Duty cycle
A few years ago there was a flurry of interest in re-powering the sophisticated aluminium chassis of the world’s many TZ250 Yamaha production racers with readily available 450 four-stroke motocross engines. After prototypes were built in several places nothing came of the idea. Why not?
Two problems stopped this apparently sensible idea. First, MX engines are designed to survive a duty cycle of many short grunts of throttle, each one occurring in the brief moments when the back tyre is on the ground. But letting one
