TECH HOTLINE
Machinery: Horsepower is derived
from a math formula. It isn’t magic
and it isn’t the end-all be-all.
A HORSEY AND A MOO-COW Dear MXA, MXA always reports dyno numbers, but I don’t always know what they mean. Can you explain horsepower and torque to me so that I might understand? We can try, but it requires some basic math skills, which we will gloss over. Every motorcycle racer wants more horsepower. It is human nature to fixate on the easiest-to-understand concept and run with it. Thus, a bike that makes 58 horsepower, like the 2014 Yamaha YZ450F, must be better than one that makes 55 horsepower, like the 2014 Kawasaki KX450F. Right? Wrong! The Kawasaki actually has better power than the Yamaha. But, just because the numbers can be misleading, that doesn’t refute the need for an easy reference number that everyone can relate to. Perhaps in the future scientists will come up with a more precise measurement of how fast a motorcycle engine is, but whether they choose kilowatts, cheval vapeur or joules, it won’t matter if they can’t get the public to go along. Horsepower has a long tradition, and it will not go gentle into that good night. People won’t want to change from 50 horse-
power to 37,285 watts. We mention watts, because horsepower as a measure of power was coined by James Watts who wanted to market his industrial steam engines to replace horses in 1782. Watt determined that a horse could turn a standard 12-foot-radius grain-mill wheel 144 times in an hour (or 2. 4 times per minute). He then extrapolated a horse’s effort to the performance of his steam engine to be able to rate it by “horse power.” The ultimate calculation was that 1 horsepower was equal to 550 foot-pounds per second, or, in simpler terms, that a horse could lift 550 pounds over a distance of 1 foot in 1 second. These numbers could be converted easily to other units of measure. Thus, 1 horsepower also equals 33,000 foot-pounds per second, 745 watts, 0.645 kilowatts or 745 joule per second. The point isn’t what unit of power we use, but whether that unit can be used in science and industry as a reliable measurement of how powerful a machine is. It is a given that 1 horsepower equals 550 foot-pounds per second. The importance of the “per second” part of the equation is that it changes horsepower from a measurement to a calculation. The time factor means that we don’t actually measure horsepower; we measure the force exerted over a distance for a given time. The resulting number is called horsepower, but the force that is being measured is torque. The power of a Honda CRF450 engine is what is used to turn the rear wheel, and the twisting force needed to turn the wheel is torque. Torque can be measured in several different units, but we’ll stick to foot-pounds. What is a foot-pound? That’s simple. If you had a 1-foot- long wrench, attached it to your front axle, and applied 1 pound of pressure to the wrench, that would be 1 foot-pound of torque. Thus, torque is a twisting force measured in foot-pounds. Now, let’s move the 1-foot wrench to the end of the engine’s crank- shaft and put 1 pound of pressure on it. For the engine to turn that wrench and its applied foot-pound of pressure, the end of the wrench would move 6.2832 feet. The end result is 6.2832 foot-pounds of work done at 1 foot-pound of torque. But, what if the engine rotates the wrench at 5252 rpm? That would be 6.2832 times 5252 rpm. The result would be that the wrench would move 33,000 feet per minute. In simple terms, torque measures the force being applied, while horsepower is a measure of how much work the force can do. How do you apply all of this? Motorcycle tuners use a dyno to determine the power an engine produces. They do this by applying a load to the engine output shaft by means of a water brake, a generator, an eddy-current absorber, or any other controllable device capable of absorbing power. The dynamometer control system causes the absorber to exactly match the amount of torque the engine is producing at that instant, then measures that torque and the rpm of the engine shaft. From those two measurements, it calculates observed power. What the dynamometer is really doing, however, is measuring the torque output of the engine. In a vehicle, torque is measured at various engine speeds (rpm). These two numbers are fed into a formula—torque times rpm divided by 5252—to arrive at horsepower. Which is more important, torque or horsepower? Most motorcycle companies advertise the horsepower and torque that their engines produce. It seems, as usual, the bigger the numbers, the better. Torque is the base number for work, and horsepower is the rate of doing more work. Thus, one can’t exist, or even have meaning, without the other.
