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What the heck does % grade mean?

 
 
roger
 
Reply Fri 30 Dec, 2005 11:46 pm
I'm asking about the signs on the hills around here. 14% grade seems pretty steep. Anyway, it's steeper that 6% grade, but percent of what?
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Type: Discussion • Score: 1 • Views: 1,440 • Replies: 11
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cicerone imposter
 
  1  
Reply Fri 30 Dec, 2005 11:49 pm
It's not percent, but degree. Think in terms of a 45 degree angle.
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timberlandko
 
  1  
Reply Fri 30 Dec, 2005 11:52 pm
Actually, I believe it refers to number of units of rise per hundred linear units.
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roger
 
  1  
Reply Fri 30 Dec, 2005 11:52 pm
Okay, so 14% is 14% of 45 degrees? It looks steeper on a bicycle, but it sure isn't 14% of vertical.

That was quick. Thanks
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cicerone imposter
 
  1  
Reply Fri 30 Dec, 2005 11:58 pm
So for a grade that's ten feet high for a 100 feet distance, that's 10 percent grade.
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roger
 
  1  
Reply Sat 31 Dec, 2005 12:01 am
Why, how clever of them. Perish the thought they should just put "6 degree grade" on those helpful little signs.
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timberlandko
 
  1  
Reply Sat 31 Dec, 2005 01:30 am
I think its sorta mental inertia, rog - they've been doing it that way since the Romans were building roads.
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Phoenix32890
 
  1  
Reply Sat 31 Dec, 2005 06:02 am
roger wrote:
Why, how clever of them. Perish the thought they should just put "6 degree grade" on those helpful little signs.


Rog- It sounds much more "official" the way that they write it. You know how it is with government. The more obtuse the better! Laughing
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ossobuco
 
  1  
Reply Sat 31 Dec, 2005 08:52 am
Roger, you can measure the amount of slope of a piece of land by using either degrees or feet of rise divided by a given length.

So, if your road goes up or down 6 feet in a length of one hundred feet, that is 6%. The distance, that is, the one hundred feet, will be level, and the road will be a diagonal line, and the amount of rise or fall will be a vertical line. What you get is an isoceles triangle... as the length of the actual road will be the hypotenuse, somewhat longer than the level length.

You can also look at this on an axis diagram and measure it by degrees.
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ossobuco
 
  1  
Reply Sat 31 Dec, 2005 08:59 am
Handicap ramps, for example, are built to a maximum slope of 8.33% or 1 foot in 12. I don't know what degree that is, offhand.
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farmerman
 
  1  
Reply Sat 31 Dec, 2005 09:44 am
rog, a 14% grade is a hill of 8 degrees. I have this tangent function plate on my Brunton Compass (which is how the %is computed based on a 100% grade being 45 degrees)., and Ive never used the damn thing until you just posted your question. This Brunton is over 30 years old, I got it in college.
Since we do mostly drilling I always keep in mind that one degree of dip is 92 ft per mile. (That keeps us from missing our "vein" so to speak)
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roger
 
  1  
Reply Sat 31 Dec, 2005 10:47 am
Okay, I understand it now. I think Phoenix is right. After all, it is so easy to confuse obscurity with profundity. I've got some nifty Carr-Lane trig tables that will solve for this, but mostly, I was just curious about where the number was coming from.
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