When Honda first tasked engineer Eiji Adachi with the responsibility of designing the CRF250R's frame, he knew he would be elevating the state of the art to a new, higher plane. This was the time to consolidate all that had been learned over the past eight years in crafting aluminum CR frames, to create a new, fourth-generation unit tailor-made to the new 250 four-stroke engine.
"The CRF250R carries a smaller, lighter engine than the powerplant used in the CRF450R," Adachi said. "So its frame proportions are correspondingly smaller, actually closer to those we have used in our two-stroke machines. Still, we designed the CRF250R frame to be another step forward compared to both the CRF450R's and our two-stroke racing machines'.
The new era of aluminum frames
"Many riders seem to think aluminum frames are bound to feel really stiff, but the CRF250R's frame has been tuned to offer a very natural feeling," Adachi explained. "By adding a bit of thickness here and trimming a little bit off there, we have balanced lightweight agility with frame rigidity. Aluminum has a lower specific gravity compared to steel, so these adjustments are easier to achieve than with comparable steel frames."
This is one of aluminum's great advantages. Material can be added and taken away to gain strength or flexibility in specific areas. Now, add another aspect: Because the main spars of Honda's aluminum CR frames are rectangular in cross-section, engineers can work with two more variables unavailable to round-tube-steel fabricators. First, they 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 a huge way, making them resistant to up-and-down flex, while still allowing side-to-side resilience, both in precise increments.
With this fourth-generation aluminum frame, engineers took advantage of the material's amazing tunability everywhere you look. The semi-double-cradle design incorporates a forged aluminum steering head. A new forged gusset at the rear of the steering head adds strength to the entire assembly. This steering head connects to a single, tapered large-diameter downtube that has been slimmed and lightened compared to the 450's downtube. Newly designed 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. The most visible distinction of the new frame are the main spars connecting the most crucial chassis elements: the steering head and the swingarm pivot. These lightweight aluminum extrusions are shorter, narrower, and feature thicker walls than Honda's third-generation aluminum frame.
Given the lighter weight and reduced loads imposed upon the 250, these alterations change the flex characteristics of the frame to create a chassis that feels softer on bump impact and improves cornering characteristics, while still delivering the solid handling and exemplary level of durability Honda's aluminum frames are known for.
In addition to its structural changes, this fourth-generation frame uses enhanced geometry to add even more handling benefits.
"We also lowered the machine's inertial mass and moved its center of gravity forward to help achieve the desired feeling of light, nimble rideability," said Adachi. "And because four-stroke silencers are bigger and heavier than those on two-strokes, the seat rail subframe had to be made stiffer, as too much flex in this area would have impaired the bike's handling." Interestingly, the CRF250R's aluminum subframe utilizes oval-shaped frame tubes, technology that combines strength and light weight in a compact package.
The end result is a world-class 250 four-stroke racer that establishes new handling standards for riders of all abilities. As with the CRF450R, a machine that has a wide range of appeal thanks to its user-friendly handling and wide powerband, the CRF250R is designed to appeal to riders of all skill levels.
As project leader Yasuhiro Nakayama explained, "We've had top motocross racers and also riders with lesser skills evaluate the CRF250R, and the impressions have been very consistent-all very positive. I believe that once you've built in qualities that have been refined and are backed by quality and proven technology, your prototypes evolve into machines that everybody finds to be easy to ride. We think the CRF250R is just such a bike."