Horsepower or simply "HP" is the yardstick by which we measure a motorcycle's performance but we sometimes see other units such kW (for kiloWatt) and also PS (for pferdestärke). For starters the three different units of power measurement is used in different regions of the world, but let us delve a bit deeper into this, while leaving out the subject of torque for the future.
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The unit was actually created by James Watt, yes, the guy who improved then promoted his version of the steam locomotive engine.
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To make things easy in measuring the power of his locomotives, he deemed that one HP was equivalent to a horse moving a 33,000 lbs. (approximately 15000 kg) mass one foot in one minute; OR a horse lifting a 550 lb. mass 1 foot in 1 second. This way, he could directly compared the number of horsepower his locomotive produced, as the common way of moving freight and people during that period was by horse-drawn carriages.
However, the unit does not stipulate what type of horse or the size of the horse...
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Then there is also BHP, for brake horsepower. As the name suggests, an engine spinning at a desired rated is tested against a drum with water in it, for its braking force. However, manufacturers in Europe test this by attaching all the components driven by the engine, hence delivering a lower reading than in the US where they remove the ancillaries.
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As for WHP, the completed bike's rear wheel is placed on a dynamometer (dyno in short) and spun up. The readings will be some 10% lower than quoted by manufacturers as it accounts for the parasitic losses in the transmission, final drive and tyre contact patch. However, this is method measures the engine's useable power. In other words, real world horsepower.
As you can see, the horsepower unit is not uniform.
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The kW is the most uniform method of power measurement and is used by engineers worldwide. The Watt (W) is an SI (Système International in French meaning International Standard of Units) unit just like the metre, kilogram, et al. "Kilo" means one thousand, so 1 kiloWatt is 1000 Watts.
This unit measures the rate of energy transfer over time, which is what an internal combustion engine or electric motor does.
The conversion is 1 kW = 1.341 HP, or 1 HP = 0.7457 kW (745.7 W). So, if you see a spec sheet that says "20 kW," the engine produces 20 x 1.341 = 26.82 HP, which manufacturers round out to 27 HP.
Pferdestärke is a German term which combined two other words: Pferde meaning horses in plural, and stärke which translates to power. Thus, it simply means horsepower.
However, it is tweaked for the metric system, therefore doing away with foot and pounds. As such, it is calculated as the amount of power needed to lift a 75kg mass 1 metre vertically in 1 second. This is the standard power measurement for European vehicle manufacturers.
Applying the metric calculations results in 1.4 percent higher result compared to the imperial HP. Meaning, 1 PS = 0.986 HP, or 1 HP = 1.014 PS. Thus, a quoted 200 PS equates to 197.264 HP. But again, some manufacturers round it out to 200 HP.