The first 4-stroke engine motocrosser (CRF450R) in Honda was developed focussing on the fastest motocrosser for all motocross races and to enable riding without stress according to the rider’s intention. The development targets were to obtain a linear characteristic in torque and traction, which was controlled easily by adopting the 4-stroke engine, and to achieve the equivalent power-weight ratio to the 2-stroke engine motocrosser of the same race class. A lightweight forged piston with an extremely short skirt, a valve train of SOHC 4-valve engine (UNICAM system), titanium inlet valves, and a separated lubrication system for both the crankcase and the transmission were adopted in the engine as means to get a liner characteristic in torque and high power. As a result, 29.5 kg of engine mass and 41 kW of maximum power were achieved. Engine startability via the kick-starter was also improved by adopting a newly developed auto-decompression system. A total vehicle mass of 102 kg was realized by lightening each part of the frame body and suspension through techniques such as a newly shaped aluminum frame and a titanium exhaust pipe.
Article of Honda R&D Technical Review Vol.14 No.1 (21 - 30)