports the crank bearing, which improves the rigidity of the crankcase housing and better controls changes in the crank journal’s oil clearance arising from thermal expansion, while improving engine quietness at the same time. The built-up type crankshaft allows the big end of the connecting rod to use a low-friction roller bearing, and a primary balancer shaft is incorporated into this cutting-edge single-cylinder engine. It’s placed so close to the crankshaft that the balancer weight passes between the two crank weights-a design that keeps the engine as compact as possible while helping improve mass centralization. Also, crankshaft rigidity is enhanced further and quietness is improved by placing the engine counterbalancer’s driving gear on the right cover inside the clutch housing-a design that narrows the distance between the left/right crank bearings and efficiently places a load-bearing ball bearing at the tip of the crankshaft’s right side.
A Brand-New Chassis
To fulfill its dual-sport requirements, the CRF250L features a brand-new frame unique to this model. Constructed from steel, the frame’s twin oval-section main spars and semi-double cradle layout provide the strength needed for off-road riding. The slim and compact dimensions allow for a narrow rider interface that’s well suited to off-road work. A lightweight, round-section steel bolt-on subframe supports the rider plus a passenger, and guards located above the rider’s footpegs protect the frame from boot scuffs. A wheelbase of 1445mm is matched to a 27° 35′ rake with 113mm trail, balancing excellent stability and agility.
The 43mm Showa inverted fork is suitably stout for off-road work, and there’s a full 9.8 inches of travel for use over a wide range of terrain and speeds. The Honda Pro-Link rear suspension delivers 9.4 inches of rear-wheel travel; the Showa shock absorber is a single tube design with 40mm diameter cylinder. The tapered aluminum swingarm incorporates a monoblock casting, a process that allows for the creation of intricate shapes and variable thicknesses suitable for this particular application. It provides the correct balance between rigidity, controlled flex, strength and reduced mass.
The front brake uses a single 256mm disc gripped by a twin-piston caliper, while the rear incorporates a 220mm disc and single-piston caliper for strong stopping power. The lightweight brake discs feature a wave design taken from the CRF250R and CRF450R, with exceptional self-cleaning abilities in adverse conditions. Lightweight aluminum rims with straight-pull spokes further reduce unsprung mass. They are directly attached in a spoke pattern layout taken from the CRF250R and CRF450R series, a design that’s extremely rigid. Block pattern enduro-style tires (3.00-21 front, 120/80-18 rear) provide traction throughout a wide range of situations and terrain. The 21-inch front wheel and 18-inch rear enhance tracking over rough terrain and make the fitment of a wide variety of off-road-specific tires possible.
Like the thousands of dual-sport Honda 250cc four-strokes that came before, the new CRF250L capitalizes on a great idea and fulfills an open niche. Its off-road prowess tickles the imagination as riders dream up destinations off the beaten path, and its Honda quality and reliability, plus its low purchase price and operating costs make it easy to own for anyone with a sense of adventure.Google+