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Calling all Engineers / Physicists: HP question

Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Calling all Engineers / Physicists: HP question

Old 02-24-06, 02:36 PM
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jsf252
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Hello all,
As a current Engineering student (Mechanical) and noob to road cycling, I am always noticing how or why things work. So I'm reading post after post and notice that WATTAGE is the unit of measurement for power on a bicycle.

Why Wattage? Wouldn't Horsepower work the same? I'm guessing its used because its easier to see when you improve from 400w to 450w, as apposed to 1.214HP to 1.225HP.

Any thoughts?

Josh
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Old 02-24-06, 02:38 PM
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Don't think so hard....go ride your bike.
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Old 02-24-06, 02:41 PM
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Watts are the standard SI unit of power. Most bike measurements are metric these days.

1 horsepower = 745.699872 watts
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Old 02-24-06, 02:44 PM
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brake horsepower, output horsepower?

Watts is Watts. Metric makes sense.
 
Old 02-24-06, 02:53 PM
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As a former Engineering Student (now graduate) I can share in your perspective, but as you are already aware...most applications of engineering use mixed units due to long histories of innovation. Cycling is almost exclusively metric - even though some basic (mostly frame tube) measurements are actually the metric equivalent of the Imperial Standard. Handlebar clamps and Seat Posts come to mind.

Yet we still refer to a steerer tube on a fork as a 1" or 1 1/8".

Being in industrial manufacturing all I can say is..... such is life
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Old 02-24-06, 02:58 PM
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Communication is most effective when you use units that most everybody is familiar with. Horsepower and watts are both units of power. But it has become fairly common to speak of rider's power output in watts. We all know Lance can average an output of about 500 watts, etc...


Generally, pounds is used on these forums for bike weight, but most bike-related figures are in SI units.
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Old 02-24-06, 03:02 PM
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I personally just want to see a larger number, it sounds more impressive... I climb that hill with an average of 400 watts. 1.2 hp just does not sound like much.

if you think about it, we like to have units that are whole numbers in the range of 1-1000, pressure is a great example... 1 atm=760 mmHg=101,000 Pa, but we do not commonly use Pa, instead we use kPa
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Old 02-24-06, 03:32 PM
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Popular convention.

How come I measure my pop cans in mL?
WHY mL? Why not L?

Someone mentioned percieved magnitude..
You sound more of a beast with 746 watts vs. 1 hp.
Same with pop bottles - you sound like you're getting more: 591 mL versus .6 L
As humans, we like to think we're all champions and getting our money's worth..

-simplyred
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Old 02-24-06, 03:34 PM
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yeah but in horsepower, at least you sound closer to gas powered trimmer or chain saw, but in Watts, my output is way too close to a 3 way lightbulb for comfort.
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Old 02-24-06, 03:34 PM
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Thanks for all the great info guys, makes more sense now. Unfortunately I have conflicting intrests, a love for bicycles, and a love for cars. So as a "car guy" I guess I like to see HP ratings as compaired to other modes of transportation.

Thanks again
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Old 02-24-06, 03:41 PM
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I believe you should use HP if you have 20", 24", 26" or 27" wheels. If you have 650c or 700c wheels you should use Watts. You shouldn't mix US Customary and SI units.
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Old 02-24-06, 04:49 PM
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Because James Watt was Scottish and if it's not Scottish it's CRAP!
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Old 02-24-06, 04:55 PM
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Because watts is a finer measurement, ie changes look bigger.
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Old 02-24-06, 05:04 PM
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Am I the only one that thinks HP is more badass than watts? Think about it. If you're putting out 1hp during a sprint, you can brag that you're as strong as a friggn horse. You try to brag that you're putting out 750W in a sprint, most'll say "wow, 10 lightbulbs? "
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Old 02-24-06, 06:08 PM
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400 watts sounds more impressive than 1.21 horsepower.
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Old 02-24-06, 06:09 PM
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Originally Posted by caloso
Because James Watt was Scottish and if it's not Scottish it's CRAP!
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Old 02-24-06, 07:12 PM
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A "real" physicist measures power in million-electron-volts-per-second.
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Old 02-24-06, 07:17 PM
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I work with a lot European companies and they report just about everything in watts. Whereas we use horsepower, BTU/hr, etc. Cycling is a euro sport, afterall.
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Old 02-24-06, 07:20 PM
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Originally Posted by SDRider
400 watts sounds more impressive than 1.21 horsepower.

Don't you mean .5364 HP?
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Old 02-24-06, 11:13 PM
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metric>english and if you really think you are 1HP, try hooking yourself to a horse to see who can outpull;-)
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Old 02-24-06, 11:24 PM
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You think dual systems are fun???

Try explaining the difference between 1st and 3rd angle projection on prints to my heros on the shop floor.

Darn Globalization!
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Old 02-24-06, 11:24 PM
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My dial operates only in watts. I would have to ride with a calculator figure how many HP to dial it up to equal 400W when dropping posuers.

T.J.
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Old 02-24-06, 11:43 PM
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Two point two one jiggah-whats!?!?!
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Old 02-25-06, 02:08 AM
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Because saying that you are producing 1.25234324 HP seems confusing to the average person. I guess Watt's are more efficient because you can say I am producing 400W of power, or 400J/s

Plus, R600DuraAce's beastly 400w of power wont sound so cool if he said, "I am producing .65214323hp)
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Old 02-25-06, 02:09 AM
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Originally Posted by SDRider
400 watts sounds more impressive than 1.21 horsepower.
1.21 horsepower sounds alot more impressive when you convert it to watts ;P
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