Why is horsepower at the wheel different from horsepower at the crankshaft (which is generally higher)?
Crankshaft horsepower and wheel horsepower are different because they are measured at different “points” of a motorcycle. Shaft horsepower is measured directly at the crankshaft: it therefore provides a rather reliable indication of “pure” engine efficiency, without taking into account the mechanical loss due to the primary drive and final drive. The efficiency of the mechanical parts (in particular, the drive parts such as coupled toothed gears, bearings, chain links on front and rear sprockets, etc) is not perfect but always involves a certain loss (generally, by friction); in other words, a reduction in the transmitted power. The wheel horsepower is measured directly at the rear wheel sprocket, or, less accurately, on rollers driven by the rear tyre. The power measured in this way takes into account all the mechanical losses, and for this reason, it is lower than the shaft horsepower; it provides, however, a more realistic indication of the actual motorcycle road performance.
