[AT] RE: Vintage tractor pulling advice/ horsepower

Bob Seith seithr at denison.edu
Thu Jul 14 10:45:14 PDT 2005


In high school physics, we were taught to use the figure "550 
foot-pounds per second" in calculations relating to horsepower. That 
works out to 33,000 foot-pounds per minute, same as cited here. We 
probably used the smaller quantity because we were studying fractional 
horsepower motors and such.

Bob Seith

Danny Tabor wrote:

>I found this article on the website Automath. It has
>never been clear to me how HP was determined. Hope
>others find it as interesting as I have.
>
>Danny Tabor
>
>What is Horsepower?
>
>
>How do you define horsepower? Ask a car enthusiast and
>most of the time you'll get a blank look, a shrug of
>the shoulders and maybe a guess along the lines of
>"What a horse can do!".
>
>That answer begs the question: What horse? A
>thoroghbred race horse that can carry the small weight
>of a jockey with a lot of speed, or a working horse
>that can pull heavy loads albeit slowly? Obviously
>there is a more precise answer. Car manufacturers,
>despite their reputation for being creative regarding
>the horsepower ratings of their products for marketing
>reasons, require a more stable definition.
>
>Horsepower definition Horsepower is defined as work
>done over time. The exact definition of one horsepower
>is 33,000 lb.ft./minute. Put another way, if you were
>to lift 33,000 pounds one foot over a period of one
>minute, you would have been working at the rate of one
>horsepower. In this case, you'd have expended one
>horsepower-minute of energy.
>
>Even more interesting is how the definition came to
>be. It was originated by James Watt, (1736-1819) the
>inventor of the steam engine and the man whose name
>has been immortalized by the definition of Watt as a
>unit of power. The next time you complain about the
>landlord using only 20 watt light bulbs in the hall,
>you are honoring the same man.
>
>To help sell his steam engines, Watt needed a way of
>rating their capabilities. The engines were replacing
>horses, the usual source of industrial power of the
>day. The typical horse, attached to a mill that
>grinded corn or cut wood, walked a 24 foot diameter
>(about 75.4 feet circumference) circle. Watt
>calculated that the horse pulled with a force of 180
>pounds, although how he came up with the figure is not
>known. Watt observed that a horse typically made 144
>trips around the circle in an hour, or about 2.4 per
>minute. This meant that the horse traveled at a speed
>of 180.96 feet per minute. Watt rounded off the speed
>to 181 feet per minute and multiplied that by the 180
>pounds of force the horse pulled (181 x 180) and came
>up with 32,580 ft.-lbs./minute. That was rounded off
>to 33,000 ft.-lbs./minute, the figure we use today.
>
>Put into perspective, a healthy human can sustain
>about 0.1 horsepower. Most observers familiar with
>horses and their capabilities estimate that Watt was a
>bit optimistic; few horses could maintain that effort
>for long.
>
>Although the standard for rating horsepower has been
>available for over 200 years, clever car manufacturers
>have found ways to change the ratings of their engines
>to suit their needs. During the famous horsepower wars
>of the 1960s, manufacturers could get higher figures
>by testing without auxiliary items such as alternators
>or even water pumps. High ratings backfired when
>insurance companies noticed them and started to charge
>more for what they saw as a higher risk. Manufacturers
>sometimes responded by listing lower horsepower
>figures, forcing enthusiasts to look at the magazine
>test reports to determine what was going on. In the
>early seventies the SAE (Society of Automotive
>Engineers) stepped in with standardized test
>procedures and the fiqures were more consistent.
>
>Between 1922 and 1947, the Royal Automobile Club used
>a horsepower rating that was the basis for an
>automobile tax. The horsepower of an engine was
>determined by multiplying the square of the cylinder
>diameter in inches by the number of cylinders and then
>dividing that figure by 2.5. Using this dubious
>method, What we know of as a 385 horsepower motor
>found in the Z06 Corvette would be rated at only 48.67
>hp!
>
>There is a metric horsepower rating, although it is
>rarely used. The two methods are close, with one SAE
>horsepower equal to 1.0138697 metric horsepower.
>
>One horsepower also equals 745.699 watts. This means
>that if you really want to confuse people, you could
>complain about the .02682 horsepower light bulb your
>landlord has in the hallway as opposed to the mundane
>20 watt measurement.
>
>
>
>
>		
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